Aluminosilicate Mineral Profile
Mineral Name: Sillimanite
Chemical Formula: Al2SiO5
Silicate Structural Class: Nesosilicate
Diagnostics Properties:
Colour: White, Brown and Green
Transparency: Transparent to Translucent
Lustre: Silky or Vitreous
Specific Gravity: 3.2
Streak: White
Mohs Hardness: 7.5 in large crystals
Mineral Cleavage: Perfect. Good in 1 direction lengthwise
Occurrence:
Type of Rock: Aluminium-rich metamorphic rocks
Temperature: Forms at temperatures greater than 550oC(2)
Pressure Field: Sillimanite is a polymorph with two other minerals: Kyanite and Andalusite. Sillimanite forms at high pressure (about 2.5 kilobars), Andalusite forms at higher temperatures but lower pressure (less than 2.5 kilobars) and Kyanite at high pressure (higher than 2.5 kilobars) but lower temperatures(3)
Geographic Localities: Brazil, England and other European sites
Main Uses: Sillimanite is mainly used in the manufacture of spark plugs and other forms of ceramic insulation. It can be used as a gemstone although this is rare and is more often used as mineral specimens.
Sillimanite Crystals - geograph.org.uk - 746197
Sillimanite
Phase diagrams at work!Sillimanite: A compact chunk with obvious crystalline tendencies. Fibres are slightly curved and have a pearly lustre. (http://mineral.galleries.com/scripts/item.exe?ENLARGE+Minerals+Silicates+Sillimanite+SIL-1)
Andalusite-bearing Rock - geograph.org.uk - 129865...
Nederlands: Fasendiagram van aluminosilicaat (Al2S...
English: Kyanite crystals (7cm long) Italiano: Cri...