ABSTRACT
The stability fields for other polymorphs of crystalline silica lie at higher temperature and pressure (Hemley et aI., 1994). a-cristobalite and a-tridymite generally invert (via reconstructive transformation) to a-quartz over long periods of time, but exist metastably under ambient conditions. Each of the principal polymorphs, quartz, cristobalite, and tridymite, has distinct low-and high-temperature structural variants. At around 573°C and 1 bar, a-quartz undergoes a rapid, displacive transformation to a form known as j3-quartz, having a slightly expanded crystal framework. Similarly, cristobalite has a high-temperature form known as j3-cristobalite, having a higher symmetry crystal structure. Normally j3-quartz, j3-cristobalite, and j3-tridymite are regarded as unquenchable. They may
form in nature during crystallization at high temperature, but invert almost instantaneously to the 0(- forms as the temperature drops below the transition temperature. However, recent work has shown that fJ-cristobalite may be stabilized at room temperature by the presence of certain oxides (Perrotta et aI., 1989), and this form has been identified in devitrified ceramic insulation material (Young et aI., 1989; Brown et aI., 1992).
