The isothermal equation of state (EoS) of phase A, Mg7Si2O8(OH)6, has been determined using high-pressure single-crystal X-ray diffraction. A third-order Birch-Murnaghan EoS fit to pressure-volume data collected from room pressure and temperature to 7.6 GPa results in V0 = 512.56(3) Å3, K0 = 97.5(4) GPa, and K′ = 5.97(14). Compression of the hexagonal (P63) structure is anisotropic with the c axis, which is perpendicular to the distorted close-packed planes of anions, approximately 23% less compressible than the a axis: Ka = 90.1(5) GPa with Ka ′ = 5.4(2) and Kc = 116.8(9) GPa with Kc = 7.5(3). The bulk modulus of phase A is intermediate between those of brucite (Br) and forsterite (Fo) and less than those of hydroxylclinohumite and hydroxylchondrodite, in a manner that is entirely consistent with its water content and density in relation to the Fo-Br series of minerals.

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