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Sillimanite is a rare, brittle, aluminum silicate mineral that's often found in metamorphic rocks. It's a polymorph of kyanite and andalusite, meaning it has the same chemical composition but a different crystal structure.

Description

• Color: Sillimanite can be colorless, white, yellow, brown, green, or blue

• Appearance: Sillimanite is usually fibrous or massive, with needle-like crystals that are often parallel or twisted

• Luster: Sillimanite has a silky luster

• Optical properties: Sillimanite can exhibit chatoyancy, or a bright, narrow slit, similar to a cat's eye

Formation

• Sillimanite forms in high-temperature, high-pressure metamorphic rocks

• It's often found in pelitic metamorphic rocks, such as mica-schists, gneisses, and cornubianites

• It can also be found in granitic rocks and pegmatites