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I– I2/c ferroelastic phase transition in the Ca_0.2Pb_0.8Al₂Si₂O₈ feldspar as a function of temperature

¹ Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy
² Centro di Studi sulla Geodinamica delle Catene Collisionali (C.N.R.), Via Accademia delle Scienze 5, I-10123 Torino, Italy

^* E-mail: benna{at}dsmp.unito.it

Feldspar of composition Ca_0.2Pb_0.8Al₂Si₂O₈ (PbF₈₀An₂₀) was synthesized from melt and subsequently isothermally annealed at T = 960°C for 4 days. In situ HT X-ray powder spectra of PbF₈₀An₂₀ feldspar, triclinic I at room temperature, were collected in the temperature range 20–800°C, and a displacive continuous ferroelastic transition to a I2/c monoclinic phase was observed. An analysis of the symmetry-required components of the spontaneous strain tensor reveals the second order character (ß = 0.46 ± 0.02) of the transition with T_C = 680 ± 15°C. A linear coupling was observed between the e₄ and e₆ components of the spontaneous strain. The transition is analogous to those observed, with increasing temperature, along the join An–SrF and in disordered Na-rich alkali feldspars. A comparison with Ca_0.2Sr_0.8Al₂Si₂O₈ feldspar (T_C= 680°C, McGuinn and Redfern, 1997) shows that PbF₈₀An₂₀ has a higher spontaneous strain (_s = 0.028 in PbF₈₀An₂₀ vs 0.020 in Ca_0.2Sr_0.8Al₂Si₂O₈) and a higher e₄ component, possibly related to the higher distortion of the non-tetrahedral polyhedron in lead feldspar.

KEYWORDS: lead feldspar, phase transformations, HT unit-cell parameters

This article has been cited by other articles:

				F. Nestola, F. Nestola, T. B. Ballaran, P. Benna, M. Tribaudino, and E. Bruno High-pressure phase transitions in Ca0.2Sr0.8Al2Si2O8 feldspar American Mineralogist, October 1, 2004; 89(10): 1474 - 1479. [Abstract] [Full Text] [PDF]

				S. A. T. Redfern Mineral transformations Mineralogical Magazine, April 1, 2000; 64(2): 155 - 156. [Full Text] [PDF]