The shaded are is the approximately range temperatures and pressures known to occur in crustal rocks. Before discussing the occurrences of these phases, it is necessary to briefly consider the nature of the transformations that relate the various phases to one another. Displacive transformations involve only slight rearrangements of the positions of the atoms that make up the mineral structure without breaking any bonds. 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.