Tectosilicates

Tectosilicates - Systematic Mineralogy Silicates Relative...

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Silicates Relative abundance of different minerals in the earth’s crust (Fig. 1) Crystal chemistry of different silicates Coordination numbers of the different cations (Table 1) Silicate Groups I- Tectosilicates Reading: pages 475 - 490 1) SiO 2 minerals There are 9 different polymorphs of SiO 2 (Table 2; Fig. 2). Transitions from one polymorph to another: reconstructive vs. displacive Tridymite: Orthorhombic, +, 2V = 35°, n<b. Occurs as a secondary mineral in volcanic rocks. Cristobalite: Pseudoisometric. n < b, moderate relief. Occurs as a secondary? mineral in volcanic rocks (particularly in spherulites). Coesite: Monoclinic, +, 2V = 54°, characteristic of ultrahigh pressure terranes. Stishovite: Tertragonal, forms in impact craters. Varieties of Quartz: Chalcedony: microcrystalline and fibrous. Secondary mineral in igneous rocks; also in sedimentary rocks. Principal constituent of chert. Jasper: microcrystalline quartz with interstitial hematite. Red in color. Sedimentary environments. Agate: Banded chalcedony 2) Feldspars Compositions and the ternary system: Ab – Or – An (Fig. 3) Plagioclase feldspar series: albite (0 – 10% An), oligoclase (10 – 30% An), andesine (30 – 50% An), labradorite (50 – 70% An), bytownite (70 – 90% An), anorthite (90 – 100% An). Common properties: Colorless in thin section, triclinic, weak birefringence, low relief, polysynthetic twinning, 2 directions of cleavage at 83 - 90°, variable extinction angles, and 2V. Optical orientations (Fig. 4).
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Tectosilicates - Systematic Mineralogy Silicates Relative...

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