Sillimanite

Class: Silicates

In metamorphic schists, gneisses; more rarely in pegmatites (Anthony et al., 2001—2005).

Sillimanite — specimen 0113, photo © NMNHS

Original description: Bowen, G. T., 1824. Description and Analysis of Sillimanite, a new mineral — American Journal of Science and Arts 8: 113—118 [view in ‘Library’].

Type locality: Chester, Middlesex Co., Connecticut, USA.

Type material: unknown.

Etymology: in honour of Benjamin Silliman, Professor of chemistry and geology.

Distribution: of widespread occurrence, but fine examples are uncommon: Austria: Tirol, Sellraintal; Germany: Bavaria, Bodenmais; Sri Lanka: Ratnapura (Anthony et al., 2001—2005).

Chemistry

Al2(SiO4)O

Essential elements: oxygen (O), aluminium (Al), silicon (Si).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: crystals prismatic, may be acicular, with square cross sections, rounded and striated, poorly terminated, to 2 cm (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {010} (Anthony et al., 2001—2005). Fracture: uneven (Arem, 1987: 172). Tenacity: brittle, tough in aggregate (Anthony et al., 2001—2005). Hardness: 6—7.5 (Lazzarelli, 2012). Density: 3.2—3.26 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: colourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous to subadamantine (Anthony et al., 2001—2005). Refractive index: 1.653—1.685 — anisotropic [biaxial (+)] (Lazzarelli, 2012). Birefringence: 0.014—0.021. Dispersion: strong (Anthony et al., 2001—2005). Pleochroism: if coloured, X = pale brown or pale yellow; Y = brown or green; Z = dark brown or blue (Anthony et al., 2001—2005).

Material from ‘Repository’

1 specimen: 0113 — 3.74 ct, India.

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