Euclase

Class: Silicates

A product of decomposition of beryl in pegmatites, and in low-temperature alpine veins (Anthony et al., 2001—2005). Extremely rare gemstone.

Euclase — specimen 0319, photo © NMNHS

Original description: Haüy, R. J., 1792. De l’Euclase — Observations sur la Physique, sur l’Histoire Naturelle et sur les Arts 41: 155—156 [view in ‘Library’].

Type locality: Ouro Preto, Minas Gerais, Brazil.

Type material: Muséum national d’Histoire naturelle, Paris, France, Haüy 3303 (Anthony et al., 2001—2005).

Etymology: from the Greek for easily and fracture in connection with its perfect cleavage.

Distribution: Brazil: Minas Gerais; Colombia: Chivor; Germany; Mozambique: Alto Ligonha district; Russia: Ural Mountains; Tanzania: Morogoro; Zimbabwe (Anthony et al., 2001—2005).

Chemistry

BeAl(SiO4)(OH)

Essential elements: hydrogen (H), beryllium (Be), oxygen (O), aluminium (Al), silicon (Si).

Crystal data

Crystallography: monoclinic — prismatic. Crystal habit: as prismatic crystals, flattened on {100}, commonly morphologically complex, to 12 cm (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {010}, imperfect on {110} and {001} (Anthony et al., 2001—2005). Fracture: conchoidal (Arem, 1987: 91). Tenacity: brittle (Arem, 1987: 91). Hardness: 7.5 (Anthony et al., 2001—2005). Density: 3.08—3.12 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: colourless, white, pale green to deep yellowish green, greenish blue, pale blue to deep blue, rarely mottled; colourless to pale blue or pale green in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous, pearly (Anthony et al., 2001—2005). Refractive index: 1.65—1.677 — anisotropic [biaxial (+)] (Lazzarelli, 2012). Birefringence: 0.019—0.025. Dispersion: 0.016. Pleochroism: weak.

Material from ‘Repository’

1 specimen: 0319 — 0.92 ct, Brazil.

→ Gallery view