Phase relationships in the system Mg₃Al₂Si₃O₁₂–Mg₃Cr₂Si₃O₁₂ at high pressure and some mineralogical properties of synthetic garnet solid solutions

Abstract

Subsolidus phase relationships in the system Mg₃Al₂Si₃O₁₂–Mg₃Cr₂Si₃O₁₂ are studied experimentally under high pressure and high temperature conditions. Phase boundary between garnet single phase and lower pressure phases moves toward higher pressure with the increase of knorringite molecule, and is nearly coincident with that estimated by Irifune et al. (1982). The low pressure phase assemblage of garnet_ss+pyroxene_ss+spinel_ss+quartz (coesite) appears at the pressures below ca. 6.7 GPa, instead of garnet_ss+pyroxene_ss+corundum_ss which was reported by Malinovsky et al. (1975) and Irifune et al. Lattice parameters of the garnet solid solutions are unexceptionally larger than the values obtained by linear combination of those for both pyrope and knorringite, in contrast with the refractive indices which show linear change between the end-members. The absence of garnet with more than 50% of knorringite molecule in inclusions of natural diamonds and xenoliths is considered to reflect that the mantle materials below ca. 240 km had not been erupted on the surface of the earth and/or such a garnet with over 50% of knorringite molecule was never formed in the earth’s interior on account of the structural change of the silicate melt, which is expected around that depth.