Recent reports of approximately 30 wt% of sulphate within saline sediments on Mars--probably occurring in hydrated form--suggest a role for sulphates in accounting for equatorial H2O observed in a global survey by the Odyssey spacecraft. Among salt hydrates likely to be present, those of the MgSO4*nH2O series have many hydration states. Here we report the exposure of several of these phases to varied temperature, pressure and humidity to constrain their possible H2O contents under martian surface conditions. We found that crystalline structure and H2O content are dependent on temperature-pressure history, that an amorphous hydrated phase with slow dehydration kinetics forms at <1% relative humidity, and that equilibrium calculations may not reflect the true H2O-bearing potential of martian soils. Mg sulphate salts can retain sufficient H2O to explain a portion of the Odyssey observations. Because phases in the MgSO4*nH2O system are sensitive to temperature and humidity, they can reveal much about the history of water on Mars. However, their ease of transformation implies that salt hydrates collected on Mars will not be returned to Earth unmodified, and that accurate in situ analysis is imperative.
Oxidation-reduction processes within natural systems greatly influence the properties of sediments, soils and clays. The objective of this experimental study was to gather new evidence for the effects of changes in redox conditions (reduction and reoxidation) on structural properties of ferruginous smectite and to understand better the mechanisms involved. The <2 µm fraction of a ferruginous smectite (sample SWa-1), which contains 17.3 wt.% of total structural Fe, was studied by infrared (IR) spectroscopy. The pure Na-saturated clay was reduced by Na dithionite for 10 to 240 min to obtain various Fe(II):(total Fe) ratios ranging from 0 to 1.0. Selected reduced samples were then reoxidized completely by bubbling O2 gas through the suspensions for up to 12 h. Infrared spectra of the initially unaltered, reduced and reduced-reoxidized samples were collected. Reduction generated changes in the three studied spectral regions (O-H stretching, M-O-H deformation, and Si-O stretching), indicating that major modifications occurred within the clay crystal beyond merely a change in Fe oxidation state. partial dehydroxylation and redistribution of Fe, and perhaps Al, cations occurred upon reduction of SWa-1, changing the structural properties of its tetrahedral and octahedral sheets. Water molecules, probably generated by dehydroxylation within the octahedral sheet upon reduction, were tightly bound to the clay surface and were possibly trapped within the clay structure. Except for dehydroxylation and the Fe oxidation state, all these modifications were largely irreversible. The tightly bound water was not completely removed upon reoxidation and the cationic rearrangements generated during reduction were not reversed: either they were preserved as in the reduced state or cations were redistributed into a different configuration from the unreduced clay.
A set of models for estimating the enthalpy of formation, the entropy, the heat capacity and the volume of dehydrated phyllosilicates is presented. The model for entropy and heat capacity estimation is essentially based on a method of decomposition into polyhedral units, similar to that published by Holland (1989). The model for predicting the enthalpy of formation is based on the electronegativity scale, as previously developed by Vieillard (1994aVieillard ( , 1994b. For the sake of consistency, the models are parameterized using the same critical selection of thermodynamic properties from the literature. This includes a set of direct measurements especially dedicated to clay minerals that had not been taken into account in previous calculation methods. The accuracy of the predictions is tested for each property. The verification tests are also carried out for minerals that include different chemical elements than the phases used to derive the model constants, especially lithium-bearing micas. Verification tests also concern the Gibbs energy function that combines contributions from both models. Finally, the models are used in order to propose a complete thermodynamic database for clay mineral end-members. The consistency of the stability domains calculated on the basis of these thermodynamic properties is investigated by drawing relevant predominance diagrams for some chemical systems of interest. The models proposed represent a significant improvement with respect to previous works as regards the global accuracy of the estimates and because the developments were realized and tested using the same set of minerals, whose properties had been collected through a critical selection of the literature.
[1] The stability of water ice, epsomite, and hexahydrite to loss of H 2 O molecules to the atmosphere at equatorial latitudes of Mars was studied to determine their potential contributions to the measured abundance of waterequivalent hydrogen (WEH). Calculation of the relative humidity based on estimates of yearly averages of watervapor pressures and temperatures at the Martian surface was used for this purpose. Water ice was found to be sufficiently unstable everywhere within 45°of the equator that if the observed WEH is due to water ice, it requires a lowpermeability cover layer near the surface to isolate the water ice below from the atmosphere above. In contrast, epsomite or hexahydrite may be stable in many near-equatorial locations where significant amounts of WEH are observed.
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