Olivine-Garnet Transformation in a Meteorite

Abstract

Garnet has been identified for the first time as a meteorite mineral in the Coorara chondrite from Western Australia. It replaces olivine grains in a 1-millimeter veinlet traversing the body of the meteorite. The associated olivine has abnormally low birefringence, which suggests a highly shocked condition. Microprobe analyses do not distinguish the garnet from the associated olivine, which has the composition (Mg_.75Fe_.25)₂ SiO₄; the garnet may have the composition Mg₃Fe₂Si₃O₁₂ but be unresolvable from the accompanying olivine, or alternatively is nonstoichiometric. Transformation of olivine to garnet under high pressure could have significant implications for the phase composition of the Earth's mantle.