year of publication
Abstract. Fifty-two samples of inverted high-temperature quartz from volcanic rocks were investigated by Guinier-Jago powder diffractometry and differential scanning calorimetry (DSC). Quartz megacrysts from Clear Lake and Cinder Cone, California show a variability of ≈ 2.5° K in their α - β transition temperature (Tα-β). Quartz phenocrysts and quartz from crystalline rocks give a range of 0.5º K in Tα-β. Neutron activation analysis of single crystals demonstrates that Al is the principal impurity (17-380 ppm). Its concentration is inversely correlated with Tα-β. A very small variation was found in the a and c lattice parameters among the specimens of volcanic quartz studied. This variation does not correlate with Al content or transition temperature. Mean values at 22º C (a=4.1934 ±0.0004 Å c=5.4046 ±0.0006 Å) are similar to those of quartz grown at low temperatures. Enthalpy of the α-β transition (ΔHα-β), obtained
over 9.0º from DSC runs, is dependent upon sample grain size and for a crushed powder with zero hysteresis (Tα-β on heating = Tα-β on cooling) is 92 ± 1.4 cal/mol. In contrast, a smgle piece of quartz
requires ΔHα-β be 107.7 ± 1.4 cal/mol and has a Tα-β hysteresis of 1.1 ºK.
Regression of published data provides equations for the variation of the molar volume (cc/mol) of quartz with temperature.
DSC heat capacity measurements over the range 400 to 900º K were fitted to extended Maier-Kelley type expressions.
The behavior of α, β and Cp as a function of T emphasizes that structural changes which occur at the α-β transition do so over a broad temperature interval.