Sillimanite
Image: Didier Descouens (CC BY-SA 4.0)
Mineral NameSillimanite
Chemical FormulaAl2SiO5
Mohs Hardness7
Crystal Systemorthorhombic
Lustervitreous
Streak Colorwhite
Cleavageperfect on {010}
Fractureuneven
Specific Gravity3.25
Colorsgreyish-blue, white, brown, green, yellow
Transparencytransparent
Type LocalityOrissa, India
Mineral GroupSilicates
Usesrefractory materials, ceramic industry, gemstone, collector
Associated Mineralskyanite, andalusite, garnet, cordierite, biotite, muscovite, quartz, feldspar
Rarityuncommon
Description
This is a faceted, oval-cut Sillimanite gemstone, displaying a transparent, greyish-blue color with good brilliance. The surface shows some fine striations or polishing marks.
Geological Context
Sillimanite is a high-temperature, high-pressure index mineral formed during regional metamorphism of pelitic (aluminum-rich) rocks. It is one of the three aluminosilicate polymorphs, indicating specific metamorphic conditions.
