High-pressure crystal chemistry of phenakite (Be₂SiO₄) and bertrandite (Be₄Si₂O₇(OH)₂)

Abstract

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 β _a =3.61±0.08, β _b =5.78±0.13 and β _c =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.