High-temperature structural study of six olivines

Single-crystal X-ray intensity data have been collected with a manual diffractometer utilizing flat-cone geometry for Ni-olivine, Ni₂SiO₄, (25°, 300°, 600°, and 900°C), monticellite, Ca_1.00Mg_O.93Fe_0.07SiO₄, (25°, 335°, 615°, and 795°C), and glaucochroite, Ca_0.98Mn_0.87Mg_0.10Zn_0.05SiO₄, (25°, 300°, 600°, and 800°C). Weighted refinements with anisotropic temperature factors resulted in residual (R) factors ranging from 0.026 to 0.048. Examination of these data, together with published data from high-temperature refinements of forsterite, hortonolite, and fayalite, indicates that the olivine structure expands primarily as a result of bond-length expansions in M(1) and M(2) octahedra. Mean Mg-O distances exhibit the greatest thermal expansions while the mean M-О distances in Fe-containing olivines generally exhibit the smallest expansions. The M(2) cation position in all six olivines is temperature-dependent. With increasing temperature, the cation is displaced in a direction away from the triangle of shared edges. The magnitude of this displacement, measured relative to the centroid of coordinating oxygens, is greatest for non-transition metal cations (Mg²⁺, Ca²⁺).