| Coesite | |
|---|---|
 | |
| General | |
| Category | Tectosilicate minerals |
| Group | Quartz group |
| Formula | SiO2 |
| IMA symbol | Coe[1] |
| Strunz classification | 4.DA.35 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH–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 | 7.5-8 |
| 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) ofsilicon dioxide (SiO2) that is formed when very high pressure (2–3gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied toquartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at theNorton Company, in 1953.[4][5]
In 1960, a natural occurrence of coesite was reported byEdward C. T. Chao,[6] in collaboration withEugene Shoemaker, fromBarringer 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 meteoriteimpact event or of anatomic bomb explosion. It was not expected that coesite would survive in high pressuremetamorphic rocks.
In metamorphic rocks, coesite was initially described ineclogitexenoliths from themantle of the Earth that were carried up by ascendingmagmas;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, orultrahigh-pressure metamorphism).[8] Such UHP metamorphic rocks recordsubduction or continental collisions in whichcrustal 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 toquartz, 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 westernAlps of Italy at Dora Maira,[8] theOre Mountains of Germany,[9] the Lanterman Range of Antarctica,[10] in theKokchetav Massif of Kazakhstan,[11] in theWestern Gneiss region of Norway,[12] theDabie-Shan Range in Eastern China,[13][14] theHimalayas of Eastern Pakistan,[15] and in theAppalachian Mountains of Vermont.[16][17]
Coesite is atectosilicate 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 beinghexagonal in shape ("a" and "c" are nearly equal and β nearly 120°), it is inherentlymonoclinic and cannot be hexagonal. The crystal structure of coesite is similar to that offeldspar and consists of foursilicon dioxidetetrahedra arranged in Si4O8 and Si8O16 rings. The rings are further arranged into chains. This structure ismetastable within the stability field of quartz: coesite will eventually decay back into quartz with a consequent volume increase, although themetamorphic 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]
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