The low-pressure stability of phase A, Mg7Si2O8(OH)6

Article Abstract:

Thermodynamic investigations reveal the low-pressure stability of phase A, Mg7Si2O8(OH)6. Experiments involving mixtures of phase A, brucite, and forsterite enable the determination of enthalpy and entropy parameters for the reaction phase A = brucite + forsterite. The results, when compared with the established pressure-temperature paths of subducting slabs, indicate that Mg-rich phase A retains its stability at much shallower depths. A modified version of the thermodynamic dataset THERMOCALC is used in the study.

Thermodynamics, Stability

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Thermal expansivities and compressibilities of hydrous phases in the system MgO-SiO2-H2O: talc, phase A and 10-angstrom phase

Article Abstract:

A study shows that volume increases linearly with temperature for talc, the 10-angstrom phase and the phase A of the MgO-SiO2-H2O system. The thermal expansivity and compressibility are greatest along the normal direction to the talc sheets. This may be due to stronger bonds within the talc sheets compared to the interlayer bonds. The study uses powder X-ray diffraction method for the measurements. Details of the compressibility measurements at different pressures for all the three hydrous phases are given.

Expansion (Heat), Thermal expansion, Compressibility, Rocks, Ultrabasic, Ultrabasic rocks

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Experimental measurements of the graphite C-O equilibrium and CO2 fugacities at high temperature and pressure

Article Abstract:

A sliding redox sensor of (Ni,Mn)O + Ni metal is used to measure the oxygen fugacity, f(sub O2), of the graphite C-O equilibrium. The measurements are made for pressures ranging between 15-30 kbar and temperatures between 1100-1400 degrees centigrade. The fugacity measurements are used in estimating the fugacity of CO2 with an error of plus or minus 0.1 log units. Experimental details of fugacity measurements and an analysis of the results are given.

Geochemistry, Graphite

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