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Mineralogical ClassificationArgentite

17th Oct 2019 22:05 UTCJolyon Ralph Founder

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Is argentite a valid species or not?  

According to the IMA, no.

But that wasn't always the opinion, as in 

17th Oct 2019 22:07 UTCJolyon Ralph Founder

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This paper caused Schneiderhohn to say 'Hey wait, I told you all before they were the same, because both argentite and acanthite are optically anisotropic in thin section.'

He also proposed keeping the name argentite and dropping acanthite. 

That never happened either.



17th Oct 2019 22:08 UTCJolyon Ralph Founder

Does anyone know when the official decision to keep acanthite and drop argentite was made?

18th Oct 2019 11:14 UTCJohan Kjellman Expert

That is a relevant question. I followed some of the old links discussing argentite/acanthite and in these were also at least mentioned the "transparency" of IMA/CMNMC. At their site I could only fetch the newest version of the IMA list. However, I recalled the eminent "way-back machine" at archive.org. And I have now compiled a bunch of IMA-lists, six between 2004 and 2009 compiled by Nickel/Nichols and four IMA-Master lists between 2012 and 2019.
Interestingly Argentite was in the lists up until 2009 and vanished in the list 2012.

I also found a print-on-demand(?) publication published 2012-01-02, the abstract reads:
Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Argentite was primarily treated as a mineral belonging to the galena group, cubic silver sulfide (Ag2S), occasionally found as uneven cubes and octahedra, but more often as dendritic or earthy masses, with a blackish lead-grey color and metallic lustre. All these forms are structurally not argentite, but its monoclinic polymorph, acanthite. Due to instability in normal air temperature, the IMA commission decided to reject this mineral, which now is a discredited mineral species.
but I have found no proper date on IMA/CMNMC's decision.

cheers

18th Oct 2019 17:52 UTCMarco E. Ciriotti Manager

I think it was a simple decision of the IMA members (without official proposal) following the fact that the isometric phase of AgS2 is stable only above at 173° C and not found in Nature at ambient temperature.


18th Oct 2019 18:15 UTCAlfredo Petrov Manager

I would rather say it isn‘t found at the ambient temperatures in mineralogists‘ lab equipment, but there are undoubtedly numerous localities where it is found at the ambient temperatures in hydrothermal systems, such as in "black smokers" on the ocean floor, or even in drill cores at geothermal powerplants. ;)) 

18th Oct 2019 18:36 UTCDavid Von Bargen Manager

The fact that a mineral is unstable at room conditions does not negate the possibility that a material is a mineral according to the IMA rules. The problem with argentite and acanthite is that only a very small change in atomic positions changes the crystallography.

18th Oct 2019 18:44 UTCJohan Kjellman Expert

I would call it a simple-minded decision. An "ambientocentric" decision. It is obvious, by deduction and circumstantial evidence, that the mineral exists at conditions not directly observable by humans. 
By this line of reasoning all minerals found only in metamict state should also be excluded. 

18th Oct 2019 19:05 UTCUwe Kolitsch Manager

Hundreds of minerals show phase transitions to other, crystallographically distinct phases at temperatures either lower or higher than ambient temperature ...

18th Oct 2019 19:14 UTCKevin Conroy Expert

... like ice for instance.

18th Oct 2019 19:45 UTCJohan Kjellman Expert

what are you implying...?

18th Oct 2019 21:13 UTCRalph Bottrill Manager

I’m sure that using a microscope with a heating stage argentite could be readily observed by a mineralogist with no discomfort. Just because our bodies do not equilibrate to ice-forming temperatures does not mean ice is not a mineral.

18th Oct 2019 21:39 UTCFrank K. Mazdab Manager

It looks like argentite got screwed because the activation energy barrier required to preserve its metastability to room temperature was just too low... diamond lucked out on that one.

On further reflection, had argentite and acanthite just been originally named "α-argentite" and "β-argentite" (or "α-acanthite" and "β-acanthite"), akin to α-quartz and β-quartz, then we could have just called the mineral by one name (as we do with quartz), and recognized that at 173° a non-quenchable(?) phase transition occurs.  And as much of silver sulfide grows above that temperature, we would then have the pleasure of adding to our collections the room temperature paramorph of the higher T phase, labeled with a simple, non-controversial name.



19th Oct 2019 14:06 UTCJohan Kjellman Expert

It looks like beta-quartz "got screwed" as well, at least I am not seeing it in the latest IMA-list, or am I missing something?

19th Oct 2019 18:21 UTCUwe Kolitsch Manager

The α-quartz - β-quartz transition is in fact totally non-quenchable.
 

19th Oct 2019 19:56 UTCFrank K. Mazdab Manager

That's correct Johan, but that's the point.  In several of the silica minerals, and others as well (for example, α-ilvaite and β-ilvaite, I believe), the above-room-T phase of a pair that undergoes a non-quenchable phase transition doesn't get its own species designation (or its own name), but is still recognized as a distinct phase.

Nonetheless, while the quartz in our collections is technically α-quartz, I rarely if ever hear people call it that... we just call it quartz, and the only time the distinction is explicitly made outside of scientific purposes is when the lower T phases paramorphs the higher T one: the pretty hexagonal bipyramidal "α-quartz *after* β-quartz" in our collections. By using the α - β-designation rather than assigning two separate names, argentite too could have enjoyed that simplicity (with the α - β largely dropped in everyday use, for convenience), with more foresight from the IMA.

20th Oct 2019 11:30 UTCJohan Kjellman Expert

Sorry guys, 
I wasn't aware of this "nomenclature/phase rule" (?) and I apologize if I offended anyone.

cheers

20th Oct 2019 11:50 UTCFrank K. Mazdab Manager

Hi Johan,

I don't think anyone's been offended, and there's certainly nothing to apologize for.  You asked a question, and thoughtful questions are always greatly encouraged here!  :-)

In any case, I think the whole "argentite/acanthite" "is it/isn't it" a mineral thing is kind of interesting, and the various analogies to other mineral systems can be enlightening.

But now I guess I ought to just toss out those argentite crystals from Pluto I had, since the former isn't a mineral anymore and the latter isn't a planet anymore... LOL
 
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