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Geohavens: World of Rocks, Minerals, Gems and Gemmology :: Gemstone Discussion :: Rubies and Sapphires
 Chemical Composition of Ruby
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| aloysiushan |
Posted on 06-03-2009 04:30
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Super Admin Posts: 41 Joined: 08.09.08 |
I recent question on chemical composition of Ruby: Ruby is corundum coloured by Chromium chromophore. Is chromium is a trace element, or is the chromium chemically bonded with the Al2O3? This question arose from wanting to know how to write the chemical formula for ruby in order to indicate the chromium impurity. The notation "Al203::Cr" listed on the web. The double-colon notation in the international handbook on the nomenclature of inorganic chemistry (the IUPAC 'Green Book'  , although the other chemical formula symbols arethere: the dot, raised dot, double brackets, etc. Googling suggests that the double-colon can be used to indicate 'Lewis dots' (covalent bonds) but that doesn't make sense to me in this case, because it's an inorganic, ionic compound. One thing I noticed is that, on the web, people seem to use the colon, the double-colon AND the hyphen interchangeably to indicate chromium as a non-stoichiometric impurity in corundum. Given that none of these are listed in the 'Green Book', I suspect that this is some kind of informal notation. __________________________________________________________ Ruby is a solid solution of chromium ions in the solid structure of the covalent Al2O3. Chromium in corundum-alumina, ie alpha-Al2O3, is a solid substitution of the trivalent chromic ion for the similar trivalent aluminium and the amount of substitution is variable, accounting for the range of chromic contents and consequently the variability of chromium colouration in corundum, from the palest of pink sapphires to the almost black red of the deepest Thai rubies. The material (ruby) is stoichiometric in that the ratio of total trivalent chromic plus aluminium to the (divalent) oxygen is fixed according to the charges on the respective ions, but the ratio of chromium to aluminium varies substantially depending probably upon local geology etc. The variable solid-solution replacement of aluminium by chromium (or ferric ion etc), or similarly of any other pair of elements such as in any isomorphous series (cf ferrous & magnesium ions in peridot for instance, of similar ionic diameter & charge & charge density & therefore both can occupy the same site in the lattice, within limits) are often expressed as in Fleischer's "Glossary Of Mineral Species" in the form (Al,Cr)2O3 where the exchangeable elements are included in brackets to indicate that the pair of them constitute effectively a single component in the stoichiometric formula. |
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| boblalux |
Posted on 23-04-2009 11:12
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Newbie Posts: 6 Joined: 21.04.09 |
I always learnt in mineralogy (also used in Dana), that Al3+ can easily substitute for Cr3+ and thus form solid solutions in corundum/ruby, thus suggesting a formula (Al,Cr)2O3 (also in Fleischer), as opposed to Al2O3 +- X whereby X is an element not in solid solution. I've never heard of Al2O3::Cr . Bob |
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| agianto |
Posted on 23-04-2009 17:39
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Junior Member Posts: 32 Joined: 10.02.09 |
With my bit of chemistry knowledge, to become a solution (whether it is a solid or liquid) the substances would have to have certain kind of bonding. Or else they would be immiscible, or dissolution would occur - for our concern - the rutile needles in a ruby, and a mixture is formed. Solid solution is a bit different from liquid solution because the former is limited by space - crystal lattice - fixed shape and volume and of course other chemistry factors such as valency.... Edited by agianto on 23-04-2009 20:18 |
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| junyi |
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