In contrast, it is not possible to quench beta-quartz to room temperature. Thus, beta-quartz always inverts to alpha quartz during cooling of the rock to room temperature. Beta tridymite is hexagonal positive, but it inverts to orthorhombic alpha-tridymite on cooling. In those rocks, tridymite occurs as phenocrysts and as fine grained matrix minerals in the groundmass of the volcaanics. Tridymite can be distinguished from quartz by its biaxial character and very low birefringence.
Beta quartz is hexagonal, but it inverts on cooling to alpha-quartz, which is trigonal.
It occurs in a wide variety of igneous and metamorphic rocks. Quartz also occurs as an authigenic or diagenetic mineral in sedimentary rocks. Quartz can be distinguished from feldspars based on its lack of twinning and cleavage and its uniaxial character. However, cristobalite does occur in igneous rocks, where it is thought to have crystallized metastable from the vapor phase. Cristobalite can also occur as spherulites and as a fine griained groundmass mineral in volcanics.