High-pressure I2/c-P2₁/c phase transformation in SrAl₂Si₂O₈ feldspar

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 Sr_0.8Ca_0.2Al₂Si₂O₈.

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.