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Rigid unit modes in framework silicates

Published online by Cambridge University Press:  05 July 2018

Martin T. Dove ,
Volker Heine  and
Kenton D. Hammonds
Martin T. Dove
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Volker Heine
Affiliation:
Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK
Kenton D. Hammonds
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
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Abstract

We describe a model for framework silicates in which the SiO4 (and AlO4) tetrahedra are treated as perfectly rigid and freely jointed. From this model we are able to identify low-energy modes of distortion of the structure, which we call Rigid unit modes. These modes can act as soft modes to allow easy distortions at phase transition. We discuss three forces that will operate at a phase transition in conjunction with the candidate soft modes to determine which of the rigid unit modes will actually precipitate a phase transition, and illustrate these ideas by detailed discussions of the phase transitions in quartz, leucite and cristobalite. The model can also be used to estimate the transition temperature, and the theory highlights an important role for the stiffness of the tetrahedra.

Keywords

rigid unit modesphase transitionsquartzleucitecristobalite
Type
Research Article
Information
Mineralogical Magazine , Volume 59 , Issue 397 , December 1995 , pp. 629 - 639
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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