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, quartz group
Formula (repeating unit)	SiO2
IMA symbol	Coe[1]
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
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
2V angle	60–70
References	[2]

Coesite (/ˈkoʊsaɪt/)^[3] is a form (polymorph) of silicon dioxide (SiO₂) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953.^[4][5]

Occurrences

In 1960, a natural occurrence of coesite was reported by Edward C. T. Chao,^[6] 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.^[7] 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).^[8] 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 (25 kbar) 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).

Coesite has been identified in UHP metamorphic rocks around the world, including the western Alps of Italy at Dora Maira,^[8] the Ore Mountains of Germany,^[9] the Lanterman Range of Antarctica,^[10] in the Kokchetav Massif of Kazakhstan,^[11] in the Western Gneiss region of Norway,^[12] the Dabie-Shan Range in Eastern China,^[13][14] the Himalayas of Eastern Pakistan,^[15] and in the Appalachian Mountains of Vermont.^[16][17]

Crystal structure

Coesite is a tectosilicate with each silicon atom surrounded by four oxygen atoms in a tetrahedron. Each oxygen atom is then bonded to two Si atoms to form a framework. There are two crystallographically distinct Si atoms and five different oxygen positions in the unit cell. Although the unit cell is close to being hexagonal in shape ("a" and "c" are nearly equal and β nearly 120°), it is inherently monoclinic and cannot be hexagonal. The crystal structure of coesite is similar to that of feldspar and consists of four silicon dioxide tetrahedra arranged in Si₄O₈ and Si₈O₁₆ rings. The rings are further arranged into chains. This structure is metastable within the stability field of quartz: coesite will eventually decay back into quartz with a consequent volume increase, although the metamorphic reaction is very slow at the low temperatures of the Earth's surface. The crystal symmetry is monoclinic C2/c, No.15, Pearson symbol mS48.^[18]