Andalusite

Class: Silicates

A result of contact metamorphism of argillaceous sediments, also in regionally metamorphosed schists. Rare in granites and pegmatites (Anthony et al., 2001—2005).

Andalusite — specimen 0034, photo © NMNHS

Original description: ‘andalousite’ Delamétherie, J. C., 1798. Sur une pierre de l’Andalousie — Journal de Physique, de Chimie et d’Histoire Naturelle et des Arts 46: 386—387 [view in ‘Library’].

Type locality: El Cardoso de la Sierra, Guadalajara, Castile-La Mancha, Spain.

Type material: unknown.

Etymology: named for the supposed type locality, ‘Andalusia region’, Spain, although the type locality is not in Andalusia.

Distribution: widespread; some localities for good crystals: Austria: Tirol, Sellraintal; Brazil: Espírito Santo, Santa Teresa district; Germany: Bavaria, Bodenmais; Spain; Sri Lanka: Nawalapitiya; USA: California, from Fresno, Fresno Co. (Anthony et al., 2001—2005).

Chemistry

Al2SiO5

Essential elements: oxygen (O), aluminium (Al), silicon (Si).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: as euhedral crystals or columnar aggregates having nearly square cross sections, commonly elongated, to 20 cm; also fibrous, compact, massive. Twinning: on {101}, rare (Anthony et al., 2001—2005).

Physical properties

Cleavage: good on {110}, poor on {100} (Anthony et al., 2001—2005). Fracture: uneven to subconchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Arem, 1987: 42). Hardness: 7.5 (Lazzarelli, 2012). Density: 3.05—3.21 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: pink, violet, yellow, green, white, grey; in thin section, colourless to pink or green (Anthony et al., 2001—2005). Diaphaneity: transparent to nearly opaque (Anthony et al., 2001—2005). Lustre: vitreous to subvitreous (Anthony et al., 2001—2005). Refractive index: 1.627—1.65 — anisotropic [biaxial (-)] (Lazzarelli, 2012). Birefringence: 0.007—0.013. Dispersion: 0.016 (Arem, 1987: 42). Pleochroism: strong (Arem, 1987: 42).

Material from ‘Repository’

2 specimens: 0033 — 0.55 ct, Brazil; 0034 — 0.55 ct, Brazil.

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