Low- to medium-temperature phase equilibria in a gas of solar composition

Abstract

Equilibrium mineral assemblages in a gas of solar composition have been calculated in the temperature and pressure ranges of 423–1023 K and 1 to 10 −8 bar, respectively. The results include the modal percent variation of all condensates and the chemical compositions of ideal and non-ideal coexisting solid solutions. The major mineral assemblage stable over most of the pressure and temperature field is γ- and α-Fe-Ni alloys + olivine + orthopyroxene + clinopyroxene + plagioclase + ilmenite. Other minerals entering the field are orthoclase at 628 K and troilite (FeS) at 621 K both with no effect of variation in pressure. Rutile is stable in parts of the high-temperature field. A typical model percent composition above 621 K is Fe-Ni-γ 2.46, Fe-Ni-α 40.26, olivine 28.94, orthopyroxene 18.25, clinopyroxene 6.26, plagioclase 3.56 and ilmenite 0.27 ( T = 663 K, P = 10 −4 bar). Below 621 K, there is a marked drop in the average density of the material. A small amount of almandine-rich garnet appears at 533 K, the temperature of appearance decreasing slightly with decreasing pressure. At low temperatures biotite, tremolite and talc are stable at high pressures, while graphite becomes important below 10 −7 bar. Compositions of plagioclase, olivine, orthopyroxene, ilmenite and garnet are found to be potential cosmobarometers and cosmothermometers.