Coesite

(mineralogy) A high-pressure polymorph of SiO₂ formed in nature only under unique physical conditions, requiring pressures of more than 20 kilobars (2 gigapascals); usually found in meteor impact craters.

Naturally occurring coesite, a mineral of wide interest and the high-pressure polymorph of SiO₂, was first discovered and identified from shocked Coconino sandstone of the Meteor Crater in Arizona in 1960. Since then coesite has been identified from the Wabar (meteorite) Crater in Saudi Arabia, from the Ries Crater in Bavaria in southern Germany, from the Lake Bosumtwi Crater in Ashanti, Ghana, Africa, and from Lake Mien in Sweden. Coesite has also been identified from some Thailand tektites which are considered to have been formed also by an impact cratering process. The finding of natural coesite elevates it as a true mineral species. Because it requires a unique physical condition, extremely high pressure, for its formation, its occurrence is diagnostic of a special natural phenomenon, in this case, the hypervelocity impact of a meteorite. See also Meteorite.

Synthetic coesite was first produced in the laboratory by L. Coes, Jr., as a chemical compound at pressures of about 35 kilobars (3.5 × 10⁹ pascals) in the temperature range of 500–800°F (246–404°C). Coesite has also been found in synthetic diamonds and has been formed by transformation from alpha quartz by the application of shearing stress. See also Diamond.

Coesite occurs in grains that are usually less than 0.0002 in. (5 micrometers) in size and are generally present in small amounts. The properties of the mineral are known mainly from studies of synthesized crystals. It is colorless with vitreous luster and has no cleavage. It has a specific gravity of 2.915 ± 0.015 and a hardness of about 8 on Mohs scale.

Coesite has as yet no evident commercial use and therefore has no obvious economic value. As a stepping-stone in scientific research, it serves in at least two ways: where it occurs naturally, coesite is diagnostic of a past history of high pressure; the occurrence of coesite from Meteor Crater clearly suggests that shock as a process can transform a low-density ordinary substance to one of high density and unique properties. Coesite has been found in materials ejected from craters formed by the explosion of 500,000 tons (450,000 metric tons) of TNT. Research on the occurrence of coesite from rocks deformed by other energy sources, such as volcanic explosions and deep-seated tectonic movement, is continuing. The study of coesite and craters may be useful in understanding the impact craters on the Earth as well as those on the Moon.

Environment

Found only as a very fine powder, often seeming amorphous, in meteor craters, a product of heat and impact on sandstone.

Crystals resembling a common gypsum shape are known from synthetic manufacture. All the following data came from them, rather than the natural white dust or glassy cement around quartz sand grains in the several meteor craters where it has been found.

Colorless. Luster glassy; hardness 7Ɖ (estimated); specific gravity 2.93. Transparent.

Silicon dioxide, like quartz.

It was first made synthetically (and named for the maker) before being found as an almost singly refracting powder with embedded sand grains in the Barringer Meteor Crater, Arizona. Since then it has been found in the Riess-kessel meteorite impact crater in Bavaria, the Wabar crater in Al Hadida, Arabia, and in two ancient craters in Ohio (Sinking Springs) and Indiana (Kentland, Newton Co.)

This is relatively recently recognized (1960) as a natural variant of silica, though it was known to have been synthetically made many years earlier.

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) are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953.^[1][2] In 1960, coesite was found by Edward C. T. Chao^[3], in collaboration with Eugene Shoemaker, to naturally occur in the Barringer Crater, which was evidence that the crater must have been formed by an impact.

The presence of coesite in unmetamorphosed rocks may be evidence of a meteorite impact event or of an atomic bomb explosion. In metamorphic rocks, coesite commonly is one of the best mineral indicators of metamorphism at very high pressures (UHP, or ultrahigh-pressure metamorphism)^[4]. Such UHP metamorphic rocks record subduction or continental collisions in which crustal rocks are carried to depths of 70 km or more. Coesite also has been identified 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.

The molecular structure of coesite consists of four silicon dioxide tetrahedra arranged in a ring. The rings are further arranged into a chain. 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.

See also

• List of minerals
• List of minerals named after people
• Stishovite, a related mineral

External links

• Coesite page
• Barringer Meteor Crater science page

References

1. ^ Coes, L., Jr. (31 July 1953). "A New Dense Crystalline Silica". Science 118 (3057): 131-132. http://www.sciencemag.org/cgi/pdf_extract/118/3057/131. 
2. ^ Hazen, Robert M. (1999). The diamond makers. New York: Cambridge University Press.
3. ^ E. C. T. Chao, E. M. Shoemaker, and B. M. Madsen (1960) First natural occurrence of coesite. Science vol. 132. no. 3421, pp. 220-222
4. ^ Chopin C. (1984): Coesite and pure pyrope in high-grade blueschists of the western Alps: a first record and some consequences. Contrib. Mineral. Petrol. 86, 107-118

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