High-pressure phase transitions in Ca0.2Sr0.8Al2Si2O8 feldspar

The synthetic feldspar of composition An20SrF80 (space group I1 at room condition, Qod = 0.88) was investigated by in situ high-pressure single-crystal X-ray diffraction. High-P experiments were performed with a BGI design diamond-anvil cell, using a steel gasket, a 4 : 1 methanol : ethanol mixture as pressure medium and a quartz crystal as internal P standard. Unit-cell parameters were measured as a function of pressure up to 7.7 GPa at room T. Two reversible phase transitions have been observed in the investigated P range. A first-order triclinic to monoclinic (I) phase transitions was observed at P = 4.3 GPa. Several cycles through the transformation were made in order to constrain the hysteresis (~ 0.3 GPa). The discontinuous character of the transition is especially noticeable in the behavior of the gamma angle, which increases from 90.55 ° to 91.04 ° before Ptr. The Murnaghan EoS parameters obtained for the volume are: K0 = 88.7(5) GPa, K’ = 1.3(3) for the triclinic phase and K0 = 89.4(9) GPa, K’ = -1.7(6) for the monoclinic (I) phase whose high-P behavior appears, therefore, linear in the pressure range of its stability. The axial compressibility scheme for both phases is a > c > b, in good agreement with previous observations for other feldspars. It is suggested that the triclinic-monoclinic transition at P ~ 4.3 GPa is similar to the displacive ferroelastic I1 - I2/c phase transition observed for the same sample with increasing T. At P =7.3 GPa a large discontinuity in the unit-cell lattice parameters indicate a further monoclinic (I) to monoclinic (II) transition. This transformation is strongly first order with a volume change of about 1.7%.