Topaz

Class: Silicates

In veins and cavities in granite, granite pegmatite, rhyolite, and in greisen, formed from high-temperature, volatile-rich pneumatolytic hydrothermal fluids. From metamorphism of aluminous, quartz-rich, and fluorine-bearing sediments (Anthony et al., 2001—2005).

Topaz — specimen 0549, photo © NMNHS

Original description: unknown.

Type locality: unknown.

Type material: unknown.

Etymology: named after Island of Zabargad [Topasos] in the Red Sea. In antique times, the name was probably used for the gemstone that is now known as peridot.

Distribution: widespread; only a few localities for the finest specimens: Brazil: from Ouro Preto, Minas Gerais; Mexico; Mozambique: Alto Ligonha; Myanmar: Mogok district; Namibia; Nigeria: Jos district; Pakistan: near Katlang and Skardu; Russia: near Mursinka, Urals, and Nerchinsk district, Siberia; USA: California and Colorado; Zimbabwe (Anthony et al., 2001—2005).

Chemistry

Al2(SiO4)(F,OH)2

Essential elements: hydrogen (H), oxygen (O), fluorine (F), aluminium (Al), silicon (Si).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: well-formed crystals common, typically morphologically complex, long to short prismatic, to 1.2 m; commonly vertically striated; columnar, compact, massive (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {001} (Anthony et al., 2001—2005). Fracture: uneven to subconchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Arem, 1987: 188). Hardness: 8 (Arem, 1987: 188). Density: 3.49—3.57 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: colourless, yellow, pink, red, orange, brown, green, blue, violet; in transmitted light, colourless; thick sections yellow, red, or blue (Anthony et al., 2001—2005). Diaphaneity: transparent to opaque (Anthony et al., 2001—2005). Lustre: vitreous (Arem, 1987: 188). Refractive index: 1.606—1.644 — anisotropic [biaxial (+)] (Lazzarelli, 2012). Birefringence: 0.008—0.011. Dispersion: 0.014 (Arem, 1987: 188). Pleochroism: in thick sections, X = yellow; Y = yellow, violet, reddish; Z = violet, bluish, yellow, pink (Anthony et al., 2001—2005).

Material from ‘Repository’

27 specimens: 0573 — 2.59 ct, Brazil; 0055 — 6.62 ct, Brazil; 0556 — 2.51 ct, no precise locality [Africa]; 0500 — 1.93 ct, Brazil; 0503 — 0.99 ct, Brazil; 0550 — 0.72 ct, Brazil; 0567 — 0.62 ct, Brazil; 0007 — 3.64 ct, Brazil; 0560 — 1.10 ct, Brazil; 0286 — 2.90 ct, Brazil; 0562 — 0.50 ct, Brazil; 0583 — 0.37 ct, Brazil; 0579 — 0.52 ct, Brazil; 0585 — 0.53 ct, Brazil; 0593 — 0.41 ct, Brazil; 0574 — 0.63 ct, Brazil; 0058 — 15.34 ct, Brazil; 0551 — 0.83 ct, no locality data; 0293 — 1.61 ct, Brazil; 0570 — 2.16 ct, Brazil; 0549 — 2.61 ct, Brazil; 0070 — 1.35 ct, Brazil; 0253 — 1.35 ct, Brazil; 0466 — 0.68 ct, no precise locality [Africa]; 0449 — 0.78 ct, no precise locality [Africa]; 0561 — 0.53 ct, Brazil; 0037 — 3.10 ct, Brazil.

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