Coesite

Coesite crystallizes in monoclinic syngony , color is white or transparent, colorless, density 2.95-3 g / cm³, hardness 7.5-8 on the Mohs scale .

In thin sections, coesite differs well from quartz due to its higher relief and low interference colors (high refractive index and low birefringence). An important feature that distinguishes it from other modifications of silica when studied by cathodoluminescence (CR) microscopy is its bright luminescence with bluish-green light, and when directly analyzed using an electron microprobe, it has luminescence up to a bright blue glow, which is clearly different from the orange glow of quartz. Easily determined by Raman scattering .

While quartz, another polymorphic type of silica, is one of the most common minerals in the earth's crust (second only to feldspars), the coesite mineral is very rare. The fact is that it is formed at high pressures (of the order of 2-3 GPa ), where rocks with a high SiO ₂ content are relatively rare, and with a decrease in pressure, coesite turns back to quartz. Therefore, it is preserved only with a rapid rise (exhumation) of rocks to the surface.

Coesite is found in metamorphic complexes of ultrahigh pressures , mantle xenoliths and in places where meteorites fall, in mantle xenoliths of eclogites in some kimberylite tubes and in the form of inclusions in eclogite paragenzi diamonds. Such xenoliths are installed, for example, in the African Roberts Victor tube. However, mantle eclogites with coesite are much less common than in crust metamorphic complexes. Perhaps the reason for this is that mantle eclogites underwent partial melting in subduction zones, and coesite turned into andesite / trondimite melts, which served as material for the formation of the earth's crust.

In 1965, Chesnokov and Popov, studying the eclogites of the Maksutov complex (southern Urals ), drew attention to the fact that radial cracks depart from quartz inclusions in garnets, indicating an increase in the volume of inclusions in the process of metamorphic evolution. They made the assumption that an increase in volume could occur due to the polymorphic transition of coesite to quartz .

In 1984, coesite inclusions were found in garnet from pyrope quartzites of the Dora Mayra massif (western Alps ) and, at the same time, in metamorphic rocks of Norway .

The formation of coesite requires a pressure of at least 28 kbar, which is equivalent to a depth of 90-100 km from the Earth’s surface, while the thickness of the earth’s crust, even in thickened parts, does not exceed 70-80 km. Thus, primarily crustal and, moreover, meta-sedimentary rocks such as Dora Mayr quartzites were submerged to mantle depths and then returned back to the surface. Blocks of rocks with a similar tectonic-metamorphic history, when pressure at the peak of the metamorphism reached the coesite stability field, became known as ultra-high pressure metamorphic terranes (UHPM terranes).

Currently, about 20 metamorphic complexes containing coesite are known (Liou et al., 2004), or pseudomorphs of quartz according to coesite. Interestingly, it was not possible to confirm the presence of coesitis in the Maksyutovsky complex.

Coesite in the rocks of the Kokchetav metamorphic complex

The Kokchetav metamorphic complex is one of the most studied UHPM terranes in the world.

The assumption that polycrystalline quartz aggregates surrounded by radial cracks in garnets from the eclogites of the Kumdy-Kul site are pseudomorphs in terms of coesite was made as far back as 1989 (Sobolev, Shatsky 1989). Soon, coesite inclusions were found in zircon from diamondiferous garnet-biotite gneisses of the Kumdy-Kul site (Sobolev et al., 1991), in zircon from eclogites of the diamondiferous Barchinsky site (Korsakov et al., 1998), as well as within the eastern part of the metamorphic belt in garnet from quartz-garnet-fengite and talc-fengite-kyanite-garnet schists from the Kulet site (Shatsky et al. 1998, Parcinson 2000).

• Sobolev N.V. Coesite as an indicator of ultrahigh pressures in the continental lithosphere // Geology and Geophysics, 2006, v. 47, No. 1, p. 95 - 105.
• Chopin C. Coesite and pure pyrope in high-grade blue-schists of western Alps: a first record and some consequences // Contrib. Mineral Petrol., 1984, v. 86, p. 107-118.
• Smith DC, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics // Nature, 1984, v. 310, p. 641-644.
• Parcinson C. Coesite inclusion and prograde compositional zonation of garnet in whiteschist of the HP - UHP Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism // Lithos, 2000, v. 52, p. 215-233.
• Korsakov A.V., Shatsky V.S., Sobolev N.V. The first find of coesite in eclogites of the Kokchetav massif // Dokl. RAS, 1998, v. 360, No. 1, p. 77 - 81.
• Shatsky V.S., Tennisen K., Dobretsov N.L., Sobolev N.V. New evidence of superhigh pressures in mica schists of the Kulet section of the Kokchetav massif (Northern Kazakhstan) // Geology and Geophysics, 1998, v. 39, No. 8 , with. 1039-1044.
• Coes L. A new dense crystalline silica // Science, 1953, v. 118, p. 131-132.
• Chao ECT Shoemaker EM Madsen BM First natural occurrence of coesite from Meteor Crater, Arizona // Science, 1960, v. 132, p. 220-222.
• Chesnokov B.V., Popov V.A. Increase in the volume of quartz grains in eclogites of the Southern Urals // Dokl. USSR Academy of Sciences, 1965, v. 62, p. 099 - 910.
• Liou JG, Tsujimori T., Zhang RY, Katayama I., Maruyama S. Global UHP Metamorphism and continental subduction / collision: The Himalayan Model // Inter. Geology Review, v. 46, 2004. p. 1-27.

• Coesite Eclogite
• Coesit in the webmineral.com database