Geobarothermometric properties of growth twins and mathematical analysis of quartz data for a broad range of temperatures and pressures

The thermoelastic analysis of the possible stable stress-free equilibrium states of growth twins is shown to be the basis for a new technique of geobarothermometry. The detailed study of a cyclic cross-shaped twin in low quartz exemplifies the procedure. To explicitly determine the barothermometric function, the simultaneous dependence upon the external pressure and temperature of the lattice-parameter ratios c/c ₀ and a/a ₀ of quartz has been calculated; we present a five-parameter, temperature-corrected version of the equations proposed by Thurston (1967) for extrapolating to high pressures the behavior of lattice parameters of crystalline solids in isothermal compression.