Sillimanite (Fibrolite)
Image: <a href="//commons.wikimedia.org/wiki/User:Strekeisen" title="User:Strekeisen">Strekeisen</a> (CC BY-SA 4.0)
Mineral NameSillimanite (Fibrolite)
Chemical FormulaAl2SiO5
Mohs Hardness7
Crystal Systemorthorhombic
Lustervitreous
Streak Colorwhite
Cleavageperfect on {010}
Fractureuneven
Specific Gravity3.25
Colorswhite, gray, pale yellow, pale brown, pale green
Transparencytransparent
Type LocalityIndia, Sri Lanka, Myanmar, USA (e.g., Connecticut, Idaho), Brazil, Germany
Mineral GroupSilicates (Nesosilicates)
Usesrefractory material, ceramics, indicator mineral in metamorphic petrology
Associated Mineralsquartz, muscovite, biotite, garnet, cordierite, kyanite, andalusite
Raritycommon
Description
This microscopic image displays numerous elongated, needle-like to fibrous crystals of fibrolite (a variety of sillimanite) in a light-colored matrix, characteristic of its appearance in thin section under plane polarized light.
Geological Context
Sillimanite is a high-temperature, moderate-to-high pressure index mineral formed during regional metamorphism of pelitic (aluminum-rich) rocks, indicating the highest grades of metamorphism among the Al2SiO5 polymorphs.
