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High-pressure phase transitions in Ca_0.2Sr_0.8Al₂Si₂O₈ feldspar

¹ Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
² Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy
³ Istituto di Geoscienze e Georisorse, CNR, Sezione di Torino, Via Accademia delle Scienze 5, I-10123 Torino, Italy

Correspondence: ^* E-mail: piera.benna{at}unito.ital.

A synthetic feldspar of composition Ca_0.2Sr_0.8Al₂Si₂O₈ (An₂₀SrF₈₀) (space group I at room condition, Q_od = 0.88) was investigated using in situ high-pressure single-crystal X-ray diffraction. Unit-cell 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 I 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 angle, which increases from 90.55° to 91.04° before P_tr. The Murnaghan EoS parameters obtained for the volume are: V₀ = 1393.97(6) ų, K₀ = 88.7(5) GPa, K' = 1.3(3) for the triclinic I phase and V₀ = 1396.9(1) ų, K₀ = 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 ß_a > ß_c > ß_b for both phases, in good agreement with previous observations for other feldspars. A similar displacive ferroelastic I-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 fist order with a volume change of about 1.7%.

This article has been cited by other articles:

				P. Benna, F. Nestola, T. B. Ballaran, T. Balic-Zunic, L. F. Lundegaard, and E. Bruno The high-pressure structural configurations of Ca0.2Sr0.8Al2Si2O8 feldspar: The IFormula-I2/c and I2/c-P21/c phase transitions American Mineralogist, July 1, 2007; 92(7): 1190 - 1199. [Abstract] [Full Text] [PDF]