In situ high temperature single crystal X-ray diffraction study of a natural omphacite

In situ high temperature single crystal X-ray diffraction (XRD) experiments have been performed on a chemically quasi-ideal omphacite sample [(Ca0.49Na0.51)(\u3a31)(Mg0.46Al0.48Fe0.063+)(\u3a31)(Si1.97 Al0.03)(\u3a32)O6], up to 1000\ub0C. The lattice parameters were studied as a function of temperature, and their thermal expansion coefficients determined. The b and c cell edges show discontinuities as a function of temperature which are interpreted here in terms of intracrystalline cation diffusion processes. Structure refinements have been carried out using data collected at room temperature, at 800\ub0C and at ambient conditions after cooling. The structural behaviour as a function of temperature of chemically quasi-ideal omphacites is compared with those of jadeite and diopside