Talk:Corundum

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Aren't emeralds a type of corundum too? Chadloder 20:47 Jan 25, 2003 (UTC)

Don't think so. Emerald is a type of beryl (beryllium aluminum silicate) while corundum is aluminum oxide. Perhaps you were thinking of ruby and sapphire, which are types of corundum. -- Heron

Yup, you're right. Sorry. :) Chadloder 20:40 Jan 26, 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.

Emery is a rock, a mixture of corundum, hematite and magnetite. --Chris.urs-o (talk) 09:29, 2 November 2012 (UTC)

Merge suggestion

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)

Oppose merge: there is (potentially) more than enough material to justify two articles; minerals and chemical compounds are usually treated seperately on WP. Physchim62 (talk) 09:14, 9 January 2006 (UTC)

New Merge Section

(To reduce confusion with above, which is different.)

• Support as nom. {Slash-|-Talk} 07:32, 21 February 2007 (UTC)

• Oppose the merger for the same reasons as the posters in 2006. One is chemistry the other a mineral. It would be more appropriate to merge Transparent alumina into the article on aluminum oxide.Dcmacnut 20:05, 2 March 2007 (UTC)

• Support If you mean combine the transparent alumina article into this, then I'm in favour
• Oppose for the same reasons as those of the former merger, Corundum is a Mineral article, and transparent alumina is a chemical article and so are to be treated separately Kevmin 11:08, 6 March 2007 (UTC)
• Question to you mineral/chemical guys. Is Transparent alumina a variety of corundum? If not then I Oppose a merge. If it is, then I am Neutral but I would then state that somewhere in both articles it should at least state so. right now the only hint is that it is like "sapphire" and "ruby". As a gemstone guy, this is inaccurate. SauliH 11:28, 6 March 2007 (UTC)

• No, transparent alumina is not a variety of corundum. Corundum is by definition a naturally occurring mineral species as defined by the International Mineralogical Association. Transparent alumina is a artificially created chemical substance and so does not qualify for the term corundum. Kevmin 11:36, 6 March 2007 (UTC)

But it could be correctly termed synthetic corundum? SauliH 18:54, 6 March 2007 (UTC)

Oppose, there is already a section in corundum on synthetic stuff. This transparent alumina would better be merged with the aluminium oxide chemical article for its ceramic and industrial usages. Info on synthetic ruby and sapphire should (and already is I think) be in those gemstone articles. The article transparent alumina seems rather an odd term - should be redirected to aluminium oxide and pertinent material merged wherever it best fits, if not already there. Vsmith 01:08, 7 March 2007 (UTC)

I am still trying to get to grips with what this material is. Does it have the same crystal structure as corundum crystal? If it is a synthetic production of corundum with like crystal structure and chemical makeup then this should be made clear in the article. SauliH 05:27, 8 March 2007 (UTC)

From what I can find this is a man made Al₂O_s Glass. This means that I is not the same substance chemically as naturally crystalline Corundum, Al₂Os, and not classifiable as a mineral under the IMA rules, that it is not anthropogenic in origin and that it has a definable crystal structure Kevmin 10:06, 8 March 2007 (UTC)

Transparent alumina redirect not logical, Jim

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)

To mention

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)

Streaks

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)

yes, and in any case the other mineral may not always streak to white - eg galena, graphite, hematite, etc. Plantsurfer (talk) 12:39, 24 October 2008 (UTC)

Corundum

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)

corundum vs sapphire??

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)

Vsmith can answer this better. As far as I know, these are just different terms for the same solid - corundum is used in geology literature to refer to a mineral and sapphire is used in gemology/physics to refer to a gem or crystal (as in titanium-sapphire laser). Lattice is the same, impurities might be different, but it's impossible to tell that difference from the name only. Materialscientist (talk) 01:04, 22 February 2012 (UTC)

Better? nah. Seems sapphire is corundum (Al₂O₃) that has small amounts of Fe and Ti substituting in the Al site - which depending on the ratios and amounts (along with other impurities) makes the pretty colors that get gem people turned on. The sapphire laser is synthetic corundum with controlled amounts of titanium doping - to make it work. Disclaimer, don't know much about lasers ... cheers, Vsmith (talk) 02:07, 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)

Requirement of impurities

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)

Rejiggered a bit as impurities per Mindat. Vsmith (talk) 20:15, 29 September 2015 (UTC)

Deltalumite

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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Crystal system

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."

--Kent G. Budge (talk) 00:55, 20 September 2021 (UTC)