1. The Aluminosilicate Group:
Al2SiO5
ANDALUSITE
SILLIMANITE
KYANITE

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 b c
alpha
beta
gamma
volume Z

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 KT
Volumetric thermal expansion
Al1 polyhedra (1/K) 1.86x x x10-5

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