Coesite

Coesite
Crossed-polars image of coesite grain (gray) ~1 mm across in eclogite. Small colored inclusion is pyroxene. Polycrystalline rim is quartz.
General
Category	Tectosilicate
Formula (repeating unit)	SiO2
Strunz classification	4.DA.35
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 7.143; b = 12.383; c = 7.143 [Å]; β = 120.00°, Z = 16
Identification
Formula mass	60.0843 g/mol
Color	Colorless
Crystal habit	Inclusions in UHP metamorphic minerals up to 3 mm in size
Cleavage	none
Fracture	conchoidal
Tenacity	brittle
Mohs scale hardness	7.5
Luster	vitreous
Streak	white
Diaphaneity	Transparent
Density	2.92 (calculated)
Optical properties	Biaxial
Refractive index	nx = 1.594 ny = 1.595 nz = 1.599
Birefringence	+0.006
Pleochroism	none
2V angle	60–70
References

Coesite is a form (polymorph) of silicon dioxide SiO₂ that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C or 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the Norton Company, in 1953.

In 1960, a natural occurrence of coesite was reported by Edward C. T. Chao, in collaboration with Eugene Shoemaker, from Barringer Crater, in Arizona, US, which was evidence that the crater must have been formed by an impact. After this report, the presence of coesite in unmetamorphosed rocks was taken as evidence of a meteorite impact event or of an atomic bomb explosion. It was not expected that coesite would survive in high pressure metamorphic rocks.

In metamorphic rocks, coesite was initially described in eclogite xenoliths from the mantle of the Earth that were carried up by ascending magmas; kimberlite is the most common host of such xenoliths. In metamorphic rocks, coesite is now recognized as one of the best mineral indicators of metamorphism at very high pressures (UHP, or ultrahigh-pressure metamorphism). Such UHP metamorphic rocks record subduction or continental collisions in which crustal rocks are carried to depths of 70 km (43 mi) or more. Coesite is formed at pressures above about 2.5 GPa and temperature above about 700 °C. This corresponds to a depth of about 70 km in the Earth. It can be preserved as mineral inclusions in other phases because as it partially reverts to quartz, the quartz rim exerts pressure on the core of the grain, preserving the metastable grain as tectonic forces uplift and expose these rock at the surface. As a result, the grains have a characteristic texture of a polycrystalline quartz rim (see infobox figure).