Coesite

Coesite is a rarely occurring mineral from the mineral class of oxides with the molar ratio of metal: oxygen = 1: 2 and small cations. As a high-pressure modification of quartz has coesite same chemical composition SiO2 ( silicon dioxide) and is thus the silica family assigned and which in addition to the other quartz modifications Seifertit, tridymite, cristobalite, stishovite still Opal, Moganite and melanophlogite and the two hypothetical minerals beta - quartz Lechatelierite belong.

Coesite crystallizes in the monoclinic crystal system and developed only microcrystalline, granular aggregates, mainly as inclusions in other minerals. Its density of 3.01 g / cm ³ is the second highest in the silica family (compared to quartz: 2.65 g / cm ³).

Etymology and history

Coesite was named after Loring Coes, Jr. (1915-1978), an American chemist, the first time in 1953 it was possible to produce the mineral synthetic.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the coesite belonged to the mineral class of " oxides and hydroxides " and then to the Department of " Oxides with metal: oxygen = 1: 2 ," where he along with cristobalite, melanophlogite, Moganite, opal, quartz, tridymite, stishovite and the " quartz group " with the system no. IV/D.01 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the coesite also in the class of " oxides and hydroxides " and there in the department of " Metal: Oxygen = 1: 2 and comparable " one. This division, however, is further divided according to the size of the involved cations and the crystal structure or membership of a related mineral family, so that the mineral "With small cations: Silica family," according to its composition in the subdivision is to find where it is the only member of the unnamed group 4.DA.35 is.

In contrast to the Strunz 's Mineral classification predominantly common in English-speaking classification of minerals according to Dana assigns the coesite in the class of " silicates and Germanates " and there in the department of " framework silicate minerals " one. Here he is as the unnamed group 75.01.04 within the subdivision " framework silicates: with - coordinated Si tetrahedral Si - SiO2 lattice " to find with Moganite.

Education and Locations

The quartz - coesite equilibrium ( ie those pressure and temperature conditions at which high-quartz and coesite can co-exist ) has been experimentally determined very accurately. The experimental results indicate pressures in the range of about 2.5 to 3.8 GPa and at temperatures of 450 to 800 ° C, which corresponds to a minimum formation depth of approximately 75 km below the surface.

The presence of coesite, in rock samples often be detected only indirectly: by the transition of the high-pressure modification ( coesite ) in the low -pressure modification (quartz) increases the volume of the mineral: It arise radial cracks, which can be observed under a microscope. Rare find quartz grains still intact Coesitkern.

Mainly is coesite in rocks from the ultra-high -pressure metamorphism (English ultra- high-pressure metamorphism, UHPM which the mineral at the same time also defined ) found (Alps, Dabie Shan, eastern China, Himalaya ). Here comes coesite present in both basic, "traditional", eclogites and in Metasedimentiten or Metaplutoniten with acidic composition ( crustal rocks ). With the ultra - high -pressure metamorphism is usually subducted continental crust at a continent - continent collision and subjected to pressures and temperatures, allowing the formation and stability of coesite. Coesite can also be caused by impact of meteorites ( Impaktmetamorphose ). So coesite - occurrence could be detected as a result of that the Ries is an impact crater. Another important locality is the Meteor Crater in Arizona.

Coesite is also found in xenoliths leading diamond kimberlites.

Crystal structure

Coesite is metastable under normal pressures. It crystallizes in the monoclinic prismatic crystal system in the space group with the lattice parameters a = 7.14 Å, b = 12.37 Å and c = 7.17 Å, β = 120.34 ° and 16 formula units per unit cell.