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Dolomite

Class: Carbonates (Nitrates)
Group: Dolomite

Varieties | Names:

Formed by diagenesis or hydrothermal metasomatism of limestone; a major component of some contact metamorphic rocks and marbles; a gangue in hydrothermal veins; in carbonatites and ultramafic rocks (Anthony et al., 2001—2005). Faceted specimens are very rare.
Dolomite — specimen 0165
Dolomite — specimen 0165, photo © NMNHS

Original description: ‘dolomie’ Saussure, H. B., 1792. Analyse de la dolomie — Observations sur la Physique, sur l’Histoire Naturelle et sur les Arts 40: 161—173 [view in ‘Library’].

Type locality: Italy.

Type material: unknown.

Etymology: named in honour of the French geologist and naturalist Déodat de Dolomieu.

Distribution: a major rock-forming mineral, abundant worldwide; some localities for fine examples include: Brazil: Bahia, Brumado; Germany: Saxony, Freiberg; Kosovo: Kosovska Mitrovica, Trepča; Mexico: Chihuahua, Naica; Switzerland: Binntal, Lengenbach (Anthony et al., 2001—2005).

Chemistry

CaMg(CO3)2

Essential elements: carbon (C), oxygen (O), magnesium (Mg), calcium (Ca).

Crystal data

Crystallography: trigonal — rhombohedral. Crystal habit: crystals typically combinations of {1011} and {1120}, tabular on {0001}, many minor forms, may exhibit curved faces, to 20 cm; saddle-shaped aggregates, columnar, granular, massive. Twinning: with {0001}, {1010}, or {1120} as twin plane, common as simple contact twins and in combinations; lamellar on {0221} (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {1011}, perfect (Anthony et al., 2001—2005). Fracture: subconchoidal (Arem, 1987: 84). Tenacity: brittle (Arem, 1987: 84). Hardness: 3.5—4 (Arem, 1987: 84). Density: 2.8—2.95 g/cm3 (Lazzarelli, 2012). Luminescence: may fluoresce white to pink under UV; triboluminescent (Anthony et al., 2001—2005).

Optical properties

Colour: colourless, white, yellow, brown, pale pink; colourless in transmitted light (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous to pearly (Anthony et al., 2001—2005). Refractive index: 1.498—1.681 — anisotropic [uniaxial (-)] (Lazzarelli, 2012). Birefringence: 0.179—0.181. Dispersion: very strong (Anthony et al., 2001—2005). Pleochroism: none.

Material from ‘Repository’

3 specimens: 0165 — 3.73 ct, Brazil, Bahia, Brumado; 0739 — 6.42 ct, no precise locality [Africa]; 0740 — 7.15 ct, no precise locality [Africa].


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