Samples of synthetic tridymite, cristobalite and tridymite-cristobalite mixed phases, examined by differential scanning calorimetry (DSC) from 360 to 770 or 840K, showed distinct peaks in heat capacity (Cp). For “pure” tridymite the lambda transition at 390K and two peaks near 436K and 470K, which may also be lambda transitions, appear to correlate respectively with the structural changes from (MC) monoclinic (Cc) to (OP) orthorhombic P212121, (OP) to (OS) orthorhombic with a non-integral superstructure, and (OS) to (OC) orthorhombic C2221, described by Nukui et al. (1978) as occurring at 383K, 423K, and 463K respectively. It may also be possible to correlate a weak Cp effect near 653K with the (OC) to (HP) hexagonal P63/mmc inversion of Nukui et al. The DSC measurements agree with the heat-content measurements by Mosesman and Pitzer (1941) and the dynamic calorimeter data of Shahid and Glasser (1970) for tridymite, although the feature observed in both these studies at 498K was absent in our tridymite sample. This 498K discontinuity probably reflects inversion in cristobalite portions of the “tridymite” samples used in the previous studies, as suggested by our DSC examination of tridymite-cristobalite mixed phases. The present Cp data for tridymite above 500K and for cristobalite above 560K agree reasonably well with the polynomial fits presented by Robie et al. (1978, p. 217, 218).

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