Silica is thought to be present in the Earth's lower mantle in subducting plates, in addition to being a prototypical solid whose physical properties are of broad interest. It is known to undergo a phase transition from stishovite to the CaCl2-type structure at similar to 50-80 GPa, but the exact location and slope of the phase boundary in pressure-temperature space is unresolved. There have been many previous studies on the equation of state of stishovite, but they span a limited range of pressures and temperatures, and there has been no thermal equation of state of CaCl2-type SiO2 measured under static conditions. We have investigated the phase diagram and equations of state of silica at 21-89 GPa and up to similar to 3300 K using synchrotron X-ray diffraction in a laser-heated diamond-anvil cell. The phase boundary between stishovite and CaCl2-type SiO2 can be approximately described as T = 64.6(49).P -2830(350), with temperature T in Kelvin and pressure P in GPa. The stishovite data imply K-0' = 5.24(9) and a quasi-anharmonic T-2 dependence of -6.0(4) x 10(-6) GPa.cm(3)/mol/K-2 for a fixed q = 1, gamma(0) = 1.71, and K-0 = 302 GPa, while for the CaCl2-type phase K-0 = 341(4) GPa, K-0' = 3.20(16), and gamma(0) = 2.14(4) with other parameters equal to their values for stishovite. The behaviors of the a and c axes of stishovite with pressure and temperature were also fit, indicating a much more compressible c axis with a lower thermal expansion as compared to the a axis. The phase transition between stishovite and CaCl2-type silica should occur at pressures of 68-78 GPa in the Earth, depending on the temperature in subducting slabs. Silica is denser than surrounding mantle material up to pressures of 58-68 GPa, with uncertainty due to temperature effects; at higher pressures than this, SiO2 becomes gravitationally buoyant in the lower mantle.
