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Dravite

Class: Silicates
Group: Tourmaline

Dravite — specimen 0432
Dravite — specimen 0432, photo © NMNHS

Original description: ‘Dravit’ Tschermak, G., 1884. Turmalin — Lehrbuch der Mineralogie, Alfred Hölder, Wien: 472 [view in ‘Library’].

Type locality: Dobrova pri Dravogradu, Dravograd, Slovenia.

Type material: National Museum of Natural History, Smithsonian Institution, Washington, DC, USA, R18133, R17274 (Anthony et al., 2001—2005).

Etymology: for the district along the Drava River in Austria and Slovenia.

Distribution: Brazil: Bahia, Brumado; Canada; Finland; Italy: Piedmont; Kenya; Norway: Arendal; Slovenia: around Dravograd; USA (Anthony et al., 2001—2005).

Chemistry

Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH)

Essential elements: hydrogen (H), boron (B), oxygen (O), sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si).

Crystal data

Crystallography: trigonal — ditrigonal pyramidal. Crystal habit: crystals equant or short to long prismatic, to 20 cm; striated; cross sections typically triangular with curved convex sides; commonly radiating, granular, and massive. Twinning: rare, on {1011} or {4041} (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {1120}, {1011}, very poor (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 7 (Anthony et al., 2001—2005). Density: 3.03—3.18 g/cm3 (Anthony et al., 2001—2005). Luminescence: may weakly fluoresce under SW UV (Anthony et al., 2001—2005). Other: pyroelectric and piezoelectric (Anthony et al., 2001—2005).

Optical properties

Colour: brown to black, red, yellow, blue, green, colourless, white; colourless to yellow in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous to resinous (Anthony et al., 2001—2005). Refractive index: 1.612—1.661 — anisotropic [uniaxial (-)] (Anthony et al., 2001—2005). Birefringence: 0.014—0.032. Dispersion: 0.017. Pleochroism: very strong; O = pale yellow; E = colourless, yellowish, greenish, brownish (Anthony et al., 2001—2005).

Material from ‘Repository’

11 specimens: 0432 — 0.53 ct, Mozambique; 0575 — 0.15 ct, Brazil; 0538 — 0.10 ct, Brazil; 0476 — 0.55 ct, Nigeria; 0576 — 0.19 ct, Mozambique; 0429 — 0.20 ct, Tanzania; 0572 — 0.33 ct, Democratic Republic of the Congo; 0495 — 0.23 ct, Tanzania; 0536 — 0.32 ct, Mozambique; 0563 — 0.20 ct, Democratic Republic of the Congo; 0117 — 1.31 ct, Namibia.


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