Sillimanite is an aluminosilicate mineral (Al₂SiO₅) named after American chemist Benjamin Silliman, and is a polymorph of kyanite and andalusite, meaning they share the same chemical formula but have different crystal structures.

Key Characteristics:

• Chemical Formula: Al₂SiO₅
• Crystal System: Orthorhombic
• Hardness: 6-7 on the Mohs scale
• Color: Typically colorless, white, or grey, but can also be brown, yellow, green, or blue
• Habit: Often occurs as slender prismatic crystals or fine fibrous crystals called fibrolite
• Occurrence: Found in high-temperature, regionally metamorphosed clay-rich rocks like schists and gneisses
• Fibrolite: A common form of sillimanite, characterized by fibrous aggregates