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Identity HelpMetamict mineral and metamict glass?

4th Apr 2016 15:29 UTCJyrki Autio Expert

What does metamict glass actually mean?

I have seen the glass part written as "glass". Does it mean it is really not a noncrystalline material or is metamict "glass" just a shorthand for some more accurate description?

Thanks

Jyrki

4th Apr 2016 18:29 UTCRob Woodside 🌟 Manager

Metamict refers to a radiation destroyed crystal structure. Glass doesn't have a crystal structure, radiation might promote xllization and devitrify glass. "Metamict glass" is rather like "orange emerald".

4th Apr 2016 19:19 UTCJyrki Autio Expert

Naming metamict glass can be found in research papers meaning glass-like material formed after destruction of crystal structure because of radiation damage.

I have understood that it is a further state of alteration of metamict mineral. Something irreversible, meaning it can't be annealed back to original crystal structure like metamict mineral. Maybe it also requires exchange of ions with surroundings like leaching of REE and incorporating Ca and OH.

I don't know if it is a valid mineralogical term and if it really means amorphous material which is not a mixture of (crystalline) minerals.

4th Apr 2016 22:06 UTCJohan Kjellman Expert

Jyrki,

It refers simply to a glass that has become a glass (disordered) by metamictization, i.e by "self-radiation". All our metamict minerals are composed of such glass, as opposed to "glass" produced by other geologic processes: tektites, obsidian and whatnot.

There are only seven hits on this term if you google it - including this thread - so don't get too hung up on it.

cheers

5th Apr 2016 01:19 UTCOwen Melfyn Lewis

I'm with Rob 100% and 50:50 with Johan.

The source of radiation need not be the mineral species itself but an impurity in a host crystal. Indeen there is not reason I know of why the radioactive source has to have been within a metamict as opposed to merely adjacent to it for some long period.

Zircon crystals containing (e.g.) thorium will become metamict. Easy tests for zircon so altered:

- Colour usually changed to green.

- Significant drop in SG.

- The material becomes optically isotropic (losing its distinctive high birefringence and showing only one RI in the fuly metamict state).

- There is a significant drop in RI.

In a mineral such as ekanite, there can be distinctive structural damage at the boundary of the radioactive host mineral and inclusions within it. Unlike zircon, it is more common to find ekanite as metamict that it is to find it with the tetragonal crystal system. As for zircon, SG and RI drop as the metamict state is approached.

I think that heating to near melting point and careful, slow cooling will restore the crystalline state to any metamict and most if not all glasses formed by other processes.

5th Apr 2016 01:53 UTCReiner Mielke Expert

"I think that heating to near melting point and careful, slow cooling will restore the crystalline state to any metamict " But what also commonly happens is that it forms a mineral ( crystalline structure) different from the original.

5th Apr 2016 05:01 UTCDoug Daniels

Rob - do you know where I can obtain an orange emerald, or even an emerald orange?

5th Apr 2016 05:57 UTCAlfredo Petrov Manager

Back to Jyrki's original question: "What does metamict glass actually mean?" (Jyrki was only asking about the use of that specific term; He already knows what "metamict" means.)

"Metamict glass" is a poorly constructed term, not "wrong", just redundant, which is why it is very little used (as Johan pointed out). All metamict minerals are glasses, but not all glasses are formed by metamictization (obviously). Most metamict minerals (or "mineraloids", for those who think minerals need a crystal structure) are quite changed from their original composition - As Jyrki said, there is exchange of ions, water enters, Fe(II) changes to Fe(III), and Pb accumulates as U and Th desintegrate. So given the changes in chemistry, plus the fact that we don't duplicate the temperatures and pressures that the original pre-metamict mineral formed at, I've always been very suspicious of the theory that one gets the original structure back after heating. Apparently it does work, sometimes.

5th Apr 2016 08:30 UTCJyrki Autio Expert

Thank you. These answers have been helpfull and I see now the redundancy.

I have had altered allanite samples analyzed as glass and wondered why.

Should chemically altered amorphous material after metamict mineral be called mineraloid instead of glass?

Are some U and Th containing minerals more prone to form it? Allanite does not contain much of U and Th but seems to alter this way. Or am I wrong.

If the outward shape of original crystal is preserved and the mineral itself is replaced by amorphous and chemically altered material, is it called pseudomorph, cast or something else?

5th Apr 2016 09:55 UTCOwen Melfyn Lewis

Reiner Mielke Wrote:

-------------------------------------------------------

> "I think that heating to near melting point and

> careful, slow cooling will restore the crystalline

> state to any metamict " But what also commonly

> happens is that it forms a mineral ( crystalline

> structure) different from the original.

Thanks for that. True for some or all polymorphs? Not true for isomorphs?

5th Apr 2016 09:57 UTCJohan Kjellman Expert

FIRSTLY, ALFREDO THANKS FOR YOUR ELOQUENCE - I AM MUCH MORE OF A "GUT MAN"

(I first intended to answer that "metamict glass" is about as redundant as "green emerald" but I didn't want to mess up Rob's last point (which I didn't get - not your fault Rob!))

Now, Jyrki Autio Wrote:

-------------------------------------------------------

> Should chemically altered amorphous material after

> metamict mineral be called mineraloid instead of

> glass?

I call them "METAMICT+MINERAL NAME" possibly with the addition "altered (this or that way)".If I'd have to explain metamict, I'd say it is a glass formed by "selfradiation". I am perfectly OK with accepting that the radiation source could be external, but have no experience from it, at least not that I am aware of. I did not understand the Ekanite example.

> Are some U and Th containing minerals more prone

> to form it? Allanite does not contain much of U

> and Th but seems to alter this way.

YES (compare monazite which often has way higher Th content without being metamict)

> Or am I wrong.

NO

>

> If the outward shape of original crystal is

> preserved and the mineral itself is replaced by

> amorphous and chemically altered material, is it

> called pseudomorph, cast or something else?

I'd call it pseudomorph only if the material is a verified other mineral, that is readily identifiable and takes up a major volume of the original crystal. Looking at or probing metamict REE-Nb-Ta-Ti-Fe-oxides one sees often that they are partially replaced/altered along cracks and rims. These are often Ca-enriched. Again I would say as above.

And remember the alteration is not part of the metamictization process, as I understand it, but it is facilitated by it.

cheers

5th Apr 2016 11:00 UTCUwe Kolitsch Manager

"Thanks for that. True for some or all polymorphs? Not true for isomorphs?"

Depends on at which temperature you are heating the sample (including heating/cooling rate etc.).

And, in general, also very much in which atmosphere the heating takes place (air, nitrogen, argon, reducing atmospheres, ...

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