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Pyrognomity of minerals

Posted by Jyrki Autio  
Jyrki Autio November 09, 2013 03:46PM

Wikipedia mentions allanite and gadolinite being pyrognomic.
In what temperature the phenomenon would be expected?
Are there more minerals with this property?

Alfredo Petrov November 09, 2013 04:10PM
Many metamict minerals would be expected to show this phenomenon, but only once. After heat annealing completes restoration of crystallinity, they wouldn't be expected to glow again.
Jyrki Autio November 09, 2013 05:53PM
I thought about photographing or videoing the phenomenon.
Do I have to heat the sample glowing red first or should it happen before the normal glow?
And after this kind of annealing, what kind of results would be expected in XRD of (formerly) metamict mineral?
Uwe Kolitsch November 09, 2013 06:39PM
"what kind of results would be expected in XRD of (formerly) metamict mineral?"

You may get a single-phase crystalline product of annealing or a mixture of crystalline phases.
It all depends on the conditions of annealing (max. T, annealing time, heating and cooling rate, atmosphere).
Often you can't deduce what the original mineral was.
Roger Curry November 09, 2013 07:08PM
Hi Jyrki,
Please do a video, I'd like to see this.
Dean Allum November 10, 2013 01:38AM
This is only the second time I have ever read about "pyrognomity". The first was earlier today.
The term is not in the Mindat glossary.

Alfredo and Uwe, do you have any modern references to this effect? Have you ever seen it exhibited in person? What are the temperature ranges which you expect it to occur? Is it exothermic?

-Dean Allum
Reiner Mielke November 10, 2013 02:01AM
Well it must take a very high temperature for that to happen because I just heated a piece of allanite-Ce to 330C and nothing happened.
Alfredo Petrov November 10, 2013 02:14AM
I have observed it myself, heating fragments of metamict minerals in a pyrex test tube over the kitchen gas flame at night with the lights turned off. I don't know the temperature, but seems to happen just below what would cause other minerals to show a dull red incandescence, so 700 or 800 C perhaps? (just a wild ass guess)

It would have to be exothermic, because the mineral is releasing the indirectly stored up energy of the radioactive decay. After it's gone, it's over, and you'd have to wait millions of years for a "recharge", so a pyrognomic mineral would be expected to show the phenomenon only once.

The ordinary thermoluminescence of some minerals (e.g.: fluorite, diamonds) can also be related to the release of energy stored by radioactive decay (but not always from that cause - it can also be release of energy stored from ultraviolet exposure or cosmic rays), but that's a phenomenon in crystalline materials, and is gradual, not the more sudden release of energy that occurs when metamict minerals are heat annealed.

Edited 1 time(s). Last edit at 11/10/2013 03:26AM by Alfredo Petrov.
Alfredo Petrov November 11, 2013 06:51AM
Fred Pough, in his well-known Field Guide to Rocks and Minerals, had the folowing to say about flame tests on zircon:
"... some varieties glow intensely for a moment (thermoluminescent), although only one time. Fluorescent frequently enough for this to be a good test."
Pough is obviously talking about the pyrognomic property, although he seems not to distinguish this from ordinary thermoluminescence.

Anyway, Jyrki, back to your original question of which other pyrognomic species could be added to your list of gadolinite and allanite: certainly zircon, of course, and probably (although I haven't tried them myself) any other metamict mineral that is not completely opaque, like thorite, ekanite, Th-bearing titanites, formanite?..... But only from geologically ancient deposits, younger material may not be metamict yet.

Edited 1 time(s). Last edit at 11/12/2013 12:23AM by Alfredo Petrov.
Henry Barwood November 17, 2013 04:28AM
I'm curious how you would tell a pyrognomic reaction from thermoluminescence?

Henry Barwood
Troy University
Troy, Alabama USA
Rob Woodside November 17, 2013 04:51PM
Thermoluminescence can be caused by healing colour centres and recrystallization (pyrognmity) and probably other mechanisms. That's my take.:-)
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