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Class: Silicates
Group: Gadolinite-datolite

A secondary mineral in mafic igneous rocks; in geodes in tuffs; in skarns in limestones; in serpentinites and schists; from some ore veins (Anthony et al., 2001—2005). Faceted specimens are extremely rare.
Datolite — specimen 0168
Datolite — specimen 0168, photo © NMNHS

Original description: ‘Datolith’ Klaproth, M. H., 1806. Chemische Untersuchung des Datoliths — Neues allgemeines Journal der Chemie 6: 107—110 [view in ‘Library’].

Type locality: Nødebro mine, Arendal, Aust-Agder, Norway.

Type material: unknown.

Etymology: from the Greek δατεῖσθαι (dateisthai), to divide, in reference to the granular texture of massive varieties.

Distribution: Austria: Kratzenberg, Habachtal; Germany: at Sankt Andreasberg, Harz Mountains; Mexico: Charcas; Norway: near Arendal; Russia: Primorsky Krai, Dalnegorsk; USA: Lane quarry, Hampden Co., Massachusetts (Anthony et al., 2001—2005).



Essential elements: hydrogen (H), boron (B), oxygen (O), silicon (Si), calcium (Ca).

Crystal data

Crystallography: monoclinic — prismatic. Crystal habit: crystals typically short prismatic, with prominent {110}, {011}, or tabular on {010}, to 12 cm; botryoidal or globular, having columnar structure; granular to compact; cryptocrystalline (Anthony et al., 2001—2005).

Physical properties

Cleavage: none (Arem, 1987: 79). Fracture: uneven to conchoidal (Arem, 1987: 79). Tenacity: brittle (Arem, 1987: 79). Hardness: 5—5.5 (Arem, 1987: 78). Density: 2.9—3 g/cm3 (Lazzarelli, 2012). Luminescence: commonly fluoresces blue under SW UV (Anthony et al., 2001—2005).

Optical properties

Colour: colourless or white, commonly with a greenish tinge; may be greyish, yellow, green, red, pink; in thin section, colourless (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent, rarely opaque (Anthony et al., 2001—2005). Lustre: vitreous (Arem, 1987: 78). Refractive index: 1.621—1.675 — anisotropic [biaxial (-)] (Lazzarelli, 2012). Birefringence: 0.044—0.045. Dispersion: 0.016 (Arem, 1987: 79). Pleochroism: none (Arem, 1987: 79).

Material from ‘Repository’

1 specimen: 0168 — 1.12 ct, USA, Connecticut, Hartford County, Farmington.

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