A transmission electron microscope investigation of the C2/c -> P2(1)/c phase transition in clinopyroxenes along the diopside-enstatite (CaMgSi2O6-Mg2Si2O6) join

A transmission electron microscope investigation was performed on a series of synthetic clinopyroxenes with compositions between Ca0.7Mg1.3Si2O6 and C a0.5Mg1.5Si2O6. For samples with Ca content lower than 0.6 atoms per formul a 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 P2(1)/c symmetry at room temperature. Antiph ase domains induced by the transition could be imaged, with irregular bound aries and size decreasing with increasing Ca content. The antiphase domain size and shape appears unrelated to the non-periodic mottled texture that c ould be imaged in samples with Ca lower than about 0.65 apfu. A comparison with data reported in the literature suggests a different beha vior in the cell parameters of the P2(1)/c clinopyroxenes along the diopsid e-enstatite (Di-En) join. For compositions between Di(60)/En(40) to about D i(40)En(60), cell parameters deviate little with respect to the trend shown by Ca-rich C2/c clinopyroxenes, whereas for compositions richer in Mg than about Di(40)En(60), a significant deviation is present, which is marked by discontinuities in the c and beta cell parameters.