Coesite - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Monoclinic
    Chemistry : SiO2
    Rarity : Rare

Coesite is a rare form of silica that forms at high pressures (between 20 and 80 kbar). These conditions of genesis imply that we only encounter this mineral in geological environments subjected to strong pressures (meteoritic impacts, in the company of stishovite, but also eclogites), or in rocks from the deep mantle, where there is a very high pressure (kimberlites from South Africa). It owes its name to the American chemist Loring Coes who carried out the first synthesis of this mineral before its discovery in nature. Coesite never forms crystals but colorless grains of very small size (3 mm at most), frequently included in clinopyroxenes or diamonds.

Main photo : Coesite from Dora-Maira Piedmont, Italy © Giovanni Scapin

Coesite in the World

The best specimens come from Meteor Crater (Arizona), where the Canyon Diablo meteorite crashed more than 12,000 years ago.

Coesite in France

This mineral is not present in the French underground.


A lamellar twin is known on {100}.

Fakes and treatments

No fake identified for this mineral species.

Hardness : 7.5 to 8
Density : 2.92
Fracture : Sub-conchoidal
Streak : White

TP : Translucent to transparent
RI : 1.593 to 1.604
Birefringence : 0.004 to 0.005
Optical character : Biaxial +
Pleochroism : None
Fluorescence : None

Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None