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Benitoite

Class: Silicates
Group: Benitoite

In natrolite veins cutting glaucophane schist in a serpentine body (San Benito Co., California, USA); in a magnesio-riebeckite-quartz-phlogopite-albite dike cutting serpentinite (Japan) (Anthony et al., 2001—2005). Faceted specimens are very rare.
Benitoite — specimen 0248
Benitoite — specimen 0248, photo © NMNHS

Original description: Louderback, G. D., Blasdale, W. C., 1907. Benitoite, a new California gem mineral — University of California Publications. Bulletin of the Department of Geology 5: 149—153 [view in ‘Library’].

Type locality: Dallas gem mine, Diablo Range, San Benito Co., California, USA.

Type material: unknown.

Etymology: after its type locality in San Benito County, California, USA.

Distribution: USA: California, San Benito Co. (Anthony et al., 2001—2005).

Chemistry

BaTiSi3O9

Essential elements: oxygen (O), silicon (Si), titanium (Ti), barium (Ba).

Crystal data

Crystallography: hexagonal — ditrigonal dipyramidal. Crystal habit: flat pyramidal crystals, tabular on {0001}, triangular or hexagonal in outline, to 6 cm. Twinning: by rotation about [0001] (Anthony et al., 2001—2005).

Physical properties

Cleavage: poor on {1011} (Anthony et al., 2001—2005). Fracture: conchoidal to uneven (Arem, 1987: 50). Tenacity: brittle (Arem, 1987: 50). Hardness: 6—6.5 (Arem, 1987: 50). Density: 3.61—3.68 g/cm3 (Lazzarelli, 2012). Luminescence: fluoresces blue under SW UV; cathodoluminescent (Anthony et al., 2001—2005).

Optical properties

Colour: blue, white to colourless, pink; commonly bicoloured (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.757—1.804 — anisotropic [uniaxial (+)] (Lazzarelli, 2012). Birefringence: 0.047. Dispersion: 0.039—0.046. Pleochroism: O = colourless; E = purple, indigo, greenish blue (Anthony et al., 2001—2005).

Material from ‘Repository’

1 specimen: 0248 — 0.08 ct, USA, California, San Benito.


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