Si-Disordering in MgAl2O4-Spinel under High P-T Conditions, with Implications for Si-Mg Disorder in Mg2SiO4-Ringwoodite

Abstract

A series of Si-bearing MgAl2O4-spinels were synthesized at 1500–1650 °C and 3–6 GPa. These spinels had SiO2 contents of up to ~1.03 wt % and showed a substitution mechanism of Si4+ + Mg2+ = 2Al3+. Unpolarized Raman spectra were collected from polished single grains, and displayed a set of well-defined Raman peaks at ~610, 823, 856 and 968 cm−1 that had not been observed before. Aided by the Raman features of natural Si-free MgAl2O4-spinel, synthetic Si-free MgAl2O4-spinel, natural low quartz, synthetic coesite, synthetic stishovite and synthetic forsterite, we infer that these Raman peaks should belong to the SiO4 groups. The relations between the Raman intensities and SiO2 contents of the Si-bearing MgAl2O4-spinels suggest that under some P-T conditions, some Si must adopt the M-site. Unlike the SiO4 groups with very intense Raman signals, the SiO6 groups are largely Raman-inactive. We further found that the Si cations primarily appear on the T-site at P-T conditions ≤~3–4 GPa and 1500 °C, but attain a random distribution between the T-site and M-site at P-T conditions ≥~5–6 GPa and 1630–1650 °C. This Si-disordering process observed for the Si-bearing MgAl2O4-spinels suggests that similar Si-disordering might happen to the (Mg,Fe)2SiO4-spinels (ringwoodite), the major phase in the lower part of the mantle transition zone of the Earth and the benchmark mineral for the very strong shock stage experienced by extraterrestrial materials. The likely consequences have been explored.