Sillimanite is an aluminum silicate mineral that occurs in clay-rich (argillaceous) rocks that have been metamorphosed at high temperature.
Pronunciation: SILL•im•in•ite (accent on capitalized syllable)
Color: brown, pale green, white
Luster: vitreous
Diaphaneity: translucent, but may appear opaque in hand specimen
Hardness: 6-7
Specific gravity: 3.23
Cleavage/fracture: 1 direction
Other distinguishing properties: Sillimanite occurs as slender, commonly fibrous crystals in certain high-temperature metamorphic rocks.
Klein, C., and Hurlbut, C.S., Jr., 1999, Manual of Mineralogy (after James D. Dana) [21st edition, revised]: New York, John Wiley & Sons, 682 p.
Los Angeles County Museum of Natural History, The Photo Atlas of Minerals: nhm.org/pam/
Return to the Minerals page.