Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Tetragonal
    Chemistry : SiO₂
    Rarity : Common

Cristobalite is one of the polymorphs of silica, like stishovite, coesite, trydimite and quartz. It exists in two forms : a high temperature, cubic, and a low temperature, pseudocubic tetragonal. It is a mineral in volcanic rocks that is found in the vacuoles of rhyolites, trachytes and andesites but also in meteorite impact glasses. It owes its name to its location of discovery : Cerro San Cristobal in Mexico. High temperature cristobalite occurs in small translucent octahedra, white to grayish in color, sometimes twinned on {111} ; low temperature forms build cryptocrystalline aggregates, sometimes fibrous or spheroidal which incorporate certain opals (opal-C and opal-CT), or by devitrification integrate certain volcanic glasses or impact glasses. A small part of French silica production is supplied by cristobalite and tridymite. However, these are synthetic minerals obtained by calcination of flint pebbles: between 900°C and 1100°C, we obtain a product containing 55% tridymite and 45% cristobalite, and between 1450°C and 1600°C , the product contains between 80 and 90% cristobalite. It is then used as an extra-white filler for prestigious structural concretes, and as an extra-white filler in paints.

Main photo : Cristobalite from Bellerberg volcano, Germany © Edgard Müller

Cristobalite sometimes twins on {111}.

No fakes listed for this mineral species, but easy to confuse with hematite without chemical analysis...