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

Abstract

An in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca_0.40Mg_1.60Si₂O₆, Ca_0.52Mg_1.46Al_0.05Si_1.98O₆, Ca_0.59Mg_1.41Si₂O₆ and Ca_0.70Mg_1.30Si₂O₆, up to 850 °C using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca_0.52Mg_1.46Al_0.05Si_1.98O₆ and Ca_0.59Mg_1.41Si₂O₆, 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 Ca_0.52Mg_1.46Al_0.05Si_1.98O₆ shows a tricritical behaviour [T _c =547(16)]. Comparison with the transition behaviour in other samples with lower Ca contents along the join diopside–enstatite indicates that a decrease in T _{c ,} 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.