What is Sillimanite?
Sillimanite is an orthorhombic aluminium silicate and a high-temperature metamorphic mineral, found as fibrous aggregates, prisms and occasional gem material.
Mineralogy and crystal structure
Reference formula: Al2SiO5. Sillimanite is one of the Al2SiO5 polymorphs, alongside andalusite and kyanite. Fibrolite is the fibrous variety; the selected model represents crystalline sillimanite. Sillimanite is classified with silicates and has a orthorhombic crystal system. Typical hardness is 6.5–7.5 on the Mohs scale and specific gravity is approximately 3.2–3.3.
Colour, lustre and habit
Typical colours include colourless, white, grey, brown, blue, green. The usual fibrous, prismatic, massive habit and vitreous, silky lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Sillimanite forms in or is associated with high-grade metamorphic rocks such as gneiss, schist and hornfels. Important documented localities include India, Sri Lanka, Myanmar, the United States, Germany and Australia.
A polymorph that records metamorphic heat
Sillimanite is valuable as a geological indicator and as a textural specimen. Its fibrous and prismatic habits show how pressure-temperature history changes the same Al2SiO5 chemistry.
Collecting and care
Protect fibrous surfaces from abrasion and clean gently. Do not confuse fibrolite aggregates with a separate species or use soft fibrous material in high-wear jewellery.
