Coesite
| Coesite | |
|---|---|
 |
|
| General and classification | |
| chemical formula | SiO 2 |
|
Mineral class (and possibly department) |
Oxides - Metal: Oxygen = 1: 2 and comparable - With small cations: Silica family |
|
System no. to Strunz and to Dana |
4.DA.35 ( 8th edition : IV / D.01) 75.01.04.01 |
| Crystallographic Data | |
| Crystal system | monoclinic |
| Crystal class ; symbol | monoclinic prismatic; 2 / m |
| Room group (no.) | C 2 / c (No. 15) |
| Lattice parameters |
a = 7.14 Å ; b = 12.37 Å; c = 7.17 Å β = 120.34 ° |
| Formula units | Z = 16 |
| Physical Properties | |
| Mohs hardness | 7.5 to 8 |
| Density (g / cm 3 ) | 3.01 |
| Cleavage | indistinct |
| colour | colorless |
| Line color | White |
| transparency | transparent |
| shine | Glass gloss |
| Crystal optics | |
| Refractive indices |
n α = 1.593 to 1.599 n γ = 1.597 to 1.604 |
| Birefringence | δ = 0.004 to 0.005 |
| Optical character | biaxial positive |
| Axis angle | 2V = 54 ° to 64 ° |
Coesite is a rarely occurring mineral from the mineral class of oxides with the molar ratio metal: oxygen = 1: 2 and small cations. As a high-pressure modification of quartz , Coesite has the same chemical composition SiO 2 ( silicon dioxide ) and is therefore assigned to the silica family, to which, in addition to the other quartz modifications seifertite , tridymite , cristobalite , stishovite , opal , mogánite and melanophlogite as well as the two hypothetical minerals beta-quartz and Lechatelierit .
Coesite crystallizes in the monoclinic crystal system and develops only microcrystalline, granular aggregates, predominantly 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 who first succeeded in producing the mineral synthetically in 1953.
classification
In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the coesite belonged to the mineral class of "oxides and hydroxides" and there to the department of "oxides with metal: oxygen = 1: 2", where together with cristobalite , melanophlogite , Mogánite , opal , quartz , stishovite and tridymite the "quartz group" with the system no. IV / D.01 .
The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns the Coesite to the class of “oxides and hydroxides” and there to the department of “metal: oxygen = 1: 2 and comparable "a. However, this section is further subdivided according to the size of the cations involved and the crystal structure or membership of a related mineral family, so that the mineral can be found according to its composition in the sub-section "With small cations: Silica family", where it is is the only member of the unnamed group 4.DA.35 .
In contrast to the Strunz mineral systematics, the systematics of minerals used mainly in the English-speaking world, according to Dana , assigns the coesite to the class of "silicates and germanates" and there to the department of " framework silicate minerals ". Here it can be found as in the unnamed group 75.01.04 within the subdivision “ Framework silicates: tetrahedral Si lattice, SiO 2 with [4] -coordinated Si ” with mogánite.
Education and Locations
The quartz-coesite equilibrium (i.e. those pressure and temperature conditions under which high quartz and coesite can coexist) was determined very precisely experimentally. The experimental findings indicate pressures in the range of about 2.5 to 3.8 GPa and temperatures of 450 to 800 ° C, which corresponds to a depth of at least 75 km below the earth's surface. Coesit is therefore metastable at normal pressure .
The presence of coesite can often only be determined indirectly in rock samples: the transition from the high pressure modification (coesite) to the low pressure modification (quartz) increases the volume of the mineral: Radial cracks arise that can be observed under a microscope. Quartz grains with a coesite core are found less often.
Coesite is mainly found in rocks of the ultra-high- pressure metamorphism (UHPM, which also defines the mineral at the same time) ( Alps , Dabie Shan in East China , Himalaya ). Coesite occurs here in basic, “classical”, eclogites as well as in metasedimentites or metaplutonites with an acidic composition (crustal rocks). In ultra-high pressure metamorphism, continental crust is mostly subducted during a continent-continent collision and exposed to pressures and temperatures that enable the formation and stability of coesite. Coesite can also be formed by the impact of meteorites ( impact metamorphosis ). For example, based on the occurrence of Coesite, it was possible to prove that the Nördlinger Ries is an impact crater . Another important site is the Meteor Crater in Arizona .
Coesite is also found in xenolites in diamond-leading kimberlites .
Crystal structure
Coesite crystallizes in the monoclinic crystal system in the space group C 2 / c (space group no. 15) with the lattice parameters a = 7.14 Å , b = 12.37 Å, c = 7.17 Å and β = 120.34 ° and 16 formula units per unit cell .
See also
literature
- SR Bohlen, DH Lindsley: Thermometry and Barometry of Igneous and Metamorphic Rocks . In: Annual Review of Earth and Planetary Sciences . tape 15 , no. 1 , 1987, pp. 397-420 , doi : 10.1146 / annurev.ea.15.050187.002145 .
- Kunal Bose, Jibamitra Ganguly: Quartz-coesite transition revisited: Reversed experimental determination at 500-1200 ° C and retrieved thermochemical properties. American Mineralogist, Volume 80, 1995, pp. 231-238 (PDF file; 754 kB).
Web links
Individual evidence
- ↑ Webmineral - Coesite (Eng.)
- ^ A b c Louise Levien, Charles T. Prewitt: High-pressure crystal structure and compressibility of coesite . In: American Mineralogist . Volume 66, 1981, pp. 324-333 (PDF file; 1.03 MB).
- ↑ Martin Okrusch, Siegfried Matthes: Mineralogie: An introduction to special mineralogy, petrology and deposit science . 7th edition. Springer Verlag, Berlin / Heidelberg / New York 2005, ISBN 3-540-23812-3 , pp. 110-115 .
- ↑ a b Coesite at mindat.org (engl.)
- ↑ Patent US2876072 : Coesite Silica. Published on February 3, 1959 , inventor: Loring Coes Jr ..
- ^ SR Bohlen, DH Lindsley: Thermometry and Barometry of Igneous and Metamorphic Rocks . In: Annual Review of Earth and Planetary Sciences . tape 15 , no. 1 , 1987, pp. 397-420 , doi : 10.1146 / annurev.ea.15.050187.002145 .