The high-temperature P2/c1?C2/c phase transition in Fe-free Ca-rich P21/c clinopyroxenes

An in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca0.40Mg1.60Si2O6, Ca0.52Mg1.46Al0.05Si1.98O6, Ca0.59Mg1.41Si2O6 and Ca0.70Mg1.30Si2O6, up to 850 °C using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca0.52Mg1.46Al0.05Si1.98O6 and Ca0.59Mg1.41Si2O6, evidence of for the P21/c-C2/c displacive phase transition was seen in changes in lattice parameters at T ≈ 550 and 300 °C respectively. Landau modelling of the phase transition behaviour for the sample with composition Ca0.52Mg1.46Al0.05Si1.98O6 shows a tricritical behaviour [Tc=547(16)]. Comparison with the transition behaviour in other samples with lower Ca contents along the join diopside–enstatite indicates that a decrease in Tc, and a switch from first-order to tricritical behavior occurs with increasing Ca content. The change in the transition behaviour was related to an interaction with the antiphase domains at the nanoscale.