In situ, high-pressure single-crystal X-ray diffraction experiments were performed on a feldspar with composition SrAl2Si2O8 (space group I2/c). The measurements were performed at 19 different pressures up to 7.9 GPa. The unit-cell parameters and the unit-cell volume show a strong discontinuity at 6.6 GPa indicating a first-order phase transition. The systematic absences clearly show that the transition occurs from I2/c to P21/c space group symmetry, as for the transition recently reported for a feldspar with composition Sr0.8Ca0.2Al2Si2O8.
Our results suggest that the P21/c phase is softer than the I2/c one. Moreover, within the I2/c symmetry two pressure–volume trends are evident due to a volume softening above 4.2 GPa. The softening did not allow fitting the data to a single equation of state, and only the data up to 3.67 were fitted with a third-order Birch-Murnaghan equation of state.
Based on our and previous experimental results we propose a possible composition-pressure stability field for the Sr-rich side along the CaAl2Si2O8-SrAl2Si2O8 join.