Thermodynamic behaviour of the high-temperature P(1)over-bar-I(1)over-bar phase transition along the CaAl2Si2O8-SrAl2Si2O8 join

In situ high-temperature synchrotron radiation powder diffraction patterns were taken from room temperature to T = 740 degrees C from synthetic feldspars along the join CaAl2Si2O8-SrAl2Si2O8( An - SrF). Three samples of composition An(95)SrF(5), An(90)SrF(10) and An(85)SrF(15) were investigated, and the evolution of cell parameters with T was determined by Rietveld analysis of powder X-ray diffraction patterns. The high-temperature P (1) over bar -1- I (1) over bar phase transition, previously observed with T-c = 241 degrees C in anorthite, was found in An(95)SrF(5), An(90)SrF(10) and An(85)SrF(15) feldspars at T-c = 233(5)degrees, 195(2)degrees and 174(2)degrees C respectively. The transition was revealed by the disappearance of critical reflections and variations in the rate of change of cell parameters with temperature. A significant, although small ( between 0.0025 and 0.0012 at room temperature), spontaneous strain could be measured, allowing the thermodynamic behaviour of the transition to be modelled. A second-order trend for An(90)SrF(10) and An(85)SrF(15) [beta = 0.504(7) and 0.505(7) respectively] or nearly second-order for An(95)SrF(5)[beta = 0.458(4)] was observed in contrast with tricritical behaviour of end member anorthite. An extrapolation of the T-c versus composition to room temperature indicates that the critical composition for the P (1) over bar phase is An(60)SrF(40).