Sillimanite

Real mineral · common · Al₂SiO₅

One of three minerals sharing the formula Al₂SiO₅, and the one that marks the hottest metamorphism; kyanite and andalusite hold the same atoms at lower temperatures. Its fibrous masses cut fine cat's-eyes.

Identity

Formula

Al₂SiO₅

Chemical class

silicate

Also known as

Fibrolite

Crystallography

Crystal system

orthorhombic

Habits

fibrous, prismatic, columnar

Fracture

splintery

Physical properties

Mohs hardness

7

Density

3.24 g/cm³

Luster

vitreous

Colors

blue, brownish green, colorless, gray, gray green

Streak

white

Transparency

transparent, translucent

Geology

Origins

metamorphic

Host rocks

gneiss, schist

Associated minerals

Kyanite, Garnet, Corundum, Quartz

Geography

India, Sri Lanka, USA

Classification

Rarity

common

Gem quality

Yes

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