Thermodynamic behaviour of the high-temperature P\bar 1{\text{-}}I\bar 1phase transition along the CaAl2Si2O8–SrAl2Si2O8join

In situ high-temperature synchrotron radiation powder diffraction patterns were taken from room temperature to T = 740°C from synthetic feldspars along the join CaAl2Si2O8–SrAl2Si2O8 (An–SrF). Three samples of composition An95SrF5, An90SrF10 and An85SrF15 were investigated, and the evolution of cell parameters with T was determined by Rietveld analysis of powder X-ray diffraction patterns. The high-temperature \(P\bar 1{\text{-}}I\bar 1\) phase transition, previously observed with Tc = 241°C in anorthite, was found in An95SrF5, An90SrF10 and An85SrF15 feldspars at Tc = 233(5)°, 195(2)° and 174(2)°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 An90SrF10 and An85SrF15 [β = 0.504(7) and 0.505(7) respectively] or nearly second-order for An95SrF5[β = 0.458(4)] was observed in contrast with tricritical behaviour of end member anorthite. An extrapolation of the Tc versus composition to room temperature indicates that the critical composition for the \(P\bar 1\) phase is An60SrF40.