Grossular

Class: Silicates
Group: Garnet

Varieties | Names:

In contact and regionally metamorphosed calcareous rocks, or rocks which have undergone calcium metasomatism; in some schists and serpentinites (Anthony et al., 2001—2005).

Grossular — specimen 0118, photo © NMNHS

Original description: Hoffmann, C. A. S., 1811. Grossular — Handbuch der Mineralogie, Craz und Gerlach, Freiberg: 479—481 [view in ‘Library’].

Type locality: Akhtaragda River mouth, Vilyuy River Basin, Sakha Republic, Far Eastern District, Russia.

Type material: unknown.

Etymology: originally named ‘kanalstein’, cinnamon stone and renamed grossularite for the green colour of gooseberries, Ribes grossularium, because the original specimens were this colour.

Distribution: many localities: Austria: Tirol, Zillertal; Canada: Quebec; Italy: Piedmont; Kenya; Mexico: Chihuahua; Russia: at the confluence of the Vilyuy and Akhtaragda Rivers, Sakha Republic; Tanzania: Merelani (Anthony et al., 2001—2005).

Chemistry

Ca3Al2(SiO4)3

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

Crystal data

Crystallography: isometric — hexoctahedral. Crystal habit: commonly in dodecahedra or trapezohedra, up to 15 cm, with striated faces; also granular, compact, and massive (Anthony et al., 2001—2005).

Physical properties

Cleavage: parting on {110} (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 7—7.5 (Lazzarelli, 2012). Density: 3.55—3.73 g/cm3 (Lazzarelli, 2012). Luminescence: may fluoresce weak golden yellow under UV (Anthony et al., 2001—2005).

Optical properties

Colour: yellow-green, pale to dark green; golden yellow, pink, red, orange, brownish red, yellowish brown; colourless, white, grey, black; may be sectored: colourless in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to opaque (Anthony et al., 2001—2005). Lustre: vitreous to resinous (Anthony et al., 2001—2005). Refractive index: 1.73—1.76 — isotropic (Lazzarelli, 2012). Dispersion: 0.02. Pleochroism: none.

Material from ‘Repository’

17 specimens: 0118 — 0.62 ct, Tanzania; 0459 — 0.35 ct, Nigeria; 0103 — 0.95 ct, Mali; 0504 — 0.38 ct, Mali; 0026 — 0.51 ct, Mali; 0185 — 0.78 ct, Mali; 0027 — 0.72 ct, Mali; 0089 — 6.99 ct, South Africa; 0635 — 2.14 ct, South Africa; 0659 — 1.33 ct, Sri Lanka; 0210 — 1.04 ct, Mexico, Baja California; 0639 — 1.44 ct, Tanzania; 0642 — 2.84 ct, Sri Lanka; 0142 — 0.73 ct, Tanzania; 0094 — 0.57 ct, Kenya; 0638 — 0.10 ct, Kenya, Coast Province, Taita-Taveta, Tsavo; 0178 — 0.22 ct, Kenya, Coast Province, Taita-Taveta, Tsavo.

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