the silica polymorphs
Silica, silicon dioxide (SiO2), is built entirely from SiO4 tetrahedra, each silicon bonded to four oxygens, with every oxygen shared between two tetrahedra. There are many different ways to link those corner-sharing tetrahedra into a three-dimensional network, and each way is a distinct crystalline polymorph of silica, plus one important non-crystalline form, silica glass.
The three main crystalline forms appear in order as you heat silica: quartz is stable up to about 867 degrees C, tridymite from there to about 1470 degrees C, and cristobalite from 1470 degrees C up to melting near 1713 degrees C. Each of these also has a low-temperature (alpha) and high-temperature (beta) version related by a quick displacive flip. Under high pressure, denser polymorphs appear, coesite and stishovite, the latter so squeezed that its silicon becomes six-coordinate rather than four.
These transformations dominate the behaviour of silica refractories and of quartz in ceramic bodies. The alpha-to-beta quartz inversion at 573 degrees C carries a roughly 1 percent volume jump that can crack ware during firing and cooling; cristobalite has an even more disruptive low-temperature inversion around 200 to 270 degrees C. Honest caveat: the transformations among quartz, tridymite and cristobalite are reconstructive and therefore sluggish, so cristobalite and tridymite routinely survive metastably at room temperature; it is the fast alpha-beta inversions within each form, not the sluggish conversions between forms, that cause the sudden, damaging volume changes.
Silica firebricks in a coke oven are mostly tridymite and cristobalite, deliberately converted from quartz during their first firing, because those forms have gentler, more predictable thermal expansion in service than raw quartz with its abrupt 573 degrees C inversion.
Quartz to tridymite to cristobalite with rising temperature; fast alpha-beta inversions cause the damaging volume jumps.
The dangerous volume changes are the fast alpha-beta inversions inside quartz or cristobalite, not the slow quartz-to-cristobalite conversion. Because the latter is reconstructive, high-temperature silica forms persist metastably long after cooling.