Compression of synthetic hydroxylclinohumite [Mg9Si4O16(OH)2] and hydroxylchondrodite [Mg5Si2O8(OH)2

Abstract

The isothermal equations of state (EoS) of synthetic hydroxylclinohumite, Mg9Si4O16(OH)2, and synthetic hydroxylchondrodite, Mg5Si2O8(OH)2, have been determined using high-pressure singlecrystal X-ray diffraction, carried out in a diamond anvil cell under hydrostatic conditions. Both humites are monoclinic (space group P21/b with a unique): a = 4.7490(3) Å, b = 10.2861(4) Å, c = 13.6991(11) Å and α = 100.649(6)° for hydroxylclinohumite, and a = 4.7449(2) Å, b = 10.3464(2) Å, c = 7.8990(6) Å, and α = 108.681(3)° for hydroxylchondrodite. A third-order Birch-Murnaghan EoS was determined from unit-cell volume data to 8.1 GPa for hydroxylclinohumite: V0 = 657.69(5) Å3, KT = 119.4(7) GPa and K' = 4.8(2). A similar analysis of hydroxylchondrodite for data collected to 7.8 GPa resulted in V0 = 367.36(3) Å3, KT = 115.7(8) GPa and K' = 4.9(2). Axial compression is anisotropic with the direction perpendicular to the close-packed anion layer, i.e., the a axis, being the least compressible. Axial moduli and their pressure derivatives are: Ka = 162(1) GPa, Ka' = 6.7(3), Kb = 97.9(5) GPa, Kb' = 4.0(1), Kc = 111.1(7) GPa and Kc' = 4.2(2) for hydroxylclinohumite. For hydroxylchondrodite: Ka = 149(1) GPa, Ka' = 6.8(3), Kb = 101.3(4) GPa, Kb' = 4.3(1), Kc = 102.4(6) GPa, and Kc' = 4.1(2). Comparison of the bulk moduli of these phases with other phases along the Mg2SiO4-Mg(OH)2 join shows that the bulk modulus increases systematically with density (ρ) and can be approximated by, KT (GPa) = 97(6) × ρ - 186(17). The bulk modulus also decreases systematically with water content: KT (GPa) = 127.9 (16) - 2.75 (11) × wt% H2O.