COMPRESSION AND COORDINATION CHANGES IN PYROXENOIDS - AN EXAFS STUDY OF MGGEO3 ENSTATITE AND CAGEO3 WOLLASTONITE

The evolution of the distortion of MgGeO3 enstatite and CaGeO3 wollastonite with increasing pressure, has been investigated using X-ray absorption spectroscopy (XAS) in a diamond anvil cell. At room temperature and low pressure (P < 7 GPa), the compressibility of the GeO4 tetrahedron is higher in MgGeO3 enstatite (K[GeO4]approximately 135 GPa) than in CaGeO3 wollastonite (K[GeO4]greater-than-or-equal-to 280 GPa). The compression mechanisms of the two compounds are different: the whole mineral compressibility of Ge-enstatite appears to be very homogeneous, in contrast to that of Ge-wollastonite which exhibits an inhomogeneous tretrahedral compressibility. This result is consistent with the variation of the Debye-Waller factors of the two compounds with increasing pressure. At higher pressures, the coordination of germanium atoms in the two compounds gradually changes from fourfold to sixfold. For CaGeO3 the coordination change starts at 7 GPa and is complete a 12 GPa, whereas it starts at about 8.5 GPa for MgGeO3 and is not complete at 31 GPa. The progressive evolution of the measured Ge-O distances as well as the modification in the X-ray absorption near-edge structure indicate two coexisting different sites rather than a progressive site modification. The transformation is found to be partially reversible in CaGeO3 wollastonite. whereas it is totally reversible in MgGeO3 enstatite.