Alumina

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Alumina is the chemical , or aluminium(III) oxide. It occurs naturally as hexagonal crystals called corundum, the second hardest mineral on Earth.[1] Used without qualification, aluminium oxide usually refers to alumina rather than the other oxides of aluminium.

Top: a heap of chemically pure activated alumina. Bottom left: a sapphire. Bottom right: a ruby.

Characteristics

The hardness of corundum by definition 9 on Mohs' scale and it has a white streak and vitreous lustre.[1]. Corundum has a specific gravity between 3.96[2] and 4.1[1]. The different varieties have refractive indices varying from between 1.759 and 1.767 to between 1.770 and 1.779.[2]
Some corundum, including all rubies, display asterism, which is to say that they reflect light in such a way as to form a six-sided star.[1][2]

Variants

The colour of a corundum crystal depends on impurities.

  • Red corundum is known as ruby.
The red color of Ruby comes from an impurity of considerabbly less than 1% Chromic oxide. The chromium is what permits the ruby to be used for a laser, producing red light. [3]
  • Blue corundum is known as sapphire. It gets its colour from iron and titanium impurities[1]
  • Colourless (pure) corundum is known as white sapphire.
  • Other varieties of corundum crystal are known by colour sapphire. These are pink, yellow, violet and green sapphire[1].

Occurence

On Earth, corundum mainly occurs in metamorphic rock, as well as in alluvial deposits.[1]

Use

Open issues

  • How common are alumina deposits on Mars?

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 B. Cairncross - Field guide to rocks and minerals of Southern Africa 2004. ISBN 978-1-86872-985-2 pp. 80-82
  2. 2.0 2.1 2.2 R. Webster & E.A. Jobbins - Gemmologists' compendium 7th ed. 2001. ISBN 978-0-7198-0291-1 p. 36.
  3. McGRAW-HILL ENCYCLOPEDIA OF Science & Technology 8th Edition (c) 1997, vol 15, page 688.