The thermal expansion of monticellite and olivine

The thermal expansion of two natural monticellite (CaMgSiO4) samples was measured by X-ray powder diffraction between 298 and 1250 K. Axial expansion in monticellite is anisotropic with α(c) > α(b) ≥α(a), with the volume expansion nearly equivalent to that of synthetic forsterite as measured in the same laboratory, although forsterite axes expand as α(b) > α(c) > α(a). Kroll physical and Fei empirical models have been used to describe the volume and axial thermal expansion of both minerals. High-temperature structural data for these minerals show that the Ca-occupied M2 polyhedron in monticellite expands less than the Mg-occupied site in forsterite, resulting in comparatively lower expansion along the b crystallographic axis. But greater expansion of the M1 polyhedron in monticellite accounts for greater expansion along its c axis and, in turn, the nearly equivalent volume expansion relative to that of forsterite.