Magnesite

Class: Carbonates (Nitrates)
Group: Calcite

Varieties | Names:

A primary mineral in igneous or sedimentary rocks; formed by metamorphism or alteration of serpentine and peridotite; rare in carbonatites (Anthony et al., 2001—2005). Faceted specimens are very rare. The largest faceted stone is a 134.5 carat from Brazil (Arem, 1987: 127).

Magnesite — specimen 0164, photo © NMNHS

Original description: ‘Magnesit’ Karsten, D. L. G., 1808. Erd-und steinarten. Magnesit — Mineralogische Tabellen, Berlin: 48 [view in ‘Library’].

Type locality: Bettolino, Baldissero Canavese, Province of Turin, Piedmont Region, Italy.

Type material: unknown.

Etymology: for the magnesium in its composition.

Distribution: some well-studied localites include: Austria: Styria; Brazil: Bahia, Brumado; China: Liáoníng; Greece: Magnesia; Italy: Piedmont, Baldissero Canavese; USA: California (Anthony et al., 2001—2005).

Chemistry

MgCO3

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

Crystal data

Crystallography: trigonal — hexagonal scalenohedral. Crystal habit: uncommon as crystals, to 30 cm, with large {1011} or {0112}, modified by {1010}, {1120}, or tabular on {0001}; typically earthy, fibrous, granular, massive (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {1011}, perfect (Anthony et al., 2001—2005). Fracture: conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Arem, 1987: 127). Hardness: 3.5—4.5 (Arem, 1987: 127). Density: 2.92—3.12 g/cm3 (Lazzarelli, 2012). Luminescence: may exhibit pale green to pale blue fluorescence and phosphorescence under UV; triboluminescent (Anthony et al., 2001—2005). Other: slightly soluble in water with the solubility increasing with the presence of sodium chloride, sodium sulfate, or carbon dioxide.

Optical properties

Colour: colourless, white, pale yellow, pale brown, faintly pink, lilac-rose; colourless in transmitted light (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.509—1.72 — anisotropic [uniaxial (-)] (Lazzarelli, 2012). Birefringence: 0.19—0.202. Dispersion: very strong (Anthony et al., 2001—2005). Pleochroism: none.

Material from ‘Repository’

2 specimens: 0164 — 4.86 ct, Brazil; 0130 — 15.60 ct, no locality data.

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