NATURAL HIGH-PRESSURE POLYMORPH OF MERRILLITE IN THE SHOCK VEINS OF THE SUIZHOU METEORITE

Abstract

A new high-pressure polymorph of merrillite with the structure of trigonal γ-Ca3(PO4)2 was found in the shock-produced veins of the Suizhou meteorite, where it coexists with ringwoodite, majorite, NaAlSi3O8-hollandite, and majorite-pyrope garnet. The crystallographic nature of this natural γ-Ca3(PO4)2 phase was characterized by Raman spectroscopy and X-ray diffraction, and all data compare favorably to the same data obtained from γ-Ca3(PO4)2 synthesized at 14 GPa and 1400°C. The cell parameters of this new high-pressure mineral are a = 5.258(1) angstroms and c = 18.727(3) angstroms, space group R-3m, and density = 3.447 (g/cm3), where the number in parentheses are standard deviations in the last significant digits. The natural occurrence of the γ-Ca3(PO4)2 phase together with other high-pressure minerals constrains the pressure of the shock veins at about 23 GPa. The Suizhou meteorite provides the first naturally occurring example of γ-Ca3(PO4)2 polymorph.