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Ways to make synthetic corundum?
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Author:  Lefty [ Wed Apr 12, 2017 3:52 pm ]
Post subject:  Ways to make synthetic corundum?

I came across the school's mineral specimen kit yesterday. It is tiny and uninteresting and I think the kids will be much more interested in the specimens I had packed from my own collection to loan to the teacher who was doing geology with his class (the next day, the remains of a cyclone hit and the schools were closed and the school holidays started the next day so he missed out).

As always, the kit contains bauxite samples and other stuff from our local alumina refinery. Standard stuff. But with the samples from the alumina plant was a container of what appeared to be synthetic, colourless corundum. They were crystals, some of which were transparent and clean enough to be faceted. They obviously had a high RI and were very brightly reflective and showed the hexagonal crystal growth pattern.

I didn't realize there was a technique for growing synthetic corundum that produced the proper hexagonal crystal pattern seen in natural sapphire. Then again, this stuff is from an alumina refinery so I assume the crystals are a by-product of some sort.

The container is sealed and there is no one else there to ask permission at the moment to open it up and more closely examine the material but it sure gives the visual impression of corundum to me.

Does anyone know if corundum crystals are produced in the alumina refining process?

Author:  Stephen Challener [ Wed Apr 12, 2017 11:27 pm ]
Post subject:  Re: Ways to make synthetic corundum?

The hydrothermal growth method does produce nice hexagonal crystals, so that would be a likely candidate.

Author:  Lefty [ Thu Apr 13, 2017 1:48 am ]
Post subject:  Re: Ways to make synthetic corundum?

Stephen Challener wrote:
The hydrothermal growth method does produce nice hexagonal crystals, so that would be a likely candidate.


I'd only ever heard of flame-fusion being used to produce synthetic sapphire. Is it more difficult to determine that a hydrothermally grown crystal is synthetic, given that it possesses the proper crystal shape?

Author:  Alan F. [ Thu Apr 13, 2017 1:47 pm ]
Post subject:  Re: Ways to make synthetic corundum?

I haven't heard of hydrothermal sapphire being widely produced, but given the amount of synthetic, clear sapphire used to produce scratch resistant watch glasses, there must be a lot about and I don't think that flame fusion would do the trick. Anyone else any further information?

Author:  Isi [ Thu Apr 13, 2017 2:46 pm ]
Post subject:  Re: Ways to make synthetic corundum?

I think corundum used for watch glasses is of the flame fusion type.

Flux grown and hydrothermal are 2 other types of synthetic corundums, and yes they are harder to identify, at least when cut.

But flame fusion corundum is infinitely more widespread (because easier, faster and less expensive to make).

Hydothermal corundum is the rarest of all. It is the most expensive to produce, and given the abundancy of natural corundum, it is not really worth it.

I have seen roughs of flux grown corundums, they are hexagonal too, still they don't look like their natural counterpart. Never seen hydrothermal rough for corundums, only for emeralds.

Author:  AlBalmer [ Thu Apr 13, 2017 10:00 pm ]
Post subject:  Re: Ways to make synthetic corundum?

Morion has corundum produced by several methods: http://www.morioncompany.com/Products_1.htm

Author:  steinfroilein [ Fri Apr 14, 2017 2:57 am ]
Post subject:  Re: Ways to make synthetic corundum?

Lefty wrote:
Does anyone know if corundum crystals are produced in the alumina refining process?

Rubis RSA France (Dalloz group) is producing his own alumina powder by calcining ammonium alun at 1200°C; which is then screened to become a Verneuil feed powder. http://www.rubisrsa.com/index.adml?r=118

More details by The Aluminum Association http://www.aluminum.org/industries/prod ... a-refining

Author:  Isi [ Fri Apr 14, 2017 4:38 am ]
Post subject:  Re: Ways to make synthetic corundum?

I once visited the Dalloz plant. It was quite fascinating to see the "boules" in their growing process. All sizes of them.

Author:  Lefty [ Fri Apr 14, 2017 5:15 pm ]
Post subject:  Re: Ways to make synthetic corundum?

Interesting links there, thanks.

I might ask if I can take a piece and see if it can be faceted, just for the hell of it :)

Author:  cascaillou [ Mon Apr 17, 2017 6:26 pm ]
Post subject:  Re: Ways to make synthetic corundum?

Both flux and hydrothermal processes will yield crystals (with facets).

Author:  Hans Durstling [ Mon Apr 17, 2017 10:08 pm ]
Post subject:  Re: Ways to make synthetic corundum?

Two Ramaura flux grown ruby crystals; hexagonal growth lines are visible on the larger one.

Cheers,
Hans Durstling
Moncton, Canada

Attachment:
Ramaura-for-GO-3901 copy 2.jpg
Ramaura-for-GO-3901 copy 2.jpg [ 215.19 KiB | Viewed 2938 times ]

Author:  Lefty [ Wed Apr 19, 2017 3:42 pm ]
Post subject:  Re: Ways to make synthetic corundum?

Well I opened the container of crystals and took one out. Taking a crappy old quartz crystal, I attempted to make a scratch on the unknown specimen - I pressed the point of the quartz crystal against the specimen....which promptly shattered into half a dozen pieces.

I picked up one of the bits and squeezed it hard between my finger and thumb and succeeded in crushing it further. I am now convinced that while the specimen may indeed be crystalline alumina, it's not in the form that we would call corundum :)

I'll bet this is what alumina looks like after extraction from the bauxite - colourless hexagonal crystals. I was going to facet a piece and put it back in the display but since it's looks like it would probably be almost as challenging as halite I don't think I'll bother.

Author:  Faceting 101 by Arya [ Sat Apr 22, 2017 4:58 pm ]
Post subject:  Re: Ways to make synthetic corundum?

Lefty wrote:
I'd only ever heard of flame-fusion being used to produce synthetic sapphire. Is it more difficult to determine that a hydrothermally grown crystal is synthetic, given that it possesses the proper crystal shape?

Corundum can actually be grown by pretty much every method. Flame fusion is the cheapest, pulled methods (like Czochralski and Kyropoulos) are used for scientific instruments and things like iPhones, flux-grown (like Chatham rubies) gives crystal habits and more "proper" pleochroism, and hydrothermal (like RusGem blue sapphires) gives the closest to natural. Floating-zone...exists and is possible but isn't really done outside of weird research, and I haven't seen anything in the literature about growing it via skull melt.

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