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I1̄ – I2/c ferroelastic phase transition in the Ca0.2Pb0.8Al2Si2O8 feldspar as a function of temperature

Published online by Cambridge University Press:  05 July 2018

P. Benna ,
M. Tribaudino  and
E. Bruno
P. Benna*
Affiliation:
Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy Centro di Studi sulla Geodinamica delle Catene Collisionali (C.N.R.), Via Accademia delle Scienze 5, I-10123 Torino, Italy
M. Tribaudino
Affiliation:
Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy
E. Bruno
Affiliation:
Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy Centro di Studi sulla Geodinamica delle Catene Collisionali (C.N.R.), Via Accademia delle Scienze 5, I-10123 Torino, Italy
*
*E-mail: benna@dsmp.unito.it
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Abstract

Feldspar of composition Ca0.2Pb0.8Al2Si2O8 (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 I1̄ 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 (β = 0.46 ± 0.02) of the transition with TC = 680 ± 15°C. A linear coupling was observed between the e 4 and e 6 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 Ca0.2Sr0.8Al2Si2O8 feldspar (TC = 680°C, McGuinn and Redfern, 1997) shows that PbF80An20 has a higher spontaneous strain (εs = 0.028 in PbF80An20vs 0.020 in Ca0.2Sr0.8Al2Si2O8) and a higher e 4 component, possibly related to the higher distortion of the non-tetrahedral polyhedron in lead feldspar.

Keywords

lead feldsparphase transformationsHT unit-cell parameters

Information

Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 2 , April 2000 , pp. 285 - 290
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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