High-pressure crystal chemistry of phenakite (Be2SiO4) and bertrandite (Be4Si2O7(OH)2)
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Compressibilities and high-pressure crystal structures have been determined by X-ray methods at several pressures for phenakite and bertrandite. Phenakite (hexagonal, space group R\(\bar 3\)) has nearly isotropic compressibility with β=1.60±0.03×10−4 kbar−1 and β=1.45±0.07×10−4 kbar−1. The bulk modulus and its pressure derivative, based on a second-order Birch-Murnaghan equation of state, are 2.01±0.08 Mbar and 2±4, respectively. Bertrandite (orthorhombic, space group Cmc21) has anisotropic compression, with βa=3.61±0.08, βb=5.78±0.13 and βc=3.19±0.01 (all ×10−4 kbar−1). The bulk modulus and its pressure derivative are calculated to be 0.70±0.03 Mbar and 5.3±1.5, respectively.
Both minerals are composed of frameworks of beryllium and silicon tetrahedra, all of which have tetrahedral bulk moduli of approximately 2 Mbar. The significant differences in linear compressibilities of the two structures are a consequence of different degrees of T-O-T bending.
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