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Class: Silicates
Group: Epidote

Characteristic of several facies of regionally metamorphosed rocks and some contaminated felsic igneous rocks; in contact zones between igneous and calcareous sedimentary rocks; from alteration of plagioclase (Anthony et al., 2001—2005). Faceted specimens are rare.
Epidote — specimen 0054
Epidote — specimen 0054, photo © NMNHS

Original description: Haüy, R. J., 1801. Epidote, (m.) c’est-à-dire, qui a reçu un accroissement — Traité de Minéralogie 3: 102—113 [view in ‘Library’].

Type locality: Bourg d’Oisans, Isère, Rhône-Alpes, France.

Type material: Muséum national d’Histoire naturelle, Paris, France (Anthony et al., 2001—2005).

Etymology: from the Greek επιδοσιζ (еpidosis), ‘increase’, in allusion to the crystal characteristic of one longer side at the base of the prism.

Distribution: Austria: Knappenwand, Salzburg; France: Bourg d’Oisans, Isère; Italy: Piedmont; Namibia; Norway: Arendal; Pakistan: Tormiq (Anthony et al., 2001—2005).



Essential elements: hydrogen (H), oxygen (O), aluminium (Al), silicon (Si), calcium (Ca), iron (Fe).

Crystal data

Crystallography: monoclinic — prismatic. Crystal habit: crystals prismatic, to 35 cm, commonly elongated and striated; fibrous, coarse to fine granular, massive. Twinning: on {100}, contact, lamellar, common (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {001}, imperfect on {100} (Anthony et al., 2001—2005). Fracture: uneven (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6—7 (Lazzarelli, 2012). Density: 3.25—3.5 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: pistachio-green to pale green, yellow, yellowish green, greenish yellow, greenish black; yellow-green in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to nearly opaque (Anthony et al., 2001—2005). Lustre: vitreous, pearly, somewhat resinous (Anthony et al., 2001—2005). Refractive index: 1.729—1.768 — anisotropic [biaxial (-)] (Lazzarelli, 2012). Birefringence: 0.014—0.045. Dispersion: 0.03. Pleochroism: strong; X = colourless, pale yellow, pale green; Y = greenish yellow; Z = yellowish green (Anthony et al., 2001—2005).

Material from ‘Repository’

1 specimen: 0054 — 0.52 ct, Sri Lanka.

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