Prehnite

Class: Silicates

A secondary or hydrothermal mineral in veins and cavities in mafic volcanic rocks; less commonly in granite, gneiss or syenite (Anthony et al., 2001—2005).

Prehnite — specimen 0540, photo © NMNHS

Original description: ‘Prehnit’ Hoffmann, C. A. S., 1789. Mineralsystem des Herrn Inspektor Werners mit dessen Erlaubnis herausgegeben von C. A. S. Hoffmann — Bergmannisches Journal 1: 389 [view in ‘Library’].

Type locality: Karoo dolerites, Cradock, Eastern Cape Province, South Africa.

Type material: unknown.

Etymology: in honour of the Dutch Colonel Hendrik von Prehn, who discovered the mineral.

Distribution: Australia; Canada; Germany; India; Italy; Namibia; South Africa; USA (Anthony et al., 2001—2005).

Chemistry

Ca2Al2Si3O10(OH)2

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

Crystal data

Crystallography: orthorhombic — pyramidal. Crystal habit: uncommon as crystals, tabular or prismatic to steep pyramidal, to 4.5 cm; aggregates are fan-like, reniform, globular; granular, compact. Twinning: fine lamellar (Anthony et al., 2001—2005).

Physical properties

Cleavage: good on {001}, poor on {110} (Anthony et al., 2001—2005). Fracture: uneven (Arem, 1987: 152). Tenacity: brittle (Arem, 1987: 152). Hardness: 6—6.5 (Arem, 1987: 151). Density: 2.8—2.95 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: light to dark green, white, yellow, grey, pink; colourless in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.611—1.663 — anisotropic [biaxial (+)] (Lazzarelli, 2012). Birefringence: 0.021—0.039. Dispersion: none. Pleochroism: none (Arem, 1987: 152).

Material from ‘Repository’

6 specimens: 0108 — 2.58 ct, South Africa; 0109 — 2.55 ct, South Africa; 0309 — 1.88 ct, South Africa; 0090 — 1.92 ct, South Africa; 0160 — 1.78 ct, South Africa; 0540 — 4.59 ct, Mali.

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