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A synthetic feldspar of composition Ca 0.2 Sr 0.8 Al 2 Si 2 O 8  (An 20 SrF 80 ) (space group I1̄ at room condition, Q od  = 0.88) was investigated using in situ high-pressure single-crystal X-ray diffraction. Unitcell parameters were measured as a function of pressure to 7.7 GPa at room T. Two reversible phase transitions were observed in the investigated pressure range. A first-order triclinic I1̄ to monoclinic I2/c phase transition was observed at P ∼ 4.3 GPa. Several cycles through the transformation were made to constrain the hysteresis (∼0.3 GPa). The discontinuous character of the transition is especially noticeable in the behavior of the γ angle, which increases from 90.55° to 91.04° before P tr . The Murnaghan EoS parameters obtained for the volume are: V 0  = 1393.97(6) Å 3 , K 0  = 88.7(5) GPa, K' = 1.3(3) for the triclinic I1̄ phase and V 0  = 1396.9(1) Å 3 , K 0  = 89.4(9) GPa, K' = -1.7(6) for the monoclinic I2/c phase whose high-P behavior appears, therefore, linear in the pressure range of its stability. The axial compressibility scheme is βa &gt; βc &gt; βb for both phases, in good agreement with previous observations for other feldspars. A similar displacive ferroelastic I1̄-I2/c phase transition was observed for the same sample with increasing temperature. At P ∼ 7.3 GPa a large discontinuity in the unit-cell parameters indicates a further transition from monoclinic I2/c phase to monoclinic-II phase. This transformation is strongly first order with a volume change of about 1.7%.

High-pressure phase transitions in Ca 0.2 Sr 0.8 Al 2 Si 2 O 8 feldspar