Forsterite

Class: Silicates
Group: Olivine

Varieties | Names:

In mafic and ultramafic igneous rocks and thermally metamorphosed impure dolomitic limestones (Anthony et al., 2001—2005).

Peridot — specimen 0547, photo © NMNHS

Original description: Levy, M., 1824. Observations on the preceding Paper, with an Account of a new Mineral — The Annals of Philosophy 7: 59—62 [view in ‘Library’].

Type locality: Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy.

Type material: unknown.

Etymology: in honour of Adolarius Jacob Forster, English mineral collector.

Distribution: Egypt: Island of Zabargad in the Red Sea; Germany: Rhineland-Palatinate: Eifel district; Italy: Campania, at Monte Somma and Vesuvius; Myanmar: Mandalay: Mogok district; Pakistan: Khyber Pakhtunkhwa, Sapat Gali; Russia: Kola Peninsula, Kovdor massif; Ural Mountains, Zlatoust; Tanzania: Mpapwa, Kingiti; USA: Arizona, Gila Co., southwest of San Carlos (Anthony et al., 2001—2005).

Chemistry

Mg2SiO4

Essential elements: oxygen (O), magnesium (Mg), silicon (Si).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: crystals typically thick, with striations, with wedge-shaped terminations, to 17 cm; commonly granular or compact massive. Twinning: оn {100}, {011}, {012} (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {010}, {100}, imperfect (Anthony et al., 2001—2005). Fracture: conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6.5—7 (Lazzarelli, 2012). Density: 3.23—3.26 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: green, yellowish, lemon-yellow, white, greyish, blue-grey; 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: 1.635—1.671 — anisotropic [biaxial (+)] (Lazzarelli, 2012). Birefringence: 0.034—0.035. Dispersion: no data. Pleochroism: none.

Material from ‘Repository’

10 specimens: 0307 — 0.35 ct, no locality data; 0310 — 0.37 ct, no locality data; 0311 — 0.35 ct, no locality data; 0312 — 0.34 ct, no locality data; 0340 — 0.64 ct, no locality data; 0547 — 0.74 ct, Thailand; 0584 — 0.27 ct, USA; 0590 — 0.29 ct, Pakistan; 0592 — 0.34 ct, Pakistan; 0545 — 0.27 ct, no locality data.

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