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Cyclosilicates - V Cyclosilicates A Beryl Composition...

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V- Cyclosilicates A- Beryl Composition : Be 3 Al 2 Si 6 O 18 Structure : Classified as a “tectosilicate” in some recent Mineralogy texts. Optical properties : Hexagonal, Colorless in thin section but thick grains show weak colors and pleochroism, prismatic crystals, hexagonal in cross section, moderate relief, n > b, weak birefringence, Uniaxial negative, lengthfast. Optical orientation : c- elongated. Occurrence : Common in granitic pegmatites, usually as an accessory mineral. Also in granites and syenites. B- Tourmaline Composition : (Na,Ca)(Li,Mg,Al) 3 (Al,Fe,Mn) 6 (BO 3 ) 3 (Si 6 O 18 )(OH) 4 Optical properties : Hexagonal, blue, green, yellow, or pink in thin section, depending on composition; strongly pleochroic with maximum absorption in the direction of ϖ (i.e. when c axis is parallel to the analyzer direction since it is a negative mineral), hexagonal in cross section, high relief, n > b, strong birefringence, Uniaxial negative, lengthfast.
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