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Corundum

Corundum is the hardest mineral After diamond on Earth. The name corundum comes from the Sanskrit kuruvinda, meaning “ruby” the name given to red corundum. Ruby and sapphire are gem varieties of corundum. An aluminum oxide, corundum is commonly white, gray, or brown, but gem colors include red ruby and blue, green, yellow, orange, violet, and pink sapphire. Colorless forms also occur. Ruby forms a continuous color succession with pink sapphire; only stones of the darker hues are considered to be ruby. Corundum crystals are generally hexagonal, either tabular, tapering barrel-shaped, or dipyramidal. Corundum can also be massive or granular. It forms in syenites certain pegmatites and in high-grade metamorphic rocks. It is concentrated in placer deposits.

Name: Probably from the Sanskrit kurivinda, for ruby, through the Tamil kurundam

Mineral Group: Hematite group.

Chemical Properties of Corundum

Chemical Classification Oxide
Chemical Composition Al2O3

Physical Properties of Corundum

Color Typically gray to brown. Colorless when pure, but trace amounts of various metals produce almost any color. Chromium produces red (ruby) and combinations of iron and titanium produce blue (sapphire).
Streak Colorless (harder than the streak plate)
Luster Adamantine to vitreous
Diaphaneity Transparent, translucent to opaque
Cleavage None. Corundum does display parting perpendicular to the c-axis.
Mohs Hardness 9
Specific Gravity 3.9 to 4.1 (very high for a nonmetallic mineral)
Diagnostic Properties Hardness, high specific gravity, hexagonal crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture
Crystal System Hexagonal
Tenacity Brittle
Parting Rhombohedral and basal parting {0001}, sometimes perfect but interrupted; also on {1011} due to exsolution (Boehmite), observed on large blocks (Georgia, USA).
Fracture Irregular/Uneven, Conchoidal
Density 3.98 – 4.1 g/cm3 (Measured)    3.997 g/cm3 (Calculated)

Optical Properties of Corundum

Type Anisotropic
Color / Pleochroism Not Visible
Twinning Polysynthetic twinning common
Optic Sign Uniaxial (-)
Birefringence Low, first-order greys and whites.
Relief High

Occurrence

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, high-grade metamorphic aluminous rocks, and some advanced argillic and potassic hydrothermal alteration assemblages; detrital in placers.

Uses Area

  • Used as gemstone.
  • It is used as abrasive because of its hardness.
  • It is used for polishing and sanding of optical glasses.
  • It is also used in refractories due to its high melting point (2,040 ° C or 3,700 ° F).
  • Artificial corundum can be produced as a special product with slow deposition and controlled growth on a coil in an oxyhydrogen flame, as for jewelry use. This procedure is known as the Verneuil process

Association

Andesine, oligoclase, nepheline, scapolite (syenites); spinel, rutile, chondrodite, “hornblende”, phlogopite, calcite (metamorphosed limestones); kyanite, sillimanite, dumortierite, chlorite (schists); pyrope-rich garnet, spinel, phlogopite, omphacitic clinopyroxene, kyanite, rutile, graphite, diamond (eclogitic xenoliths).

Distribution

Numerous localities.

  • In the USA, from Chester, Hampden Co., Massachusetts; the Cortland district, Westchester Co., New York; at Franklin, Sussex Co., New Jersey; large crystals from Hogback Mountain, Jackson Co., and Buck Creek, Clay Co., North Carolina; and from the Laurel Creek mine, Rabun Co., Georgia.
  • At Bancroft and Haliburton, Ontario, Canada.
  • On Naxos and Samos Islands, Greece.
  • Large crystals from around the Soutpansberg, Transvaal, South Africa.
  • Red gems from: the Mogok district, Myanmar (Burma).
  • In the Ratnapura district, Sri Lanka.
  • Around Mysore, India.
  • In the Jegdalek marble, near Sorobi, Laghman Province, Afghanistan.
  • At Merkestein, near Longido, and the Morogoro district, Tanzania. From Ampanihy, Madagascar.
  • Blue, green, and yellow gems from: Chanthaburi and Trat, Thailand. Around Bottambang and Pailin, Cambodia.
  • From the Zanskar district, Kashmir, India.
  • In the Umba Valley, Tanzania.
  • From around Andranondambo and Antsiermene, Madagascar.
  • At Anakie, Queensland, Australia.
  • From Yogo Gulch, 25 km southwest of Utica, Fergus Co., Montana, USA.