HIGH-TEMPERATURE FERROELASTIC STRAIN BELOW THE I2 C-I(1)OVER-BAR TRANSITION IN CA1-XSRXAL2SI2O8 FELDSPARS/

Lattice parameters of feldspars of composition Ca1-xSrxAl2Si2O8, (x = 0.85, 0.80, and 0.75) have been measured as a function of temperature (T) from room-T to 950 degrees C. At room-T each is triclinic (<I(1)ov er bar>), undergoing a continuous ferroelastic displacive transition t o a monoclinic phase (I2/c) on heating. The observed critical temperat ure (T,) increases as Ca content increases. The thermodynamic characte r of the transition also changes, from near-second-order at x = 0.85, approaching tricritical as Ca content increases. Curvature of the tran sition line through composition (X)-T space suggests the importance of high-order coupling between X and the order parameter. Strain-tails i n alpha and gamma close to T-c are interpreted in terms of inhomogenei ty of the M-cation distribution, related to tweed microstructures note d in previous investigations. A Landau model is derived which describe s the observed transition-related phenomena in the system Ca1-xSrxAl2S i2O8 as a function of T and X.