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Aren't emeralds a type of corundum too? Chadloder 20:47 Jan 25, 2003 (UTC)
The Emery article says its mohs hardness is 8, but this article says "Emery is an impure and less abrasive variety, with a Mohs hardness of 9.0." Shouldn't this be changed to 8.0? The hardness of pure corundum is 9.0.
No! This is a mineralogy article about the naturally occurring mineral, aluminium oxide is a chemistry article about the chemical. Vsmith 15:28, 8 January 2006 (UTC)
Oppose merger I agree with the above poster. Would an article about diamond or charcoal be merged with one about carbon?-- 24.217.183.224 08:23, 9 January 2006 (UTC)
(To reduce confusion with above, which is different.)
Transparent Alumina is an invention ( 2004, 3M, a glass; according to the Aluminium oxide article) where as Corundum seems to be about the basic mineral from which Alumina and such can be derived (and associated gems). I would have expected a reference to Star Trek (which had something of that order in one or two of it's films) and the chemical process (assuming a trade secret does not exist that stops us knowing or telling), "rare earth elements" used and industrial uses of said Transparent Alumina. As such arriving on the Corundum page is both confusing and misleading. -- Lord Matt ( talk) 22:00, 23 November 2007 (UTC)
could there be a note somewhere in here saying about the fact it is sometimes used as a callibration material for some X-ray diffraction equipment? —Preceding unsigned comment added by 148.168.40.4 ( talk) 09:23, 5 February 2008 (UTC)
Isn't corundum the second hardest mineral (diamond being the first... roughly four times harder than corundum)? Gingermint ( talk) 05:08, 16 November 2009 (UTC)
Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the other mineral.
Surely it only leaves a streak on the things it can not scratch? Is the quoted sentence just poorly worded? 86.139.146.127 ( talk) 12:20, 24 October 2008 (UTC)
Would it be important to add in that corundum gets dirty easily since microscopic bits flake off most minerals but it doesn't because of hardness so dirt gets stuck easily? Scientific Alan ( talk) 01:11, 4 February 2012 (UTC)
Hi, i cant find the difference between corundum and sapphire. (Im personally interested in the physical perspective, not the jewellers one, is there any physical difference? i.e. is a sapphire laser a corundum laser and vice versa?) — Preceding unsigned comment added by 83.134.145.21 ( talk) 00:55, 22 February 2012 (UTC)
Corundum is polycrystalline form of alumina (Al2O3), while sapphire or ruby is the single crystal of alumina, often with very small amount of doping to have different colors. — Preceding unsigned comment added by 99.29.18.17 ( talk) 01:03, 29 January 2013 (UTC)
The opening section "Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium" indicates that iron, titanium and chromium must be present in order for it to be corundum. I have found other sources which make no mention of these elements, one of which lists the essential components as just aluminium and oxygen. As far as I can tell, in scientific circles, corundum refers to a phase of Al2O3 just like diamond is a phase of carbon.
The same reference for corundum requiring traces of iron, titanium and chromium also state that diamond is "nearly pure carbon", meaning (if the references is taken to mean corundum must contain traces of these elements) it would need something else to be diamond and pure carbon can't be diamond; it states that quartz is "SiO2 with traces of other elements", again indicating pure silicon dioxide can't be quartz. It also lists 2 contradictory compositions for anatase which would make it impossible for anything to be anatase. It appears to be stating the composition of particular samples rather than the composition of corundum in general.
Are there any objections to removing the implicit requirement of it containing these trace elements and instead saying it commonly has these traces? Black.jeff ( talk) 10:58, 29 September 2015 (UTC)
Deltalumite is another natural polymorph of Al2O3. And there are more (see https://www.mindat.org/min-1136.html) Eudialytos ( talk) 12:37, 18 June 2017 (UTC)
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Corundum belongs to the R3c (No. 167) space group, per these sources. This is a trigonal space group, not hexagonal, per this source and Wikipedia's own page and sources cited therein. Corundum is identified as trigonal in this reference also:
Duval, E.; Champagnon, B.; Lacroix, R. (1976). "Jahn-Teller effect in trigonal symmetry : 3dn ions in corundum". Journal de Physique. 37 (12): 1391–1407. doi: 10.1051/jphys:0197600370120139100.
There is a lot of confusion about this even in standard references; for example, Nesse (2000) lists corundum as "Hexagonal (trigonal)" which only adds to the confusion. But perhaps the final word is:
Borchardt-Ott, Walter; Kaiser, E. T. (1995). Crystallography (2nd ed.). Berlin: Springer. p. 230. ISBN 3540594787.
"In corundum, every third octahedral hole is vacant, but in the ideal structure, all holes are equivalent. This results in a loweringt of the space group symmetry to R3c. Thus, corundum is trigonal, while in NiAs, the symmetry of the hexagonal closest packing (P63/mmc) is retained."