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

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 K T0 = 54.3(±1.5) GPa and its pressure derivative K′ T0 = 4.7(±0.2) at 300 K, and K T = 48.8(±1.0) GPa and K T ′ = 4.74(±0.12) at 600 K. The temperature derivative of the bulk modulus, (∂ K T /∂ K T p , is −0.018(±0.008) GPa/K, assuming that K T 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 + 0.087 P ) −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 α c 0 = 59(±4) × 10 −6 K −1 and the linear incompressibility K c0 = 75(±5) GPa, is about 5 times more expansible as well as compressible than the a axis with α a0 = 11(±3) × 10 −6 K −1 and K a0 = 388(±10) GPa. The present equation of state for Mg(OH) 2 and previously determined equations of state for MgO and H 2 O ice VII were used to calculate the breakdown pressures of Mg(OH) 2 into MgO plus H 2 O ice VII in the stability field of H 2 O 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 H 2 O. 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 H 2 O, derived from shock compression and molecular dynamics calculations, may overestimate the volume of fluid H 2 O at pressures above 50 GPa.