Clinopyroxene / High and low temperature X-ray diffraction / Crystal structure Abstract.A single crystal of a synthetic C2/c clinopyroxene of composition Cao.8Mg 1 . 2 Si 2 0 6 (Di 80 En 2 o) has been used for X-ray intensity data collection at Τ = -130°, 25°, 400° and 700° C. A residual in electron density along the diad axis, indicating a M2' split position, has been observed up to 700° C. Structure refinements according to the M2-split model led to unweighted R values between 2.9% and 4.9%. The most important feature observed in the average structures was the deformation of the M2 polyhedron with increasing temperature, leading to a decrease in Ca coordination from 8-to 6-fold as reported for other chain silicates. The temperature increase causes a M2 polyhedral deformation opposite to that observed at room temperature, when there is an increase in the mean ionic radius of cations in M2 (chemical substitution). A possible interpretation is that at high temperature the M2 cavity retains the expansion along the diad axis imposed by octahedral expansion and by changes of the tetrahedral chain. Expansion of the M2 polyhedron becomes too great for the cations that occupy the cavity; tetrahedral chain shifting and consequent M2 polyhedral distortion allow reasonable bond lengths to be maintained through release of two of the four 03. Thermal ellipsoids analysis shows that at low and room temperature the average structure is affected by a significant positional disorder, while at higher temperature thermal vibration prevails.