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Class: Silicates

Produced by regional or contact metamorphism of magnesium carbonate, mafic, or ultramafic rocks (Anthony et al., 2001—2005). Faceted specimens are extremely rare.

Original description: ‘Actynolite’ Kirwan, R., 1794. 16th Species: Lamellar Actynolite — Elements of Mineralogy, Elmsly, London: 167 [view in ‘Library’].

Type locality: unknown.

Type material: unknown.

Etymology: from the Greek words ακτίνα (aktina) for ‘ray’ and λίθος (lithos) for ‘stone’, in allusion to the fibrous nature of the original specimens.

Distribution: Austria: Zillertal; China: Kunlun Mountains; Russia: Urals; Switzerland: from Zermatt, Valais (Anthony et al., 2001—2005).



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

Crystal data

Crystallography: monoclinic — prismatic. Crystal habit: as bladed crystals, to 15 cm; columnar, may be kinked; granular to massive. Twinning: simple or lamellar (Anthony et al., 2001—2005).

Physical properties

Cleavage: good on {110}; parting on {100} (Anthony et al., 2001—2005). Fracture: uneven. Tenacity: brittle (Arem, 1987: 37). Hardness: 5.5—6 (Lazzarelli, 2012). Density: 3.03—3.07 g/cm3 (Lazzarelli, 2012). Luminescence: none (Arem, 1987: 37).

Optical properties

Colour: bright green to greyish green; in thin section, colourless, pale green to deep green (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.6—1.63 — anisotropic [biaxial (-)] (Lazzarelli, 2012). Birefringence: 0.03. Dispersion: none. Pleochroism: weak; X = pale yellow, yellowish green; Y = pale yellow-green, green; Z = pale green, deep greenish blue (Anthony et al., 2001—2005).

Material from ‘Repository’

3 specimens: 0260 — 1.12 ct, Tanzania; 0261 — 0.96 ct, Tanzania; 0075 — 1.48 ct, Sri Lanka.

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