A simple and generalised P–T–V EoS for continuous phase transitions, implemented in EosFit and applied to quartz

PETROLOGICAL INTEREST 105113 Crystallography CONSISTENT THERMODYNAMIC DATA 530 Continuous phase transition 01 natural sciences 105116 Mineralogy 105113 Kristallographie Geochemistry and Petrology EosFit HIGH-PRESSURE Continuous phase transition; Elasticity; EosFit; Equation of state; Quartz; Geophysics; Geochemistry and Petrology Geophysic 105116 Mineralogie 0105 earth and related environmental sciences Equation of state LAMBDA-TRANSITION Quartz SPONTANEOUS STRAIN EQUATION-OF-STATE Elasticity MOLECULAR-DYNAMICS BETA-QUARTZ THERMAL-EXPANSION SINGLE-CRYSTAL QUARTZ
DOI: 10.1007/s00410-017-1349-x Publication Date: 2017-04-11T11:11:50Z
ABSTRACT
Continuous phase transitions in minerals, such as the α–β transition in quartz, can give rise to very large non-linear variations in their volume and density with temperature and pressure. The extension of the Landau model in a fully self-consistent form to characterize the effects of pressure on phase transitions is challenging because of non-linear elasticity and associated finite strains, and the expected variation of coupling terms with pressure. Further difficulties arise because of the need to integrate the resulting elastic terms over pressure to achieve a description of the P–T–V equation of state. We present a fully self-consistent simplified description of the equation of state of minerals with continuous phase transitions based on a purely phenomenological adaptation of Landau theory. The resulting P–T–V EoS includes the description of the elastic softening occurring in both phases with the minimum number of parameters. By coupling the volume and elastic behaviour of the mineral, this approach allows the EoS parameters to be determined by using both volume and elastic data, and avoids the need to use data at simultaneous P and T. The transition model has been incorporated in to the EosFit7c program, which allows the parameters to be determined by simultaneous fitting of both volume and elastic data, and all types of equation of state calculations to be performed. Quartz is used as an example, and the parameters to describe the full P–T–V EoS of both α- and β-quartz are determined.
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