High-pressure crystal chemistry of chrysoberyl,Al2BeO4: Insights on the origin of olivine elastic anisotropy

High-pressure crystal structure refinements and axial compressibilities have been determined by x-ray methods for the olivine isomorph chrysoberyl, Al₂BeO₄. Unlike silicate olivines, which are more than twice as compressible along b than along a, chrysoberyl (space group Pbnm) has nearly isotropic compressibility with β _a =1.12±0.04, β _b =1.46±0.05, and β _c =1.31±0.03 (all×10^?4 kbar^?1). The resultant bulk modulus is 2.42±0.05 Mbar, with K′ assumed to be 4. The axial compression ratios of chrysoberyl are 1.00:1.30:1.17, compared to axial compression ratios 1.00:2.02:1.60 for forsterite. These differences in compression anisotropy arise from differences in relative bond compressibilities. In chrysoberyl the average aluminum-oxygen and beryllium-oxygen bond compressibilities are similar, yielding nearly isotropic compression, but in silicate olivines octahedral cation-oxygen bonds are significantly more compressible than Si-O bonds, so that compression parallel to a is much more restricted than that parallel to b. The inherent anisotropy of the olivine structure is not, by itself, sufficient to cause anisotropic compression. It appears that in the case of olivine the distribution of cations of different valences, in conjunction with the structure type, leads to anisotropies in physical properties.