High-temperature ferroelastic strain below the I2/c-I1 transition in Ca (sub 1-x) Sr (sub x) Al (sub 2) Si (sub 2) O (sub 8) feldspars

Cell parameters of plagioclases of composition Ca (sub 1) (sub x) Sr (sub x) Al (sub 2) Si (sub 2) O (sub 8) (x = 0.85, 0.80 and 0.75) have been measured as a function of T from room-T to 950 degrees C. At room-T each is triclinic (I<$E 1 back 40 up 40 ->), undergoing a continuous ferroelastic displacive transition to a monoclinic phase (I2/c) on heating; the critical T increases as the Ca content increases. Curvature of the transition line through composition (X-T space suggests the importance of higher-order coupling between X and the order parameter. Strain-tails in alpha and Gamma close to T (sub c) are interpreted in terms of inhomogeneity of the M-cation distribution, related to tweed microstructures. A Landau model is derived which describes the transition-related phenomena in this system as a function of T and X.