Gemmology
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Cordierite

Class: Silicates

In thermally metamorphosed argillaceous sediments and regionally metamorphosed schists, gneisses; in mafic igneous rocks and granites (Anthony et al., 2001—2005). The gemstone bears the name iolite.

Original description: Lucas, J. A. H., 1813. XIII. Cordierite (f.). (Iolithe (m)) — Tableau Méthodique Espèces Minérales, D’Hautel, Paris: 219—222 [view in ‘Library’].

Type locality: Großer Arber, Zwiesel, Bavaria, Germany.

Type material: unknown.

Etymology: in honour of Pierre Louis Antoine Cordier, French mining engineer and geologist.

Distribution: Australia; Canada: Manitoba; Finland; Germany: Bavaria, Bodenmais; India; Madagascar: south of Antsirabe; Namibia: around Namib; Norway; Sri Lanka; Zimbabwe (Anthony et al., 2001—2005).

Chemistry

Mg2Al4Si5O18

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

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: crystals short prismatic, striated, to 18 cm; typically granular to compact, massive. Twinning: common on {110}, {130}, simple, lamellar, cyclical (Anthony et al., 2001—2005).

Physical properties

Cleavage: fair on {100}, poor on {001} and {010} (Anthony et al., 2001—2005). Fracture: subconchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Arem, 1987: 70). Hardness: 7—7.5 (Arem, 1987: 70). Density: 2.56—2.66 g/cm3 (Lazzarelli, 2012). Luminescence: none (Arem, 1987: 70).

Optical properties

Colour: blue, smoky blue, bluish violet; greenish, yellowish brown, grey; colourless to very pale blue in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Arem, 1987: 70). Refractive index: 1.542—1.578 — anisotropic [biaxial (+/-)] (Lazzarelli, 2012). Birefringence: 0.008—0.012. Dispersion: 0.017 (Arem, 1987: 70). Pleochroism: X = pale yellow, green; Y = violet, blue-violet; Z = pale blue (Anthony et al., 2001—2005).

Material from ‘Repository’

7 specimens: 0025 — 0.21 ct, Tanzania; 0123 — 0.21 ct, Tanzania; 0214 — 0.18 ct, Tanzania; 0380 — 0.19 ct, Tanzania; 0381 — 0.19 ct, Tanzania; 0389 — 0.21 ct, Tanzania; 0390 — 0.20 ct, Tanzania.


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