Thermodynamics of the lambda transition and the equation of state of quartz

The published data on the alpha–beta transition in quartz are reanalyzed using Pippard’s theory and modern computer techniques. Volume data confirm the difficulties in reaching equilibrium in alpha quartz near the transition, explaining the low and irreproducible heat capacity measurements in this region. The pressure dependence of the transition temperature and the best X-ray volume measurements are used to evaluate entropy and enthalpy changes in the transition region. At 1000 K the entropy and enthalpy of beta quartz are 116.215±0.1 J mol⁻¹ K⁻¹ and −865256±70 J mol⁻¹ relative to the codata selections for alpha quartz at 298.15K. These values are in excellent agreement with the completely independent selections of Richet et al, but not with USGS Bulletin 1452. Full equation of state data for quartz up to 1900 K and 4000 MPa are given graphically and by equations valid for designated portions of the P–T plane. The SiO₂ phase diagram has been calculated including cristobalite, coesite, and liquid regions. The existence of an intermediate incommensurate phase between alpha and beta quartz is discussed, but not included explicitly in the calculations.