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A fibrous or prismatic aluminum silicate mineral. Sillimanite was named in honor of Benjamin Silliman, a professor of mineralogy at Yale. It was previously called fibrolite. Sillimanite is found in South Africa, India, Germany (Bavaria), France, the Czech Republic, Scotland, United States (California, New York, Pennsylvania, South Carolina, Delaware, Connecticut, Massachusetts) and southeast Asia. Its color is usually a brown or gray, but stones found in southeast Asia are a gem quality sapphire blue. Sillimanite is a refractory material with high heat resistant. It is used for kiln brick, spark plugs, and laboratory items.

Synonyms and Related Terms

fibrolite; aluminum silicate; bucholzite; Sillimanit (Deut.);

Other Properties

Orthorhombic crystal system; usually occurs as fibrous mass. Perfect cleavage in one direction.

Fracture = uneven or splintery. Luster=vitreous. Streak = colorless.

Composition Al2SiO5
Mohs Hardness 6.0 - 7.5
Melting Point 1900
Density 3.23-3.24
Refractive Index 1.65; 1.66; 1.68


Click on any of the charts below for more information:

Additional Information

Mineralogy Database: Sillimanite

Sources Checked for Data in Record

  • Robert Fournier, Illustrated Dictionary of Practical Pottery, Chilton Book Company, Radnor, PA, 1992
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979

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