Thermal expansion and the high-low transformation in quartz. I. High-temperature X-ray studies

The (100) and (101) interplanar spacings of 400 mesh quartz powder were measured by high-temperature X-ray diffractometry from 22 to 1400°C. Expansion through the high-low transformation was reversible and continuous, with evidence of 1-2° hysteresis; expansion from 574 to 1000° is zero, with a net decrease from 1000 to 1400°. Maximum linear coefficients of expansion at the transformation were 400±50 x 10^-6 deg^-1 across the (100) plane and 140±30 x 10^-6 deg^-1 across the (101) plane. The maximum volume coefficient of expansion was 100±20 × 10^-5deg^-1 between 572 and 574°. Differential thermal analyses provided ambiguous data, indicating a latent heat of transformation of 160±40 cal/mole, in apparent contradiction with thermal expansion measurements.