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 −5 deg −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.