The α-β Inversion in quartz: A coherent phase transition under nonhydrostatic stress

Experiments on oriented cores from a single crystal of quartz show that the temperature of the α-β transition is raised 10.6±0.4°C/kb and 5.0±0.4°C/kb by uniaxial compression perpendicular and parallel, respectively, to the optic axis at a confining pressure of 3 kb. Approximately the same results are indicated for confining pressures ranging from 1 to 5 kb. There is no detectable curvature of the phase boundary for uniaxial stresses σ between 0 and 10 kb: |(d2Tα-β/dσ2)| < 0.05°C/kb2. The increase of transition temperature with hydrostatic pressure of 25.8±0.3°C/kb between 1 and 5 kb also determined in these experiments is consistent with previous determinations. These results and others for quartz are analyzed assuming the α-β inversion is either a λ transition or a first-order transition characterized by a small reversible transformation strain. Although both hypotheses are roughly consistent with most of the results, the hysteresis in the transition suggests that the second may be preferable, and a theory is developed that describes the effect of general nonhydrostatic stress on such transitions.