Phenakite

Class: Silicates
Group: Phenakite

In pegmatites, greisens, and alpine and hydrothermal veins (Anthony et al., 2001—2005).

Phenakite — specimen 0492, photo © NMNHS

Original description: ‘Phenakit’ Nordenskjöld, N., 1834. Beskrifning på Phenakit, ett nytt Mineral från Ural — Kongliga Svenska Vetenskaps-Akademiens Handlingar 1833: 160—165 [view in ‘Library’].

Type locality: Izumrudnye Kopi area, Malysheva, Yekaterinburg, Sverdlovsk District, Urals, Russia.

Type material: Mining Institute, Saint Petersburg, Russia, 617/22 (Anthony et al., 2001—2005).

Etymology: from the Greek for ‘deceiver’, in allusion to its being mistaken for quartz.

Distribution: Brazil: Bahia, northwest of Salvador; Minas Gerais, east of Belo Horizonte; Madagascar: Anjanabonoina; Namibia; Russia: Ural Mountains, Tokovaya River, near Yekaterinburg; Switzerland: Valais, near Reckingen; USA: Colorado, Mt Antero (Anthony et al., 2001—2005).

Chemistry

Be2SiO4

Essential elements: beryllium (Be), oxygen (O), silicon (Si).

Crystal data

Crystallography: trigonal — rhombohedral. Crystal habit: crystals rhombohedral, tabular to prismatic, less commonly long prismatic to acicular, to 20 cm; in columnar aggregates, as spherulites, and granular. Twinning: penetration twins, by rotation about [0001] (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {1120}, distinct; on {1011}, imperfect (Anthony et al., 2001—2005). Fracture: conchoidal (Arem, 1987: 149). Tenacity: brittle (Arem, 1987: 149). Hardness: 7.5—8 (Arem, 1987: 149). Density: 2.9—2.98 g/cm3 (Lazzarelli, 2012). Luminescence: cathodoluminescent (Anthony et al., 2001—2005).

Optical properties

Colour: colourless, yellow, pink, brown; may be mottled (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Arem, 1987: 149). Refractive index: 1.65—1.695 — anisotropic [uniaxial (+)] (Lazzarelli, 2012). Birefringence: 0.016. Dispersion: 0.015 (low). Pleochroism: none.

Material from ‘Repository’

4 specimens: 0490 — 0.41 ct, Madagascar; 0491 — 0.41 ct, Madagascar; 0492 — 0.64 ct, Madagascar; 0493 — 0.78 ct, Madagascar.

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