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The Mineral sillimanite

Fibrous While Sillimanite

Sillimanite is named in honor of Benjamin Silliman (1779-1864), noted chemist and the earliest professor to teach mineralogy at Yale University. Silliman was also the father in law of noted mineralogist and author James Dwight Dana.

Chemical Formula

Al2SiO5

Color

Gray, brown, yellowish-gray, white. Rarely colorless, light pink, or light purple.

Crystal System

Orthorhombic

Properties

Streak
Colorless
Hardness
6.5 - 7.5
Transparency
Transparent to translucent
Specific Gravity
3.2 - 3.3
Luster
Vitreous, silky
Cleavage
1,1
Fracture
Uneven, splintery
Tenacity
Brittle

Crystal Habits

Most often fibrous, sometimes with radiating crystal sprays embedded in matrix. Also columnar, massive, compact, and as rounded waterworn crystals and crystal masses. Rarely as prismatic crystals.

Additional Information

Composition
Aluminum silicate
In Group
Silicates; Nesosilicates
Striking Features
Crystal habits and hardness
Environment
In metamorphosed schists and gneisses. Rarely in granite pegmatites.
Rock Type
Metamorphic

Other Names

Fibrolite

Uses

The rare transparent Sillimanite crystals from Burma, Sri Lanka, and India are valued as important rare collector stones, and are occasionally cut into exquisite gemstones for collectors. Compact masses of Sillimanite were once used by Native Americans of the American Southwest to fashion tools.

Noteworthy Localities

Unusual prismatic and waterworn transparent crystals of Sillimanite come from Mogok, Burma (Myanmar); and from the Ratnapura District, Sri Lanka. Orissa, India, is another uncommon source for transparent crystallized Sillimanite. 

In the U.S., brownish fibrous crystal groupings of Sillimanite have come from Chester, Middlesex Co., Connecticut. Also in Connecticut are the old classic locality of Norwich, New London Co.; and Willimantic, Windham Co., where small crystals in matrix were found. Fibrous masses of Sillimanite come from Brandywine Springs, New Castle Co., Delaware; and dark brown crystals crystal sprays from Oconee Co., South Carolina. Gray Sillimanite masses are found in Natrona Co., Wyoming.

Common Mineral Associations

Quartz, Biotite, Almandine, Plagioclase

Distingushing Similar Minerals

Tremolite - Lower hardness, often fluorescent.
Chrysotile - Much lower hardness, greasy feel, and flexible.
Dumortierite - Usually in a more blue or violet color.

sillimanite Photos



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