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Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.
The isothermal equations of state (EoS) of synthetic hydroxylclinohumite, Mg9Si4O16(OH)2, and synthetic hydroxylchondrodite, Mg5Si2O8(OH)2, have been determined using high-pressure single-crystal 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 
= 100.649(6)° for hydroxylclinohumite, and a = 4.7449(2) Å, b = 10.3464(2) Å, c = 7.8990(6) Å, and = 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 (
) and can be approximated by, KT (GPa) = 97(6) x – 186(17). The bulk modulus also decreases systematically with water content: KT (GPa) = 127.9 (16) – 2.75 (11) x wt% H2O.
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