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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.);

Physical and Chemical Properties

  • Orthorhombic crystal system; usually occurs as fibrous mass or prismatic crystals
  • Perfect cleavage in one direction
  • Fracture = uneven or splintery
  • Luster = vitreous to silky
  • Streak = colorless or white
  • Pleochroism = colorless to pale brown
  • Fluorescence = generally inert
Composition Al2SiO5
Mohs Hardness 6.0 - 7.5
Melting Point 1900 C
Density 3.23-3.24 g/ml
Refractive Index 1.65; 1.66; 1.68
Birefringence 0.015 - 0.021

Resources and Citations

  • Mineralogy Database: Sillimanite
  • Gem Identification Lab Manual, Gemological Institute of America, 2016
  • 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
  • Encyclopedia Britannica, Comment: "Sillimanite." Accessed 18 Aug. 2004
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979
  • Wikipedia: Sillimanite (Accessed Nov. 9, 2005 and Jan 2023)