A silicate is a drifting salt of the Silice (SiO). In Chemistry, they are chemical compounds. In Mineralogy, the silicates designate an important family of mineral.
The silicates constitute 97% of the Earth's crust. There exists many variétés about it;:
- silicates ferromagnésiens: Chlorite S, serpentine S, Amphibole S, Pyroxene S, Peridot S or Olivine S, Garnet-red S;
- aluminosilicates, for which the Aluminum can replace the Silicium: Feldspar S, rich in Silica, Zeolite S, Mica S;
- aluminosilicates: Garnet-red Tourmaline, S.
Classification of silicates
The silicates, are minerals whose skeleton is primarily formed by Oxygen and Silicon tetrahedrons added with aluminum, magnesium, iron, calcium, potassium, sodium and other elements. They can be classified according to several criteria, but the two classifications most employed in mineralogy are based on the sequence of the tétraèdres :- the topochemical classification (classification of Machatski-Bragg) is based on the sequence of the tetrahedrons where silicon and aluminum can give place to substitution isomorphe ; the tetrahedrons where only aluminum is present are considered hétéro-tétraèdres ;
- the topological classification (classification of Zoltai) is based on the sequence of the tetrahedrons centered by any cation.
The subcategories defined by these two criteria are the same ones and, in the majority of the cases, the end result is also the same one. However, in the case of aluminosilicates of which the structure comprises tetrahedrons centered by anions other than silicon and aluminum, two classifications divergent.
Independent groups of aluminosilicates (and the their minerals most representative) sont :
- the Tectosilicate s : the tetrahedrons have all their tops communs : quartz, Tridymite, Cristobalite, Feldspar S, Feldspathoïde S, Zeolite S, Scapolite S.
- the Phyllosilicate s : the tetrahedrons share three tops out of four and form a layer bidimensionnel : Chlorite S, Mica S, Talc, Pyrophyllite, Serpentine, Kaolinite ;
- the Inosilicate s : the tetrahedrons are associated in chains or ribbons. Pyroxene S, Amphibole S, Pyroxénoïde S, Amphiboloïde s ;
- the Cyclosilicate s : the structure is made up of rings isolated from 3,4 or 6 tetrahedrons. Tourmaline, Beryl, Cordierite, Dioptase ;
- the Sorosilicate s : two tetrahedrons share a top and form a group 6 : epidote S, Mélilite, Thortveitite, Hémimorphite (Calamine)
- the Nésosilicate S (or orthosilicates) : the tetrahedrons do not have any common top, but only with others polyèdres ; the anions all belong to the tetrahedrons. Zircon, Olivine S, Garnet-red s ;
- Nésosubsilicate s : as for the nésosilicates, the tetrahedrons do not have a common top, but here the structure comprises in addition to the anions which do not belong to tétraèdres : Disthene, Andalusite, Sillimanite, Staurolite, Topaz, Titanite (Sphène).
The principal differences between topochemical classification of topological Machatski-Bragg and that of Zoltai are given in the following table.
Silicates in astronomy
Some star S surrounded by Matter circumstellaire “ froide ” - in on this side temperature of sublimation of silicates, in the neighborhoods of 1 500 K - presents grains of dust made up of silicates. Their presence is revealed by spectral lines broad, called features of silicates, in the spectral bands Infrarouge S NR and Q, with 10 and 20 μm respectively.Spectral profile of these features , which depends on the type of silicate, of the geometry of the grains of dust and the additional presence of others made up (e.g. the Glace with less 300 K), gives indications on the conditions physico - chemical of the studied environments.
The presence of silicate is frequently attested around the evolved/moved young stars and stars, in particular in the discs of accretion and the envelopes circumstellaires.
See too
Related articles
silicate|silicate- Geology
- Glossary of the minerals
External bonds
- Site presenting the silicates
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