Andradite

Class: Silicates
Group: Garnet

Varieties | Names:

In skarns from contact metamorphosed impure limestones or calcic igneous rocks; in chlorite schists and serpentinites (Anthony et al., 2001—2005).

Demantoid — specimen 0143, photo © NMNHS

Original description: Dana, J. D., Brush, G. J., 1868. Lime-Irongarnet; Andradite — A System of Mineralogy, John Wiley and Sons, New York: 268—270 [view in ‘Library’].

Type locality: Drammen, Buskerud, Norway.

Type material: unknown.

Etymology: in honour of José Bonifácio de Andrada e Silva, Brazilian mineralogist.

Distribution: Mexico: Chihuahua; Norway: Arendal; Russia: from the Bobrovka River, Nizhny Tagil, Urals; South Africa: Cape Province, near Kuruman; Switzerland: Valais, Zermatt; USA: Alaska, on Prince of Wales Island; Arkansas, Hot Spring Co., Magnet Cove; California, San Benito Co. (Anthony et al., 2001—2005). Good facetable material from Namibia.

Chemistry

Ca3Fe2(SiO4)3

Essential elements: oxygen (O), silicon (Si), calcium (Ca), iron (Fe).

Crystal data

Crystallography: isometric — hexoctahedral. Crystal habit: commonly well-crystallised dodecahedra, trapezohedra, or combinations, to 5 cm; also granular to massive (Anthony et al., 2001—2005).

Physical properties

Cleavage: none. Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6.5—7 (Lazzarelli, 2012). Density: 3.7—4.1 g/cm3 (Arem, 1987: 104). Luminescence: none (Arem, 1987: 104).

Optical properties

Colour: yellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; greyish black, black; may be sectored (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: adamantine to resinous, dull (Anthony et al., 2001—2005). Refractive index: 1.88—1.94 — isotropic (Lazzarelli, 2012). Dispersion: 0.057 (Arem, 1987: 104). Pleochroism: none.

Material from ‘Repository’

8 specimens: 0143 — 0.13 ct, Namibia; 0240 — 0.15 ct, Namibia; 0067 — 1.79 ct, South Africa; 0515 — 0.68 ct, South Africa; 0517 — 0.52 ct, South Africa; 0634 — 0.54 ct, no precise locality [Africa]; 0641 — 0.80 ct, South Africa; 0645 — 0.96 ct, South Africa.

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