a mineral, aluminum silicate, Al 2 SiO 5 , occurring in the form of long, slender, and often fibrous crystals.
Also called fibrolite.

1825–30; named after B. Silliman; see -ite1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sillimanite (ˈsɪlɪməˌnaɪt)
a white, brown, or green fibrous mineral that consists of aluminium silicate in orthorhombic crystalline form and occurs in metamorphic rocks. Formula: Al2SiO5
[C19: named after Benjamin Silliman (1779--1864), US chemist]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
sillimanite   (sĭl'ə-mə-nīt')  Pronunciation Key 
A usually white, pale-green or brown orthorhombic mineral occurring as long, slender, fibrous crystals in metamorphic rocks. It is a polymorph of andalusite and kyanite, but can form at higher temperatures than either of these. Chemical formula: Al2SiO5.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Older parts of unit are characterized by an abundance of feldspar and local sillimanite.
Rare minerals such as garnets, sillimanite, staurolite or andalusite can be common in places.
Rare minerals such as garnets, staurolite, or sillimanite can be abundant locally.
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