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 0.5 ° 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 22 ° 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 9.0 ° 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 92.0 ±1.4 cal/mol. In contrast, a single piece of quartz requires ΔHα-β be 107.7±1.4 cal/mol and has a Tα-β hysteresis of 1.1 ° K. Regression of published data provides equations for the variation of the molar volume (cc/mol) of quartz with v. These equations imply a ΔVα-β of 0.205±0.031 cc/- mol. Expressions are also provided for the temperature dependence of the thermal coefficient of expansion, α, the compressibility, β, and (∂/gb/∂T)p (which is identically -(∂α/∂P)T). DSC heat capacity measurements over the range 400 to 900 ° K were fitted to extended Maier-Kelley type expressions to give: {Mathematical expression} (851 to 900 ° K), which together with the values of ΔHα-β measured over the range 842-851° K give 7875.3 cal/mol for H900-H400. 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. © 1979 Springer-Verlag.
