Home VIII. Silicates (Grouping according to mineralogical systematics)

Silicates show silicon atoms in the oxidation state of 4+ (tetravalent), which are coordinated in a tetrahedron by four oxygen atoms.  Such [SiO4]4--tertahedrons can exist as indepentend units, however they can be linked together by their corners in order to form larger units in a way that two tetrehedrons share one oxygen atom. The tetrahedrons are linked together be their corners only, structures are not realized by linking edges or faces. The units formed by the linkage of these tetraherdons are used for the systematic classification of the silicates. SiO4-Tetrahedron

Nesosilicate Nesosilicates [neso (gr.) = island] or independent tetrahedral silicates: The isolated [SiO4]4--tetrahedrons exist as independent Units wihich are linked together by cations like Fe2+, Mg2+ or Zr4+. Examples for nesosilicates are olivine (Fe,Mg)2[SiO4], zircon Zr[SiO4], but also the minerals of the garnet group like almandine Fe2+3Al2[SiO4].

Sorosilicate Sorosilicates [soro (gr.) = group] or double tetrahedral silicates: Two [SiO4]4--tetrahedrons are linked together by sharing on oxygen atom forming a [Si2O7]6--ion. This structure is realized in e.g. thortveitite Sc2[Si2O7] .

Cyclosilicate Cyclosilicates [cyclo (gr.) = ring] or ring silicates: Three [SiO4]4--tetrahedrons are linked together  forming a ring with the formula  [Si3O9]6- as in benitonite BaTi[Si3O9]. A further variation is possible by forming a ring consisting of six [SiO4]4--tetrahedrons as an [Si6O18]12--ion. The beryl with the chemical formula Al2Be3[Si6O18] can be named as an example.

Inosilicates [ino (gr.) = thread] or chain silicates: Chain silicates are realized by linking [SiO4]4--tetrahedrons in a way to form continous chains. They may be represented by a composition of  [SiO3]2-. A typical example is diopside CaMg[Si2O6], in which the "endless" chains als hold together by Ca2+- and Mg2+-ions. Inosilicate (single chain)

Double chain silicates: Two silicate chains of the ionsilicates are linked by the corners forming double chains yielding [Si4O11]6- -ions as realized in glaucophane Na2Mg3Al2[(OH,F)|Si4O11]2. Double chain silicates are commonly grouped with the inosilicates. Inosilicate (double chain)

Phyllosilicates [phyllo (gr.) = sheet] or sheet silicates: They are formed if the above described [SiO3]2-chains are linked together to form continuous sheets with the chemical formula [Si2O5]2-. Sheet silicates are for example pyrophyllite Al2(OH)2[Si4O10] or talc Mg3(OH)2[Si4O10]. Phyllosilicate

Tectosilicates [tecto (gr.) = framework] or framework silicates: Tectosilicates are formed by a [SiO4]4--tetrahedron, which is linked together with four tetrahedrons in a three-dimensional framework in such a way, that the tetraherdons share one oxygen atom. This yield a ratio of (Si,Al):O=1:2, where silicon may be replaced by aluminium. Tectosilicates are among others feldspars like orthoclase K[AlSi3O8] as well as zeolites like natrolite Na2[Al2Si3O10]·2H2O. Tectosilicate

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© 2004 Büro für angewandte Mineralogie · Dr. Stephan Rudolph · D-47918 Tönisvorst
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