Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Orthorhombic
    Chemistry : SiO₂
    Rarity : Uncommon to fairly common

Tridymite is a common high-temperature phase of silica. It has several polymorphs (stishovite, coesite, cristobalite, and especially quartz). The crystal system of tridymite is a complex subject because it changes with temperature, but often retains the forms acquired at high temperature. Monoclinic (or sometimes triclinic) at 25°C, it becomes orthorhombic (often pseudohexagonal) at medium temperature and then hexagonal at high temperature. The lattice parameters change. One of the two orthorhombic forms seems to be the most widespread, and its parameters are presented here. It is a mineral found in the cavities of acid and intermediate lavas (trachytes, andesites, rhyolitic tuffs). It is also formed by devitrification of obsidian, forming whitish spherules in a black matrix. Note however that the spherules of this decorative stone, marketed under the name of flake obsidian, are more frequently cristobalite. Its name comes from the Greek tridumos (triple), because of its frequent twins of 3 crystals. It is a translucent to transparent mineral with a vitreous luster; it occurs in thin pseudohexagonal tabular crystals, colorless or white in color, sometimes grouped in a twin of two or three individuals, more rarely in rosette aggregates. The crystals are in reality low-temperature tridymite which has externally retained the high-temperature form. It has no industrial use, however synthetic tridymite is used as an extra-white mineral filler in paints and in prestigious building concrete.

Main photo : Tridymite from Emmelberg, Eifel, Germany © Jean-Paul Fayard

Multiple contact twins are known on {10-16} and on {30-34}

No fakes listed for this mineral species.