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 ∼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 ∼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 K0′ = 5.24(9) and a quasi-anharmonic T2 dependence of –6.0(4) × 10–6 GPa·cm3/mol/K2 for a fixed q = 1, γ0 = 1.71, and K0 = 302 GPa, while for the CaCl2-type phase K0 = 341(4) GPa, K0′ = 3.20(16), and γ0 = 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.

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