High-pressure crystal chemistry of LiScSiO₄: An olivine with nearly Isotropic compression

Single-crystal X-ray diffraction data have been obtained at several pressures to 5.6 GPa for synthetic LiScSiO₄ olivine. The bulk modulus is 118 ± 1 GPa, assuming K′ = 4. This value is smaller than that of forsterite because compressibility of Li-O bonds in the M1 octahedral site is approximately twice that of the M1²⁺-O bonds in other isomorphs. Compressibilities of a, b, and c orthorhombic axes are 2.70, 2.80, and 2.61 (all × 10^-3 GPa^-1), respectively. This nearly isotropic compression (axial compression ratios = 1.00: 1.04:0.97) contrasts with that of forsterite (1.00:1.99:1.55), fayalite (1.00:2.83:1.22), monticellite (1.00:1.85:1.10), and chrysoberyl (1.00:1.30:1.17). These differences arise from the distinctive distribution of cations of different valences and consequent differences in M1-O and M2-O bond compressibilities. The Li M1 octahedron displays a significant decrease in polyhedral distortions with pressure, a behavior not observed in other olivines.