Grandidierite

Class: Silicates

A rare accessory mineral in aluminous boron-rich rocks, regionally metamorphosed under low pressure; in pegmatites, gneisses; in xenoliths (Anthony et al., 2001—2005).

Grandidierite — specimen 0744, photo © NMNHS

Original description: Lacroix, A., 1902. Note préliminaire sur une nouvelle espèce minérale — Bulletin de la Société Française de Minéralogie 25: 85—86 [view in ‘Library’].

Type locality: Cap Andrahomana, Tôlanaro, Anosy, Toliara, Madagascar.

Type material: Harvard University, Cambridge, Massachusetts, USA, 86382 (Anthony et al., 2001—2005).

Etymology: in honor of Alfred Grandidier, French explorer and naturalist.

Distribution: in large crystals from Toliara, Madagascar (Anthony et al., 2001—2005).

Chemistry

(Mg,Fe2+)(Al,Fe3+)3(SiO4)(BO3)O2

Essential elements: boron (B), oxygen (O), magnesium (Mg), aluminium (Al), silicon (Si), iron (Fe).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: anhedral, elongated, to 8 cm (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {100}, perfect; {010} (Anthony et al., 2001—2005). Fracture: no data. Tenacity: no data. Hardness: 7.5 (Anthony et al., 2001—2005). Density: 2.98—2.99 g/cm3 (Anthony et al., 2001—2005). Luminescence: none.

Optical properties

Colour: bluish green, greenish blue; increasingly greener with greater Fe content; colourless to blue or green in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent, translucent (Anthony et al., 2001—2005). Lustre: vitreous, pearly (Anthony et al., 2001—2005). Refractive index: 1.590—1.639 — anisotropic [biaxial (-)] (Anthony et al., 2001—2005). Birefringence: 0.033—0.037. Dispersion: r < v; very strong (Anthony et al., 2001—2005). Pleochroism: X = deep blue to blue-green; Y = colourless; Z = light greenish blue to deep green (Anthony et al., 2001—2005).

Material from ‘Repository’

1 specimen: 0744 — 0.20 ct, Madagascar, Toliara, Anosy, Tranomaro.

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