Beta quartz

Beta quartz is what quartz becomes when heated past 573 °C. The change is not melting or recrystallisation: the silicon-oxygen framework stays connected and the tetrahedra rotate slightly, moving the structure from trigonal to hexagonal symmetry. It happens in both directions, instantly, at the same temperature, and it cannot be prevented by cooling quickly.

That instantaneous reversibility is what makes the transition matter industrially. Quartz occupies about one per cent more volume above the transition than below it, so any object containing quartz that passes through 573 °C changes size abruptly. In a kiln, that is when things crack.

Beta quartz cannot be examined at room temperature, because it reverts the instant it cools below the transition. Everything known about its structure comes from diffraction performed in a heated stage, which is why the structure determinations for it are a distinct and smaller body of work than those for the low form.

What survives cooling is shape. A crystal grown above 573 °C grows with hexagonal symmetry and keeps that outline when the internal structure reverts, so a stubby hexagonal bipyramid in a volcanic rock indicates the crystal formed as beta quartz — a paramorph, with alpha structure in a beta habit.

Beta quartz is stable between 573 °C and about 870 °C at atmospheric pressure, above which the silica reorganises into tridymite. It is therefore the form that crystallises from high-temperature volcanic melts, and the quartz phenocrysts in rhyolite and granite porphyry commonly grew as it.

Every one of those crystals is now alpha quartz. The structure inverted as the rock cooled through the transition, leaving the external form as the only record of how it grew.

Beta quartz is not used, because it cannot be kept. What matters industrially is the transition itself, and specifically avoiding damage from it.

Ceramic and refractory firing schedules slow down through 573 °C in both directions, because the abrupt volume change cracks ware that is heated or cooled too fast — the effect is well enough known in the trade to be called quartz inversion. Foundry sands face the same problem: silica sand expands at the transition and can buckle a mould, which is one reason olivine and chromite sands are used for demanding castings.