Class: Oxides & hydroxides
Group: Hematite
Characteristic of Al-rich, Si-poor geological environments; in
syenite and
monzonite, and some quartz-free
pegmatites; primary or a reaction product in
eclogitic xenoliths in
kimberlites. In regional or contact
metamorphic aluminous rocks, and some advanced
argillic and potassic hydrothermal alteration assemblages (
Anthony et al., 2001—2005). Famous specimens:
Black Star of Queensland, 733 carats from Australia, now owned by an unknown private party;
Logan sapphire, 422.99 carats from Sri Lanka, in National Museum of Natural History, Smithsonian Institution, Washington.
Original description: unknown [prehistoric].
Type locality: unknown [prehistoric].
Type material: unknown.
Etymology: derived from the Sanskrit, kuruvinda (ruby).
Distribution: many localities. Afghanistan: Jegdalek marble, near Sorobi, Laghman Province; Australia: Queensland, Anakie; Cambodia: around Bottambang and Pailin; Canada: Ontario, Bancroft and Haliburton; Greece: Naxos and Samos Islands; India: Kashmir, Zanskar district; around Mysore; Madagascar: Ampanihy, around Andranondambo and Antsiermene; Myanmar: Mogok district; South Africa: Transvaal, around the Soutpansberg; Sri Lanka: Ratnapura district; Tanzania: near Longido, and the Morogoro district; Umba Valley; Thailand: Chanthaburi and Trat; USA: Georgia, Rabun Co., Laurel Creek mine; Massachusetts, Hampden Co., Chester; Montana, Fergus Co., 25 km southwest of Utica, Yogo Gulch; New Jersey, Sussex Co., Franklin; New York, Westchester Co., Cortland district; North Carolina, Clay Co., Buck Creek, Jackson Co., Hogback Mountain (
Anthony et al., 2001—2005).
Chemistry
Al2O3
Essential elements: oxygen (O), aluminium (Al).
Crystal data
Crystallography: trigonal — hexagonal scalenohedral.
Crystal habit: crystals hexagonal, prismatic or steeply dipyramidal, tabular, rhombohedral, rarely acicular, typically rough, to 1 m; sectorially striated on {0001} || {1011}. Also granular, massive.
Twinning: common lamellar || {1011}, may be an exsolution phenomenon. Contact or penetration twins on {0001} or {1011}, rare (
Anthony et al., 2001—2005).
Optical properties
Colour: colourless, grey, brown; pink to pigeon-blood-red, orange, yellow, green, blue to cornflower blue, violet; may be colour zoned, asteriated; colourless, pale bluish or reddish in transmitted light (
Anthony et al., 2001—2005).
Diaphaneity: transparent to translucent (
Anthony et al., 2001—2005).
Lustre: adamantine to vitreous (
Anthony et al., 2001—2005).
Refractive index: 1.758—1.78 — anisotropic [uniaxial (-)] (
Lazzarelli, 2012).
Birefringence: 0.008—0.009.
Dispersion: 0.018 (low) (
Arem, 1987: 71).
Pleochroism: weak to none.
Material from ‘Repository’
14 specimens: 0147 — 0.02 ct, Mozambique;
0318 — 0.11 ct, Myanmar;
0276 — 0.61 ct, Tanzania, Ruvuma, Songea;
0629 — 0.12 ct, Tanzania, Ruvuma, Songea;
0654 — 0.13 ct, no precise locality [East Africa];
0263 — 0.18 ct, Sri Lanka;
0326 — 0.20 ct, Sri Lanka;
0640 — 0.20 ct, Madagascar;
0668 — 0.31 ct, Thailand;
0646 — 0.27 ct, Madagascar;
0505 — 0.11 ct, Sri Lanka;
0223 — 0.36 ct, Madagascar, Ihorombe, Ilakaka;
0364 — 0.12 ct, no locality data;
0148 — 0.18 ct, no locality data.
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