Experiments were performed on amorphous silica at temperatures of 1000¿--2850 ¿C and pressures of 9--14 GPa using a 2000-t uniaxial split-sphere multianvil apparatus (USSA-2000). The melting curve of coesite was extended from 8 to 13.7 GPa, and the melting temperatures indicate a slope that is almost independent of pressure. Melting of stishovite was observed at 2850 ¿C and 14 GPa. The triple point for coexisting coesite, stishovite, and melt was located at 2800 ¿C and 13.7 GPa. the coesite-stishovite phase transition was determined between 2000¿ and 2800 ¿C and is described by the equation P (GPa)=5.8(4)+0.0028(2)T(¿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 cannot be predicted by existing thermodynamic data (e.g., Fei et al., 1990), which suggests some uncertainties in the extrapolation of heat capacities of coesite and stishovite at high temperatures. Thermodynamic extrapolations of the metastable melting curves of quartz and coesite show a strongly negative slope and thus are consistent with the proposal that the room temperature amorphization of quartz and coesite results from crossing of the metastable extension of their melting curves. ¿ American Geophysical Union 1993