Na, P, Ti and coordination of Si in garnet from peridotite and eclogite xenoliths

Abstract

THEORY and experiment show that some cations are surrounded by extra oxygen anions at elevated pressure: for example, Si changes from 4-coordination in quartz and coesite to 6-coordination in stishovite. Sobolev and Lavrent'lev¹ suggested that substitution of Na in garnet derived from the mantle is coupled with 6-coordinated Si according to the equation Ca^VIII+Al^VI→Na^VIII+Si^VI. We show here from sensitive electron-microprobe analyses that Na in garnets from ultramafic and eclogite xenoliths can be explained by coupled substitutions with Ti and P without invoking 6-coordinated Si.