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ANDALUSITE

The Aluminosilicate Group: Al 2 SiO 5. ANDALUSITE. KYANITE. SILLIMANITE. Aluminosilicates. Formula Al 2 SiO 5 (Al 2 O 3 · SiO 2 ) Edge-sharing chains of AlO 6 octahedra Occur in high pressure and/or high temperature environments

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ANDALUSITE

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  1. The Aluminosilicate Group: Al2SiO5 ANDALUSITE KYANITE SILLIMANITE

  2. Aluminosilicates • Formula Al2SiO5 (Al2O3·SiO2) • Edge-sharing chains of AlO6 octahedra • Occur in high pressure and/or high temperature environments • Uses: geological barometer, ceramics, rarely as gemstone

  3. Unit cell properties andalusite sillimanite kyanite space group Pnnm Pbnm P-1 a 7.80 7.49 7.13 b 7.90 7.68 7.85 c 5.56 5.78 5.57 alpha 90 90 90.06 beta 90 90 101.04 gamma 90 90 105.75 volume 342.4 332.3 293.7 Z 4 4 4

  4. Andalusite a-b plane

  5. Andalusite b-c plane

  6. Sillimanite a-b plane

  7. Sillimanite b-c plane

  8. Kyanite a-b plane

  9. Kyanite b-c plane

  10. Topaz b-c plane

  11. Topaz b-c plane

  12. Other physical properties of the aluminosilicate group andalusite sillimanite kyanite Bulk Modulus KT0 151 171 193 Volumetric thermal expansion Al1 polyhedra (1/K) 1.86x10-5 3.36x10-5 3.29x10-5

  13. PT phase relations of the aluminosilicate group Disorder in sillimanite between the Al and Si tetrahedral sites may cause shifting of the triple point to higher P and T (Greenwood 1972)

  14. Occurrences of aluminosilicates • All are generally produced from metamorphism of silica-poor sediments. Associated with corundum in severely silica-depleted rocks • Kyanite: high pressure, low temperature. Occurs in regionally metamorphosed pelitic rocks, eclogites, kimberlites • Andalusite: low pressure, high temperature. Generally occurs in contact metamorphic zones, may occur with kyanite in high pressure rocks because of slow reaction • Sillimanite: very high temperature, low to high pressure. Occurs in high temperature contact metamorphism, schists, and gneisses

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