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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 H2O 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 Fe2+ in olivine to Fe3+ in magnetite, and perhaps in smectites provided a potential energy source for organisms. [ABSTRACT FROM AUTHOR]

We present an X-ray diffraction and multi-nuclear (²H and 43Ca) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H2O interface. Variable-temperature 43Ca MAS NMR and controlled humidity XRD indicate that Ca2+ 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 43Ca 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 ²H 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 2H2O molecules experiencing anisotropic motion describable using the ²H C2/C3 jump model we proposed previously. ²H T1 relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for ²H2O dynamics, yielding GHz-scale rotational reorientation rates compatible with the C3 component of the C2/C3 hopping model. The apparent activation energy for ²H2O 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 Ca2+ for H2O. The results support the general principle that the dynamic mechanisms of proximity-restricted H2O 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. [ABSTRACT FROM AUTHOR]

Abstract: Martian layered deposits and regolith at Gale Crater may contain multiple hydrated mineral phases, tentatively identified as hydrated Mg-sulfate minerals and smectites. We have used humidity buffer experiments to assess the stability of hydrated Mg-sulfate minerals in the presence of smectites in order to improve our understanding of the probable behavior of Mg-sulfate minerals within multiphase geological materials on Mars. A series of long-term experiments employed temperature (−25 to +23°C) and relative humidity (RH) conditions (7–100%) that emulate near-equatorial martian surface conditions. Our results indicate that the hydration state of Mg-sulfate minerals is affected by the presence of RH-sensitive clay minerals (i.e., smectites). The formation of gypsum and bassanite in dry mineral mixtures via cation exchange between Ca-bearing smectite and Mg-sulfate minerals indicates that Ca-sulfate minerals may be useful indicators of H2O and metal mobility at Mars-relevant temperatures (−25 to +23°C). The presence of smectites also suppresses deliquescence of Mg-sulfate minerals at 100% RH and low, but non-freezing, temperatures. Co-existence of smectites and Mg-sulfate minerals appears to buffer RH within mixtures of these minerals, which can result in production or preservation of Mg-sulfate phases that are inconsistent with measured values of atmospheric RH. Consequently, hydrated Mg-sulfate minerals may persist beyond their expected T–RH equilibrium fields on longer timescales within smectite–MgSO4 mixtures than in the pure MgSO4–H2O system. Dehydration of highly hydrated Mg-sulfate minerals appears to slow in the presence of smectite, which may have important implications for long-term preservation of organic biosignatures within Mg-sulfate crystals on Mars. Together, these observations suggest that Mg-sulfate mineral behavior (and thus cycling and bioavailability of H2O and metals) may be impacted by the presence of smectites within mineralogically complex martian layered deposits and regolith. [Copyright &y& Elsevier]

Although the transition of the blueschist to greenschist facies has been modeled by the reaction glaucophane (Gln) + paragonite (Pg) + water (V) = clinochlore (Chl) + albite (Ab) + quartz (Qtz), we have found in the system Na2O-MgO-Al2O3-SiO2-H2O that smectite (Smec) forms repeatedly and persistently through the approximate reaction Gln + Chl = Smec ± Ab. The reaction was verified using both synthetic and natural Gln and Chl, where in all cases complete reaction of Chl occurs in the presence of Gln to yield Smec. We have, therefore, investigated the reaction Gln + Pg + Qtz ± V = Smec + Ab, as a possible alternative to the first reaction. The reaction Gln + Pg + Qtz ± V = Smec + Ab was studied over the P-T range of 600--800 °C and 1.4--2.0 GPa keeping the water content in the range of 0--4 wt% to avoid the direct reaction of Gln to Smec + Ab at water contents above 4 wt%. The was experimentally reversed at 600 °C and 1.60(5) GPa, 700 °C and 1.70(5) GPa, and at 800 °C and 1.90(5) GPa (at 0 wt% water content), but with partial melting at 800 °C and pressure ≤2.0 GPa with water ≥3 wt%. The compositions of the phases and the state of A1-Si disorder in the albite were determined. The amphibole showed minor increase in Al and decrease in Mg and Si relative to the glaucophanic amphibole starting material, whereas paragonite showed minor enrichment in Mg and Si at the highest temperatures, complementary to the change in amphibole composition. Albite showed only limited ordering with decrease in temperature. Smectite at 600--700 °C has the average composition Na0.25Mg2.21A11.19Si3.44 O10(OH)2·nH2O and shows slight Mg depletion and Al enrichment at 800 °C. Evidence for at least some Mg enrichment in the interlayer cation sites was supported by powder X-ray diffraction with in situ heating stage analyses using changes in the d001 reflection at 200 °C as an approximate indication of Mg for Na substitution. The composition of smectite formed in this study is quite similar to that of other natural trioctahedral phyllosilicates reported in the literature. This, together with the experimental reversibility of the reaction observed here, indicates that smectite can form as a progressive metamorphic mineral at greenschist- and amphibole-facies conditions, particularly at the expense of chlorite and glaucophane. [ABSTRACT FROM AUTHOR]

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 02l diffraction bands peaking at 4.59 A, 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 02l 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+ΣFe) = 65-70%, lacks tetrahedral Fe3+ and octahedral Al3+, and has Fe3+ /ΣFe from 64 to 93%. Its chemical composition is consistent with a fully tri-octahedral smectite, but the abundance of Fe3+ gives a nominal excess charge of +1 to +2 per formula unit. The excess charge is likely compensated by substitution of O2- 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 Fe2+ and was oxidized later. X-ray diffraction shows a sharp 001 peak at 15 Å, 00l peaks, and a 02l diffraction band at the same position (4.59 A) 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 mu m, arising from H2+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. [ABSTRACT FROM AUTHOR]

Abstract: The recent detection of up to ∼10 wt% water-equivalent H heterogeneously distributed in the upper meter of the equatorial regions of the martian surface and the presence of the 3-μm hydrations feature across the entire planet raises the question whether martian surficial dust can account for this water-equivalent H. We have investigated the H2O and CO2 adsorption properties of palagonitic dust (<5 μm size fraction of phyllosilicate-poor palagonitic tephra HWMK919) as a martian dust analog and two smectites under simulated martian equatorial surface conditions. Our results show that the palagonitic dust, which contains hydrated and hydroxylated volcanic glass of basaltic composition, accommodates significantly more H2O under comparable humidity and temperature conditions than do the smectites nontronite and montmorillonite. [Copyright &y& Elsevier]

Abstract: Interlayered clays are common constituents of several soil orders. The presence of interlayering is indicated by modified thermal stability of soil vermiculites and smectites. In most studies the clays have been reported as Al-hydroxy-interlayered minerals. Despite the abundance of organic compounds in many soils, the potential existence of soil organo-clay intercalations has rarely been considered. In the present study three clay subfractions separated from the eluvial (E) horizon of one podzol from the Tatra Mts., Poland, were studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and elemental CHNS analysis, in the natural state and after being subjected to organic matter removal, free iron oxides removal, saturation with different cations, and heating at 330°C and 550°C. All fractions contain a “~14Å clay”. In the natural state the “~14Å clay” shows high thermal resistance, manifested by only a partial shift to 10Å after heating at 330°C. This behavior strongly suggests the presence of interlayering. Saturation with K+ shifted the ~14Å reflection to ~13–11Å, which suggests displacement of a charged species from the interlayer. Clay sample treatment with H2O2 and NaOCl reduced the stability of the interlayered phase because 330° C heat treatment shifted the ~14Å phase to ~10Å, which suggests that the interlayered phase might be organic. Furthermore, the 13–18% organic carbon content of the clays and near disappearance of infrared peaks at 2855cm−1, 2926cm−1, 1738cm−1, ~1600cm−1, and ~1400cm−1 after H2O2 or NaOCl treatment suggests that the clay interlayer phase is mostly organic. Both Al-hydroxy and organic interlayers can be evaluated using a combination of X-ray diffraction, elemental analysis, and infrared spectroscopy and should be considered as possible interlayer phases during the study of interlayered clays. [Copyright &y& Elsevier]