High-temperature ferroelastic strain below the I2/c-I1̄ transition in Ca_1-xSr_xAl₂Si₂O₈ feldspars

Lattice parameters of feldspars of composition Ca_1-xSr_xAl₂Si₂O₈, (x = 0.85, 0.80, and 0.75) have been measured as a function of temperature (T) from room-T to 950°C. At room-T each is triclinic (I1̄), undergoing a continuous ferroelastic displacive transition to a monoclinic phase (I2/c) on heating. The observed critical temperature (T_c) increases as Ca content increases. The thermodynamic character of the transition also changes, from near-second-order at x = 0.85, approaching tricritical as Ca content increases. Curvature of the transition line through composition (X)-T space suggests the importance of high-order coupling between X and the order parameter. Strain-tails in α and γ close to T_c are interpreted in terms of inhomogeneity of the M-cation distribution, related to tweed microstructures noted in previous investigations. A Landau model is derived which describes the observed transition-related phenomena in the system Ca_1-xSr_xAl₂Si₂O₈ as a function of T and X.