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. In Angel et al 2017 we present a simplified description of the equation of state of minerals with continuous phase transitions based on a purely phenomenological adaptation of Landau theory. This “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.

(Image credit to Contributions to Mineralogy and Petrology)