Experimental study of Cr incorporation in pargasite

Pargasite compositions corresponding to the nominal structural formula NaCa2 (Mg4Al1-xCrx)(Si6Al2)O22(OH)2, with 0 £ x £ 1, were investigated at 900 °C and 3 and 20 kbar PH2O. The aims of this work were to determine the extent of the solid solution, i.e., the maximum x value, as a function of experimental conditions, and to characterize the M3+ (Al3+ and/or Cr3+) distribution over the M sites. A first series of experiments at 900 °C and 3 kbar showed the presence of large amounts of Cr6+ as sodium chromate (Na2CrO4) and dichromate (Na2Cr2O7) in the final fluid phase, which promotes the extraction of Cr from the bulk solid (up to 45 mol%). The oxidizing agent was the oxygen from water, liberated by diffusion of hydrogen from water through the Pt tube wall. To prevent this oxidation, a new double-chamber tube was designed, one chamber containing powdered metallic Cr to trap the excess of oxygen and the second chamber initially containing the starting gel and water. Using this tube, (chromium)-pargasites were produced with a maximum Cr content around 0.43 apfu from EMP analyses (based on 23 O atoms), close to the maximum Cr solubility observed in naturally occurring pargasites. In FTIR spectra of Cr-free pargasite, two intense OH-stretching bands are observed at 3714 and 3681 cm–1, corresponding to OH groups adjacent to Mg3 and to Mg2Al, respectively, and pointing toward a filled A-site. A third band is observed at 3656 cm–1, which can be assigned to OH groups pointing toward vacant A-sites. Along the Al-Cr pargasite join, significant modifications are observed in the OH-stretching region: a new band appears at 3660 cm–1, which can be assigned to OH groups adjacent to Mg2Cr, and pointing toward a filled A-site. Its relative intensity increases regularly with the Cr content, showing that Cr3+ enters M site(s) adjacent to an OH group. Combining EMP and FTIR data, we conclude that up to 0.27 Cr per formula unit occupies the M3 site. However, these are more octahedral trivalent cations than expected, and these are balanced by lower tetrahedral Al and lower occupancies of the A and M4 sites. This charge arrangement, with important deviations from ideal stoichiometry, is apparently the only stable one under the conditions applied. Results were confirmed at 900 °C and 20 kbar PH2O, indicating that increasing pressure does not affect Cr solubility in pargasite

Extending the prediction of the thermodynamic properties of clay minerals to the trapping of trace elements

The thermodynamic properties of clay minerals, which control the stability of these minerals in solution, are still a matter of debate in spite of recent advances (Gailhanou et al., submitted). This is especially the case for the minerals that may structurally include trace elements and potential radionuclides such like Ni, Cd, Co, Cr, Mn, Pb, ... The usual methods developed in order to predict thermodynamic properties are parameterised using a given set of minerals. For clay minerals, the latter are mainly composed by Si, Al, Fe and Mg, apart from the alkalis elements (Chermak and Rimstidt, 1989), which means that predictions are limited to minerals whose layers are composed by Si, Al, Fe and Mg. At the vicinity of H&ILW disposal cells, the possible interactions between clay rock or engineered barrier and waste degradation products can result in the appearance of clay minerals that may structurally include radionuclides within an irreversible trapping process. This work aims at proposing a method for predicting the thermodynamic properties of such minerals. Theoretical principle and selection of calibration phases Vieillard (1994) has developed a methodology of estimation based on the difference of electronegativity by considering three scales of values of the parameter HO=(Mz+clay) in the three sites of phyllosilicates. We have considered the work of Vieillard (1994) that originally applies to the estimate of H0f and extended it to the estimate of Cp(T), S0 and V. Some popular estimate methods (Chermak and Rimstidt, 1989) are based on the hypothesis that the thermodynamic property of a mineral can be obtained by combining the properties of its components. An improvement of this principle had consisted in decomposing minerals into their polyhedral components (Chermak and Rimstidt, 1989). Now, we can write the fictive solution equilibrium with a basic polyhedral component MxOy as: and assumming the entropy of this fictive reaction is zero, we can define a SO= parameter as: . The value for the oxide analog of the polyhedral unit is obtained by implementing S0 of the oxide in the S0(MxOy) term. We have also defined, from the same reasoning, similar parameters for heat capacity and volume of the basic polyhedral components: ; . Results and discussion On Figure 1, we have displayed, for entropy, the correlation obtained between calculated values of SO= for the polyhedral unit and for the oxide analog. A straight line and a second-order function are obtained, for the interlayer and octahedral cations, respectively, with a good correlation coefficient. Fig. 1 - Development of predictive capacity for entropy estimates The implementation of the derived semi-empirical, first or second order relations allows to estimate the thermodynamic properties of a clay mineral, MX80 (Na0.409K0.024Ca0.009(Si3.738Al0.262)(Al1.598Mg0.214Fe3+0.173Fe2+0.035)O10(OH)2) in the present case, loaded by 6 radionuclides and to compare the values with the results obtained by Gailhanou et al. (submitted).The results can be expressed in terms of the concentrations for the elements Ni, Cd, Co, Cr, Mn and Pb and in terms of energetic potential with respect to the measurements performed by Gailhanou et al. (submitted)

Water-bearing minerals on mars: source of observed mid-latitude water?

The Odyssey spacecraft documented the existence of heterogeneously distributed hydrogen at martian mid-latitudes, suggesting that large areas of the near-equatorial highlands contain near-surface deposits of 'chemically and/or physically bound H20 and/or OH' in amounts up to 3 .8% equivalent H20. Shallow occurrences of water ice are not stable near the martian equator, making the hydrogen deposits at these latitudes somewhat enigmatic. Clay minerals and zeolites have both been proposed as possible water-bearing constituents on Mars, and both are common terrestrial alteration products of hydrovolcanic basaltic ashes and palagonitic material comparable to those that may be widespread on Mars. Smectites within martian meteorites, attributed to hydrous alteration on Mars rather than on Earth, provide direct evidence of clay minerals from Mars. In addition, new thermal emission spectrometer (TES) data provide good evidence for unspecified zeolites in martian surface dust [6] . The nature of the hydrogen-containing material observed in the equatorial martian regolith is of particular importance to the question of whether hydrous minerals have formed in the past on Mars. Also, whether these minerals exist in a hydrated (i .e., containing H2O molecules in their structures) or dehydrated state is a crucial question . The existence of hydrous minerals is also important in connection with their possible role in affecting the diurnal variation of the martian atmosphere, in their potential role in unraveling the paleohydrology and paleobiology of Mars, and in their possible use as a water resource to support exploration of the martian mid-latitudes

Characterization of the nighttime low‐latitude water ice deposits in the NASA Ames Mars General Circulation Model 2.1 under present‐day atmospheric conditions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94673/1/jgre2617.pd