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Corundum

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Kathy Feick

Corundum Al2O3

Corundum is a very hard, tough and stable mineral. For all practical purposes, it is the hardest mineral, second only to diamond. It is unaffected by acids and most environments. The name corundum was first derived either from the Sanskrit word Kuirvinda or from the Indian name for corundum, Kauruntoka.

Formation:

Corundum forms in a variety of geological settings. Generally, these settings are aluminum rich and silica poor. It often appears in light-coloured igneous rocks such as desilicated pegmatites, syenites and Nepheline syenites. Corundum can occur as an accessory mineral in metamorphic rocks, derived from aluminous or carbonate sediments such as crystalline limestone and marbles, mica schist’s and gneisses. Furthermore, it can be found in the contact zone between igneous rocks and limestone’s.

many pink corundum pieces

Corundum image source

Varieties:

 Corundum has the chemical formula Al2O3. Pure corundum is rare in nature, and is completely colourless. Small amounts of metallic elements such as Cr, Fe and Ti can substitute for aluminum in the structure, which gives rise to many colour variations. Some corundum even owes its colour to irradiation, or the elements vanadium, cobalt or nickel. Ruby and sapphire, two varieties of corundum are among the oldest known gems to man, dating back many thousands of years. They have always been held in very tight regard.

  • Ruby: Ruby gets its name from the Latin word ”Ruber,” meaning red. It is pinkish red to red in colour. Ruby owes its colour to corundum impurities.

raindrop shaped red gemstone; Ruby

Ruby image source

  • Padparadscha Sapphire: A Rare orange-pink variety of corundum. It owes its colour to both iron and chromium inclusions.

orange-red round gemstone; Padparadscha Sapphire

Padparadscha Sapphire image source

  • Sapphire: The name sapphire comes from the Latin word for blue, “Sapphires,” which is also thought to have been used in ancient times to refer to lapis lazuli. The name sapphire is the general tem for all other colours of corundum (blue, pink, green, yellow, violet, purple, orange, brown, white, gray, black and colourless). Sapphires owe their colour to Fe2+, fe3+ and Ti4+ inclusions.

round blue gemstone; sapphire

  • Emery: A black massive variety of corundum. Colour is caused by a mixture of magnetite, hematite and spinel.

large grey specimen of emery corundum that ressembles a rock

Emery image source

Uses:

  • Gemstone (occurs in all colours)
  • As an abrasive (generally synthetically manufactured from bauxite)
  • There are some electrical uses for non gem quality material

Notable occurrences:

  • Burma
  • Sri Lanka
  • North Carolina and Montana, USA
  • Canada
  • Many African localities
  • Several localities in India
  • Middle Eastern and Southeast Asian countries

Craigmont:

In 1876, a girl named Annie and her father Henry Robillard discovered a mass deposit of corundum in a mountainous area. At the time, corundum was used as an abrasive, to cut and polish steel, as a hardener in the making of steel wheels for locomotives and train cars, and finally for grinding optical lenses. This discovery attracted attention and ultimately led to the settlement Craigmont.

In 1900, mining operations began by the Canadian Corundum Company. The first mill was a pilot and produced approximately 20 tonnes of ore per day. In 1904, a larger mill was opened which produced the outstanding amount of 300 tonnes of ore per day. This crushing mill was the heaviest and most powerful ever built in North America.

The town of Craigmount grew to a peak of about 600 persons. It was the world’s largest Corundum producing town at the time. Unfortunately, in 1913, a fire destroyed the mill. This resulted in job loss and gradual decline of residents. Ultimately, by 1921, Craigmont was a ghost town.

References:

Corundum

Corundum: mineral information page

The mineral Corundum

Minerals Zone

Corundum and mining