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Crystal system Orthorhombic
Transparency Transparent to translucent
Luster Vitreous to sub-adamantine
Fracture Uneven
Cleavage Perfect pinacoidal
Specific Gravity 3.24
Hardness 6 to 7.5
Optical Character Biaxial+ Double Refractive
Refractive index 1.659 - 1.680
Birefringence 0.15 t 0.021
Dispersion 0.015
Fluorescence red in transparent blue stones
Pleochroism Strong; colorless, pale yellow, blue.
Chemical Formula Al2SiO5

Sillimanite (SILL-ah-man-ite) is the third member of the trio, including andalusite and kyanite, that share the same chemical composition: aluminum silicate (Al2SiO5). In England, it is known as fibrolite (FY-bro-lite), because it is ordinarily fibrous and massive. Sillimanite was named after Benjamin Silliman. The mineral is usually semi-translucent to opaque and varies from gray to white or brownish green, colors that may resemble inferior qualities of jadelite.

Actually, it is less likely to resemble jade in appearance than it does in properties. In addition, there is a rarely encountered transparent to translucent variety in a violetish to grayish-blue color that may or may not display chatoyancy. Transparent stones without chatoyancy are usually pale blue, somewhat resembling Ceylon sapphire and iolite.

Sillimanite occurs in the orthorhombic system, usually in elongated striated crystals. The massive type has a hardness of
6 to 7, but the crystals increase to 7 1/2. The cleavage is excellent; therefore, transparent material is not very tough. The toughness of the fibrous material, however, is comparatively good. The luster is vitreous to subadamantine, the fracture is uneven, and there are no characteristic inclusions. The mineral is not attacked by acids nor is it fusible before the blowpipe or the jeweler's torch.

Sillimanite usually occurs in metamorphic deposits. Gem-quality material has been discovered only as pebbles in the gravels of Burma and Sri Lanka. Massive mottled and fibrous material comes from Idaho.

Transparent sillimanite is rather easily separated from both sapphire and iolite by the wide difference in R.I. and S.G. The R.I. is approximately 1.659 - 1.680, the birefringence is .021, and it is biaxial positive, with the beta index near 1.660. Pleochroism in the blue variety is strong, showing colorless, pale yellow and blue. Dispersion is weak (.015). The paler colors bear a marked resemblance to euclase and may be distinguished from it readily only by the fact that its S.G. (approximately 3.22 to 3.26) is distinctly higher than that of euclase (3.10). Stones that resemble jadeite in appearance are rather difficult to separate, for the R.I. and other properties of the two minerals are similar; however, it is possible to distinguish between them by the somewhat lower S.G. of sillimanite.