Quartz

Abstract: With very few exceptions the numerous polymorphs of SiO2 all consist of SiO4 tetrahedra linked through their corners, thus forming three-dimensional framework structures. The topology of the tetrahedral linkage and the efficiency of space filling are different for the polymorphs. For a given type of framework, for example, that of quartz or cristobalite, space filling can be improved by so-called displacive transformations from a more open high-temperature form (e.g., “high”, “h”, or β “quartz”) to a denser form stable at lower temperatures (“low” or “α quartz”). These transformations do not change the topology of the framework, i.e., chemical bonds in a crystal can be deformed, but are not broken and rearranged.

Abstract: Equilibrium constants for oxygen isotope exchange between quartz and water have been measured from 195°C (1000 ln α = 12.0) to 750°C (1000 ln α = 0.4). Over limited temperature ranges the behavior of fractionation with temperature can be approximated by 1000 ln α = 3.38 (106 T−2) − 3.40 for 200°–500°C and by 1000 ln α = 2.51 (106 T−2) − 1.96 for 500°–750°C. The results of measurements in the quartz-water system can be combined with analogous results from other mineral systems to make mineral-pair isotopic thermometers for application to problems of petrogenesis.