Space Sci Rev
DOI
10.1007/s11214-012-9905-1
Characterization
and Calibration of the CheMin Mineralogical Instrument on Mars Science
Laboratory
David
Blake · David Vaniman · Cherie
Achilles · Robert Anderson · David
Bish · Tom
Bristow · Curtis Chen · Steve
Chipera · Joy Crisp · David Des
Marais ·
Robert T. Downs · Jack
Farmer · Sabrina Feldman · Mark
Fonda · Marc Gailhanou · Hongwei
Ma · Doug W.
Ming · Richard
V. Morris · Philippe Sarrazin · Ed
Stolper · Allan Treiman · Albert
Yen
Received: 15 November 2011
/ Accepted: 22 May 2012
© The Author(s) 2012. This
article is published with open access at Springerlink.com
D. Blake ( )
Exobiology
Branch, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA e-mail: david.blake@nasa.gov
D. Vaniman
Planetary Science
Institute, 1700 E. Fort Lowell, Tucson, AZ 85719-2395, USA
C. Achilles
ESCG/Hamilton Sundstrand,
2224 Bay Area Blvd., Houston, TX 77058, USA
R. Anderson · C. Chen · J. Crisp · S. Feldman · A. Yen
Jet
Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr.,
Pasadena, CA 91109-8099, USA
D. Bish · H. Ma
Dept. of
Geological Sciences, Indiana University, 1001 East Tenth St., Bloomington, IN
47405, USA
T. Bristow · D. Des Marais
Exobiology
Branch, MS 239-4, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA
S. Chipera
Chesapeake Energy Corp.,
6100 N. Western Ave., Oklahoma City, OK 73118, USA
R.T. Downs
Department of Geosciences,
University of Arizona, Tucson, AZ 85721-0077, USA
J. Farmer
School of
Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
M. Fonda
Space
Science Division, MS 245-1, NASA Ames Research Center, Moffett Field, CA
94035-1000, USA
M.
Gailhanou
Faculté des
Sciences et Techniques, Avenue Escadrille Normandie Niemen, Service 262, 13397
Marseille Cedex 20, France
Abstract A
principal goal of the Mars Science Laboratory (MSL) rover Curiosity
is to identify and characterize past habitable environments on Mars.
Determination of the miner-alogical and chemical composition of Martian rocks
and soils constrains their formation and alteration pathways, providing
information on climate and habitability through time. The CheMin X-ray
diffraction (XRD) and X-ray fluorescence (XRF) instrument on MSL will return
accurate mineralogical identifications and quantitative phase abundances for
scooped soil samples and drilled rock powders collected at Gale Crater during Curiosity’s
1-Mars-year nominal mission. The instrument has a Co X-ray source and a cooled
charge-coupled device (CCD) detector arranged in transmission geometry with the
sample. CheMin’s angu-lar range of 5◦ to 50◦ 2θ with < 0.35◦ 2θ resolution is sufficient to identify and
quantify vir-tually all minerals. CheMin’s XRF requirement was descoped for
technical and budgetary reasons. However, X-ray energy discrimination is still
required to separate Co Kα from Co Kβ and Fe Kα photons.
The X-ray energy-dispersive histograms (EDH) returned along with XRD for
instrument evaluation should be useful in identifying elements Z > 13 that are contained in the sample. The
CheMin XRD is equipped with internal chemical and min-eralogical standards and 27
reusable sample cells with either Mylar® or Kapton® windows
to accommodate acidic-to-basic environmental conditions. The CheMin flight
model (FM) instrument will be calibrated utilizing analyses of common samples
against a demonstration-model (DM) instrument and CheMin-like laboratory
instruments. The samples include phyl-losilicate and sulfate minerals that are
expected at Gale crater on the basis of remote sensing observations.
Keywords X-ray
diffraction · Mineralogy
· Mars
habitability · Mars
science laboratory · Planetary
science ·
Spacecraft instruments
1 Introduction
1.1 The Mars Science Laboratory Mission
The
overall science objective of the Mars Exploration Program for the Mars Science Lab-oratory
(MSL) mission is “To explore and quantitatively assess a local region on the
Mars surface as a potential habitat for life, past or present.” Specific
science objectives are: (1), to assess the biological potential of at least one
target environment identified prior to MSL or discovered by MSL; (2), to
characterize the geology and geochemistry of the landed region at all
appropriate spatial scales (i.e., ranging from micrometers to meters); (3), to
investigate
D.W. Ming · R.V. Morris
Lyndon B.
Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058-3696, USA
P. Sarrazin
SETI
Institute, 189 Bernardo Ave., Mountain View, CA 94043, USA
E. Stolper
MC 206-31,
California Institute of Technology, Pasadena, CA 91125, USA
A. Treiman
Lunar and Planetary
Institute, 3600 Bay Area Blvd., Houston, TX 77058-1113, USA
Table 1 Composition of |
|
|
|
|
|
|
|
Wt.% |
Keystone |
Keel Davis |
Watchtower |
Pequod |
Paros |
|
|
Watchtower-class rocks at
Gusev |
|
||||||
|
|
|
|
|
|
|
|
Crater (in oxide wt.%) as |
|
|
|
|
|
|
|
SiO2 |
46.9 |
45.2 |
42.4 |
46.0 |
46.6 |
|
|
determined by Alpha
Particle |
|
||||||
X-ray
Spectrometry (APXS) |
TiO2 |
1.96 |
1.94 |
2.21 |
1.92 |
1.37 |
|
(Gellert et al. 2006) |
|
||||||
Al2O3 |
13.61 |
12.07 |
12.33 |
13.1 |
13.73 |
|
|
|
|
||||||
|
Cr2O3 |
0.05 |
0.04 |
0.00 |
0.05 |
0.02 |
|
|
FeOa |
10.5 |
10.9 |
13.2 |
11.1 |
11.4 |
|
|
MnO |
0.27 |
0.22 |
0.22 |
0.20 |
0.17 |
|
|
MgO |
8.48 |
8.64 |
10.00 |
8.42 |
7.91 |
|
|
CaO |
6.36 |
6.71 |
7.44 |
7.13 |
6.45 |
|
|
Na2O |
3.44 |
3.60 |
2.67 |
3.48 |
3.42 |
|
|
K2O |
0.56 |
0.37 |
0.74 |
0.38 |
0.37 |
|
|
P2O5 |
2.41 |
2.51 |
4.50 |
2.83 |
2.31 |
|
|
SO3 |
4.15 |
6.43 |
3.34 |
4.29 |
4.97 |
|
|
Cl |
1.23 |
1.28 |
0.80 |
0.98 |
1.05 |
|
aAll
Fe is calculated as FeO |
Total |
99.42 |
99.91 |
99.85 |
99.88 |
99.79 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
planetary
processes of relevance to past habitability including the role of water; and
(4), to characterize the broad spectrum of surface radiation, including galactic
cosmic radiation, solar proton events, and secondary neutrons.
1.2 Science Goals of the CheMin Mineralogical
Instrument
The CheMin
(Chemistry and Mineralogy) instrument directly addresses the Mars Explo-ration
Program objectives by seeking to identify and characterize past or present
habitable environments as recorded in the chemical and mineralogical
composition of sediments and rocks. CheMin will principally address science
objective 2 above, but it will also support goals 1, 3 and 4. To meet these
objectives, CheMin will utilize X-ray diffraction (XRD) to perform
mineralogical analyses of rocks, sediments, dusts and soils to assess the
involve-ment of water in their formation, deposition or alteration, and to
search for potential mineral biosignatures, energy sources for life, or
indicators of past habitable environments.
1.3 The Importance of Mineralogy to MSL Science
Goals
Minerals
are natural crystalline materials of uniform inter-atomic structure and limited
ranges of chemical composition. The century-old technique of X-ray diffraction
(the oper-ating principle of CheMin) is exquisitely sensitive to that structure
and secondarily sensitive to chemical composition. Specific minerals can form
and persist only through limited ranges of temperature, pressure, and ambient
chemical conditions (i.e., humidity, water activity, Eh, pH, etc.). A material
of a given chemical composition (like a basalt) may be transformed to a wide
range of mineral assemblages depending on the physical and chemical conditions
that it experienced (Spear 1993).
Thus, knowledge of the minerals present in a sample provides insight into the
physical and chemical conditions under which it was formed or altered.
Tables 1 and 2 show the
elemental composition (measured by Alpha Particle X-ray Spec-troscopy (APXS);
Gellert et al. 2006) and iron mineralogy (measured by Mössbauer
spec-troscopy; Morris et al. 2006)
of basaltic rocks of the Watchtower class, investigated by the
Table 2 Iron mineralogy of Watchtower-class
rocks at Gusev Crater (in oxide wt.%) as determined by Mössbauer Spectroscopy
(Morris et al. 2006)
aFe3+ /FeT is required
for
norminative
calculations. Norms also assume equilibrium crystallization under anhydrous
conditions. Chemistry assumes no H2O/OH is
present
Mineral |
Keystone |
Keel Davis |
Watchtower |
Pequod |
Paros |
|
|
|
|
|
|
Pyroxene |
47 |
13 |
7 |
2 |
1 |
Olivine |
0 |
4 |
7 |
4 |
3 |
Garnet |
4 |
0 |
12 |
12 |
11 |
Magnetite |
10 |
9 |
0 |
0 |
0 |
Hematite |
15 |
40 |
31 |
14 |
18 |
Ilmenite |
8 |
8 |
3 |
6 |
2 |
npOx |
17 |
27 |
39 |
62 |
66 |
0.43 |
0.73 |
0.83 |
0.88 |
0.94 |
MER-A
rover Spirit at Gusev Crater. These rocks have nearly identical chemical
composi-tions (Table 1), but have wildly differing amounts of particular
iron-bearing minerals reflec-tive of changes of oxidation state (Table 2). The observed mineralogical changes are inferred
to represent progressive alteration and oxidation, something that is not
evident from com-positional data alone. The Mössbauer instruments on the MER
rovers are very sensitive to iron-containing minerals but indifferent to
others. On MSL, the CheMin instrument will be able to identify iron-containing
as well as iron-free minerals. With a knowledge of the full suite of minerals
present, a more complete story can be made of alteration and the history of
rock-water interactions on Mars.
1.4 Integration of CheMin with Other MSL
Instruments
CheMin is
one of ten instruments on the MSL rover. The other instrument housed with
CheMin in the analytical laboratory within the rover’s body is SAM (“Sample
Analysis at Mars”), a gas chromatography (GC)-mass spectrometry (MS)-tunable
laser spectrometry (TLS) instrument. Integration of CheMin and SAM analyses of
splits of the same samples will maximize the information obtained by combining
specific mineralogy with results from gas chromatography, mass spectrometry,
evolved gas analysis, and tunable laser spectrom-etry. In addition,
complementary quantitative chemical composition for elements Na and heavier
(via XRF-equivalent APXS analysis) will be collected on the raw or brushed
sur-faces of samples, including those spots drilled to collect powder for
CheMin and SAM. These analyses will be supplemented by hand-lens resolution
investigation of sample spots with MAHLI (Mars Hand Lens Imager, with up to 14
µm resolution) before and after brush-ing. Remote chemical analyses by the
ChemCam instrument using laser induced break-down spectroscopy (LIBS) will
provide guidance to appropriate sampling spots as well as data on elements that
will not be measured by either APXS or CheMin (e.g., H, C, N). All analyses
will be supported by high-magnification Remote Micro-imager (RMI) images
co-located with the ChemCam remote spot analyses, by mast-mounted camera
surveys, and by engineering camera data. CheMin data, comprising XRD patterns
suitable for quantitative analysis and elemental energy-dispersive histograms
(EDH) suitable for identification of el-ements present in the sample 13 <
Z < 42, are particularly synergistic with the APXS and ChemCam
analyses and the SAM GC-MS-TLS data. The challenge in integration of these
datasets will be in identifying and accommodating sample selection differences
between instruments, which are:
(1) CheMin will analyze only the < 150 µm
size fraction of drilled or scooped samples.
(2) SAM can analyze
splits of the same < 150 µm
material as CheMin, but it can also accommodate splits < 1 mm.
(3)
APXS
will analyze 1.7 cm diameter areas on rock or soil, either unmodified or
brushed.
(4)
ChemCam
will obtain rapid compositional data on spots up to ∼
7 m distant and ∼ 200 to 400 µm in diameter, with the ability to
raster across the sample surface, produce linescans, or to excavate several
hundred micrometers into a sample.
Because each MSL instrument has a
distinct analytical area, volume and dataset, mean-ingful integration of the
whole MSL dataset will require an understanding of these sampling issues. Dust
cover and surficial alteration that might not be penetrated by either ChemCam
or APXS is likely to have minimal influence on the deeper drill samples that
will be delivered to SAM and CheMin. Grain-size heterogeneity may result in
differences between < 1 mm
splits passed to SAM and < 150 µm
splits analyzed by CheMin. Viewing a sample at such different scales can be a
powerful method for improving understanding, but multiple types and scales of
analysis can be confusing. In order to address the sampling issue, the MSL
rover is equipped with an Observation Tray where splits of the sieved samples delivered
to SAM and CheMin can be analyzed by APXS and MAHLI. This permits chemical
analysis of both pre-drilled and drilled material, as well as allowing
high-magnification imaging of the sample to examine grain properties (UV
fluorescence, angularity, size range, color, and “hand lens” mineral
identification). In addition, samples on the Observation Tray can be examined
by day or by night (with either visible or UV illumination) to determine sample
stability over a range of temperatures.
1.5 CheMin Instrument
Overview
CheMin is part of the Analytical Laboratory of the
MSL rover Curiosity, located inside the main body of the rover. CheMin
will analyze powdered rock and soil samples delivered to it by the Sample
Analysis/Sample Processing and Handling (SA/SPaH) and Collection and Handling
for In-Situ Martian Rock Analysis (CHIMRA) systems. CheMin is capable of
analyzing up to 27 pristine samples and as many as 74 samples with reuse of
sample cells. The total number of analyses will be determined by “drive” vs.
“analyze” decisions made by the MSL Science Operations Working Group (SOWG) as Curiosity
progresses towards Gale Crater’s central mound. Each CheMin analysis can take
up to 10 hours of analysis time over two or more Martian nights, although some
samples may require only one night for analysis. CheMin measures the mineralogy
of crushed or powdered rock samples and/or soil through X-ray diffraction. The
geometry of CheMin is shown in Fig. 1.
During an analysis, a collimated
X-ray beam from an X-ray tube source is directed through powdered or crushed
sample material. An X-ray sensitive CCD imager is positioned on the opposite
side of the sample from the source and directly detects X-rays diffracted or
fluoresced by the sample (Fig. 1,
left). The CCD detector is operated in single-photon count-ing mode (the
detector is read out sufficiently often that most pixels contain either no
charge or charge derived from a single photon). When operated in this manner,
the CCD can be used to measure the amount of charge generated by each photon
(and hence its energy). Diffracted primary beam characteristic X-rays strike
the detector and are identified by their energy (for CheMin, with a Co X-ray
tube, Co Kα X-rays
having an energy of 6.929 keV are selected), producing a two-dimensional image
of Co Kα X-rays
that constitutes the X-ray diffraction pattern. At incremental radii this
pattern is summed circumferentially about the central nondiffracted beam to
yield a one-dimensional plot of 2θ versus
intensity compara-ble to conventional diffractometer data (Fig. 1,
upper right).
Fig. 1 Geometry of
the CheMin instrument. (Left) overall geometry of CheMin; (above
right) XRD 2θ plot obtained
by summing diffracted photons from the characteristic Kα line of the X-ray source (Co Kα is colored magenta in Fig. 1 (left); (below right) X-ray
energy-dispersive histogram (EDH) obtained by summing all of the X-ray
photons detected by the CCD (fluoresced photons from the sample shown
schematically in green and red in Fig. 1 (left)
From this diffraction pattern, one can (in theory) determine
the minerals present in a sample and their relative abundances. Each mineral
has a unique diffraction pattern, and the pattern of an unknown mineral or
mineral mixture can be identified by comparison with stan-dard patterns
compiled for wide ranges of natural and synthetic materials. Two such
com-pilations are from the International Centre for Diffraction Data (ICDD, Newtown
Square, PA) and the Crystal Structure Database (Downs and Hall-Wallace 2003). Abundances of minerals can be retrieved by
several methods that seek to model both diffraction peak posi-tions and
intensities. Among these methods are Rietveld refinement (Bish and Post 1993), the Reference Intensity Ratio (RIR) method (Chung
1974) and full-pattern fitting methods like FULLPAT
(Chipera and Bish 2002). The last is particularly useful because it is
well adapted for the processing of CheMin data products to quantify abundances
of both crys-talline and amorphous materials, a challenging task in remote XRD
analysis. The CheMin team includes several clay mineralogists familiar with
these and other methods for quantify-ing difficult phases such as
poorly-crystalline clay minerals and X-ray amorphous materials. The diffraction
data, including as-received and processed diffraction files, will be archived
in NASA’s Planetary Data System for public access, where researchers around the
world will be able to download, reprocess and reanalyze the data. The legacy of
CheMin opera-tions for MSL will be many years of improving mineral analysis as
new researchers and new methods revisit the CheMin data archive.
Some of the X-rays that impinge on the sample are absorbed,
and portions of their en-ergies are re-emitted (i.e., fluoresced) as
characteristic X-rays of elements in the sample (e.g. Ca Kα x-radiation). These fluoresced X-rays
provide information about the elemen-tal composition of the sample. These data
are compiled into a histogram of the number of CCD-detected X-rays versus X-ray
energy (Fig. 1, lower right). This energy-dispersive his-togram
(EDH) is used by to monitor CheMin’s performance (see Sect. 1.9)
and to select the energy window for diffracted Co Kα photons from the CCD, minimizing background
from fluoresced X-rays. Raw histograms of the intensities of fluoresced X-rays
are used to eval-uate CCD performance and to aid in mineral identification by
observations of the presence
or absence
of elements expected to be in the mineral(s). However, because the sensitivity
of the CCD detector changes as a function of time, temperature and radiation
damage during the mission, these data cannot be accurately quantified.
1.6 The CheMin Instrument
1.6.1 The X-Ray Source
CheMin
has a microfocus X-ray tube with a Co anode and a beam-defining final aperture
placed at some distance from the tube, which together produce a collimated
X-ray beam. The source produces continuum and characteristic X-radiation from a
50 µm diameter spot on the Co anode. The tube has a nominal operating voltage
of 28 keV with a filament current of 1.5 A and cathode output of 100 µA. The
photons emanate from the X-ray tube in a cone of radiation that exits the tube
through a beryllium window. At a specific distance from the tube, the pinhole
aperture intersects the beam, creating a nearly parallel, collimated ∼
70 µm diameter beam of X-radiation directed at the center of the sample cell.
After passing through the sample, the direct beam is stopped by a beam trap
mounted on the edge of CCD detector.
The X-ray power supply is enclosed in a housing pressurized with
a mixture of sulfur hexafluoride and nitrogen gas, mixed so that the
condensation temperature of the gas is below the minimum expected temperature
in the body of the rover (to ensure that the gas never liquefies).
1.6.2 The CheMin Sample Handling System
The
CheMin sample handling system consists of a funnel, a sample wheel (which
carries 27 reusable sample cells and 5 permanent reference standards), and a
sample sump where ma-terial is dumped after analysis (Fig. 2).
CheMin receives sieved and portioned drill powders and scoop samples from the
SA/SPaH system (Jandura 2010) through the CHIMRA drill, scoop, and sorting
assembly (Sunshine 2010). A maximum of 76 mm3 of sample material is
delivered to the piezoelectrically vibrated funnel system that penetrates
through the rover deck. When CheMin is not receiving samples, the CheMin inlet
is protected by a cover. The material received through the funnel passes into a
sample cell that consists of a 10 mm3 active cell volume and a ∼
400 mm3 reservoir above the cell.
Excess material, should it be delivered, will remain in the reservoir during
analysis. The funnel contains a 1 mm mesh screen to keep larger grains from
entering the CheMin sample handling system. Grains that cannot pass through the
screen will remain there for the duration of the mission, although no material
is expected because the sample is pre-sieved to < 150 µm in the CHIMRA
sorting chamber to prevent clogging of the CheMin funnel screen. Any grains smaller
than 1.0 mm but larger than 150 µm will pass into the upper reservoir portion
of the sample cell, where they will remain until the cell is inverted and they
are dumped into the sump. For the life-time of the mission, nominally one Mars
year, CheMin is required to accept and analyze material delivered from SA/SPaH
with no more than 5 % internal contamination between samples. Self-generated
contamination may originate from material that has remained in the funnel from
previously delivered samples (and delivered along with subsequent samples), or
from material that has remained in previously used analysis cells (CheMin will
be capa-ble of reusing each cell two to three times to accommodate 74 or more
analyses during an extended mission). Cells are emptied after an analysis by
rotating the sample wheel 180◦ (to invert the cell) and vibrating the cell
so that the sample material is emptied into a sump at the bottom of the
instrument. If contamination is suspected either from the funnel or from a
pre-viously used cell, CheMin can reduce sample-to-sample contamination by
dilution. Aliquots
Fig.
2 Schematic
diagram of the CheMin sample wheel. Individual sample cells can hold as much as
400 mm3, but only ∼
10 mm3 is required to fill the
active analytical volume of a cell. Twenty-seven reusable cells are available
for analysis of drilled or scooped samples sieved to <150 µm. Sample cells are
filled at the top and dumped at the bottom. Cells in red hold standard
materials that are accessible during the mission for calibration purposes
of sample
material can either be dumped into the funnel and delivered directly to the
sump through a shunt in the wheel without entering a sample cell (to entrain
and remove funnel contamination), or a previously used sample cell can be
filled, shaken and emptied to the sump prior to receiving a second aliquot of
sample for analysis (to entrain and remove sam-ple cell contamination). These
processes will require coordination with SA/SPaH to deliver more than one
aliquot of a given sample.
The collimated ∼ 70 µm diameter X-ray beam
illuminates the center of a sample cell having 6 µm thick Mylar® or 10 µm Kapton® windows. The sample
introduced into the funnel consists of ≤ 76 mm3 of powdered material with
a grain size of < 150 µm. Only about 10 mm3 of material is required to
fill the active volume of the sample cell, which is a disc-shaped volume with
an 8 mm diameter and 175 µm thickness. The remaining sample
Fig. 3 CheMin cell
geometry. (Left) Exploded view of dual-cell assembly, showing windows,
tuning-fork assembly and piezodriver. (Right) Assembled cell,
ready for testing (note yellow Kapton® window on
left, clear Mylar® window
on right)
material
occupies the reservoir above the cell (see Fig. 3).
During filling, analysis and sam-ple dumping, the sample cell is shaken by
piezoelectric actuators (“piezos”). The modes in which the piezos will be
driven are still under test and may vary from sample to sample, de-pending on
sample-dependent characteristics such as grain cohesion (e.g., clay-rich
samples versus samples that lack fine particles). In CheMin testbeds and
prototype instruments, the frequency of the piezo-actuator is varied so that
during a part of the cycle the sample holder is at resonance, at which time the
sample exhibits bulk convective movement similar to a liquid, delivering sample
grains in random orientation into the volume exposed to the beam. In the CheMin
Flight Model (FM) and the equivalent Demonstration Model (DM) a nomi-nal
resonant frequency of 2150 Hz is maintained during analysis, and shaking
amplitude is varied to adjust the intensity of grain motion.
During the moderate shaking that produces granular convection,
it is possible that phase separation will occur as a result of size, density or
shape differences between individual mineral grains. To mitigate this problem
CheMin at intervals uses episodically larger shaking amplitudes (i.e., “chaos
mode”) to re-homogenize the material in the sample chamber.
The CheMin sample cells are paired in dual-cell “tuning-fork”
assemblies with a single horizontally driven piezoelectric actuator in each
assembly (Fig. 3). Sixteen of the dual-cell assemblies are mounted
around the circumference of the sample wheel (Figs. 2 and 4).
Five of the cells will be devoted to carrying standards; the other 27 cells are
available for sample analysis and can be reused by emptying samples into the
sump after analysis. Cells are filled and analyzed at the top of the wheel. A
cover assembly sits directly above the sample cell during analysis to keep
material in the sample reservoir from being ejected during vibration. Inlets to
the cells holding standards are sealed with HEPA filters to prevent the
material from falling out as the wheel is moved, while allowing for pressure
equalization within the cells.
Both Mylar®- and Kapton®-windowed cells are mounted on the wheel. The
rationale for using two types of cell windows is based on our experience with
CheMin prototype instru-ments. Mylar® windows exhibit a very flat diffraction
background across the full range of 2θ . However MylarTM is less durable than
Kapton® under severe vibration and
is susceptible to destruction if highly acidic samples (e.g., copiapite; a
hydrated iron sulfate) are encoun-tered. What is known about mineralogy on Mars
suggests that acidic conditions are not unlikely (e.g., determination of
jarosite occurrences at both MER landing sites). Kapton® windows are more durable
and are not susceptible to acid attack, but they have a small diffraction
contribution at ∼ 6–7◦ 2θ that could interfere with
the 001 diffraction peak from
Fig. 4 The CheMin
sample wheel. The wheel is shown inverted in this picture. A shroud (gold
colored metal) covers the lower part of the wheel so that as
filled cells are inverted, material that falls out of the cells prematurely
will be directed into the sump
some clay
minerals. For security and to assure that CheMin can handle a wide range of
sam-ple types on Mars, both Kapton® windows
(in 13 cells) and Mylar® windows (in 14 cells) are used in
the reusable cells. None of the remaining 5 cells, loaded with standards, have
any concern related to clay-mineral diffraction so Kapton® is the
norm for standard cells, but one of the standards (amphibole), useful for both
XRD and EDH analysis, is loaded into a Mylar®-windowed
cell to provide access to at least one standard with this window design
throughout the duration of the mission.
Once data from an observation are sent to ground and accepted,
the analyzed material is emptied from the cell and that cell is ready to be reused.
CheMin does not have the capability to store previously analyzed samples for
later re-analysis once the wheel has been moved to receive another sample or to
analyze one of the standards.
1.6.3 The CCD Detector
CheMin uses a 600 × 1182 pixel E2V CCD-224
frame transfer imager operated with a 600 × 582 pixel data collection area (Fig. 5). Once the 600 × 582 pixel active area is exposed to X-ray photons
for a brief period (5–30 seconds), it can be rapidly transferred into a 600 × 600 pixel shielded area
(the “transfer frame”). This allows data collection to take place continuously
without the use of an X-ray shutter or electronic beam blanking. The pixels in
the active portion of the array are 40 × 40 micrometers square, and the active region of
deep depleted silicon is 50 µm thick. The front surface passivation layer is
thinned over a substantial fraction of the active pixel area. This imager is a
modern version of the E2V CCD-22 that was specially built for an X-ray
astronomy application (Kraft et al. 1994).
The large size of the individual pixels (relative to those present in
conventional CCD imagers) causes a greater percentage of X-ray photons to
deposit their charge inside a single pixel rather than splitting the charge
between pixels. The enhanced deep depletion zone results Characterization and
Calibration of the CheMin Mineralogical Instrument
Fig. 5 The CheMin
CCD. The active 600 × 582 pixel region
of the CCD is the exposed portion in the center. The frame transfer
region of the CCD is the shielded portion to the right of the active area. The
beam stop can be seen in the center of the upper edge of the active area (held
with two screws). The CCD is cooled with an active cryocooler that is attached
to the back of the CCD by a thermal strap. The CCD support/attachment hexapod
structure (seen below the CCD assembly) supports but thermally isolates the CCD
assembly from its attachment plate
in improved
charge collection efficiency for high energy X-rays. The thin passivation layer
makes the CCD sensitive to relatively low-energy X-rays (down to Si and in some
samples, down to Al). The frame transfer region of the CCD (the portion of the
CCD in Fig. 5 that is covered by a metal bar)
has much smaller pixels than the active region because they are only meant to
hold the charge associated with an X-ray photon, not to absorb it. An
individual pixel in the array can hold hundreds of thousands of electrons, many
more than the few thousand electrons deposited by a single X-ray photon. The
active portion of the CD has larger pixels because the charge cloud of
electrons that is deposited from an absorbed photon can be tens of microns in
size.
In order to keep the CCD from being exposed to photons in the
visible energy range (from X-ray induced optical fluorescence from the sample)
during analysis, a 150 nm Al film supported on a 200 nm polyimide film is
suspended on a frame placed in front of the detector. The detector itself is
cooled by a cryocooler to approximately 48 ◦ C below
the temperature of the Rover Avionics Mounting Platform (RAMP) that is used as
a heat sink by the cryocooler. The RAMP is expected to be between 0 ◦ C and +26 ◦ C during the
mission, yielding CCD temperatures of −48 ◦ C to −22 ◦ C. By
cooling, dark current in the CCD is reduced.
A number of interactions occur between the sample and the
X-ray beam to produce pho-ton fluxes detected by the CCD. The two interactions of
importance to the CheMin experi-ment are elastic interactions in which primary
beam photons are absorbed and re-emitted by sample atoms (i.e., diffraction
events), and inelastic interactions in which sample atom inner shell electrons
are ejected from the nucleus, resulting in the emission of a characteristic
pho-ton from the sample (i.e., fluorescence events). In X-ray diffraction,
elastic interactions of
Fig. 6 Left: Schematic
diagram of a vibrating sample holder for granular convection. Right:
Cu Kα diffrac-tion
pattern of the 75–150 μm size fraction of a crushed quartz crystal sample
in a laboratory instrument with convective sample holder. Upper: sample
with vibration (ICDD database intensity reference values shown by red
triangles); Lower: same sample without vibration
primary
beam photons with sample atoms constructively or destructively interfere,
resulting in maxima in discrete angular directions (“Laue cones” or “Debye rings”)
representative of the crystal structure of the sample. In fluorescence,
sample-generated photons are emitted uniformly in all directions and the
energies of individual photons can be used to identify the elements present in
the sample.
1.7 Refinement of CheMin Instrument Performance
1.7.1 Analysis of Grain Motion in Vibrated Sample
Cells
X-ray
diffraction analysis consists of the measurement of the directions and
intensities at which crystalline matter diffracts X-rays. Placing an individual
crystal in fixed orientation in a monochromatic X-ray beam will at most lead to
a single diffracted beam, and most likely no diffraction at all. To accurately
identify a crystalline phase, it must be exposed to the X-ray beam in all
orientations to record all possible diffracted beams within the angular range
covered. Powder XRD achieves this condition by using powdered materials—or
solid polycrystalline materials—to create a sample that offers all possible
crystalline orientations within the analytical volume. In laboratory powder-XRD
instruments, fine-grained samples (ideally <10 μm grainsize) offer a very large
number of crystallites in random orientations in the analytical volume.
Miniaturized XRD instruments have even more stringent grain-size constraints
because their analytical volume is scaled down. In an instrument like CheMin,
an ideal sample would have a submicron grain size, difficult to achieve by
grinding. When coarser than ideal powders must be analyzed, a means of
increasing crystalline orientation statistics is necessary. A common method
used in laboratories is to spin the sample in a thin glass capillary. For
CheMin, a new approach was developed based on granular convection of the sample
in vibrated cells. The analytical volume represents a small fraction of the
sam-ple cell volume (∼ 1/6000) but the internal flow of material through
granular convection ensures that the entire sample is analyzed in random
orientation over time (Fig. 6). Gran-ular
convection is obtained by vibrating the sample holder to fluidize the powder.
Gravity combined with the interaction of the powder with the oscillating cell
walls leads to con-vective flow. In addition to allowing quality XRD to be
obtained with large grain sizes (up
to 150
μm), the vibrations facilitate sample insertion and removal. Conventional
methods of XRD sample handling would have required CheMin to be fitted with a
grinder, a cell filling mechanism and possibly a cell spinner, increasing
mechanical complexity, cost and risk. The benefits of the convective cells in
producing XRD data suitable for quantitative analysis have been demonstrated in
a number of CheMin precursor and laboratory analog instruments.
A convective sample holder must ensure an intense vibration in
the cell, thousands of m s−2 (hundreds
of g) being typically required to obtain reliable convection. This is achieved
by using a balanced mechanical resonator similar to a tuning fork. A sample
cell is integral to each arm of the tuning fork, and both cells are vibrated at
the same time. The balanced design ensures minimal transmission of the
vibration to the structure of the instrument; the energy is kept in the
resonator. The vibration is induced by a piezoelectric actuator placed at the
base of the tuning fork and driven at the resonance frequency of the assembly.
The intensity of vibration is pulsed at about 1 Hz. A short period of intense
shaking ensures that convection is initiated and a period of lower intensity
maintains the motion while preventing the bed of powder from expanding and
losing bulk density which would be detrimental to the XRD measurement.
Granular convection is a well-known phenomenon that finds few
practical applications. It has been studied principally with reference to large
beds of mm-sized particles, but little was known about the physics of granular
convection in the micrometer range of the CheMin samples, or in the thin bed
geometry of the CheMin sample cell. Granular flow in CheMin cells was modeled
by Dr. C. Campbell (USC) using discrete particle computer simulations, and
studied empirically by Particle Image Velocimetry (PIV). In PIV, contrast
features (not necessarily individual grains—in fact the grains can be below the
resolution of the obser-vations) are identified and tracked from frame to frame
in a sequence of captured video frames of particle motion. Vectors are drawn
between the features to calculate granular flow velocities and bulk powder
movement.
Both PIV and discrete particle computer simulations were
carried out to better understand granular flow phenomena and predict the
operation of the system at Mars surface conditions (lower gravity and lower
pressure). The study of XRD data quality versus convection speed demonstrated
that the velocity of convection has no real influence on quality of diffraction
as long as the bed of powder is kept moving constantly. A pause in the motion
will cause large grains to stay in a diffraction condition for an extended
period of time, resulting in bright diffraction spots on the detector that
anomalously increase the relative intensity of the corresponding XRD peaks. The
main requirement for granular convection therefore, is that is must be robust.
Using PIV measurements, the velocity of granular flow was
evaluated as a function of shaking conditions. It was found that above a
specific vibration amplitude threshold, granu-lar velocities are linear with
amplitude for a given frequency (Fig. 7-left).
The onset thresh-old for granular convection is reduced at higher frequencies
(Fig. 7-right).
The granular flow model was used to evaluate the effect of
Mars gravity. Simulations run under Earth (E) and Mars (M) gravity (Fig. 8) show that at slightly higher velocities, flow patterns
at Mars gravity are equivalent to those at Earth gravity. This is at odds with
what is known about conventional granular convection in large beds of
particles, for which the vibration required for a given velocity scales
linearly with gravity. This difference in behavior is due to the predominance
of grain-wall interactions in the thin cells of the CheMin instrument, as
opposed to the predominance of inertial effects in thick beds.
The effect of gravity was also studied
experimentally by making PIV measurements of granular convection in a vibrated
cell aboard a Piper Cherokee aircraft while performing
Fig. 7 Left: PIV
velocity averaged over entire cell as a function of peak-peak shaking amplitude
(960 Hz), Right: Plot of vibration amplitude threshold necessary
to initiate convection, as a function of frequency
Fig. 8 Granular
flow pattern simulations at Earth gravity (E, left) and Mars
gravity
(M, right)
showing similar flow patterns and slightly higher velocities at Mars gravity
a series
of mini parabolic flights over the Pacific Ocean. Periods of 3–4 s at Mars
gravity could be obtained. Slightly faster convection was observed under Mars
gravity conditions. It was shown that only a small reduction of the vibration
amplitude is required to maintain the same convection velocity from Earth to
Mars gravity. For example a 30 μm peak-to-peak vibration at Earth gravity
is equivalent to 25–28 μm at Mars gravity. Practically speaking,
therefore, granular convection in Earth gravity is roughly equivalent to
granular convection in Mars gravity.
The effect of the reduced atmospheric pressure on Mars was
studied with the model and tested in a chamber at Mars pressure under dry
conditions. No substantial effect could be associated with reduced pressure,
although the transition from non-dry Earth conditions to low-pressure Mars
conditions substantially slowed the convection. The transient state is due to
water adsorbed on grains that increases the grain-grain cohesion when the
pressure in reduced. Once the adsorbed water is removed, dry samples showed
convection similar to that observed at ambient conditions.
The effect of electrostatic charging on granular flow under
dry Mars conditions was also evaluated. A dedicated study was conducted by both
modeling and experimental measure-ment of granular convection in a vibrated
cell placed in a Mars atmospheric chamber. PIV measurements were made and
charge accumulation on the windows measured. It was ob-served that granular
flow initially creates a voltage on the windows by tribocharging, but this
charge does not accumulate or persist. Most likely the charge is transported
away by grain flow. PIV measurements showed no influence of this charge on
granular convection. Based on this research, convective flow in vibrated sample
cells on Mars should be similar to that observed on Earth.
Vibration is known to cause grain
segregation when grains vary in size, density, shape or some other feature.
Grain segregation in the CheMin vibrated cells is a concern because the
analysis is performed in a very small volume in the middle of the cell.
Preferential settling of a phase in any region of the cell would affect the
measurement of mineral composition. Segregation in vibrated beds has been
observed in particular when vibration amplitude is re-duced to the point the
convection no longer happens or happens very slowly, finer or denser particles
are allowed to settle in lower regions of the cell. With higher vibration
levels, the convective flow mixes the materials inside the cell at higher speed
than segregation occurs, resulting in a cancellation of the effects of
segregation in most cases. Segregation is still observed in the presence of
convection in some extreme cases such as prepared mixtures of coarse grained
materials with substantially different density (example: quartz +
corun-dum). In order to limit this bias, the vibration profile includes a
periodical shaking at very high amplitude for periods of a few seconds. This
intense shaking is referred to as “Chaos mode” for the fast unstructured motion
resulting in the cell, and has the effect of mixing and homogenizing the
material. A case of apparent segregation in a beryl-quartz calibration sample
data set from the flight instrument is presented in Sect. 4.2.1, thought to be due to the lack of chaos mode
pulses during this particular measurement.
Another possible bias in the measurement
of mineral composition is agglomeration of particles in the cell. This is
observed at the edges of the cell in particular when the sample contains fines.
The agglomerates are wedged between the polymer windows where they can least
flex. These agglomerates grow over time as additional particles are jammed into
them. As an example, crushed sandstone is likely to show a preferential
agglomeration of the fine-grained cement matrix phases at the edge of the
window, resulting in an apparent enrichment of the coarse sand particles
(quartz) in the middle of the cell where the analysis is performed. The chaos
mode also proves effective at limiting this agglomeration as long as the
intense shaking pulses are applied frequently to break agglomerates in an early
state of formation. If agglomerates are allowed to grow and compact over tens
of minutes or hours, their compactness and resulting cohesion can make them
difficult to break up and disperse with the chaos mode.
While segregation and
agglomeration are of concern, methods have been found to mit-igate or cancel
them, and such methods have been validated by accurate quantitative anal-yses
obtained with CheMin derived XRD instruments used at NASA Ames Research
Cen-ter, NASA Johnson Space Center, Jet Propulsion Laboratory, Los Alamos
National Lab-oratory, and Indiana University (see for example, Bish et al. 2008; Blake et al. 2009;
Treiman et al. 2010; Achilles et al. 2011; Taylor et al. 2012).
A good metric to evaluate the presence or absence of either segregation or
agglomeration is to monitor the measured composition as a function of time
during an analysis. Selective segregation or agglomeration progressively
increases the proportion of one or more specific phases during an analysis. In
such cases, more accurate compositional data are obtained at the beginning of
the analysis, assuming the cell has not been shaken for an extended period
between loading and start of the analysis.
1.7.2
Optimization of the CheMin Geometry for X-Ray Flux and 2θ Resolution
The
first optimization of the CheMin X-ray geometry from an XRD performance
standpoint was performed using a dedicated ray-tracing program called CheminRay
that can simulate diffraction geometry based on the following components:
(5)
A microfocused X-ray tube with adjustable spot
size. The spectral characteristics of this source include Kα1 , Kα2 and Kβ lines with their specific peak width, shape
and relative intensity, and bremsstrahlung radiation.
(2)
An elliptical (or circular ) pinhole with an
adjustable radius and thickness or a slit colli-mator with adjustable size (x–y) and
distance to the X-ray source.
(3)
A flat sample of a perfect randomly oriented powder
in transmission with adjustable angle to the X-ray beam, distance to the
collimator, thickness, compactness and compo-sition (mineral or mixture of
minerals). The interaction with the powder is calculated as a “reflectivity”
(probability of diffraction) for each photon, as it depends on its energy and
direction. When a photon is diffracted its path through the sample is modeled
and an absorption probability is calculated. Any photon may be diffracted,
transmitted or absorbed.
(4)
A CCD of adjustable dimensions, pixel size,
angular orientation to the direct beam, and x, y position relative to the direct beam. An energy
range can be selected taking into
account
the energy resolution capability of the CCD (allowing simulation of Kα data, for example).
All parameters can be set
manually to simulate a specific geometry. Up to 3 parameters can be set as
variables, scanned within a grid to study a range of configurations, and any
set of parameters can be used as variables for optimization with a genetic algorithm
ap-proach. The data products of CheminRay include a 2-D image representing the
distribution of diffracted photons on the CCD and a 1-D diffractogram resulting
from the circumferential integration of intensity around the diffraction rings.
An automatic line fitting tool imbedded in the application produces XRD peak
intensities, peak shapes and peak positions which can be used for the
optimization.
CheminRay is a virtual
prototyping and testing tool. The advantage of ray tracing over other modeling approaches
is that numerous parameters can be taken into account, yielding a numerical
simulation with high fidelity to the real-world experiment. A drawback however,
is that such simulations are time-consuming because as with real experiments
the signal to noise ratio depends on the number of photons used in the model,
hence the amount of computation time.
The starting point of the
geometry optimization was the CheMin III instrument (a pre-cursor to CheMin IV,
the principal testbed of the CheMin flight instrument) available in the
laboratory. The accuracy of CheminRay was first verified by comparing
experimental data from CheMin III to simulated data of the CheMin III geometry
as shown in Fig. 9. The CheMin III collimator
design was then optimized using both a genetic algorithm and a grid scanning
approach. Genetic algorithms apply techniques inspired by evolutionary bi-ology
to optimize a result by evolution from generation to generation of a population
of potential solutions. In each generation, the fitness of the whole population
is evaluated and individuals are selected and combined with a fitness-dependent
probability to form a next generation of parameters. The formulation of
“fitness factors” and “evolutionary rules” is critical in achieving an actual
parameter optimization. The objective of the optimization of CheMin was to
achieve the best resolution with the highest flux intensity. A number of
fit-ness factor definitions were tested based on a mathematical combination of
resolution and intensity, but all definitions proved to be too restrictive,
leading to a particular throughput / resolution compromise. The genetic
algorithm was modified to maximize the throughput of classes of XRD resolution
as defined as the maximum FWHM observed in the XRD pattern. Computed
populations of configurations were then reported in throughput versus
resolution. After selecting a target resolution, the optimum configuration (in
this case, a refinement of pinhole diameter and distance to the source) was
obtained from the best configurations in the corresponding resolution class.
Studies utilizing a grid scan led to similar results. The optimized geometry,
which results in a resolution of 0.3◦ 2θ was chosen for implementation
Fig. 9 Experimental
Kα 2-D
patterns from the CheMin III instrument (left) compared to CheminRay
results for the same geometry (right). The patterns differ mostly
in background intensity and residual Kβ rings
present in the experimental data
Fig. 10 Comparison
of performance of CheMin III (blue) and optimized CheMin IV geometry (red)
using CheminRay simulations. Left: computed XRD pattern of
Jarosite showing a 20× increase in integral intensity
for the CheminRay optimization. Right: Resolution curve for the
optimized geometry. While CheMin III offers higher resolution at low 2θ angles, for minerals, such high resolution at
low 2θ is not required. The chosen
optimization is based on high photon throughput with a flat resolution curve
across 2θ
in the
CheMin IV instrument. A 20× increase in the XRD throughput
(X-ray flux) was cal-culated compared to the CheMin III geometry, with a
uniform 2θ resolution
along the full range of 2θ (e.g., see
Fig. 10).
Similar optimization approaches with more parameters were
taken to explore the po-tential of different instrument parameters for the
flight instrument. In particular, optimized geometries were proposed based on a
dual X-ray tube layout as specified in the early stage of the flight instrument
development. After the second X-ray source was descoped, an in-strument
geometry was proposed based on a slight evolution of the CheMin IV design,
having a single X-ray tube, and sample and CCD detector both perpendicular to
the beam. CheminRay was used to study the effect of parameters such as pinhole
diameter or sample thickness on resolution, intensity, peak asymmetry and peak
shift. Ray tracing results were backed by experimental verification of selected
geometries using a dedicated breadboard instrument based on commercial grade
components.
Once the geometry of the system was established, a second
model was developed based on a mathematical description of the diffracted
intensity at the CCD taking into account all critical parameters of the
geometry. Unlike the ray tracing model which could virtually ex-plore all
possible configurations but required extensive computation time, this model
limits its scope to the chosen geometry through a number of approximations, but
requires far less computation time. The output of the mathematical model was
verified with ray tracing re-sults as well as experimental data from breadboard
instruments. This model was a critical engineering tool for the design of the
flight instrument, allowing detailed investigation of the effect of small
variations of design parameters on the resulting patterns. The model was used
primarily to link engineering requirements to MSL level II science objectives.
Small but significant adjustments were made to the geometry using this tool.
1.7.3 Fundamental-Parameters Modeling of Profiles
Produced by CheMin
Line profile fitting is the basic approach to
obtain phase contents and structure information from X-ray powder diffraction data.
Typically, two methods are used for line profile fit-ting: Constrained or
unconstrained analytical profile fitting, and the fundamental parameters
method.
In the first method, empirical instrument profile functions
(mathematical functions) are used to simulate the observed diffraction pattern.
In some cases, these mathematical func-tions are convolved with other empirical
descriptions of instrument- and sample-related con-tributions to generate a
simulated diffraction pattern. The disadvantage of this method is that
refinable variables relating to the instrument profile functions have no clear
physical mean-ing; they are simply empirical mathematical functions. Profile
parameters resulting from a refinement are generally not sensible, even though
refinements converge well, giving accept-able residual factors. In such cases,
it is often common to apply mathematical constraints to the refinement to
obtain reasonable results.
In the case of fundamental parameter descriptions of profiles,
all of the geometrical pa-rameters of the entire instrument are mathematically
described and are convolved together. In this case, results of refinement have
definite physical meanings. By convolving the indi-vidual instrument parameter
functions together, an instrument profile can be generated, and if these
functions accurately describe the instrument, the simulated instrument profile
can be very accurate (as determined through the use of profile standards such
as NIST SRM 660a LaB6 ). This instrument profile can
then be convolved with sample-related profiles to simu-late the experimental
diffraction pattern. Generally the individual instrument contributions to the
observed profile are well known, for example, the diffraction geometry, slits,
radius, etc., employed in a Bragg-Brentano powder diffractometer. Some
instrument-related param-eters are not precisely known, and refinement of these
within a limited range can ultimately provide instrument functions that have
physical meaning, in contrast to the analytical profile functions used in the
empirical line profile fitting method. As a result, structure and phase
information resulting from a refinement are more realistic and accurate.
The geometry of the CheMin instrument is shown schematically
in Fig. 1. X-rays are generated by a Co
radiation source, they are shaped by a collimator, and they are ultimately
diffracted by the sample. The instrument profile for this configuration can be
simulated by convolving the radiation source spectrum of Co with a function for
the beam size aberration, a function for the collimator, and a function
accounting for sample thickness. The source emission profile is generated by
convolving seven Co Kα lines and
six Co Kβ lines
with a Lorentzian function (Hölzer et al. 1997).
The Kβ lines in the diffraction
pattern originate from radiation leakage (e.g., Kβ photons that have lost charge (“split
events”) and fall into
Fig. 11 X-ray paths
in the sample for normal-beam-transmission diffraction. S is the cross sectional area of
the X-ray beam after being shaped by the pinhole collimator. Parameter t is sample thickness, X is the distance the X-rays travel before
diffraction, and d is the
path of the diffracted beam in the sample. Parameter dx is the thickness of the volume element, and 2θ is the diffraction angle. The total diffracted
intensity at any angle 2θ is the integration of the volume element over
the sample thickness
the Kα energy window), and the intensities are thus allowed
to be refinable, maintaining their relative intensities. The finite X-ray beam
size effect is approximated by a Gaussian function rather than an impulse
function (Young 1993), and the collimator is
described by a circular function.
Absorption
Correction for the Normal-Beam-Transmission Diffraction Experiment The diffraction
geometry for the CheMin instrument is a normal-beam-transmission geome-try. As
shown below, diffraction intensities vary systematically with diffraction angle
2θ for this instrument
geometry resulting from sample absorption (which is not the case in
Bragg-Brentano geometry). Thus an absorption correction must be applied to
obtain ac-curate diffraction intensities for Rietveld refinement, in order to
provide reasonable phase contents and structural details. With reference to
Fig. 11, I0 is the
diffraction intensity of a unit volume sample at angle 2θ under the condition of zero absorption, S is the cross-sectional area of the direct beam at
position x in a
sample of thickness t, x and d are the path lengths of the X-ray in the sample
before and after diffraction, and μ is the
linear ab-sorption coefficient of the sample. The diffracted intensity from the
volume element Sdx with diffraction angle, 2θ , is:
dI
=
I0
e −μ(x+d)Sdx
=
I0
Se−μ[x+(t
−x)
sec 2ϑ ]
dx (1)
The total diffracted
intensity at diffraction angle 2θ is the integration of dI at the same angle over the
range of sample thickness x ∈
[0,
t ].
I2ϑ |
|
t dI |
|
t I0 Se−μ[x+(t −x) sec
2ϑ ] dx |
|
|
|
|
= |
0 |
|
= |
0 |
|
|
|
|
I Se−μt
sec 2ϑ |
|
|
|||
|
= μ(01 |
− |
sec 2ϑ ) ₃1 − e −μt
(1−sec 2ϑ ) |
(2) |
|
||
|
|
|
|
|
|
|
Relative
intensities are used instead of absolute intensities in Rietveld refinement, in
both the analytical profile fitting and fundamental parameter method, and
therefore the intensities at diffraction angle 2θ can be normalized to the intensities at 2θ = 0:
I2θ =0 = I0
Ste −μt (3)
The normalization
factor for the intensity correction is
f |
|
I2ϑ |
|
eμt
(1−sec 2ϑ) − 1 |
(4) |
|
2ϑ = |
I2ϑ =0 |
= μt
(1 − sec 2ϑ ) |
|
|||
|
|
|
Beryl-Quartz
Mixture Results Diffraction data were obtained in the CheMin FM instru-ment
for a 97 % beryl, 3 % quartz mixture by integration of the Debye rings on the
2-D diffraction image. Intensities were corrected for sample absorption
according to Eq. (4). During Rietveld
refinement, data in the range 5–8◦ 2θ were excluded to remove the con-tribution
from the Kapton® window. Data above 53◦ 2θ were also excluded, as data in this region
originate from the edges of the CCD detector and resultant Debye rings are
poorly represented. The quality of the fit for the four minor peaks at high
diffraction angles is com-paratively poor, probably for the same reason. Both
structural parameters and instrumental parameters were refined using the FM
instrument diffraction data and the fundamental pa-rameters method. Sample linear
absorption coefficients, sample thickness, and sample pack-ing factors were
also refined. The refinement converged to a weighted profile or agreement
factor (“Rwp”) of 12.6 %. Beryl is the major phase in the sample and quartz is
only a mi-nor phase, with a refined wt.% of 1.9. These data were also fit using
the analytical profile fitting method, using the popular modified
Thompson-Cox-Hasting pseudo-Voigt function (Young 1993).
The refinement converged to a larger Rwp factor of 18.2 %, with a large dif-ference
between calculated and experimental intensities, particularly for the first
beryl peak. Although the phase content of quartz resulting from this refinement
(2.6 %) is more accurate than the result obtained with the fundamental
parameters method, the fit quality is signif-icantly inferior and it is not
possible to obtain information on sample-related broadening effects. For this
reason, the fundamental parameter approach will be used in the refinement of
the CheMin instrumental broadening function and the analysis of CheMin data.
1.8 Measurement of the Energies of Individual
Photons
The
CCD-224 directly detects individual X-ray photons that are absorbed by the
active sili-con, producing a number of electron-hole pairs equal to the energy of
the X-ray in electron volts, divided by 3.65 (the energy of an electron-hole
pair in the silicon structure). For exam-ple, a Co Kα X-ray with an energy of 6.93 keV will
produce 1,899 electron-hole pairs. XRD and XRF data products are described in
1.9 below. The X-ray fluorescence requirement was descoped from the CheMin
instrument for technical and budgetary reasons. However, en-ergy discrimination
is still required for the energy binning that will segregate Co Kα and other energies critical to XRD. Although
of secondary importance to the energy resolution of primary Co X-rays,
qualitative elemental information from sample-generated secondary X-ray
fluorescence will also be important for supporting mineral identification by
pinpoint-ing the elements to be included or excluded in mineral search/match
routines.
1.9 XRD and XRF Data Products
In order
to retain energy information from individual X-ray photons, it is necessary to
op-erate the CCD in “single photon counting mode.” This is accomplished by exposing
the CCD to the X-ray flux for only a brief interval between read cycles. During
a nominal 5–30 second single-frame exposure, the likelihood is low that more
than one photon (character-istic, Bremsstralung or fluorescent) will be
collected in any single pixel of the array (most of the nearly 360,000 pixels
will contain background). Raw data consist of 600 × 582
ar-rays that store charge collected from individual CCD frames. The data are
stored as “Digital
Fig. 12 2-D diffractograms
from the Gore Mountain amphibole (obtained from the CheMin Flight Model (FM)
during ThermoVac testing). Left: A single 30-second frame of raw data
containing all detected photons (characteristic and continuum radiation from
the X-ray tube plus sample-generated secondary fluorescence). Middle:
Minor frame of energy-selected Co Kα data,
summed from 200 single frames. Right: Major frame of
energy-selected Co Kα data,
summed from 7 minor frames
Numbers”
or “DN” that are transformed to energy (in keV) subsequent to downlink. During
a CheMin analysis, individual CCD images are stored in memory (up to a total of
2,730 images or frames of 600 × 582 pixels). Ideally, all
raw frames would be transmitted to ground for processing. However the data
volume is too large for this to be feasible. The raw CCD frames are therefore
processed into a number of higher-level data products by the Rover Compute
Element (RCE) and selected data products are subsequently transmitted to
ground. However, a small number of individual raw frames will be downlinked to
monitor background before the X-ray tube is powered on, at intervals during the
analysis, and after the X-ray tube is powered off. These raw frames will be
utilized to assess the health of the CCD over time and to choose DN values
suitable for background and high and low DN limits for energy-selected
diffraction products (such as CoKα). If for whatever reason,
the background or high and low limit DN values have changed between analyses or
even be-tween individual minor frames, the quality of the higher-level data
products will have been compromised. In this event, the correct DN values can
be determined on the ground from the raw frame data and uplinked to the spacecraft
on a following Sol. The full complement of raw frames from an analysis are
retained in CheMin’s flash memory and can be repro-cessed by the RCE using the
correct DN values, and a corrected set of higher-level data products can be
retransmitted to ground. Ten to 200 raw frames are collected and processed by
the RCE to produce “Minor Frame” data products (described below). Five
to 20 minor frames collected within a single sol, typically at night, are
referred to as a “Major Frame.” A complete analysis of an individual
sample, which may require one or more sols, is called an “Observation.”
Major frames and Observations are assembled on the ground from down-linked
Minor Frame products.
Figure 12 shows
data obtained from the CheMin FM instrument during ThermoVac test-ing.
Background-subtracted single raw frames (Fig. 12,
left) contain all photons detected by the CCD during a single 30-second
exposure. Various minor frame products are gener-ated from raw frames by the
RCE and transmitted to ground. Figure 12
(middle) shows an energy-selected Co Kα data product, summed from ∼
200 single frame images. Figure 12 (right)
shows a major frame product, assembled on the ground from 7 minor frame images.
Minor frame and major frame 1-D patterns are constructed on
the ground by summing the images circumferentially around the central beam,
resulting in a 1-D diffractogram similar to that obtained by conventional XRD
instruments (e.g., Fig. 13). Phase
identification and quantitative analysis are performed on the 1-D data.
Fig. 13 1-D
diffractograms from the Gore Mountain amphibole. At incremental radii 2-D
patterns as in Fig. 12 are summed
circumferentially about the central nondiffracted beam to yield a
one-dimensional plot of 2θ versus
intensity comparable to conventional XRD data. Left: Minor frame Co Kα diffractogram. Right: Major frame Co Kα diffractogram. Intensity in arbitrary units
CheMin’s XRF detector was descoped from the instrument due to perceived
cost and risk factors. However, limited XRF data from the CCD detector will be
provided to the extent they are available. Figure 14
shows single frame, minor frame and major frame XRF data from the Gore Mountain
amphibole. The instrument is only sparingly sensitive to elements below atomic
number 19 (K) as a result of sample self-absorption, the aluminized light
shield, and Mars atmosphere absorption. However, even the limited XRF data
provided by the CCD detector will be useful in the discrimination of mineral
phases.
“Fully
Processed” Data “Fully processed” data are comprised of 600 × 582
two-dimensional (2-D) arrays of energy-selected Co Kα and Co Kβ X-rays,
plus a one-dimensional (1-D) Energy-Dispersive Histogram (EDH). The integer
value in a particular x, y position in
a Co Kα or Co Kβ array represents the number of Co Kα or Co Kβ
X-rays that were detected in the corresponding x, y pixel of the CCD array (a DN value in the array
is considered to represent a Co Kα or Co Kβ photon if its background-subtracted value
fits within upper and lower values ground-specified for Co Kα or Co Kβ events).
Co Kα and Co Kβ diffraction events can be treated separately;
in practice the Co Kα pattern is
signifi-cantly stronger and will be the primary product. The 2-D patterns are
transformed into 1-D diffractograms on the ground by the CheMin science team by
summing circumferentially about the central beam. The resulting 1-D patterns
are normalized for number of pixels ver-sus 2θ and
normalized to account for variable arc length. The energy histograms provide
performance-check information on CCD function and may be used to aid in
constraining search/match procedures in XRD analysis. Several ground-selectable
Minor Frame products that utilize the energy-discriminating capability of the
CCD can be calculated by the RCE and transmitted to ground. These include:
“XRD energy-selected mode”:
If the background-subtracted DN value in an x,
y
pixel falls between ground-specified low and high values, a 2-D counting number
array is incre-mented by one at that x,
y
location in the counting number array. The result is a 2-D array that
represents all of the detected X-ray events that lie within a specified energy
window. Windows may be chosen for the energies of Co Kα and Co Kβ photons
(these constitute Co Kα or Co Kβ diffraction patterns); as noted above the
general case will be selection of Co Kα. Two
further refinements of this mode can be selected:
Fig. 14 XRF
Energy-Dispersive Histograms (EDH). Left: Single frame EDH from
Gore Mountain amphi-bole. Right: Minor frame EDH, sum of 200 single
frame EDH. Middle: Major frame EDH, sum of 7 minor frame EDH. Elements
detected from the sample include Fe, Ti and Ca. Cobalt originates from the
X-ray tube, argon from the 7 Torr argon atmosphere of the test chamber, silicon
from self-fluorescence of the CCD detector and aluminum from the CCD light
shield, and only in part from the amphibole
“Single pixel mode”: If the
background-subtracted DN value in an x,
y
pixel falls between ground-specified low and high values, and nominal
zero values are present in the four pixels immediately above, below, right and
left of the x, y pixel of interest,
a 2-D counting number array is incremented by one at that x, y location in the counting number array. This mode
differs from the general case in that X-ray “split pixel” events, in which
charge from a single photon is shared within two or more adjacent pixels, will
not be counted. Single pixel mode Co Kα patterns
have fewer peak artifacts in their patterns because in normal energy-selected
mode, Co Kβ photons
which split a small amount of charge into an adjacent pixel may be counted as
Co Kα photons.
“Summed Split Pixel mode”:
If the background-subtracted sum of the DN values of an x, y pixel plus the four pixels immediately
above, below, right and left of the x,
y
pixel of in-terest falls between ground-specified low and high values, a 2-D
counting number array is incremented by one at that x, y location in the counting number array. This mode
attempts to count single pixel events as well as split pixel events. Summed
split pixel mode patterns will maximize the number of Co Kα counts while removing Co Kβ photons which have split a small amount of
charge into an adjacent pixel.
“XRF (all photons) mode”:
All of the background-subtracted DN values in the 600 × 582 CCD
array are summed into a 1-D histogram of DN value vs. number of counts. This
array amounts to an energy-dispersive histogram (EDH) of the sample, plus
characteristic
and Bremsstrahlung photons from the X-ray tube.
XRF all photons mode EDH have a higher background and poorer energy resolution
due to split pixel events that broaden the energy distribution of observed
peaks. Two further refinements of this mode can be ground-selected:
“XRF single pixel mode”:
Background-subtracted DN values from each x, y pixel that have nominal
zero values in the four pixels immediately above, below, right, and left of the
pixel of interest are summed into a 1-D histogram of DN value vs. number of
counts. This array amounts to an energy-dispersive histogram (EDH) of the
sample, plus char-acteristic and bremsstrahlung photons from the X-ray tube,
excluding split pixel events. XRF single pixel mode EDH provide for the lowest
background and highest energy reso-lution because split pixel events, which
broaden the energy distribution of observed peaks, are removed.
“XRF split pixel mode”:
Background-subtracted DN values from each x,y pixel plus the DN values in the
four pixels immediately above, below, right and left of the pixel of interest
are summed into a 1-D histogram of DN value vs. number of counts. This array
amounts to an energy-dispersive histogram (EDH) of the sample, plus
characteristic and bremsstrahlung photons from the X-ray tube, including
reconstituted split-pixel events. XRF split pixel mode processing will provide
the maximum count rate but will provide poorer resolution because of split
events, which when summed, show some energy losses relative to the charge
deposited by the incoming photon.
“Modified
Raw” Patterns Modified raw patterns are produced by background
subtracting a frame, setting CCD pixels below a specified DN value to
zero and run-length encoding the data. The DN cutoff can be near zero or just
below the most useful energies (Fe Kα to Co Kβ ). Following downlink, the 2-D patterns are
reconstructed, transformed into 1-D diffractograms, and treated as in the fully
processed mode.
“Film-Mode”
Patterns In film mode, background values are subtracted from each frame
and the DN values in the frame are then summed into a 600 × 582 counting
number ar-ray. The resulting Minor Frame acts similarly to a piece of
photographic film, detecting all photon events, including diffraction,
fluorescence, bremsstrahlung, etc. This data product maximizes the number of
counts that can contribute to the pattern, but background can be raised
substantially if the sample exhibits strong fluorescence. There is a higher
background associated with film-mode patterns and interpretation of film-mode
diffraction is compli-cated by presence of both Co Kα and Co Kβ
characteristic diffraction, as well as diffraction from tungsten L-line
radiation that is produced as the cathode of the X-ray tube sputters onto the
anode (this problem increases in severity as the tube ages). An additional
complication arises when poor grain movement in the sample cell results in
“spottiness” in the pattern due to multiple photon detections in the same pixel
from grains that remain in the diffrac-tion orientation. Indeed, film-mode
images are used to diagnose grain movement problems if anomalous peak
intensities are suspected in 1-D patterns. The 2-D patterns that are
down-linked in film mode are transformed into 1-D patterns as in the previous
data types but the multiple diffraction energies produce a pattern that is more
difficult to interpret uniquely. Mixed intensities and multiple peaks for each
d-value can preclude accurate phase identi-fication or quantification in some
samples. Film mode generally will not be used because of these problems and
because of the high and irregular background, which degrades detec-tion limits
and removes any capability of detecting and quantifying amorphous components.
However, under certain circumstances (e.g., extensive neutron damage from the
RTG) the energy-dispersive function of the CCD may degrade to the point where
separation of Co Kα
Table 3 Critical source and detector
flight requirements for the CheMin flight instrument
Parameter |
Source and Detector |
|
|
|
Characteristics |
|
|
|
|
|
|
2θ range |
5–50◦ 2θ |
|
|
2θ resolution |
≤ |
0.35◦ 2θ |
|
|
|
|
|
Operating
voltage |
28 keV |
|
|
Total flux |
5.6E5 |
|
|
CCD energy range |
1–15 keV |
|
|
CCD energy resolution @
6.93 keV |
≤ 250 eV |
|
and Co Kβ XRD patterns is not possible. In such cases,
XRD will still be possible in film mode.
Once a 1-D plot of 2θ versus intensity is obtained, standard
methods of analysis for labo-ratory XRD data are applicable. The baseline
approach for MSL XRD data will be to use the mineral identification utilities
in programs such as Jade® (MDI, Pleasanton, CA), TOPAS
(Bruker Corp.), and Xpowder® (Martin 2004), typically using ICDD or AMCSD library data and
operator knowledge to identify minerals based on analysis of the 1-D
diffraction pattern. Qualitative chemical information obtained from the EDH can
be used to include or exclude certain elements, refining the search. Following
identification of all minerals detectable in the sample, mineral abundances
will be quantified. Quantification will be ac-complished using either a program
such as FULLPAT (Chipera and Bish 2002) or
Rietveld methods (reconstruction of the full 1-D pattern from fundamental
crystallographic proper-ties of all phases present). The presence of amorphous
material will be determined from broad scattering reflecting the radial
distribution properties of the amorphous material.
2 CheMin
Science and Measurement Requirements
2.1 CheMin
Science Requirements
The overarching science requirement of the
CheMin instrument is that it be able to “. . . utilize X-ray Diffraction to
establish the mineralogy of rocks and soils, allowing the MSL science team to
infer the formation and alteration histories of samples acquired by the
mission.” The CheMin measurement objectives are to provide: (1), mineral
identification and quantitative mineralogy of rock and soil samples utilizing
powder X-ray diffraction; (2), qualitative elemental composition of rocks and
soils utilizing energy dispersive histograms (EDH); and (3), analyses of as
many as 74 samples supplied by SA/SPaH CHIMRA during the nominal one-Mars-year
surface mission.
2.2 CheMin
Measurement Requirements
The CheMin measurement
requirements are listed below. Critical source and detector re-quirements
necessary to achieve these measurements are shown in Table 3.
•
Detection limit of 3 wt.%.
“The CheMin instrument shall have the ability to achieve a detection
limit of better than 3 wt.% for crystalline phases (demonstrated by showing a
minimum detection limit (MDL) of less than or equal to 3 weight percent
abundance of quartz in a quartz-beryl mixture).”
The
rationale for this requirement is based on MER A (Spirit) observations of
min-eralogy at Gusev Crater (Ming et al. 2006; Morris
et al. 2006, 2008).
With a 2 % detec-tion limit, CheMin would not detect halides (NaCl, KCl, MgCl2 ), indicators of
ground-water action or deposition by acid fog (∼ 2 % abundance at Clovis
outcrop). With a 5 % detection limit, CheMin would not detect goethite (FeOOH),
a positive indicator of basalt altered by water at oxidizing conditions (∼
5 % abundance at Clovis outcrop) and hematite (Fe2 O3 ), a probable indication
of aqueous alteration (∼ 4 % abundance at Paso Robles outcrop). With a 10
% detection limit, CheMin would not detect sulfates (gyp-sum/anhydrite and
magnesium sulfates), positive indicators of aqueous alteration and soil forming
processes (Clovis outcrop).
With a 3 % MDL, the phases
observed by CheMin are sufficient to identify the suite of sulfates at Clovis
as well as phosphates, with local information bearing on outcrop vs. soil
origins. In concert with APXS results, inferences could be made about the
halide associations.
•
Measurement accuracy of 15 % relative and
precision of 10 % relative. “The CheMin instrument shall have
the ability to determine the abundance of crystalline mineral phases present in
the sample at 4 times the MDL to an accuracy of 15 % relative (1σ ) and a precision of 10 % relative. This
will be demonstrated by determining the abundance of quartz in a quartz-beryl
mixture having 12 wt.% quartz (4 times the required MDL).” This test is
specifically written for the FM, but extended calibration using the DM will use
a broad suite of natural and synthetic mineral mixtures of known abundance.
The rationale for this
requirement is as follows: 15 % relative accuracy and 10 % rel-ative precision
for the measurement of 12 % quartz in a quartz-beryl mixture is represen-tative
for many mineral mixtures and is a realistic goal for quantitative XRD in
polyphase mixtures. However, the presence of amorphous or X-ray amorphous
materials, or poorly crystalline materials such as clay minerals and
Fe-Ox-hydroxide phases is expected to reduce both the precision and accuracy of
analyses. This level of accuracy and precision in mineralogical measurements
will be critical for mass-balance calculations of both ig-neous and sedimentary
systems on Mars. There is an extensive literature of mass balance
determinations for soils and sediments based solely on chemical (XRF or APXS)
data using “assumed” mineral mixtures; a quantum improvement in these
calculations will be attained when both mineral presence and abundance are
constrained.
•
Distinguish minerals at abundances above
detection limits in a rock matrix. “The CheMin instrument
shall, with an integration time of 10 hours, have the ability to distinguish
unique minerals at abundances above detection limits in a rock matrix (e.g.,
calcite, gyp-sum and jarosite in an evaporite; apatite in a basalt). This will
be demonstrated by mea-
surement of the full-width at half-maximum (FWHM)
intensity showing angular (2θ )
resolution of better than or equal to 0.35◦ over the 2θ range from 24◦ to 45◦ (Co Kα), for
quartz and beryl peaks in a
mixture of 12 wt.% quartz and 88 wt.% beryl.”
The rationale for this requirement can be
illustrated using a model of the complex mineralogy thought to be present in
Peace Outcrop, Columbia Hills (case 2 from Table 5
of Ming et al. 2006) on the basis of seven
minerals modeled from APXS data. The mod-eled mineralogy includes forsterite,
augite, labradorite, magnetite, chlorapatite, kieserite and gypsum. In Fig. 15 (upper) the principal lines for these seven minerals
are shown over the 2θ range detectable by the CheMin instrument.
There are clearly many close or overlapping lines in the important range from
30◦ to 45◦ 2θ . Figure 15 (lower) shows the same stick diffractogram of 7
minerals, but with patterns that have been given artificial resolutions ranging
from 0.30◦ to 0.60◦ 2θ . Clearly, the decrease in
2θ resolution yields cases in which some minerals will not be
identified in the analysis. Figure 16 shows the
Fig. 15 (Upper):
Positions of the major diffraction peaks of 7 minerals inferred from
APXS data to be present in the Peace Outcrop, Columbia Hills by Ming et al. (2006), over the 2θ range
detected by the CheMin instrument. Positions and intensities of the peaks were
obtained from the ICDD powder diffraction database. Lower: as above, but
showing the effect of instrumental 2θ
resolution on modeled patterns. Resolution varies from 0.30◦ to 0.60◦ 2θ
Fig. 16 A portion
of the diffractogram shown in Fig. 15 (lower),
showing the region between 30◦ and 32◦ 2θ . As 2θ resolution
is degraded below 0.30◦ 2θ , the ability to identify kieserite is lost.
The presence of kieserite is suspected but not confirmed if the FWHM is worse
than 0.35◦ 2θ
range from 30◦ to 32◦ 2θ , a region in which labradorite, kieserite,
apatite and augite peaks overlap. Although the kieserite (−111) peak
is still identifiable at 21.5◦ 2θ , identification of the kieserite triplet at
30◦ to 32◦ 2θ is lost as the FWHM goes from 0.35◦ to 0.40◦ . The
presence of kieserite is suspected but not confirmed if the FWHM is worse than
0.35◦ . The
presence of kieserite vs. gypsum in an otherwise igneous rock has implications
for the activity of activity of water, and the salinity of the hydrous fluid
involved.
•
Provide an energy-dispersive histogram (EDH)
with specified energy range and resolu-tion. The CheMin instrument
shall have the ability to return an energy-dispersive his-togram (EDH) over the
0–15 keV range with energy resolution sufficient to separate Fe Kα, Co Kα and Co Kβ . This requirement can be met by analysis of
an Fe-containing amphibole standard for 10 hours showing energy resolution of
250 eV at Fe Kα or Co
Kα.
The rationale for this requirement
is as follows: The full 0–15 keV energy range is needed for characterization of
split events and to diagnose the quality of sample cell vibra-tion by
characterization of the incidence of double Co Kα photon detections. A FWHM energy resolution < 250 eV is a first priority requirement, needed to
separate diffracted primary Co Kα photons
from fluoresced Fe Kα (iron will
be present as a major element in virtually all samples). As the FWHM of the
detector increases above 250 eV, there will be an increase in the number of Fe
Kα photons that are included
in the energy window chosen for Co Kα detection.
This will result in a decrease in the peak/background ratio in
Fig. 17 (Left): 2-D image of diffraction
pattern from a synthetic clay-bearing evaporite sample. The direct beam
is at the lower center of the image; 2θ increases
radially with distance from this point. Right: 1-D 2θ diffractogram derived from the 2-D image. The
low angle detection capabilities of the CheMin IV instrument (which match the 2θ range and 2θ resolution
of the CheMin flight instrument) are capable of detecting the basal spacing of
all known natural phyllosilicates
diffraction patterns, and a degradation of the
minimum detection limit. As a second level priority, while XRF analysis has
been descoped, a simple determination of the presence or absence of an element
will greatly aid in search/match mineral determinations.
2.3 List of Available Flight, Demonstration, and
Testbed Instruments and Components
In
addition to the Flight Model (FM) and Demonstration Model (DM) CheMin
instruments, several prototype and testbed instruments or components have been and
are being used dur-ing the calibration and characterization process. These are
discussed below with a brief description of how each will be used in
calibration and characterization.
2.3.1 CheMin IV Testbeds
CheMin
IV instruments, fitted with a 1200 × 1152 pixel E2V 5530 CCD
rather than the CCD-224 used in the FM and DM, but having comparable 2θ resolution, are currently
op-erational at Ames Research Center, Jet Propulsion Laboratory and Johnson
Space Center. These testbeds were used for early evaluation of candidate FM
standards and evaluation of quantitative XRD performance using synthetic rocks,
and are presently in use for the devel-opment of an XRD pattern library
appropriate to the CheMin configuration. For example, Fig. 17 and Table 4 show
CheMin IV results obtained from a synthetic mixture of phyllosil-icate and
evaporite minerals. In Fig. 17 (left), the ∼
15 Å d-value from nontronite (7◦ 2θ ) is seen as the first
spatially distinct bright ring of intensity around the undiffracted beam (a
silhouette of the beam stop is seen at the center of the lower edge of the
image). Fig-ure 17 (right) shows the
conventional 1-D diffractogram that results from summing the 2-D pattern
circumferentially around the central undiffracted beam. Table 4 shows a quantitative analysis of this sample using
the program FULLPAT. These results and others were used to establish and
validate the required mineral detection level, analytical precision and
accuracy of mineralogical results from the flight instrument.
Table 4 Quantitative XRD analysis
of a synthetic clay-bearing evaporate (CheMin IV data)
Phase |
Measured |
Known |
relative |
Comment |
|
wt.% |
wt.% |
difference, |
|
|
|
|
wt.% |
|
|
|
|
|
|
Nontronite |
69 |
61 |
13.1 |
within 15 % |
Gypsum |
22.6 |
25 |
−9.6 |
within 15 % |
Halite |
2.8 |
8.7 |
−67.8 |
not
within 15 % |
Hematite |
5.7 |
5.3 |
7.5 |
within
15 % |
|
|
|
|
|
2.3.2 Inel Testbed
A commercial
Inel MPD® laboratory XRD (Inel, Inc., Artenay, France) with
a position-sensitive detector is housed in the Planetary Mineralogy and
Spectroscopy Laboratory at Ames Research Center. The instrument is configured
to analyze Mars analog rocks in a diffraction geometry nearly identical to the
CheMin flight instrument. The instrument is equipped with a monochromated Co
source and a 120◦ position-sensitive
detection system capable of collecting XRD patterns with resolutions in excess
of the spacecraft instrument (but which can be degraded to MSL CheMin
resolution for comparison and pattern match-ing). A Mars atmosphere chamber is
installed with a 12-sample carousel, MSL-style funnel and a CheMin transmission
sample cell that can be filled, piezoelectrically shaken during analysis and
dumped many times, all under Mars pressure. The Inel instrument has been used
to measure the candidate FM standards under Mars atmosphere conditions, to
measure specimen-to-specimen contamination in tests of sample cell reuse, and
to measure “real” rocks under simulated Mars conditions, whether those rocks
are obtained from SA/SPaH tests or from field collections intended for the
CheMin XRD library. There are significant differences between the experiments
as they were conducted on the Inel instrument for val-idation and verification
(V&V) of requirements, and as they will be conducted on either the MSL DM
or FM instruments. These differences include:
(1)
The Inel instrument was operated at 10 mA and 30
keV, yielding a much higher count rate than the MSL instrument.
(2)
The Inel machine utilizes a doubly monochromated
source, so there is no Bremsstrahlung or Co Kβ radiation
in the beam.
(3)
The
actual diameter of the beam at the sample is ∼ 300 μm, much larger that
in the MSL instrument. The distance from the sample to the detector is larger,
so that the instrumental resolution is not critically dependent on beam
diameter.
(4)
The piezo is driven by a controller that ramps its
frequency through resonance in ½– 5 second cycles. At 60-second intervals, the
controller pulses the piezo at resonance at a higher excitation to produce a
“chaos mode” in the sample holder. The CheMin FM piezos are driven by a
controller that locks onto sample cell resonance and varies amplitude during an
analysis.
There are
similarities between the experiments as they were conducted on the Inel XRD for
V&V, and as they will be performed on the CheMin DM or FM instruments.
These similarities principally involve the sample cell and the environment in
which the sample is analyzed. Of all of the CheMin testbed instruments
available (with the exception of the DM), only the Inel instrument can collect
diffraction patterns utilizing an MSL cell in a 7 torr atmosphere. The
principal use of the Inel machine is to study, evaluate and understand grain
motion during diffraction. The following experiments were conducted in the Inel
testbed during V&V.
Table 5 Sequential analyses of a 65
mm3 aliquot of
88:12 beryl:quartz mixture (by weight) to test for sample homogeneity over
time. Analyses performed under Mars atmospheric pressure, using a CheMin FM
sample cell. Sample size = 65 mm3
Cumulative Analysis time |
Beryl % |
Quartz % |
|
|
|
30 minutes |
90.6 ± 1 |
09.4 ± 1 |
60
minutes |
87.6 ± 1.2 |
12.4 ± 1.2 |
90
minutes |
88.2 ± 1.2 |
11.8 ± 1.2 |
120
minutes |
89.8 ± 1 |
10.2 ± 1 |
150
minutes |
90.6 ± 1 |
09.4 ± 1 |
Average |
|
10.64 |
Std. Dev. |
|
1.39 |
Rel. Accuracy |
|
11.33 % |
Rel. Precision |
|
13.05 % |
|
|
|
|
|
|
|
|
|
Table 6 Sequential analyses of a 45
mm3 portion of
88:12 beryl:quartz mixture (by weight) to test for sample homogeneity over
time. Sample size =
45 mm3 |
||
|
|
|
Cumulative Analysis time |
Beryl % |
Quartz % |
30
minutes |
87.7 ± 1.2 |
12.3 ± 1.2 |
60
minutes |
87.2 ± 2.6 |
12.8 ± 3.3 |
90
minutes |
88.4 ± 2.6 |
11.6 ± 2.6 |
120
minutes |
89.3 |
10.7 |
Average |
|
11.85 |
Std. Dev. |
|
0.79 |
Rel. Accuracy |
|
1.25 % |
Rel. Precision |
|
6.66 % |
|
|
|
|
|
|
Precision
and Accuracy of Quantification of Mineral Mixtures Beryl and
quartz standards were crushed and sieved to < 150 μm
grain size. Mixtures were made of 88 % beryl:12 % quartz (by weight), and 65 mm3 aliquots
of the mixture were loaded in the sample carousel and kept at 7 Torr atmosphere
for several days. Samples were delivered through the funnel into the sample
cell and analyzed to obtain a series of measurements of the same sample over
several hours, to see if phase segregation (and observed changes in
composition) had occurred. Quantitative analyses were obtained with the MDI
program Jade® , using Rietveld refinement. Table
5 shows the results of five analyses, obtained
after 30, 60, 90, 120 and 150 minutes of accumulated data.
Delivery to
CheMin of Less than the Specified Quantity of Sample Material A second
test was performed with the Inel instrument to evaluate the effect on
quantification (if any) caused by the delivery of less than the nominal 76 mm3 volume of
a 12:88 quartz:beryl mixture. Table 6 shows the
results of replicate analyses of the beryl:quartz sample, collected over
several hours, using a 45 mm3 portion (2/3 fill).
Sample-to-Sample
Contamination Tests An additional requirement of the CheMin sample handling
system is that there should be no more than 5 % contamination between
successive analyses. A test was performed to evaluate the sample-to-sample contamination
when suc-cessive samples are analyzed. Two sources of contamination
exist—material that remains in the funnel and is delivered to the sample cell
with a subsequent sample, and material that remains in a sample cell after it
is dumped, contaminating a subsequent sample when that sample cell is reused.
There is no sample shunt in the Inel testbed as there is on the CheMin
Table
7 Results
of sample-to-sample contamination test
|
Beryl % |
Quartz % |
|
|
|
No dilution |
99.69 |
0.31 |
1 dilution |
99.76 |
0.24 |
2 dilutions |
99.89 |
0.11 |
|
|
|
sample
wheel (see Fig. 2), so that funnel-generated
contamination cannot be evaluated sep-arately from sample cell-generated
contamination.
Using the
sample carousel, the Inel sample cell was loaded with powdered quartz (half < 45 μm
and half 45–150 μm) and run for sufficient time to acquire a major frame.
The
quartz
was then emptied and a load of beryl (half < 45
μm and half 45–150 μm) was an-alyzed. This was followed by two
separate experiments in which one and two purge loads of beryl were loaded,
vibrated for ten minutes and dumped, followed by analysis of a post-purge beryl
load to test for < 5 % quartz contamination. If
quartz was detected, the amount present was quantified using Rietveld
refinement. It was found that the % contamination of quartz in beryl decreased
as additional purges of beryl were applied. All three analyses passed the < 5 %
requirement (Table 7).
2.3.3 Mini-CheMin “Terra” Testbeds
Several
Terra® instruments (a commercial spinoff instrument of
CheMin, manufactured by inXitu, Inc., Campbell, CA) are in use at Ames Research
Center, Jet Propulsion Laboratory, Los Alamos National Laboratory, Johnson
Space Center and Goddard Space Flight Cen-ter. They are battery-operated,
field-portable instruments designed with enough fidelity to the CheMin FM to
allow practical evaluation of how the CheMin design functions in field
situations and for the development of XRD libraries. The Terra®
instrument at GSFC is be-ing used for direct comparisons of mineral and rock
analyses between SAM and CheMin testbeds. The Terra instrument at JSC is likewise
being used for direct comparisons of min-eral and rock analyses between CheMin
and the ChemCam testbed at LANL.
2.3.4 Prototype 1 Funnel-Wheel-Sump Assembly and
Stand-Alone Funnel Assembly
A test assembly (“Prototype
1”) consisting of the CheMin funnel, a sample wheel with mounted cells and
piezovibration actuators was used to evaluate sample handling and grain motion
properties of the CheMin sample handling system. The assembly does not include
the dust shroud that will cover the bottom part of the wheel or the sump as in
the FM and DM. The entire assembly was operated in a Mars-atmosphere chamber
(pumped to 2 torr and back-filled with 7 torr CO2 ) at cold temperatures
(approximately −40 °C); samples were loaded
into a carousel above the assembly for delivery to the cells on the sample
wheel. The goal of these tests was to determine the sample loading, movement,
and sample dumping characteristics of the CheMin design using a range of
materials expected on Mars or selected to test sample handling for extremes of
size, cohesion, hydration, hardness and electrostatic properties. Samples used
in these tests are listed in Table 8. A Column
marked with an “X” denotes the particular characteristic that is being
investigated with a specified sample.
In addition
to the Prototype 1 assembly, a separate funnel assembly was used to evaluate
funnel functions. The funnel assembly was used to examine grain movement and
amount of remnant contamination with a variety of samples. The stand-alone
funnel was tested at
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Table 8 Samples for CheMin |
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sample |
fine |
cohesion |
platy or |
hard |
electrostatics |
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Funnel-Wheel-Sump
assembly |
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size |
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acicular |
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or magnetics |
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tests, and properties
basis for |
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selection |
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JSC
MARS-1a |
X |
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arkose |
X |
X |
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basalt (Saddleback) |
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X |
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quartz |
X |
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X |
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kaolinite |
X |
X |
X |
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a JSC MARS-1 is a natural |
hematite (red ocher) |
X |
X |
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palagonite
sample provided by |
smectite |
X |
X |
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NASA-Johnson Space Center |
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angles up
to the maximum tilt design for CheMin FM operation (up to 20◦ with the
rover parked). The funnel assembly tilt tests were done in the same Mars
chamber used to test samples with Prototype 1.
2.3.5 Flight Model (FM)
The flight
model (FM) has elemental and XRD standards in five permanently loaded sam-ple
cells (discussed below in Sect. 3.3). Prior to
integration with the rover, these standards were analyzed in a simulated Mars
atmosphere at a range of RAMP temperatures to ob-tain baseline function data
from the FM, for (1) defining initial pre-mission functionality and (2)
cross-calibration with identical samples that are loaded in the demonstration
model (DM). Results of FM analyses of the permanently loaded standards are
shown in Sect. 4.
2.3.6 Demonstration Model (DM)
The
demonstration model (DM) was built from components identical or equivalent to
those used in constructing the FM. Identical standards to the FM are loaded in
five permanent cells and are used to evaluate any changes in functionality of
the FM that might be related to ATLO, launch, flight, and landed operations.
The DM will be cross-calibrated to the FM using these common standards. DM
testing prior to and during the mission with a variety of samples is critical
to interpreting FM data, because no end-to-end testing was performed in which
samples were loaded through the funnel and analyzed in one of the 27 open
sample cells of the FM (doing so would have exposed the inside of the
instrument to contamination from sample powder with unknown consequences during
launch and spaceflight).
The DM
will be kept on-site at JPL for purposes of (1) continuing evaluation of DM
performance relative to results returned by the FM; (2) prototyping possible
changes in function (analytical sequence, piezovibration operation, temperature
cycling, etc.) before applying such changes to operation of the FM on Mars; and
(3) analyzing selected known mineral and rock samples in open cells, which
could not be performed in the FM.
2.3.7 Cell Window Assemblies for Durability Tests
Sample holders have been
subjected to long-duration vibration tests to determine whether the sample cell
windows will degrade after repeated and prolonged vibration. Table 9 sum-marizes test results for cell window durability
in MylarTM and KaptonTM cells.
Table 9 Window durability tests using 75–150
μm quartz and intense (∼ 100 μm amplitude)
shaking in CheMin FM sample cells
a Used in FM,
DM and testbed instruments
material |
thickness (μm) |
result |
|
|
|
Mylar® |
3.6 |
50 % failure after 16
hours |
Mylar® |
6a |
50 % failure after 168
hours |
Kapton® |
7.5a |
no failures at >170 hours |
3 Calibration and Standards
3.1 Calibration of X-Ray Diffraction Patterns
(1)
Effects of 2θ Statistics, 2θ Range, 2θ FWHM and Instrumental Broadening Function on
Calibration
Detection limits for
individual phases as well as the precision and accuracy of mineralogic analysis
are influenced by geometrical factors. Because the CCD is off-center and has a
fi-nite size, diffraction rings are not fully captured by the CCD. The
proportion of each ring captured by the CCD is a function of 2θ . At high 2θ values, less of the ring
is captured. At low 2θ , the effective radius of the diffraction
ring is so small that relatively few pixels define the ring. A diffracted peak
at very high or very low 2θ will consequently have
poorer statis-tics than one at moderate 2θ . Of equal importance, the
CCD has a specific Charge Collection Efficiency (CCE) for Co Kα and Co Kβ photons (i.e., as a
function of photon energy, only a fraction of the photons that strike the CCD
will be absorbed by the silicon), as well as a characteristic energy-dependent
and device-dependent ratio of single events vs. split events (photons that are
of higher energy and are absorbed deeper in the silicon have a tendency to form
an electron charge cloud that diffuses across pixel boundaries). Each of these
factors will influence the statistics of the final diffraction pattern.
Crystalline
standards having diffraction maxima across the range of observed 2θ are sufficient to determine and calibrate 2θ , 2θ range, and
2θ resolution values for the
DM and the FM. Of importance to this calibration (and to all diffraction data
from CheMin) is the random orientation of crystallites presented to the beam
during an analysis. For these measurements, a standardized powder size range is
used (45–90 μm grain size; this same size range is used in loading all of
the standards in the FM and DM) as well as a standard protocol for shaking the
sample. Under normal circumstances, the grain size distribution of the sample
will be a function of the SA/SPaH sampling system as well as the nature of the
sample.
An
important parameter in quantitative XRD is the instrumental broadening function
(described in Sect. 1.7.3). In a sample of
finite thickness, diffraction can take place any-where along the beam path
through the sample. Once diffracted, photons can be reabsorbed or rediffracted
along their path inside the sample. In weakly absorbing samples, diffracted
photons originating from near the front surface of the sample (X-ray source
side) will con-tribute proportionally more to the pattern than in strongly
absorbing samples. This results in an asymmetry in the individual diffraction
peaks that is a function of the linear absorption coefficient of the material
for Co Kα and Co Kβ photons—and the thickness of the sample.
Absorption by
the sample of transmitted Co Kα and Co Kβ photons will vary as a func-tion of
composition. In addition, the mass thickness of the sample will vary depending
on the proportion of solid material and void space present in the beam as the
sample is shaken and different components of the powder pass through the beam.
Granular standards are used to ensure that these materials are as much like the
unknowns as possible, to account for such effects.
3.1.2 X-Ray Diffraction Calibration for 2θ
Resolution (FM and DM)
A mixture
of two minerals with several closely spaced diffraction maxima is used to
cal-ibrate 2θ resolution
at low, medium, and high ranges of 2θ . The
minerals selected were beryl (a synthetic emerald donated by Chatham Created Gems,
Inc., San Francisco, CA) and quartz. Grain size distribution is controlled
(45–90 μm) to minimize grain size effects and cell loading is standardized
to control particle mass thickness along the pathways of diffracted photons.
3.1.3 X-ray Diffraction Calibration for Mineral
Identification (FM and DM)
Known
standard minerals are used to calibrate both Co Kα and Co Kβ
diffraction patterns for peak locations and relative peak heights. Minerals
chosen were amphibole, beryl, quartz, and arcanite. Grain size distribution is
controlled (45–90 μm) to minimize grain size effects and cell loading is
standardized to control particle mass thickness along the pathways of
diffracted photons.
•
X-Ray Diffraction Calibration for Mineral
Quantification (FM and DM and Testbeds)
Artificial
mixtures of mineral standards are used for quantitative XRD (QXRD)
calibra-tion. In the FM and DM, 3:97 and 12:88 quartz:beryl mixtures are used
for determining the accuracy and precision of quantitative analysis, peak
positions and peak FWHM across CheMin’s 5–55◦ 2θ range. Mineral mixtures allow tighter
mineral composition control than possible with natural rocks. For the DM and
testbeds, in addition to the quartz/beryl mix-tures, known standard minerals
will be physically admixed to simulate basalts, evaporites, and other
lithologies, particularly those relevant to the Gale Crater site (e.g.,
mixtures in-cluding clay and evaporite minerals; see Sect. 3.4.2 and Table 10).
During the mission, the DM instrument will be used for troubleshooting
off-nominal behaviors of the FM, and for collecting XRD patterns and XRF
spectra from analog samples for comparison with flight data. The quartz:beryl
standards will be available throughout the mission to evaluate instru-ment
performance (instrument profile, peak position, peak FWHM, etc.).
3.1.5 Quantitative Techniques for X-Ray
Diffraction Analysis
For
mineral identification, the search/match utilities in commercial XRD analysis programs
such as Jade® (MDI, Inc., Pleasanton, CA) and
Xpowder® (Martin 2004)
will be used. Dur-ing the mission, “Payload Downlink Leads” will analyze CheMin
data in a tactical time-frame (minutes) to produce “quicklook” data products
for use in planning the next Sol’s activities (these qualitative analyses will
use search-match programs such as Jade® ).
Follow-ing mineral identification, quantification will be accomplished using
one or more of several established quantitative analysis methods on a non-tactical
timeframe (weeks). For samples in which constituent minerals do not have known
crystal structures, or minerals and compo-nents without well-defined structures
(e.g., glasses and many clay minerals) a program such as FULLPAT will be used.
In FULLPAT analyses, individual library standard patterns for all phases
believed to be present in the sample are simultaneously fit to the observed
pattern using a least-squares refinement. These library patterns can be
calculated from crystal struc-tures, simulated from ICDD data, or directly
measured on the DM or testbed instruments. A version of FULLPAT tailored
specifically to CheMin instrument parameters is being de-veloped by co-I Steve
Chipera. Several hundred mineral, rock and soil samples, including
Table 10 Composition of four synthetic
rock mixtures to be tested in the CheMin DM and CheMin testbed instruments
Component |
Origin |
Wt.% |
|
|
|
Synthetic Basalt (BST-01) |
|
|
Bytownite |
Sonora, Mexico |
45.0 |
Augite |
Ceder Butte, OR |
31.0 |
Forsterite |
San Carlos, AZ |
16.8 |
Fluorapatite |
Wilberforce, ONT |
7.1 |
Synthetic Altered Basalt
(BST-20) |
|
|
Bytownite |
Sonora,
Mexico |
43.2
% |
Augite |
Ceder Butte, OR |
32.2 % |
Hematite |
Cleator Moor, England |
16.7 % |
Chlorite (clinochlore) |
Clay Minerals Soc. Ref.
CCa-1 |
7.9 % |
Synthetic Evaporite
(EVAP-01) |
|
|
Gypsum |
WY Selenite |
48.3 % |
Halite |
Morton Salt |
33.6 % |
Arcanite |
synthesized from solution |
18.0 % |
Synthetic
Clay-mineral-bearing Evaporite (EVAP-02) |
|
|
Nontronite |
Clay Minerals Soc. Ref.
NAu-2 |
61 % |
Gypsum |
Wy Selenite |
25.0 % |
Halite |
Morton Salt |
8.7 % |
Hematite |
Cleator Moor, England |
5.3 % |
|
|
|
phyllosilicates,
carbonates and evaporites, are being measured in the CheMin IV instrument at
Ames Research Center, for use in FULLPAT analysis. A subset of this sample
suite will be analyzed on the CheMin DM instrument at JPL. For samples composed
of minerals with well-known crystal structures, full-pattern fitting and
Rietveld refinement will be used, in conjunction with Jade® or TOPAS® software
to determine mineralogical compositions. For Rietveld refinements, patterns are
calculated from first principles for all the phases in the sample, using known
crystal structures. The calculated patterns are used to simulate the observed
pattern and the quantitative abundances are obtained directly from scale
factors. There are advantages to both methods. FULLPAT can be used for phases
that are not fully characterized or whose three-dimensional structures are
poorly constrained, such as many clay minerals and X-ray amorphous materials.
The Rietveld method is potentially more ac-curate but requires a full
description of the crystal structures.
3.2 Calibration of Energy Dispersive Histograms
Determination
of Energy-Dispersive Peak Statistics, Energy Range, Peak FWHM, and Sam-ple Mass
Absorption Coefficients The quality of the energy dispersive
histogram (EDH) provided by the CCD is determined in part by the energy
distribution and the total flux of the continuum and characteristic fluorescent
radiation that strikes the sample, the FWHM of individual X-ray peaks, the
geometrically corrected mass absorption coefficient of the sample, and CCD
parameters such as charge collection efficiency (CCE) and the ratio of single
events versus split events. Each of these factors will influence the statistics
of the final energy-dispersive histogram. A single multi-element standard,
homogeneous at the spatial resolution of the X-ray beam, is sufficient to
determine detection sensitivity and FWHM (in eV) of detected elemental peaks as
a function of photon energy and CCD temperature, both
for the FM
and DM. These properties may change as the mission progresses, the CCD ages,
and cells are reused. In the FM and the DM multi-element standards include
amphibole and a ceramic containing high concentrations of most elements
reported in APXS data from ear-lier missions. The arcanite sample provides a
standard for sulfur, an element of particular interest at the Gale Crater site.
Energy
Dispersive Histogram Performance Checks and Instrument Calibration (FM, DM, and
Testbeds) For EDH performance checks (energy resolution and energy
response as a function of CCD temperature, age, evaluation of neutron
effects on the CCD, etc.) and for calibration, several standards are used. These
have compositions that span a concentration range from light elements to heavy
elements (Sect. 3.3). The standards provide
fluorescence maxima that represent the energy range up to characteristic Co Kβ radiation (7.65 keV) and beyond (e.g., Sr Kα). Within the five sealed sample cells of the
FM and DM, minerals and a doped ceramic evaluated for this purpose are listed
in Sect. 3.3.3. For all standards in the sealed
cells, grain size distributions are controlled (45–90 μm), primarily to
avoid powder clumping or packing in permanent standard cells that must be used
several times during the mission and for minimizing grain-size effects that can
induce asymmetry in diffraction peaks. In other cases, specific to the DM and
testbeds, grain sizes will reflect the standard SA/SPaH powder collection and
sieving protocol, to examine powdered materials as similar to the unknown
samples as possible.
•
Standards
Mounted Permanently in the Sealed Cells of the FM and DM for XRD and EDH
Calibration and Evaluation
3.3.1 Requirements of XRD Standards
XRD Standards will be used
as checks on the following:
1. Peak 2θ position calibration (Peaks that span ∼
5 to 50◦ 2θ , Co Kα radiation).
•
Peak intensity calibration (Check on instrument,
sample vibration, and data processing effects)
•
Peak shape characterization (Check on instrument
response required for data processing —e.g., FULLPAT and Rietveld analyses).
This is important in CheMin because peak shapes are affected by sample
absorption (diffracted photons near the exit side of the sample are more likely
to reach the CCD than those produced near the entrance side of the sample).
•
Calibration of 2θ
resolution. This is particularly important in CheMin; although the ge-ometry is
fixed when the instrument is fabricated, factors such as aging of cell windows
with repeated use on Mars could increase the “drumming” effect during
vibration, effec-tively increasing sample thickness and degrading 2θ resolution.
3.3.2 Requirements of EDH Standards
EDH standards will be used
to check the following:
•
CCD energy resolution for Fe Kα, Co Kα, and Co Kβ . Knowledge of how well these three energies
are separated is critical for determination of whether Fe fluorescence is
ef-fectively separated from Co Kα
diffraction, and whether Co Kβ
diffraction events might be misinterpreted as Co Kα diffraction.
2.
Baseline split-event characterization for the CCD
and follow-on characterization as the CCD ages (e.g., accumulated neutron damage)
or as supporting components age (e.g., cryocooler).
3.
Energy
resolution across the effective range of elements detected by the CCD. This is
important because the search-match methods to be used for mineral
identification are greatly enhanced by the ability to exclude or include
particular elements in the search routine. This is especially important in
CheMin because 2θ resolution is poorer than in standard
laboratory diffractometers and even qualitative chemical information can
im-prove mineral discrimination. Of lesser importance, but nevertheless of
interest, is the potential to perform XRF analysis on a “best effort” basis
using the EDH data.
3.3.3 Standards for Calibration of XRD and EDH
Performance in the FM
Several potential standards
were examined in laboratory diffractometers, testbeds, and by other methods
(e.g., thermogravimetric analysis; XRF analysis; electron microprobe analy-sis)
to determine suitability as standard materials. Five were selected for use
(Table 11). Two cells in the FM and DM carry
quartz:beryl mixtures (3:97 and 12:88) that are required for calibration and
verification and validation of the FM. Three other cells are loaded with
stan-dards chosen for varied fluorescence with diverse chemical composition,
namely amphibole (Fe-containing, to measure Fe Kα and Co Kα separation in
energy-dispersive histograms), arcanite (a sulfate mineral), and a synthetic
ceramic containing a wide range of elements (to understand FWHM of individual
elemental peaks as a function of energy and CCD temper-ature in the EDH, as
well as split-pixel effects).
Table 11 lists the five standards that were chosen. Column
1 lists an amphibole standard (from Gore Mountain, New York) with both an
acceptable range of diffraction maxima and varied chemical composition that
includes high Fe content. Columns 2–3 list the two stan-dards selected
specifically for strong diffraction over a range of 2θ (beryl:quartz). Columns 4–5 list a
high-fluorescence S-rich composition (arcanite), and the chemically diverse
doped ceramic. Table 11 also lists the hardness
and density for each of the standards selected. Den-sity is an important factor
in determining which minerals can be mixed together in a single cell (e.g.,
beryl and quartz); minerals of comparable density are less likely to exhibit
grain segregation during piezovibration of the sample cell. Hardness is also a
factor if different minerals are mixed together, as the harder minerals will
tend to abrade softer minerals.
The
amphibole has several closely-spaced diffraction maxima that provide a means of
evaluating 2θ resolution and peak-profile parameters from
low 2θ (∼
12◦ ) to 48◦ 2θ . This standard also has a
relatively complex composition (magnesiohastingsite or pargasite, de-pending on
ferrous/ferric ratio), useful for evaluation of CCD EDH performance;
composi-tion determined by INAA and electron microprobe is shown in Table 11. It has a significant Fe content that can be
monitored to evaluate any overlap of Fe Kα into the energy range
selected for analysis of the diffracted primary Co Kα used for X-ray
diffraction. There are no known stability issues with use of amphibole as a
diffraction standard.
The beryl used in the DM
and FM is a synthetic emerald supplied by Thomas Chatham of Chatham Created
Gems, Inc. The sample from Chatham used in the DM and FM contains 0.73 weight
percent Cr2 O3 as determined by INAA;
other constituents (Be, Al, Si) will not be detected or are poorly determined
by EDH with CheMin. Beryl has three well-defined and separated peaks of high
intensity (I > 60) across a 2θ range from 13◦ to 36◦ in Co Kα radiation. It also has a
high reference intensity ratio (RIR of 2.1), providing a strong signal after
relatively short exposures. There are no known stability issues with the use of
beryl as a diffraction standard. Beryl has diffraction maxima that are
relatively simple and widely
Table 11 Flight model (FM) and
demonstration model (DM) standards: composition (weight %), linear absorption
coefficients for Co Kα, reference
intensity ratio, density, and mohs hardness
*Light-element components in red italics are not be detectable
by CheMin and are shown for completeness only
**RIR is the reference intensity ratio: the ratio of the
strongest peak of the phase of interest to that of the (113) corundum peak in a
1:1 mixture (50-50 by weight)
spaced, providing
a capability for evaluation of peak profiles. The small amount (3 % and 12 %)
of quartz mixed with this sample for V&V purposes will not interfere with
XRD calibration based solely on the beryl component.
Arcanite
in the DM and FM is synthetic, prepared by crystal growth from saturated
solu-tions of reagent K2
SO4 . The stoichiometric
composition is listed in Table 11. The arcanite
has high K and S content that will produce fluorescence at 3.31 and 2.31 keV,
respectively, creating fluorescence background in the diffraction pattern in a
manner comparable to some of the sedimentary samples likely to be examined by
the MSL rover at Gale Crater. In addi-tion, arcanite has diffraction maxima
that will provide a means of evaluating 2θ resolution and peak-profile
parameters from ∼ 25◦ to 48◦ 2θ . It has the highest
sulfate content among the standards. Arcanite is rare among sulfate salts in
that it does not readily hydrate; as such it provides an exceptionally stable
sulfate calibration standard. It is also very stable on heating.
The doped
ceramic used in the DM and FM is synthesized from 41 % of the Clay Min-erals
Society nontronite NAu-2 (pre-fired to 1000 ◦ C to avoid
vessiculation during later firing) mixed with smaller portions of calcium
phosphate, anhydrite, potassium bromide, ru-tile, zinc sulfate, Mn-sulfate,
rubidium chloride, strontium chloride, chromium oxide, nickel chloride, and 9.5
% Li-tetraborate flux. This mix was ball-milled, pressed, and fired in a
porcelain crucible to 800 ◦ C (firing
temperature is kept low to minimize sulfur loss). The ceramic produced is a
fine-grained mixture of complex synthetic phases with a small amount of borosilicate
glassy matrix. This material was crushed and sieved to 45–90 μm to provide
a polycrystalline sample containing most of the elements of interest to the MSL
mission that are also within the detection range (13 <
Z < 42) of the CheMin CCD. The analysis listed in Table 11 is calculated for the as-fired ceramic before
crushing and sieving.
3.3.4 Stabilities of Standards to Be Flown in the
FM
The
candidate standards must be able to survive thermal and vacuum testing,
decontamina-tion bakeout, and several months at space vacuum, followed by two
Earth years (one Mars year) in the MSL rover body on Mars. Of these conditions,
decontamination bakeout at 80 ◦ C and 10 −5 torr is most severe.
Unstable minerals, especially hydrous phases that read-ily exchange water, are
not suitable. Quartz and beryl contain no water, hydroxyl, or other potentially
volatile constituents and will remain stable. To assess stability of the
arcanite, ceramic and hornblende, thermogravimetric data were collected to 800 ◦ C with dry N2 flush (overheating to 10× the
expected maximum temperature compensates for lack of vacuum conditions). Of the
standards to be flown, arcanite showed no weight loss. Dehydroxylation of
amphibole occurs at high enough temperature (> 400 ◦ C) that stability should
not be an issue. A small weight loss begins at ∼ 400–500 °C in the ceramic
and it too will be stable throughout the mission.
3.4
Cross-calibration
of the FM and DM, and Samples to Be Tested in the CheMin DM and CheMin IV
3.4.1 Blind Samples and Test Samples
Cross-calibration
of the CheMin DM and FM is based on the five standards carried in and common to
both. Extended calibration using the DM and other instruments, principally
CheMin IV, is planned and results will be reported later. The CheMin DM and the
CheMin IV instruments will be used to analyze blind samples provided to the
CheMin science team by the MSL Project. The Project will make available several
different samples for a round-robin exercise. XRD results on the blind samples
using the CheMin DM and CheMin IV instruments will be compared to the
conventional analyses as a check on CheMin calibra-tion and performance.
In
addition to the blind samples provided by the MSL project, the CheMin science team
is analyzing its own suite of 140 blind samples provided by CheMin Co-I Dick
Morris of Johnson Space Center as a round robin exercise. Many of these samples
are also being analyzed by other MSL instrument testbeds such as ChemCam and
SAM. Once the samples are analyzed using the CheMin IV testbed instruments at
JPL, NASA/ARC and NASA/JSC, the compiled analyses will be compared with results
obtained by XRD in the laboratory of CheMin Co-I David Bish of Indiana
University. Selected samples from this suite will be analyzed in the DM as
deemed appropriate.
Table 12 Minerals to be examined in CheMin testbed systems
(potential “problem minerals” in red)
*Solid-solution minerals of known composition used to examine capability
of the CheMin design for using unit-cell parameters to constrain composition
3.4.2 Other Tests
Sample
Preparation to Test Grain-Size Effects (Preliminary CheMin IV Test, Followed by
DM Test) The transmission geometry of CheMin is complex in that much of
the elastic and inelastic photon interaction will occur from particle
edges and along a dynamic pathway, where both the source particle and particles
in the path of a fluoresced or diffracted photon can have absorption and
secondary fluorescence effects influenced by particle size and by the
distribution of minerals in polymineralic grains. To evaluate effects other
than polymineralic interactions, crushed and sieved samples will be prepared of
three homogeneous minerals (amphibole, sylvite, and chromite) in several grain
sizes.
Preparation of
Synthetic Rocks to
Test QXRD (CheMin
DM and CheMin
IV Instruments)
Synthetic
rocks have been prepared from well-characterized mineral stocks to evaluate
ig-neous and sedimentary lithologies representative of potential MSL targets.
These include a basalt, an altered basalt, a synthetic evaporite, and a
synthetic clay-mineral-bearing evapor-ite. The descriptions shown in Table 10 include the minerals used in synthesis, their
source, and the weight percentages used.
Determination
of RIR Factors in the CheMin IV Instrument for FULLPAT Analysis A suite
of 100+ purified minerals including a variety of clay
mineral standards will be analyzed in the CheMin IV instrument at Ames Research
Center and Johnson Space Center; a subset of these samples will also be
analyzed in the DM. Reference Intensity Ratio (RIR) factors will be compiled in
order to perform accurate FULLPAT quantification of minerals such as
phyllosilicates that cannot be quantified using Rietveld refinement.
A
Representative “Problem Mineral” Suite It is possible that certain
minerals could occur on Mars that would cause problems if introduced
into the CheMin funnel or sample cells. These minerals generally fall into the
category of hydrous silicates or salt hydrates that could release moisture or
“ball up” if excavated from shallow cold conditions at high humidity and
introduced into the warmer and drier interior of the rover where CheMin resides.
Some sulfates may also be aggressive in chemical attack, on either metal or
cell window materials. Potential problem minerals that will be tested in
breadboards are listed in red in Table 12.
Representative
Solid-Solution Series Determination
of accurate unit-cell parameters in solid-solution series can provide an
ability to constrain mineral chemical composition. For
Fig. 18 Difference
in peak position for end-members of K–Na jarosite (red o’s) and Fo–Fa
olivine (black x’s), plotted against average 2θ position for each peak (Co Kα). Dashed line shows resolution (FWHM)
for the flight model of CheMin
most
solid-solution series the attainable accuracy depends directly on the accuracy
of unit-cell parameters refined using measured data. Unit-cell parameters
determined from Rietveld refinements use complete diffraction patterns and do
not depend on only a single diffraction peak. Because the Rietveld method uses
entire diffraction patterns, estimation of compo-sition within a solid solution
is possible even when differences based on single diffrac-tion peaks are small,
on the order of the instrument resolution. Not all solid solutions have
variation in unit-cell parameters sufficient to provide meaningful estimates of
composition when using estimates based on single peaks. For example, Fig. 18 compares olivine Fo–Fa mixtures and jarosite K–Na
mixtures in terms of the range in 2θ (⑩2θ
) between
the end-members, shown on the vertical scale. The horizontal scale shows the
average 2θ position for each peak. All 2θ values are for Co Kα. The dark dashed line
labeled “FWHM” is the modeled full-width half-maximum peak resolution expected
of the CheMin flight model, which varies with 2θ . When considering single
diffraction peaks, the range of ⑩2θ must be above this dashed
line if the peaks listed are to be used to resolve pure end-members, and well
above it for any meaningful scaling of Fo–Fa or K–Na composition to be
possible. All of the olivine peaks have ⑩2θ values well above the
expected FWHM resolution, but only three of those for jarosite will be
resolvable as distinct between the Na and K end-members; two of those are
redundant along the c-axis and the third one has mostly a c-axis
component. However, refinement of a set of unit-cell parameters using
the full diffraction pattern will facilitate discrimination between
compositionally similar materials. Jarosite presents addi-tional complications due
to the possibility of hydronium substitution for Na and K, which results in
significant difficulties in determining composition from unit-cell parameters,
even when using laboratory diffraction data. Data in Fig. 18 show that Fo–Fa composition can be estimated well
with CheMin, but the ability to determine jarosite composition from unit-cell
parameters will be limited due to complex substitutions. To examine performance
of the CheMin design in determining composition from solid-solution series,
both olivine and jarosite of known Fo–Fa or K–Na composition will be examined
using the CheMin IV in-struments. Any compositional estimates will be made
using refined unit-cell parameters rather than single diffraction peaks.
Analysis
of X-Ray Amorphous Materials X-ray amorphous materials have
broad X-ray scattering spread over several degrees 2θ , without the sharper diffraction peaks that
result from well-ordered crystalline solids. Among the amorphous phases
anticipated on Mars are volcanic glass, impact glasses (including maskelynite),
some amorphous sulfates, and fully amorphous silica (opal-A). In addition to
X-ray amorphous materials, poorly crystalline materials including Fe-oxides
such as ferrihydrite and some sulfates can have very broad and poorly resolved
diffraction peaks, although the presence of multiple poorly resolved peaks
reveal these as poorly crystalline and not fully X-ray amorphous. In some cases
that
may occur
in actual MSL operations, only a general “amorphous constituent” will be fit
and other information (ChemCam, MAHLI “hand lens” information, etc.) must be
used to infer whether the amorphous component is volcanic glass, opal-A, or
some other amorphous material.
Analysis
of the Organic Check Material (OCM) The MSL rover will carry an
organic check material (OCM) to be analyzed as an organic blank by the
SAM instrument. The OCM consists of amorphous silica in a solid form, mounted
on the rover, for drilling and periodic analysis by SAM after passage through
SA/SPaH. Splits of this material will be characterized by the CheMin DM and
CheMin IV to provide a baseline characterization. This will provide some
capability for recognizing the OCM as a contaminant should rem-nants appear in
CheMin cells along with subsequent sample deliveries through SA/SPaH.
Conversely, analyzing OCM in a previously used CheMin sample cell will provide
a quan-titative assessment of remnant contamination from previously analyzed
samples.
3.4.3 Performance Checks
Instrument
XRD and EDH performance checks were conducted on the FM during Ther-moVac
testing and will be conducted during the deployment of the FM on Mars, to
eval-uate instrument degradation and instrument drift. Instrument performance
checks will be conducted using the same algorithms developed for routine data
analysis (Sect. 1.9). Per-formance checks will
be conducted in concert with routine calibration, approximately once every 40
sols, or as needed should data downlink indicate off-normal operation.
Monitoring
in X-Ray Diffraction Mode XRD performance checks will use one or more of
the mineral standards, which include a number of strong lines between 5 and
50 degrees 2θ . From
these minerals a suite of 2θ positions,
2θ FWHM values versus 2θ position, and peak intensities versus ICDD
powder file peak intensity will be determined. Plots will be made of radial
distance from the primary beam versus known 2θ (2θ calibration), 2θ versus 2θ FWHM, and
measured intensity of peaks versus baseline values developed in the FM during
ThermoVac testing, and in the DM. An instrumental broadening function will be
determined based on the known composition and thickness of the sample, for
reference to prior calibration.
Monitoring
in Energy-Dispersive Mode Energy-dispersive performance checks will use
the mineral standards to calibrate elemental peak energies versus measured
energy, FWHM of element peak versus energy, and total intensity of individual
peaks versus time (mission duration). Relative intensities of the peaks will be
compared with known concentrations in the standards. The quantity and nature of
split events will be evaluated and monitored.
4 Characterization of the FM During ThermoVac
(TVAC) Testing
4.1 Overview
In early 2009,
the CheMin FM was evaluated under a variety of environmental conditions during
MSL project ThermoVac testing. RAMP temperatures were varied from −40 °C to +26 °C
under both vacuum and Mars atmospheric pressure (modeled with 7 torr Argon)
conditions.
XRD and XRF processing modes were evaluated for all of the standards, plus
empty Mylar® and Kapton® cells. A
partial description of the FM ThermoVac results is shown below. Two off-nominal
performance issues were discovered which will potentially impact the quality of
returned Mars data:
(1)
The
performance of the CCD for energy discrimination and the peak to background of
detected XRD patterns is a function of CCD temperature. Under vacuum, the CCD
could be
cooled to the baselined temperature of −60 °C
(required to reduce background noise in the CCD) regardless of RAMP
temperature. However, under Mars atmosphere conditions, the minimum attainable
CCD temperature was found to co-vary with RAMP temperature. On further testing,
it was found that, under Mars atmosphere conditions, the CCD can be cooled to
an ultimate temperature only 48 degrees lower than the RAMP temperature. This
dependence of CCD temperature on RAMP temperature is thought to be the result
of a parasitic heat load on the cryocooler created by thermal conduction
be-tween cryocooler components and cryocooler support structures in the
instrument under Mars atmosphere conditions. Several attempts were made to
remedy this problem, but none were found to be satisfactory. Removing the
cryocooler support structures is not a possible solution because they are
necessary in order for the cryocooler’s mechanical components to survive launch
vibration loads. Because the anticipated RAMP temper-
ature at
Gale crater is predicted to be 0 °C to +26 °C
during the times when CheMin will be operating, this will have a moderately
negative impact on the quality of the EDH data. However, data collection
strategies have been devised and tested which can largely overcome this problem.
(2)
Quantitative
analyses of the beryl:quartz 88:12 mixture in the FM varied dramatically
between minor frames, causing the instrument to fail a principal requirement
that it be
capable of
quantifying mineralogical mixtures with an accuracy of ±15 % of
the amount present. There are three possible reasons for this off-nominal
result that are discussed below (Sect. 4.2.1).
4.2 XRD Characterization
4.2.1 XRD Mineral Detection and Quantification:
Quartz/Beryl Analyses
Quantification of Mineral
Mixtures XRD
mineral detection and quantification in the FM was evaluated during
ThermoVac utilizing the sealed beryl:quartz 88:12 and 97:3 standards.
Measurements were made in vacuum and in 7 Torr Ar at a RAMP temperatures
ranging from −40 °C to +26 °C. In vacuum, the CCD
could be cooled to its ultimate temperature of −60 °C regardless of RAMP temperature.
However, with the test chamber filled with 7 Torr of Argon to simulate a Mars
atmosphere, the CCD could be cooled to only 48◦ below ramp temperature,
with the result that at RAMP temperatures above −12 °C the preferred CCD operating
temperature of −60 °C could not be
achieved. RAMP temperatures of 0 °C to 26 °C are predicted for Gale Crater
during MSL’s nominal mission, with the consequence that some reduction in
ultimate mineral detection limits will be experienced at the higher
temperatures, as described below.
Figure
19 shows a major frame Co Kα XRD pattern of the
beryl:quartz 88:12 standard, compiled from 230 30-sec. frames (∼
2 hrs.) of FM data at 0◦ RAMP in a 7 Torr Ar at-mosphere (−48◦ CCD). The design-specified
range of 5◦ to 50◦ 2θ is self-evident in the
figure. Table 13 lists the measured values of
the observed peaks. The positions of the peaks, their deviation in 2θ from reference values, and
the measured 2θ FWHM are all within
Fig.
19 XRD of
beryl:quartz 88:12 mixture. This mixture will be analyzed at regular
intervals on the Mars surface to characterize the performance of CheMin. The
FWHM of the 100 peak of beryl at ∼ 12.95◦ 2θ and the mid-range beryl
and quartz peaks between 20 and 40◦ 2θ will be used as a figure
of merit for the resolution of the instrument
Table 13 CheMin FM data obtained during Thermo-Vac
testing. Observed 2θ values, d -values, ⑩d (obs.– ref.), peak ID, peak rel. intensity
and peak FWHM for Beryl:Quartz 88:12 standard mixture shown in Fig. 19
2θ (obs.) |
d (obs., Å) |
d (ref., Å) |
⑩d (Å) |
Peak hkl |
I % |
FWHM (°) |
|
|
|
|
|
|
|
12.95 |
7.95 |
7.98 |
0.03 |
Be 100 |
100 |
0.275 |
22.54 |
4.58 |
4.60 |
0.02 |
Be 002, 110 |
37.7 |
0.274 |
24.41 |
4.24 |
4.255 |
0.016 |
Qtz 100 |
3.5 |
0.190 |
26.06 |
3.97 |
3.99 |
0.02 |
Be 200 |
43.3 |
0.304 |
31.17 |
3.33 |
3.343 |
0.013 |
Qtz 101 |
12.9 |
0.294 |
32.04 |
3.24 |
3.254 |
0.014 |
Be 112 |
71.2 |
0.313 |
34.68 |
3.00 |
3.015 |
0.015 |
Be 210, 202 |
23.9 |
0.315 |
36.52 |
2.854 |
2.867 |
0.013 |
Be 211 |
74.1 |
0.325 |
41.72 |
2.512 |
2.523 |
0.011 |
Be
212 |
20.1 |
0.322 |
45.97 |
2.289 |
2.293 |
0.004 |
Be 004, 302 |
5.2 |
0.383a |
|
2.272 |
2.282 |
0.010 |
Qtz
012 |
1.7 |
|
|
2.227 |
2.236 |
0.009 |
Qtz 111 |
0.6 |
|
47.89 |
2.204 |
2.213 |
0.009 |
Be 310 |
1.6 |
0.367a |
|
2.200 |
2.208 |
0.008 |
Be
104 |
2.1 |
|
49.32 |
2.143 |
2.152 |
0.009 |
Be 213, 311 |
9.3 |
0.352 |
|
2.118 |
2.128 |
0.010 |
Qtz 020 |
0.8 |
|
51.78 |
2.048 |
2.058 |
0.010 |
Be 114, 222 |
3.2 |
0.321 |
53.57 |
1.984 |
1.993 |
0.009 |
Be 312, 204 |
17.7 |
0.395a |
a Closely spaced peaks
contributing to a single peak maximum |
|
|
|
requirements—most
importantly, the FWHM values, which figure prominently in analyzing complex
mineral mixtures. The only observed maxima which exceed the 0.35 2θ FWHM design specification
occur where several closely spaced peaks overlap each other.
Minor frames from the Beryl:Quartz 88:12 standard data were
quantified using Rietveld refinement (Table 14).
The results are not within the specified limits of precision and accu-
Table 14 Quantitative analysis of
minor frames in Beryl:Quartz 88:12 standard mixture
Minor Frame # |
Frame # |
Beryl % |
Quartz % |
|
|
|
|
1 |
1–50 |
92.9 |
7.1 |
2 |
51–100 |
95.5 |
4.5 |
3 |
101–150 |
96.1 |
3.9 |
4 |
151–200 |
96.8 |
3.2 |
5 |
201–230 |
97.1 |
2.9 |
Major Frame (Sum) |
1–230 |
96.2 |
3.8 |
Fig.
20 XRD of
beryl:quartz 88:12 mixture, emphasizing the quartz 101 and beryl 112
maxima. Individually summed 50-frame minor frame analyses are shown.
Quantitative analyses of the 5 minor frames by Rietveld refinement, which
utilizes all the peaks in the pattern, are shown in Table 13. However, the reduction in quartz 101 peak
intensity during the course of analysis clearly shows a phase segregation
problem
racy
called out in the CheMin instrument requirements. Figure 20 illustrates this graphically, showing progressive
reduction of the intensity of the 101 peak of quartz vs. the 112 peak of beryl
during the analysis. Experiments using the CheMin testbeds are being performed
to determine why this off-nominal behavior occurred. Several possibilities are
being inves-tigated:
1.
The standard materials were all sieved to a
grainsize of 45 to 90 μm. This differs from most of the known and unknown
materials tested in the CheMin testbeds, which were crushed powders < 150
μm (although the Inel tests used 90–45 μm powders).
2.
The standards in the FM are all sealed in the cells
with HEPA filters, to keep the material in the cell for the duration of the
mission. There could be electrostatic effects due to friction of the quartz
grains with the HEPA material that cause phase segregation during shaking.
Moreover, the HEPA filter material is mounted on rigid frames epoxy-bonded in
the throat of the cell; this changes the center of mass and vibration modulus
of the cell.
3.
The style of piezovibration of the FM cells is
different from that implemented in the CheMin testbeds. In the FM, the
piezovibration frequency is locked onto the sample cell resonance, and only the
amplitude of shaking is adjusted during an analysis. In the CheMin testbeds,
the piezovibration frequency is ramped through a range that briefly includes
the resonant frequency of the cell. There was insufficient time during FM
Ther-moVac testing to experiment with variations in shaking intensity.
Fig. 21 Film mode
XRD of beryl:quartz 97:3 mixture, showing detection of quartz 100 and
101 peaks (Kα) and
quartz 101 (Kβ )
During the
operation of the CheMin testbeds and precursor instruments, we were well aware
that the style and intensity of piezovibration can cause phase separation in
complex mixtures. Fortunately, by understanding the nature of the samples,
conditions of analysis can be varied and phase separation minimized or
mitigated. To understand these problems in the CheMin FM, a test chamber is
being built at Ames Research Center to study grain motion, utilizing an FM cell
driven by FM electronics. Similar experiments will be performed in the CheMin
DM instrument.
Minimum Detection Limits
in Mineral Mixtures |
Figure
21 shows a film mode XRD pattern |
|
||||
of
the Beryl:Quartz 97:3 standard (RAMP |
− |
30
◦ C, CCD |
− |
60◦ , 7 Torr Ar, consisting of 640 |
|
|
|
|
|
|
|
30-sec. frames). In film mode, there is increased background
resulting from Bremsstrahlung radiation and sample-induced fluorescence (minor in
this case), and CoKβ
diffraction is
not removed.
Quartz 101 Kα and
Kβ peaks
can clearly be
seen at |
∼ |
31◦ |
and |
∼ |
28◦
2θ , |
|
||
|
|
|
|
|
|
|
||
in addition
to quartz 100
at |
∼ |
24◦ 2θ . This
detection validates the
MDL of 3
% listed in |
|
|||||
|
|
|
|
|
|
|
|
the
instrument specifications. In real geological materials having many phases,
especially X-ray amorphous materials and minerals with broadened peaks such as
clay minerals, this detection limit will probably not be achieved, and MDLs
will have to be evaluated based on the nature of the sample.
4.2.2 XRD and EDH Tests: Gore Mountain Amphibole
Cross-Platform
Comparison of XRD Patterns During ThermoVac testing of the CheMin FM,
only standards in sealed cells were analyzed since samples put into any of the
27 open cells would contaminate the instrument during launch and spaceflight.
Cross-platform com-parisons were made between the CheMin FM, CheMin IV and
Terra with samples of the Gore Mountain Amphibole. Figure 22 shows a comparison of patterns from each of these
instruments, obtained for equivalent counting times.
Because
Mars is an iron-rich planet, many of the materials analyzed by CheMin will exhibit
strong Fe Kα
fluorescence from the sample. In film mode XRD patterns, this is manifested as
increased background (and therefore decreased peak to background ratio) across
the full 2θ range.
Energy-selected Co Kα mode
patterns, which exclude photons having energies outside of a specified Co Kα “energy window” are able to eliminate this
background. In order to discriminate Co Kα (6.93 KeV)
from Fe Kα (6.40 KeV)
in iron containing samples, the CheMin instrument has a requirement to maintain
the FWHM of Fe
Fig. 22 Comparison
of Gore Mountain amphibole XRD patterns between the CheMin FM, CheMin IV and
a Terra field instrument. Pink = CheMin FM,
consisting of 1450 30-second frames of data (725 minutes). Blue = CheMin IV,
consisting of 48,000 seconds of acquisition time, or 1500 30-second frames of
data. Red = Terra,
consisting of 10,000 seconds of acquisition time, or 1000 10-second frames.
Terra has approxi-mately 4X the flux of the other two instruments
Fig. 23 Gore
Mountain amphibole EDH for a series of RAMP temperatures predicted for MSL
operations at Gale Crater. RAMP temperatures: red = 5 °C, dark
blue = 10 °C, green
= 15 °C and
light blue = 20 °C
Kα and Co Kα in EDH < 250 eV. The Gore Mountain amphibole (which
contains 12 % FeO) was included as a reference standard in the CheMin FM in
part to evaluate Fe Kα and Co Kα FWHM energy resolution during the mission.
The flight instrument can easily meet this requirement when the CCD is cooled
to −60 ◦ C;
however, as was described above, at the predicted RAMP 0◦ C to +26 °C
temperatures at Gale Crater, the CCD temperatures will vary between −48 °C and −22 °C.
Figure 23 shows EDH spectra for the Gore Mountain amphibole
taken over a range of temperatures that are predicted for the MSL RAMP during
operations at Gale Crater. Above 0 ◦ C (RAMP),
the Fe Kα and Co Kα FWHM exceed the < 250 eV requirement. However, even at 20 ◦ C RAMP,
energy-selected Co Kα patterns
of the Gore mountain amphibole meet the required 2θ resolution and peak to background ratio of
the instrument (Fig. 24).
Fig. 24 Diffraction
patterns from amphibole standard collected at 5 ◦ C and 20
◦ C RAMP
temperatures. Diffraction data meet required 2θ resolution and peak/background even at the
higher RAMP temperatures expected at Gale Crater
Many
additional samples containing a range of Fe concentrations will be tested in
the DM to establish the detection limits of the FM instrument during the course
of the mission.
4.3 X-Ray Fluorescence Characterization (CCD
Temp. = −60 °C)
The XRF
requirement for the CheMin instrument was descoped early in the instrument fabrication
for technical and budgetary reasons. However, energy discrimination is still
re-quired in order to separate Co Kα, Co Kβ and Fe Kα for
energy-selected XRD patterns. Furthermore, energy discrimination is important
for producing other minor frame 2-D XRD images such as “single pixel,” “split
pixel” and “modified raw” patterns. Lastly, even with a degraded energy
discriminating capability, information as to which elements are present in a
sample will be invaluable in phase identification. All of the standard samples
(whose compositions are shown in Table 11) are
useful for characterizing the energy discriminating capability of the CheMin
instrument: The ceramic sample contains most of the elements of geologic
interest above Z = 13, the
Gore Mountain amphibole contains major Fe (to evaluate Fe Kα FWHM and Fe Ka energy separation from Co Kα), the arcanite contains K and S, and the
beryl in the beryl:quartz mixtures plus the doped ceramic contain 0.64 to 2.05
wt.% Cr. Figure 25 shows an EDH pattern for the
ceramic standard, collected with CCD temperature of −60 °C to
minimize background noise.
4.4 CheMin Beginning of Life (BOL) and End of
Life (EOL) Performance
During the
nominal MSL mission of one Mars year, the CheMin CCD will be damaged by high-energy
neutrons from the nuclear power source. Strategies have been developed and
tested to minimize the effect of this degradation by utilizing on-chip binning
and re-duced CCD exposure times. Tests performed during ThermoVac show that BOL
instru-ment performance can be maintained throughout MSL’s nominal mission
using these strate-gies.
Fig. 25 Energy-dispersive histogram (EDH)
of the ceramic standard, RAMP −30 ◦ C, CCD −60 ◦ C,
simu-lated mars atmosphere
5 Example Patterns from Mars Analog Rocks and
Minerals
5.1 Basaltic and Ultramafic Rocks
5.1.1 Basaltic Rocks and Their Weathering Products
Mars is a
basaltic planet and much of the mineral alteration that will be seen there will
be of igneous precursors. The nature of past alteration processes on the Mars
surface can be eluci-dated by a better understanding of alteration mineralogy.
Depending on conditions, volcanic glass can alter to a variety of mineral
assemblages, including zeolites, smectites, kaolin minerals, hydrated volcanic
glass, and opaline silica. For example, Zolotov and Mironenko (2007) suggested that amorphous silica, goethite, and
kaolinite would form early under acid alteration conditions, whereas zeolites
and carbonates would form later under more alkaline conditions. Yen et al. (2007) suggested that recently observed silica
deposits on Mars could have formed from hydrothermal alteration or from acidic
vapors with small amounts of liq-uid water. Formation of both smectites and
zeolites from volcanic ash is common on Earth, with smectite formation
occurring in below-neutral pH conditions and zeolites forming in alkaline
conditions. Thus, detection of secondary zeolites would strongly imply the
occur-rence of alkaline conditions but detection of both smectites and zeolites
would indicate a much more persistent and evolved hydrogeologic system (Bish et
al. 2008). Kaolin miner-als usually form under
more acidic conditions, often with high water:rock ratios and they may be
accompanied by amorphous silica deposits when formed hydrothermally. Figure 26 shows CheMin IV XRD results from an altered
basaltic tephra found near the summit of Mauna Kea in Hawai’i. Table 15 compares a quantitative analysis of the Mauna Kea
sample using CheMin IV data with an analysis using data from a commercial
laboratory instrument. The identification of alunite and kaolinte as well as a
large quantity of amorphous material in this case suggests alteration by acidic
hydrothermal solutions.
5.1.2 Ultramafic Rocks
In this section, analyses of a suite of ultramafic rocks
similar to those identified on Mars (Poulet et al. 2009;
Ehlmann et al. 2009) are shown. We evaluated
the ability of the Terra
Fig. 26 CheMin IV
XRD pattern of volcanic tephra from near the summit of Mauna Kea Volcano, Hawai’i
(Hamilton et al. 2008) Alteration phases
include alunite, smectite, kaolinite and amorphous phases formed by
hydrothermal aqueous alteration of basaltic tephra. Peak markers: blue = alunite, red
= albite, green
= kaolinite;
smectite not labeled, but 001 peak prominent at 7◦ 2θ . Large amorphous component
evi-dent as broad scattering hump centered at about 27◦ 2θ
Table 15 Rietveld
refinement of XRD data shown in Fig. 26.
Analyses obtained from CheMin IV data are compared to quantitative
results from a laboratory Siemens D500 XRD instrument. Differences in relative
wt.% are compared with accuracy requirements for the CheMin flight instrument
Phase |
CheMin IV (wt.%) |
D500
(wt.%)CheMin IV – D500 (rel. wt.%) |
Accuracy |
|
|
|
|
|
|
Alunite |
30 |
34 |
−12 |
Within 15% |
Albite |
11 |
12 |
−8 |
Within
15% |
Amorphous |
45 |
29 |
+55 |
need
to developa |
Smectite |
7 |
12 |
−42 |
<4X MDLb |
Hematite |
1 |
1.3 |
−23 |
<4X MDL |
Kaolinite |
5 |
12 |
−58 |
<4X MDL |
a Accuracy for measurement of
amorphous components depends on use of appropriate standards and instrument-specific data from
those standards; this is yet to be done for CheMin IV and for the CheMin DM
b MDL = minimum detection limit for accurate
quantification within CheMin FM requirements
instrument
to identify and quantify minor minerals in ultramafic mantle xenoliths and we
evaluated the accuracy of analyses by comparing XRD results with thin section
point count-ing measurements (optical petrography and electron microprobe
analysis). Mantle xenoliths were collected on the 2008 AMASE (Astrobiology Mars
Analog Svalbard Expedition) ex-
Table 16 Mineral proportions in mantle
xenoliths by petrographic point counting (wt.%, calculated by mul-tiplying area
% by phase density) and quantitative XRD using the Terra instrument (wt.%)
Xenolith |
UI-2B |
|
|
UI-3 |
|
|
UI-5 |
|
|
UI-21 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Pet% |
XRD% |
|
Pet% |
XRD% |
|
Pet% |
XRD% |
|
Pet% |
XRD% |
|
|
|
|
|
|
|
|
|
|||
Ol |
65 |
66 |
10 |
8 |
76 |
83 |
63 |
76 |
|||
Opx |
21 |
25 |
8 |
26 |
18 |
16 |
22 |
21 |
|||
Cpx |
10 |
6 |
75 |
59 |
2.6 |
1 |
8.6 |
3 |
|||
Sp |
2.0 |
1.4 |
3.1 |
2.9 |
1 |
0 |
1 |
0 |
|||
Pl |
∼ 0.2 |
0.6 |
|
∼ 2.4 |
4.1 |
|
∼ 0.4 |
0 |
|
∼ 0.8 |
0 |
Am |
0 |
0 |
1.5 |
0 |
0 |
0 |
0 |
0 |
|||
Altn |
∼ 1.5 |
0 |
0 |
0 |
1.6 |
0 |
4.5 |
0 |
|||
PM |
1.1 |
– |
14.4 |
– |
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Methods:
Pet = petrographic mapping; XRD = Terra XRD w/Rietveld
refinement. Minerals: Ol = olivine; Opx = orthopyroxene; Cpx = augite; Sp = spinel; Pl = plagioclase; Altn = “aqueous alteration”
(carbonates, zeolites, clays); Am = amphibole; PM = “partial melt” (∼ 2/3 olivine, 1/6 plagioclase, 1/6 pyroxene)
pedition
to Svalbard (Steele et al. 2008), from basalt outcrops
on the Sverrefjell volcano (Skjelkvale et al. 1989).
Each xenolith was split into several fragments (a few gm each) for thin
sectioning, XRD, and petrographic analyses.
Mineral
identifications were first made by optical microscopy. Mineral proportions were
derived from optical images of the whole thin section area, manually annotated
for mineral species and measured by area in an image-processing code (NIH
Image; http://rsbweb.nih.gov/ij/). Weight
percent was calculated from area percent by multiplying
area percent by assumed phase densities. We believe that the proportion
of total pyroxenes is accurate, but it was difficult in some samples to
distinguish orthopyroxene from clinopy-roxene. Mineral compositions were
determined by wavelength-dispersive electron micro-probe analysis (EMP) using
well-characterized natural and synthetic standards (Treiman et al. 2010).
X-Ray Diffraction Analysis X-ray diffraction analyses
were obtained with a Terra XRD. Qualitative analysis for mineral species
identification was performed by comparison with the ICDD database, and mineral
proportions were calculated via Rietveld refinement using the commercial MDI
program JadeTM .
The
xenoliths span a range of compositions and degrees of alteration. From EMP
analysis and optical petrography, it was determined that all but one are spinel
lherzolites (Table 16) with olivine of Fo87−91 ,
orthopyroxene of Wo01 En90−91 and augite
of Wo46 −47 En49 (Treiman
et al. 2010). Spinels vary widely in Cr and Al contents.
Xenolith UI-3 is a pyroxenite (web-sterite), with abundant augite (Wo47 En47 ), and
lesser orthopyroxene (Wo01 En86 ), spinel,
and amphibole.
The xenoliths contain two
types of secondary material: partial melts and products of aqueous alteration.
Partial melts are concentrated near and around the spinels and are com-posed of
small (10’s of μm) crystals of olivine, plagioclase, pyroxenes and spinel
with glass and void spaces. The aqueous alteration products are veinlets and
space fillings (Treiman et al. 2002) that
constitute less than a few percent of the xenoliths; the alteration materials
in-clude zoned spherules of (Mg,Fe)CO3 (Treiman et al. 2002),
smectitic clays, silica, zeolites, and hematite replacing the carbonate and
clay.
The
mineral identifications and proportions determined by Terra XRD and by optical
pet-rography are very similar. Terra detected all of the major minerals in the
xenoliths, namely olivine, orthopyroxene, augite, and spinel. Mineral proportions
are close to, but not identi-cal to those determined petrographically. Most of
the differences can be ascribed reasonably to heterogeneity in the
xenoliths—the portion exposed in thin section may not be identical to that in
the fragment powdered for XRD. Terra XRD and petrography gave nearly iden-tical
proportions of spinel. For some lherzolites, XRD gave slightly greater
proportions of olivine than did petrography, but likely within counting
uncertainties of the latter. Propor-tions of orthopyroxene versus augite were
commonly different by the two methods; sample heterogeneity is the likely
cause. Terra XRD detected plagioclase feldspar in several xeno-liths. Before
petrographic examination, the occurrence of plagioclase in these rocks was
thought unlikely. However, the xenoliths do contain plagioclase in the partial
melt material, and the measured proportions in thin sections are close to those
from Terra XRD. Similarly Terra XRD detected olivine in UI-3, although none was
seen as large crystals. However, the partially melted material includes
olivine, which was discovered by EMP analyses.
XRD
Detection Limits The
Terra XRD was developed to be able to detect minerals at the 1 % level,
and our results indicate that this instrument meets and can exceed that
spec-ification for common ultramafic rock types. CheMin flight instrument
analyses will have similar detection limits and accuracy/precision of
quantification. Abundances of spinel in these xenoliths demonstrate detection
limits within error: abundances above 1 % (UI-2B & UI-3) were detected at
the same levels by XRD and petrography but spinel was not detected by XRD in
xenoliths (UI-5 & UI-21) where abundances measured in thin section are
below 1 %. Amphibole in UI-3, present in petrographic analyses at 1.5 % by
vol., was not detected by Terra. Carbonate, smectite, hematite and zeolites
were not detected by Terra XRD, but all are at abundance levels ₃
1 %.
The
Terra XRD (and CheMin by implication) will be useful for identifying minerals
more abundant than ∼ 1 % by volume, and the instrument is capable of
producing data that can be used to retrieve mineral proportions in ultramafic
rocks. It may be possible to re-duce detection limits further through the
judicious optimization of data collection strategies. With further analysis and
calibration, CheMin XRD data will be useful in constraining the compositions of
minerals in mafic and ultramafic rocks (e.g., Butterworth et al. 2006).
5.2 Phyllosilicate Minerals
Observations
from the OMEGA imaging spectrometer aboard Mars Express (Bibring et al. 2004; Chicarro et al. 2004)
have revealed a rich diversity of mineralogy on the surface of Mars, including the hydrated clay mineral nontronite
in older cratered terrains (Bibring et al. 2005),
along with montmorillonite and other clay minerals (Poulet et al. 2005). Recent observations of hydrated silicate
minerals by the CRISM instrument on Mars Reconnais-sance Orbiter have validated
these observations and provided an even more detailed picture of the
distribution of hydrous phyllosilicates on the Martian surface (Mustard et al. 2008; Bishop et al. 2008).
OMEGA and CRISM data also suggest the presence of diverse clay minerals,
including Fe,Mg,Al-phyllosilicates, kaolin group minerals, chlorites, and
serpen-tine minerals, along with a variety of hydrous sulfate minerals. Clay
minerals, including likely nontronite (Milliken et al. 2010),
are among the primary targets at Gale Crater.
In XRD, discriminating
between the variety of possible silicate clay minerals is tradition-ally done
by taking advantage of the existence of distinct large repeat distances
perpendicu-lar to the phyllosilicate layers, seen as their 00l diffraction peaks. Clay
mineral discrimina-tion is based primarily on these repeat distances in their
structures.
Fig.
27 CheMin
IV Co Kα XRD pattern of kaolinite (blue
lines). Peak at ∼ 6.5◦ 2θ results from the Kapton® window and other peaks are
kaolinite Co Kβ peaks (Bish et al. 2008)
CheMin
can identify and distinguish a number of clay minerals. For example,
discrim-ination between 1:1 phyllosilicates (such as the kaolin minerals), with
repeat distances of ∼ 7 Å (e.g., Fig. 27),
and smectites (e.g., montmorillonite, nontronite, saponite), with re-peat
distances from 10–15 Å (e.g., Fig. 28) is
straightforward (Bish et al. 2008). However, it
is important to note that the variety of treatments used in terrestrial
laboratories to aid in discrimination of clay minerals will not be accessible
on Mars (e.g., saturation with ethy-lene glycol vapor, heat treatments).
Although these treatments will not be available on Mars, dehydration within the
CheMin instrument could be used to advantage in discriminating be-tween
phyllosilicate minerals that exhibit different dehydration behavior, such as
chlorite vs. smectite. In addition, it should be possible to identify the
hydrated kaolin mineral halloysite. The lowest-angle 001 diffraction peak from
10.1 Å hydrated halloysite occurs at ∼ 10.2◦ 2θ with Co radiation and is
easily detectable; the mineral may readily dehydrate to ∼
7 Å, making its identification possible based on this transition. Some
discrimination among the various smectites may be made by utilization of CheMin
EDH spectra (to distinguish Fe-smectites, for example) and correlative
elemental data from the APXS and ChemCam in-struments. Additional information
may be obtained from dehydration and dehydroxylation temperatures determined in
stepwise heating within the SAM instrument.
The study
of clay mineralogy in situ on Mars will begin with the arrival of CheMin
on Curiosity in 2012. Gale Crater shows evidence of sedimentary
processes along with spectral signatures of both clays and sulfates.
Clay mineralogy will be a critical component of deter-mining hydrogeologic
history and habitability and the CheMin instrument is well suited for the task.
5.3 Mg- Fe- Ca-Sulfate Hydrates
We expect to find sulfate minerals in abundance at Gale
Crater, particularly polyhydrated sulfates and kieserite (Milliken et al. 2010). The polyhydrated forms are not well
constrained
Fig. 28 CheMin IV Co Kα XRD pattern of nontronite (61
wt.%, red), gypsum (25 wt.%, blue), halite (8.7
wt.%, green) and hematite (5.3 wt.%, purple) (Bish et al. 2008)
but are
likely to include members of the Mg-sulfate system, which contains at least
seven different crystalline hydrates (all of which are readily identified by
XRD), as well as an amorphous hydrated form. The association of Mg-sulfates
with smectites that are also iden-tified at Gale Crater (Milliken et al. 2010) suggests that Ca-sulfate is also likely be present
as a result of cation exchange between smectite and Mg-sulfate brine (Vaniman
and Chipera 2006). Experience with CheMin IV
and Terra instruments has proven the capabilities of the CheMin design in identifying and quantifying complex mixtures of
Mg- Fe- and Ca-sulfates. Figure 29 shows a
relatively complex mixture of sulfates, analyzed in the field in Rio Tinto,
Spain using a CheMin IV instrument.
Field deployments of the Terra instrument have demonstrated
the capability of CheMin-like instruments to determine complex sulfate mineral
mixtures that include a range of ferric sulfate phases (e.g., Chipera et al. 2009). Figure 30
shows for example a Terra XRD anal-ysis of an efflorescent mineral sample
collected and analyzed in the field at an abandoned mine site near Leadville,
Co. The sample was found to contain a mixture of melanterite (FeSO4 ·7H2 O) and
siderotil (FeSO4 ·5H2 O) with
minor gypsum.
A significant challenge to sample acquisition and processing
by SA/SPaH—CHIMRA and analysis by CheMin could result from changes in the
hydration state of the hydrated sulfate minerals after they are collected. For
example, the sample whose XRD pattern is shown in Fig. 30
had completely altered to Siderotil when returned to the lab for XRD anal-ysis
on a conventional instrument. Other samples analyzed at the Leadville site with
Terra contained predominantly epsomite (MgSO4 ·7H2 O), but
when returned to the laboratory for XRD analysis, had completely altered to
mixtures of the 4-hydrate starkeyite and the 6-hydrate hexahydrite. End-to-end
tests of some of these challenging salt hydrates are planned with MSL testbed
systems, as well as with the CheMin DM. With experience on Mars and
interpretation based on phase stability data, we hope to understand the nature
of such trans-formations and develop methods to analyze unstable and metastable
sulfate hydrates safely
Fig. |
29 XRD |
pattern of
a complex Fe–Al
sulfate hydrate from
Rio |
Tinto, Spain.
The sam- |
ple, |
which was |
analyzed in
the field using
a CheMin IV
instrument, |
contains
20 % ferricopi- |
apite (Fe3+ 2/3 Fe3+ 4 (SO4 )6 (OH)2 ·20H2 O), 22 %
alunogen (Al2 (SO4 )3 ·17H2 O), 56 %
rhomboclase [(H5 O2 )+ Fe3+ (SO4 )2 ·2(H2 O)], and
minor quartz
Fig. 30 XRD pattern of an efflorescent
sample collected and analyzed in the field by a Terra XRD instrument at
an abandoned mine site near Leadville, CO (Chipera et al. 2009). The sample contains a mixture of 77 %
melanterite (FeSO4 ·7H2 O), 19 %
siderotil (FeSO4 ·5H2 O), and 4
% gypsum
and
accurately. Regardless of their hydration states, the identification of diverse
Fe- Mg-Ca-sulfate hydrates at Gale, especially in concert with phyllosilicates,
would be extremely interesting and informative with regard to habitability.
5.4 Carbonates
The
existence of carbonate minerals on Mars has long been postulated, based on evidence
of past water along with a CO2
atmosphere that may have been denser during the Noachian era (Gooding et al. 1978; Pollack et al. 1987).
More recently, carbonates have been identified as minor phases in Martian
meteorites (Romanek et al. 1994) and in orbital
observations (Ehlmann et al. 2008; Brown et al.
2010; Michalski and Niles 2011). Carbonates were first
Fig. 31 Terra XRD
pattern of “breccia cave carbonate” identified by Morris et al. (2010, 2011) as
the closest terrestrial analog to the Comanche carbonate in the Columbia
Hills. Rietveld refinement of the data show 95 % magnesite and 5 % dolomite.
Line markers for stoichiometric magnesite shown. Deviation from stoichiometry
is due to Ca substituting for Mg in the structure (as shown by EMP analyses;
Blake et al. 2011)
identified
in outcrops on the Mars surface by the Spirit rover in the Columbia
Hills of Gusev crater (Morris et al. 2010). The
best analogs on Earth to the ALH84001 meteorite are ultra-mafic xenoliths found
on the Spitsbergen archipelago (Treiman et al. 2002),
which contain carbonates with complex compositional zoning. However, the
carbonate globules contained in these rocks are below the detection level of
CheMin (Sect. 5.1.2). Other carbonate
occur-rences in the Bockfjord volcanic province of Spitsbergen (Blake et al. 2010) are the highest fidelity terrestrial analogs to
the Comanche carbonates in the Columbia Hills (Morris et al. 2010, 2011; Blake et
al. 2011). Occurrences of carbonate associated
with volcanic rocks on the Sverrefjell volcano
appear to be cements deposited by late-stage hydrothermal systems occupying
volcanic pipes. The carbonates are complexly zoned Ca–Fe–Mg carbonates as shown
by EMP analysis (Blake et al. 2011).
In the rhombohedral carbonate system, the position of the 104
peak can be used to de-termine Ca–Mg cation ratios and the 015 peak, when
present, provides an indication of dolomite-like ordering of cations along the c-axis
(Goldsmith and Graff 1958). The intensity of
individual peaks also provides some information about site occupancies. A
search-match of the pattern shown in Fig. 31,
for example, identifies the minerals dolomite and siderite, but the X-ray
fluorescence data show that only a small amount of Fe is present. A Rietveld
refinement of the data using siderite fits moderately well, but the site
occupancy refinement suggests that the structure is more like magnesite. The
best fit for the pattern is a somewhat disordered, fine-grained Ca-substituted
magnesite. The carbonates are Ca–Mg- and Fe- rich with zoned compositions that
are probably the result of rapidly changing fluid compositions during
deposition; thus the magnesite identification represents an averaged structure.
The complex cation substitutions that can occur in zoned rhombohedral
carbonates are challeng-ing when only XRD data are available but this is a case
in which the combination of CheMin EDH data along with XRD can assist in
characterization.
Carbonate minerals on Earth are quite commonly associated with
habitable zones such as hot springs (low-temperature hydrothermal zones),
marine or lacustrine sediments, or biominerals. Carbonate minerals are also a
common byproduct of serpentinization, the pro-cess by which ultramafic mantle
minerals are transformed into serpentine at low tempera-tures in the presence
of water (Schulte et al. 2006).
Serpentinization is an important source of hydrogen, which is used as an energy
source by chemosynthetic microbes on Earth.
6 The CheMin Mineralogical Instrument at Gale
MSL’s
rover Curiosity is scheduled to land in Gale Crater in August 2012. The
crater, which is ∼ 155 km in diameter, has a large central mound that
exhibits fine-scale layering sug-gestive of a sedimentary sequence of rocks
deposited in the presence of or beneath water. The layers at the base of the
central mound contain phyllosilicates and sulfates and these strata grade
upwards into sulfate-dominated units, as determined from orbital IR
observa-tions. Because Gale is one of the oldest and deepest craters on Mars,
the sediment pile in the central mound could have preserved a record of aqueous
mineralogy from a time in Mars’ history that parallels that of the early Earth
when terrestrial life was thought to have had its origin. The phyllosilicate
minerals and sulfates identified in the central mound of Gale are well known
from their Earthly counterparts to trap and preserve organic compounds— biogenic
or otherwise—and cementation during early diagenesis could have protected them
from oxidation or removal by subsequent aqueous fluids.
If we review the compositional and mineralogical
interpretations from the Watchtower rocks at Gusev Crater, shown in Tables 1 and 2, what
mineralogies could represent similar starting compositions at Gale, in the
absence of oxidation? The impact that produced Gale crater must have penetrated
through the Mars crust and into the upper mantle, which con-tains predominantly
ultramafic minerals such as olivine. The tremendous heat of the impact would
have caused extensive melting, and as the rocks cooled, hydrothermal systems
would have formed and could have persisted for hundreds of thousands of years
(e.g., Schwenzer et al. 2010). Serpentinization
is one process that may have taken place, by which olivine is converted to
serpentine, magnetite, hydrogen and other product phases (Schulte et al. 2006). The reaction can be self-catalyzing in that it
is exothermic and the attendant volume increase would create further cracking
to allow entry of water into unreacted rock. In terrestrial set-tings, the
serpentinization process and the hydrogen that it produces fuels chemosynthetic
ecosystems, which are thought to be among the earliest examples of life on
Earth.
In its traverse from the landing ellipse to the central mound
of Gale, Curiosity will en-counter most of the sediments deposited since
the crater’s formation. The array of sediment types is stunning and the record of
sedimentation points to multiple environments of de-position, ranging from what
appear to be cemented units of high thermal inertia within the landing ellipse
to nontronite-rich deposits near the base of the mound and a range of strata
ranging from clay mineral-rich to sulfate-rich, with the latter dominating
upper units. Eolian processing has distributed dune material rich in mafic
igneous detritus within Gale’s moat and etched yardang features from some of
the middle to higher strata; eolian processes are also inferred to have filled
and exhumed Gale crater in a manner similar to other compara-ble craters on
Mars (Malin and Edget 2000). The overall
history of sedimentation at Gale appears to be one of progressive desiccation
along with a transition from phyllosilicate-forming to sulfate-precipitating
conditions (Milliken et al. 2010).
This sequence of events is of course, highly
speculative. Many scenarios are possible, and we won’t know anything with
certainty until Curiosity lands in Gale crater and begins
surface operations. The geologic history of the
Mars surface resides in its rocks. The rocks play the dual role of recording
the history of surface and near-surface conditions, and pro-viding a geologic
context to the imaging, chemical, organic, isotopic, and other analyses that
MSL will perform.
Acknowledgements The CheMin
flight instrument could not have been realized without long-term sup-port from
NASA’s research and technology programs and institutions, including: Ames
Research Center’s Director’s Discretionary Fund, the Exobiology Instrument
Development program, the Planetary Instrument Definition and Development
program (PIDDP), the Mars Instrument Definition and Development program
(MIDDP), the Astrobiology Science and Technology Instrument Development program
(ASTID), the As-trobiology Science and Technology for Exploration of Planets
program (ASTEP), NASA’s Small Business Innovative Research program (SBIR), and
the diligent efforts engineers and scientists of the Jet Propulsion Laboratory,
California Institute of Technology under a contract with NASA. We also thank
Thomas Chatham of Chatham Created Gemstones for the donation of synthetic
emerald for the beryl:quartz standard materials.
Open Access This article is distributed
under the terms of the Creative Commons Attribution License which
permits any use, distribution, and reproduction in any medium, provided the
original author(s) and the source are credited.
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