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Anorthite

Class: Silicates
Group: Feldspar


A rare constituent of mafic plutonic and volcanic rocks; in some metamorphic rocks; in metamorphosed carbonate rocks; with corundum deposits (Anthony et al., 2001—2005).
Bytownite — specimen 0133
Bytownite — specimen 0133, photo © NMNHS

Original description: ‘Anorthit’ Rose, G., 1823. Ueber den Feldspath, Albit, Labrador und Anorthit — Annalen der Physik und der Physikalischen Chemie 73: 186—194 [view in ‘Library’].

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

Type material: unknown.

Etymology: from the Greek αν (an) and oρθός (orthos), ‘not right angle’ or ‘oblique’, in allusion to the oblique triclinic form of the crystals.

Distribution: Canada; Italy: Monte Somma and Vesuvius, Campania; Japan; Russia: Urals; Sweden; USA (Anthony et al., 2001—2005).

Chemistry

CaAl2Si2O8

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

Crystal data

Crystallography: triclinic — pinacoidal. Crystal habit: crystals commonly short, prismatic on [001], rarely on [010], to 2 cm; lamellar, coarse granular, massive. Twinning: commonly polysynthetic on the Albite law; also after the other laws (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {001}, less so on {010}, imperfect on {110} (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6—6.5 (Lazzarelli, 2012). Density: 2.77 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: white, greyish, reddish; 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: 1.575—1.588 — anisotropic [biaxial (-)] (Lazzarelli, 2012). Birefringence: 0.013. Dispersion: weak (Anthony et al., 2001—2005). Pleochroism: none.

Material from ‘Repository’

12 specimens: 0723 — 0.15 ct, USA, Oregon; 0447 — 0.53 ct, USA, Oregon; 0008 — 0.72 ct, USA, Oregon; 0038 — 0.52 ct, USA, Oregon; 0446 — 0.53 ct, USA, Oregon; 0133 — 2.00 ct, Mexico; 0284 — 2.28 ct, Mexico; 0484 — 0.62 ct, Mexico; 0677 — 3.31 ct, Madagascar; 0416 — 0.36 ct, Sri Lanka; 0571 — 0.65 ct, Sri Lanka; 0060 — 0.67 ct, India.


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