ALBERT

Description: Hydrated evaporite minerals have the ability to hold large amounts of H 2 O, making them a potential source of H 2 O in cold, low- P H 2 O environments such as the surface of Mars. Many of these hydrated evaporite minerals experience a reversible change in hydration state in response to changes in temperature ( T ) and relative humidity (RH). Such phases may thus have the potential to interact with the martian atmosphere on a daily or seasonal basis. The Na 2 Mg(SO 4 ) 2 · n H 2 O system was previously thought to contain three hydrated phases: a decahydrate ( n = 10), konyaite ( n = 5), and blödite ( n = 4). We examined this system using temperature- and RH-controlled X-ray powder diffraction (XRD) methods, as well as temperature-controlled single-crystal X-ray diffraction. When blödite was exposed to sub-freezing conditions, T ≤ –10 °C, a new phase was produced ( n = 16, 52 wt%H 2 O). Similar low-temperature behavior has been documented in the MgSO 4 · n H 2 O system, through the presence of meridianiite ( Peterson et al. 2007 ). The hydration and dehydration behavior of phases in the Na 2 Mg(SO 4 ) 2 · n H 2 O system was evaluated with powder XRD from –30 to 〉25 °C and from ~99 to near 0% RH, and single-crystal XRD data were collected for the n = 16 phase at –120 °C. The 16-hydrate is triclinic, space group P , with unit-cell parameters a = 6.5590(12), b = 6.6277(14), c = 14.441(3) Å, α = 87.456(15)°, β = 79.682(15)°, = 65.847(13)°, and a unit-cell volume of 563.3(2) Å 3 . The existence of this new phase at low temperatures, its high hydration state, and its ability to form reversibly from blödite all suggest that if phases in this system exist on the martian surface, they will participate in the Mars H 2 O cycle.

Description: We present an X-ray diffraction and multi-nuclear ( 2 H and 43 Ca) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H 2 O interface. Variable-temperature 43 Ca MAS NMR and controlled humidity XRD indicate that Ca 2+ occurs as proximity-restricted outer-sphere hydration complexes between –120 and +25 °C in a two-layer hydrate and at T ≤ –50 °C in a 2:1 water/solid paste. Changes in the 43 Ca NMR peak width and position with temperature are more consistent with diffusion-related processes than with dynamics involving metal-surface interactions such as site exchange. The 2 H NMR signal between –50 and +25 °C for a two-layer hydrate of Ca-hectorite is similar to that of Na- and other alkali metal hectorites and represents 2 H 2 O molecules experiencing anisotropic motion describable using the 2 H C 2 /C 3 jump model we proposed previously. 2 H T 1 relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for 2 H 2 O dynamics, yielding GHz-scale rotational reorientation rates compatible with the C 3 component of the C 2 /C 3 hopping model. The apparent activation energy for 2 H 2 O rotational diffusion in the two-layer hydrate is greater for Ca-hectorite than Na-hectorite (25.1 vs. 21.1 kJ/mol), consistent with the greater affinity of Ca 2+ for H 2 O. The results support the general principle that the dynamic mechanisms of proximity-restricted H 2 O are only weakly influenced by the cation in alkali metal and alkaline earth metal smectites and provide critical evidence that the NMR resonances of charge-balancing cations in smectites become increasingly influenced by diffusion-like dynamic processes at low temperatures as the charge density of the unhydrated cation increases.

Description: The Mars Science Laboratory (MSL) rover Curiosity has documented a section of fluvio-lacustrine strata at Yellowknife Bay (YKB), an embayment on the floor of Gale crater, approximately 500 m east of the Bradbury landing site. X-ray diffraction (XRD) data and evolved gas analysis (EGA) data from the CheMin and SAM instruments show that two powdered mudstone samples (named John Klein and Cumberland) drilled from the Sheepbed member of this succession contain up to ~20 wt% clay minerals. A trioctahedral smectite, likely a ferrian saponite, is the only clay mineral phase detected in these samples. Smectites of the two samples exhibit different 001 spacing under the low partial pressures of H 2 O inside the CheMin instrument (relative humidity 〈1%). Smectite interlayers in John Klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 Å, but largely remain open in the Cumberland sample with a basal spacing of ~13.2 Å. Partial intercalation of Cumberland smectites by metal-hydroxyl groups, a common process in certain pedogenic and lacustrine settings on Earth, is our favored explanation for these differences. The relatively low abundances of olivine and enriched levels of magnetite in the Sheepbed mudstone, when compared with regional basalt compositions derived from orbital data, suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine. Mass-balance calculations are permissive of such a reaction. Moreover, the Sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim. Early diagenetic fabrics suggest clay mineral formation prior to lithification. Thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants, allowing circum-neutral pH. The kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years. Mineralogical evidence of the persistence of benign aqueous conditions at YKB for extended periods indicates a potentially habitable environment where life could establish itself. Mediated oxidation of Fe 2+ in olivine to Fe 3+ in magnetite, and perhaps in smectites provided a potential energy source for organisms.

Description: Ferrian saponite from the eastern Santa Monica Mountain, near Griffith Park (Los Angeles, California), was investigated as a mineralogical analog to smectites discovered on Mars by the CheMin X-ray diffraction instrument onboard the Mars Science Laboratory (MSL) rover. The martian clay minerals occur in sediment of basaltic composition and have 02 l diffraction bands peaking at 4.59 Å, consistent with tri-octahedral smectites. The Griffith saponite occurs in basalts as pseudomorphs after olivine and mesostasis glass and as fillings of vesicles and cracks and has 02 l diffraction bands at that same position. We obtained chemical compositions (by electron microprobe), X-ray diffraction patterns with a lab version of the CheMin instrument, Mössbauer spectra, and visible and near-IR reflectance (VNIR) spectra on several samples from that locality. The Griffith saponite is magnesian, Mg/(Mg+Fe) = 65–70%, lacks tetrahedral Fe 3+ and octahedral Al 3+ , and has Fe 3+ /Fe from 64 to 93%. Its chemical composition is consistent with a fully tri-octahedral smectite, but the abundance of Fe 3+ gives a nominal excess charge of +1 to +2 per formula unit. The excess charge is likely compensated by substitution of O 2– for OH – , causing distortion of octahedral sites as inferred from Mössbauer spectra. We hypothesize that the Griffith saponite was initially deposited with all its iron as Fe 2+ and was oxidized later. X-ray diffraction shows a sharp 001 peak at 15 Å, 00 l peaks, and a 02 l diffraction band at the same position (4.59 Å) and shape as those of the martian samples, indicating that the martian saponite is not fully oxidized. VNIR spectra of the Griffith saponite show distinct absorptions at 1.40, 1.90, 2.30–2.32, and 2.40 μm, arising from H 2 O and hydroxyl groups in various settings. The position of the ~2.31 μm spectral feature varies systematically with the redox state of the octahedrally coordinated Fe. This correlation may permit surface oxidation state to be inferred (in some cases) from VNIR spectra of Mars obtained from orbit, and, in any case, ferrian saponite is a viable assignment for spectral detections in the range 2.30–2.32 μm.

Description: Sulfate minerals are important indicators for aqueous geochemical environments. The geology and mineralogy of Mars have been studied through the use of various remote-sensing techniques, including thermal (mid-infrared) emission and visible/near-infrared reflectance spectroscopies. Spectral analyses of spacecraft data (from orbital and landed missions) using these techniques have indicated the presence of sulfate minerals on Mars, including Fe-rich sulfates on the iron-rich planet. Each individual Fe-sulfate mineral can be used to constrain bulk chemistry and lends more information about the specific formational environment [e.g., Fe 2+ sulfates are typically more water soluble than Fe 3+ sulfates and their presence would imply a water-limited (and lower Eh) environment; Fe 3+ sulfates form over a range of hydration levels and indicate further oxidation (biological or abiological) and increased acidification]. To enable better interpretation of past and future terrestrial or planetary data sets, with respect to the Fe-sulfates, we present a comprehensive collection of mid-infrared thermal emission (2000 to 220 cm –1 ; 5–45 μm) and visible/near-infrared (0.35–5 μm) spectra of 21 different ferrous- and ferric-iron sulfate minerals. Mid-infrared vibrational modes (for SO 4 , OH, H 2 O) are assigned to each thermal emissivity spectrum, and the electronic excitation and transfer bands and vibrational OH, H 2 O, and SO 4 overtone and combination bands are assigned to the visible/near-infrared reflectance spectra. Presentation and characterization of these Fe-sulfate thermal emission and visible/near-infrared reflectance spectra will enable the specific chemical environments to be determined when individual Fe-sulfate minerals are identified.

Description: Changes in the mechanisms of formation and global distribution of phyllosilicate clay minerals through 4.567 Ga of planetary evolution in our solar system reflect evolving tectonic, geochemical, and biological processes. Clay minerals were absent prior to planetesimal formation ~4.6 billion years ago but today are abundant in all near-surface Earth environments. New clay mineral species and modes of clay mineral paragenesis occurred as a consequence of major events in Earth’s evolution—notably the formation of a mafic crust and oceans, the emergence of granite-rooted continents, the initiation of plate tectonics and subduction, the Great Oxidation Event, and the rise of the terrestrial biosphere. The changing character of clay minerals through time is thus an important part of Earth’s mineralogical history and exemplifies the principles of mineral evolution.

Description: Kaolin-group minerals typically form as a result of hydrothermal alteration and/or weathering processes. They occur in environments as diverse as tropical soils, continental sedimentary deposits, and altered crustal rocks. They have also been detected on the surface of Mars. Given their prevalence, they have attracted the attention of researchers in materials chemistry, environmental geochemistry, and high-pressure mineral physics. Their structure and related properties have been studied for about a century, and these studies reflect advances in experimental techniques, modeling approaches, and concepts in mineralogy. Among key features of their structure are the predominance of 2-D stacking defects and the peculiar role of H-bonding in the control of their polytypism.

Description: Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO 4 tetrahedra mimicking the hierarchy of polymerization of SiO 4 tetrahedra seen in silicate minerals. To augment previous Mössbauer studies of individual phosphate species and groups of species, this paper presents new Mössbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mössbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated Mössbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mössbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mössbauer spectra acquired on Mars.

Description: Allophane and imogolite are common alteration products of volcanic materials. Natural and synthetic allophanes and imogolites were characterized in the present study in order to clarify the short-range order of these materials and to gain an understanding of their spectral properties. Spectral analyses included visible/near-infrared (VNIR), and infrared (IR) reflectance of particulate samples and thermal-infrared (TIR) emissivity spectra of particulate and pressed pellets. Spectral features were similar but not identical for allophane and imogolite. In the near-infrared (NIR) region, allophane spectra exhibited a doublet near 7265 and 7120 cm –1 (1.38 and 1.40 μm) due to OH 2 , a broad band near 5220 cm –1 (1.92 μm) due to H 2 O + , and a band near 4560 cm –1 (2.19 μm) due to OH + . Reflectance spectra of imogolite in this region included a doublet near 7295 and 7190 cm –1 (1.37 and 1.39 μm) due to OH 2 , a broad band near 5200 cm –1 (1.92 μm) due to H 2 O + , and a band near 4565 cm –1 (2.19 μm) due to OH + . A strong broad band was also observed near 3200–3700 cm –1 (~2.8–3.1 μm) which is a composite of OH , H 2 O , and H 2 O 2 vibrations. Visible/near-infrared spectra were also collected under two relative humidity (RH) conditions. High-RH conditions resulted in increasing band strength for the H 2 O combination modes near 6900–6930 cm –1 (1.45 μm) and 5170–5180 cm –1 (1.93 μm) in the allophane and imogolite spectra due to increased abundances of adsorbed H 2 O molecules. Variation in adsorbed H 2 O content caused an apparent shift in the bands near 1.4 and 1.9 μm. A doublet H 2 O vibration was observed at 1600–1670 cm –1 (~6.0–6.2 μm) and a band due to OH bending for O 3 SiOH was observed at ~1350–1485 cm –1 (~6.7–7.4 μm). The Si–O–Al stretching vibrations occurred near 1030 and 940 cm –1 (~9.7 and 10.6 μm) for allophane and near 1010 and 930 cm –1 (~9.9 and 10.7 μm) for imogolite. OH out-of-plane bending modes occurred near 610 cm –1 (16.4 μm) for allophane and at 595 cm –1 (16.8 μm) for imogolite. Features due to Si–O–Al bending vibrations were observed at 545, 420, and 335 cm –1 (~18, 24, and 30 μm) for allophane and at 495, 415, and 335 cm –1 (~20, 24, and 30 μm) for imogolite. The emissivity spectra were obtained from pressed pellets of the samples, which greatly enhanced the spectral contrast of the TIR absorptions. Predicted NIR bands were calculated from the mid-IR fundamental stretching and bending vibrations and compared with the measured NIR values. Controlled-RH X-ray diffraction (XRD) experiments were also performed in order to investigate changes in the mineral structure with changing RH conditions. Both allophane and imogolite exhibited decreasing low-angle XRD intensity with increasing RH, which was probably a result of interactions between H 2 O molecules and the curved allophane and imogolite structures.