Difference between revisions of "Alumina"
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| − | + | '''Alumina''' is the chemical <math>Al_2O_3</math>, or aluminium(III) oxide. It occurs naturally as hexagonal crystals called '''corundum''', the second hardest [[mineral]] on Earth.<ref name=Cairncross>B. Cairncross - ''Field guide to rocks and minerals of Southern Africa'' 2004. ISBN 978-1-86872-985-2 pp. 80-82</ref> Used without qualification, '''aluminium oxide''' usually refers to alumina rather than the other oxides of aluminium. | |
| − | ''' | + | ==Characteristics== |
| + | The hardness of corundum by definition 9 on Mohs' scale and it has a white streak and vitreous lustre.<ref name=Cairncross />. Corundum has a specific gravity between 3.96<ref name=WebsterJobbins>R. Webster & E.A. Jobbins - ''Gemmologists' compendium'' 7th ed. 2001. ISBN 978-0-7198-0291-1 p. 36.</ref> and 4.1<ref name=Cairncross />. The different varieties have refractive indices varying from between 1.759 and 1.767 to between 1.770 and 1.779.<ref name=WebsterJobbins>R. Webster & E.A. Jobbins - ''Gemmologists' compendium'' 7th ed. 2001. ISBN 978-0-7198-0291-1 p. 36.</ref><br /> | ||
| + | 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.<ref name=Cairncross /><ref name=WebsterJobbins /> | ||
| − | The | + | ==Variants== |
| + | The colour of a corundum crystal depends on impurities. | ||
| + | * Red corundum is known as '''ruby'''. | ||
| + | * Blue corundum is known as '''sapphire'''. It gets its colour from iron and titanium impurities<ref name=Cairncross /> | ||
| + | * 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<ref name=Cairncross />. | ||
| + | |||
| + | ==Occurence== | ||
| + | On Earth, corundum mainly occurs in [[metamorphic rock]], as well as in [[alluvial deposits]].<ref name=Cairncross /> | ||
| + | |||
| + | ==Use== | ||
| + | * It is used as an [[abrasive]].<ref name=Cairncross /> | ||
| + | * Used in the production of [[refractory]].<ref name=Cairncross /> | ||
| + | * Ruby [[laser|lasers]] are based around synthetic ruby crystals, portions of which are doped with [[chromium]]. | ||
| + | * Alumina can be used as an [[aluminium ore]]. | ||
| + | * Both natural and synthetic corundum is used in jewelry. | ||
| + | * As a catalyst in the [[Sabatier process]]. | ||
| + | |||
| + | ==Open issues== | ||
| + | * How common are alumina deposits on Mars? | ||
==References== | ==References== | ||
Revision as of 11:03, 16 February 2013
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.
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.
- 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
- It is used as an abrasive.[1]
- Used in the production of refractory.[1]
- Ruby lasers are based around synthetic ruby crystals, portions of which are doped with chromium.
- Alumina can be used as an aluminium ore.
- Both natural and synthetic corundum is used in jewelry.
- As a catalyst in the Sabatier process.
Open issues
- How common are alumina deposits on Mars?
