Afghanite

Class: Silicates
Group: Cancrinite

In thin veinlets cutting lazurite crystals (Anthony et al., 2001—2005). One of the rarest gemstones.

Afghanite — specimen 0283, photo © NMNHS

Original description: Bariand, P., Cesbron, F., Giraud, R., 1968. Une nouvelle espèce minérale : l’afghanite de Sar-e-Sang, Badakhshan, Afghanistan. Comparaison avec les minéraux du groupe de la cancrinite — Bulletin de la Société Française de Minéralogie et de Crystallographie 91: 34—42 [view in ‘Library’].

Type locality: ‘Sar-e-Sang, Hindou-Kouch oriental, province du Badakhshan, Afghanistan’ (Bariand, Cesbron, Giraud, 1968: 35).

Type material: Laboratoire de Minéralogie-Cristallographie, Université Pierre-et-Marie-Curie, Paris, France; École Nationale Supérieure des Mines de Paris, Paris, France; Natural History Museum, London, England, 1969, 72 (Anthony et al., 2001—2005).

Etymology: after the country of its discovery, Afghanistan.

Distribution: Afghanistan: Sar-e Sang, Badakhshan; Canada: Newfoundland; Italy; Russia: near Lake Baikal; Tajikistan: Pamir (Anthony et al., 2001—2005).

Chemistry

(Na,Ca,K)8(Si,Al)12O24(SO4,Cl,CO3)3(H2O)

Essential elements: hydrogen (H), carbon (C), oxygen (O), sodium (Na), aluminium (Al), silicon (Si), sulphur (S), chlorine (Cl), potassium (K), calcium (Ca).

Crystal data

Crystallography: hexagonal. Crystal habit: as stout to slender, thin tabular laths and as rounded grains (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {1010} (Anthony et al., 2001—2005). Fracture: conchoidal (Anthony et al., 2001—2005). Tenacity: brittle. Hardness: 5.5—6 (Lazzarelli, 2012). Density: 2.55—2.65 g/cm3 (Lazzarelli, 2012). Luminescence: none. Radioactivity: GRapi 32.54.

Optical properties

Colour: blue; colourless in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.522—1.533 — anisotropic [uniaxial (+)] (Lazzarelli, 2012). Birefringence: 0.006. Dispersion: none. Pleochroism: none.

Material from ‘Repository’

1 specimen: 0283 — 0.05 ct, Afghanistan.

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