High temperature phase equilibria in a solar-composition gas

Using recent additions to thermochemical data on minerals and information on their solid solution behavior, new equilibrium phase diagrams have been computed in a system of solar gas composition (Si, Al, Mg, Ca, Fe, Ni, Ti, Na, K, C, H, O, S, N) in the pressure and temperature ranges of 1 to 10-6 bar and 1153 to 1773 K respectively. These calculations show that Fe-Ni alloy condenses before all silicates included here (except melilite) down to a pressure of 2 x 10-4 bar below which plagioclase and clinopyroxene condense first. Orthopyroxene condenses next followed by ilmenite. Pressure-temperature variation of the chemical composition of melilite, clinopyroxene, orthopyroxene, metal alloy and plagioclase may be used for cosmothermometry and cosmobarometry for equilibrium assemblages. The major transition from the refractory oxides and melilite (the meteorite 'inclusion assemblage') to an assemblage of Fe-Ni alloy, olivine, plagioclase and pyroxenes ('planet-forming') takes place within a narrow interval of pressure and temperature. Small fluctuations of either pressure or temperature across this narrow region result in drastic changes in types and modes of minerals, which may explain the wide mineralogical varieties of meteorites. (author)