A single-crystal X-ray refinement has been performed on two synthetic P21/c clinopyroxenes of composition Ca0.15Mg1.85Si2O6 (Di15En85) and Ca0.23Mg1.77Si2O6 (Di23En77). Splitting of the M2, O2B, O3A and O3B sites was inferred by the presence of significant residuals in the difference Fourier map and by an anomalous size and orientation of the displacement parameters. A refinement performed according to a split-model showed the coexistence of two M2 polyhedral configurations, one clinoenstatite-like, the other like Caricher C2/c clinopyroxenes. The two configurations have been interpreted assuming the coexistence of Ca and Mg centred polyhedra, possibly in connection with clustering preliminary to exsolution. The difference between the two configurations increases with increasing Ca content. A TEM investigation of the sample with composition Ca0.15Mg1.85Si2O6 showed the lack of exsolution textures and antiphase domains larger than 1μm.

Analysis of the average structural configurations shows that the substitution of Ca for Mg deforms differently Ca-poorer (up to 0.2–0.3 atoms p.f.u) and Ca-richer (between 0.3 and 0.6 Ca atoms p.f.u.) P21/c iron-free clinopyroxenes. In Ca-poorer ones only minor changes in the β parameter were found, related to a lack of shift in tetrahedral chains, whereas Ca-richer undergo a significant decrease in the βparameter, possibly due to stronger chain shift.

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