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Spinel

Class: Oxides & hydroxides
Group: Spinel

Varieties | Names:

A common mineral, formed at high-temperatures as an accessory in igneous rocks, principally basalts, kimberlites, peridotites, and in xenoliths; in regionally metamorphosed aluminum-rich schists; in regionally and contact metamorphosed limestones (Anthony et al., 2001—2005). The largest specimens include: Samarian spinel, over 500 carats, stored in the Central Bank of Iran, Tehran; 398.72 carat red spinel purchased in China in 1676 and stored in the Diamond Fund, Moscow; Timur ruby, 361 carats, part of the Crown Jewels of the United Kingdom.
Spinel — specimen 0014
Spinel — specimen 0014, photo © NMNHS

Original description: unknown.

Type locality: unknown.

Type material: unknown.

Etymology: from the Latin spinella, little thorn, in allusion to the spine-shaped octahedral crystals.

Distribution: many localities. Canada: Ontario, Burgess; Quebec, Wakefield; Germany: Eifel district, and at Bodenmais, Bavaria; Italy: Campania, Monte Somma and Vesuvius; Madagascar: Andrahomana, Ambinda, Besakoa, and many other places; Myanmar: around Mogok; Russia: Sakha, Aldan Shield, Emeldjak and Katalakhu deposits; Sri Lanka: Ratnapura and Elahera districts, and elsewhere; Tajikistan: Pamir Mts, Gorondarinskoye; USA: California, Riverside Co., near Midland, and at the Crestmore quarry; New Jersey, Sussex Co., Ogdensburg, Franklin and Sterling Hill, Orange Co., at many places from Amity and Edenville, Allegany Co., New York to Andover (Anthony et al., 2001—2005).

Chemistry

MgAl2O4

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

Crystal data

Crystallography: isometric — hexoctahedral. Crystal habit: commonly as octahedra, to 30 cm, may show dodecahedron or cube faces, rarely as dodecahedra; coarse granular, rounded, massive. Twinning: on {111} as both twin and composition plane, the spinel law (Anthony et al., 2001—2005).

Physical properties

Cleavage: parting on {111} (Anthony et al., 2001—2005). Fracture: conchoidal, uneven to splintery (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 8 (Lazzarelli, 2012). Density: 3.54—3.7 g/cm3 (Lazzarelli, 2012). Luminescence: reds and pinks: crimson in LW UV and SW UV, red in X-rays, no phosphorescence; blue: inert in UV; deep purple: red in LW UV, essentially inert in SW UV, lilac in X-rays; pale blue and violet: green in LW UV and X-rays, essentially inert in SW UV.

Optical properties

Colour: colourless, brown, black; red, orange, yellow, green, blue, indigo, violet (Anthony et al., 2001—2005). Diaphaneity: transparent to nearly opaque (Anthony et al., 2001—2005). Lustre: vitreous, splendent to dull (Anthony et al., 2001—2005). Refractive index: 1.71—1.735 — isotropic (Lazzarelli, 2012). Dispersion: 0.02 (Arem, 1987: 177). Pleochroism: usually none; anomalous in some blue zincian varieties.

Material from ‘Repository’

22 specimens: 0477 — 0.56 ct, Myanmar, Mandalay, Mogok; 0496 — 0.44 ct, Myanmar, Mandalay, Mogok; 0608 — 0.49 ct, Myanmar; 0014 — 1.00 ct, Sri Lanka; 0456 — 0.37 ct, Sri Lanka; 0664 — 0.30 ct, Sri Lanka; 0041 — 0.10 ct, Tanzania; 0597 — 0.06 ct, Tanzania; 0462 — 0.32 ct, Myanmar; 0658 — 0.44 ct, Sri Lanka; 0384 — 0.13 ct, China; 0385 — 0.14 ct, China; 0463 — 1.41 ct, Thailand, Kanchanaburi, Bo Phloi; 0442 — 1.29 ct, Thailand, Kanchanaburi, Bo Phloi; 0648 — 1.08 ct, Thailand, Kanchanaburi, Bo Phloi; 0650 — 0.93 ct, Thailand, Kanchanaburi, Bo Phloi; 0666 — 1.42 ct, Thailand, Kanchanaburi, Bo Phloi; 0670 — 1.58 ct, Thailand, Kanchanaburi, Bo Phloi; 0665 — 1.88 ct, Thailand, Kanchanaburi, Bo Phloi; 0671 — 1.38 ct, Thailand, Kanchanaburi, Bo Phloi; 0672 — 1.36 ct, Thailand, Kanchanaburi, Bo Phloi; 0673 — 0.73 ct, Thailand, Kanchanaburi, Bo Phloi.


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