MELTING AND SUBSOLIDUS RELATIONS OF SIO2 AT 9-14 GPA

Experiments were performed on amorphous silica at temperatures of 1000 -degrees-2850-degrees-C and pressures of 9-14 GPa using a 2000-t uniax ial split-sphere multianvil apparatus (USSA-2000). The melting curve o f coesite was extended from 8 to 13.7 GPa, and the melting temperature s indicate a slope that is almost independent of pressure. Melting of stishovite was observed at 2850-degrees-C and 14 GPa. The triple point for coexisting coesite, stishovite, and melt was located at 2800-degr ees-C and 13.7 GPa. The coesite-stishovite phase transition was determ ined between 2000-degrees and 2800-degrees-C and is described by the e quation P (GPa) = 5.8(4) + 0.0028(2)T(degrees-C). The resulting dP/dT slope is more than twice that determined in the in situ X ray study of Yagi and Akimoto (1976). The present coesite-stishovite boundary cann ot be predicted by existing thermodynamic data (e.g., Fei et al., 1990 ), which suggests some uncertainties in the extrapolation of heat capa cities of coesite and stishovite at high temperatures. Thermodynamic e xtrapolations of the metastable melting curves of quartz and coesite s how a strongly negative slope and thus are consistent with the proposa l that the room temperature amorphization of quartz and coesite result s from crossing of the metastable extension of their melting curves.