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
Twinning
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