Static compression of Mg(OH)2 to 78 GPa at high temperature and constraints on the equation of state of fluid H2O

Abstract

Static compression data on brucite, Mg(OH)2, to 35 GPa at 300 K and to 80 GPa at 600 K were measured by synchrotron X ray diffraction with a diamond‐anvil cell. A least squares fit yielded bulk modulus KT0 = 54.3(±1.5) GPa and its pressure derivative K′T0 = 4.7(±0.2) at 300 K, and KT = 48.8(±1.0) GPa and KT′ = 4.74(±0.12) at 600 K. The temperature derivative of the bulk modulus, (∂KT/∂KTp, is −0.018(±0.008) GPa/K, assuming that KT linearly depends on temperature. The average thermal expansivity α decreases from 80(±4) × 10−6 K−1 at ambient pressure to 10(±6) × 10−6 K−1 at 80 GPa. The coefficient as a function of pressure P (in gigapascals), can be expressed by α (K−1)= 80 × 10−6(1 + 0.087P)−0.97. The thermal expansion and elastic properties of Mg(OH)2 are highly anisotropic such that the c axis with the linear thermal expansion coefficient αc0 = 59(±4) × 10−6 K−1 and the linear incompressibility Kc0 = 75(±5) GPa, is about 5 times more expansible as well as compressible than the a axis with αa0 = 11(±3) × 10−6 K−1 and Ka0 = 388(±10) GPa. The present equation of state for Mg(OH)2 and previously determined equations of state for MgO and H2O ice VII were used to calculate the breakdown pressures of Mg(OH)2 into MgO plus H2O ice VII in the stability field of H2O ice VII. This equilibrium boundary is used to constrain the dehydration curve of Mg(OH)2 at very high pressure because the intersection point between this boundary and the melting curve of ice VII must be the triple point of Mg(OH)2, MgO plus ice VII, and MgO plus fluid H2O. The calculation shows that the equilibrium boundary between Mg(OH)2 and MgO plus ice VII intersects the melting curve of ice VII at 55(±15) GPa and 850(±20) K. The calculation of the melting curve of ice VII indicates that the existing equation of state of fluid H2O, derived from shock compression and molecular dynamics calculations, may overestimate the volume of fluid H2O at pressures above 50 GPa.