A transmission electron microscope investigation was performed on a series of synthetic clinopyroxenes with compositions between Ca0.7Mg1.3Si2O6 and Ca0.5Mg1.5Si2O6. For samples with Ca content lower than 0.6 atoms per formula unit (apfu) selected-area electron diffraction (SAED) patterns showed the presence of reflections violating the C2/c space group (b type, h+k odd), indicating a transition to the P21/c symmetry at room temperature. Antiphase domains induced by the transition could be imaged, with irregular boundaries and size decreasing with increasing Ca content. The antiphase domain size and shape appears unrelated to the non-periodic mottled texture that could be imaged in samples with Ca lower than about 0.65 apfu.
A comparison with data reported in the literature suggests a different behavior in the cell parameters of the P21/c clinopyroxenes along the diopside-enstatite (Di-En) join. For compositions between Di60En40 to about Di40En60, cell parameters deviate little with respect to the trend shown by Ca-rich C2/c clinopyroxenes, whereas for compositions richer in Mg than about Di40En60, a significant deviation is present, which is marked by discontinuities in the c and β cell parameters.