The crystal structure of intermediate clinopyroxenes along the join diopside-enstatite (CaMgSi_2O_6-Mg_2Si_2O_6)

The structures of five clinopyroxenes on the diopside–enstatite join have been refined by Rietveld analysis of powder-diffraction patterns obtained with synchrotron radiation. The reliability of the present Rietveld refinements was checked against previous single-crystal data on the sample with composition Di80En20, and it showed reasonable agreement. A difference in the y/b M2 coordinate is related to the lower resolution of the powder-diffraction data of this work, not sufficient to detect the splitting at the M2 site. The Rietveld analysis allowed us to detect the strain modulations on (010) prior to exsolution. The small size of primitive antiphase domains, less than the length-scale of X-ray diffraction, prevents P21/c refinement in samples with composition Di59En41 and Di52En42CaTs2. The refined clinopyroxenes encompass the transition from P21/c to C2/c structures. In the P21/c structure, the A and B chains of tetrahedra undergo a slight straightening with increasing Ca content, and changes similar to those observed at high temperature. The results are in general agreement with large changes in the configuration of the chains at the transition, as observed during the P21/c – C2/c transition at high pressure and high temperature.