Sillimanite - Cut Locality: Orissa, India Size:, 1.53 x 1.10 x 0.67 cm 8.84Cts — Photo: Didier Descouens · CC BY-SA 4.0
Sillimanite
Also known as: Fibrolite.
The high-temperature Al₂SiO₅ polymorph — 'fibrolite' — named for chemist Benjamin Silliman.
Intro adapted from Wikipedia (CC BY-SA).
Sillimanite is an IMA-recognized mineral species in the silicate class, with the chemical formula Al₂SiO₅.
It crystallizes in the orthorhombic crystal system (space group Pbnm).
Key properties: Mohs hardness 6.5–7.5; specific gravity 3.23–3.24; typically brown or white or gray or green; White streak; Vitreous to subadamantine luster; refractive index 1.665; optically biaxial.
IMA symbol Sil, grandfathered status — known before the IMA’s founding, named in 1824.
Found in high-grade metamorphic schists, gneisses, and hornfels; more rarely in pegmatites. Also as a detrital mineral.
It is found associated with Andalusite, kyanite, potassic feldspar, almandine, cordierite, biotite, quartz.
The name refers to Professor Benjamin Silliman (1779–1864), Professor of Chemistry and Mineralogy, Yale University, New Haven, Connecticut, USA.
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