Sinhalite

Class: Borates

A rare accessory mineral formed in boron-rich skarns at the contact of limestones with granite or gneiss (Anthony et al., 2001—2005). Faceted specimens are relatively rare. The largest faceted stone is a 158 carat from Sri Lanka (Arem, 1987: 173).

Sinhalite — specimen 0150, photo © NMNHS

Original description: Claringbull, G. F., Hey, M. H., 1952. Sinhalite (MgAlBO4), a new mineral — Mineralogical Magazine 29: 841—849 [view in ‘Library’].

Type locality: Sri Lanka.

Type material: Natural History Museum, London, England, 1952,36—1952,37 and other earlier museum specimens (Anthony et al., 2001—2005).

Etymology: from Sinhala, the Sanskrit name for Sri Lanka.

Distribution: Canada: Ontario, near Bancroft; China: Liáoníng; Myanmar: Mogok; Sri Lanka: in gem gravels at the Ratnapura district and elsewhere (Anthony et al., 2001—2005).

Chemistry

MgAl(BO4)

Essential elements: boron (B), oxygen (O), magnesium (Mg), aluminium (Al).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: rare euhedral crystals, equant to prismatic [001], to 2 cm; anhedral grains (Anthony et al., 2001—2005).

Physical properties

Cleavage: no data. Fracture: conchoidal (Anthony et al., 2001—2005). Tenacity: no data. Hardness: 6.5—7 (Arem, 1987: 173). Density: 3.46—3.5 g/cm3 (Lazzarelli, 2012). Luminescence: none (Arem, 1987: 173).

Optical properties

Colour: colourless, pale yellow, yellow, greenish brown; dark brown, brownish pink, pale pink if chromian; colourless in transmitted light (Anthony et al., 2001—2005). Diaphaneity: transparent (Anthony et al., 2001—2005). Lustre: vitreous (Arem, 1987: 173). Refractive index: 1.665—1.712 — anisotropic [biaxial (-)] (Arem, 1987: 173). Birefringence: 0.036—0.042. Dispersion: 0.018 (low). Pleochroism: distinct; pale brown, greenish brown, dark brown (Arem, 1987: 173).

Material from ‘Repository’

1 specimen: 0150 — 1.43 ct, Sri Lanka.

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