Cyclosilicates Minerals with rings of tetrahedra T : O ratio = 1 : 3

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Presentation transcript:

Cyclosilicates Minerals with rings of tetrahedra T : O ratio = 1 : 3 So the silicate group is n(SiO3), where n=3,4 or 6

The cyclosilicates may contain three-, four- or six-membered rings of SiO4 tetrahera: Three-membered rings are exemplified by benitoite BaTiSi3O9 Four-membered rings are exemplified by axinite (Ca,Fe,Mn)3Al2BO3(Si4O12(OH) in which the silicate rings alternate with BO3 rings.

Six-membered rings are more common and are seen in: Tourmaline has the general formula WX3Y6(BO3)3(Si6O18)(OH,F)4 giving rise to a number of different compositions; i.e. individual species. A typical formula is (Na,Ca)(Fe,Mg,Al,Li)3Al6(BO3)3Si6O18(OH) in which BO3 rings alternate with the silicate rings.

Tourmaline

Tourmaline Al, Fe3+,Mn3+ BO3 triangle Yellow- OH and Ca, Na – blue, Li, Mg, Al - black BO3 triangle

Beryl Be3Al2Si6O18 and cordierite Mg2Al4Si5O18 are considered by some to be framework silicates, but the silicate rings are the main structural feature.

Beryl Be3Al2Si6O18

Purple – Be, green - Al

Cordierite

Cordierite (Mg, Fe)2[Al4Si5O18], orthorhombic, Cccm 6-member silica rings linked to adjacent rings those above and below by octahedrally-coordinated Mg, Fe3+ and Al. Blue dots represent sites for possible alkali metals or H2O

Some texts cite the mineral ekanite as an example of an eight- membered ring structure. However, it is composed of two four-membered rings linked by their apical oxygens. Thus, three corners of each tetrahedron are shared and the Si:O ratio is 2:5, in violation of the definition of ring silicates. This is another example of the weakness of the traditional definition of the silicate sub-classes. ThK(Ca,Na)2Si8O20