Coesite ()[ The word coesite is pronounced as "Coze-ite", after chemist Loring Coes, Jr.]
Occurrences
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
.
In metamorphic rocks, coesite was initially described in
eclogite xenoliths from the mantle of the Earth that were carried up by ascending
;
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 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,
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 tetrahedron arranged in Si
4O
8 and Si
8O
16 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.
See also
External links
