CRYSTAL-CHEMISTRY OF FERROMAGNESIAN SILICATE SPINELS - EVIDENCE FOR MG-SI DISORDER

Single-crystal X-ray diffraction data have been obtained for synthetic Fe-Mg silicate spinels, gamma-(MgxFe1-x)2SiO4 (x = 1.00, 0.40, and 0.20). The measurements on gamma-Mg2SiO4 synthesized at 20 GPa and 1400-degrees-C, compared with previous data on a specimen synthesized at 22 GPa and 1000-degrees-C, provide evidence that approximately 4% of Si(tot), enters octahedral coordination in the sample synthesized at higher temperature. Fe-bearing silicate spinels synthesized at lower pressures display no evidence for Fe-Si disorder.