sorosilicates

sorosilicates - 1 VI- Sorosilicates A- Lawsonite...

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1 VI- Sorosilicates A- Lawsonite Composition : CaAl 2 Si 2 O 7 (OH) 2 .2H 2 O Optical properties : Orthorhombic, colorless in thin section, prismatic crystals often tapered at both ends, may show one or 2 directions of cleavage in thin section, high relief, n > b, moderate birefringence, biaxial positive, lengthfast for rectangular sections. Optical orientation : c- elongated, X = c, Y = a, Z = b (Fig. 1). Occurrence : Index mineral for high P, low T, Ca – rich rocks (common in mafic and calcareous/ marly rocks). Replaced by epidote group minerals at higher temperatures. B- Epidote group minerals 1- Zoisite Composition : Ca 2 Al 3 Si 3 O 12 (OH) Structure : Sometimes classified as a “nesosilicate” as it consists of both isolated tetrahedra (SiO 4 ) -4 as well as double tetrahedra (Si 2 O 7 ) -6 . Optical properties : Orthorhombic, Colorless in thin section, prismatic crystals, high relief, n > b, weak to moderate birefringence, but interference colors are usually anomalous Berlin blue, Biaxial positive, 2V = 0 – 60°, lengthfast or lengthslow (Fig. 2).
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sorosilicates - 1 VI- Sorosilicates A- Lawsonite...

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