FMF - Friends of Minerals Forum, discussion and message board

[Carles Millan]

Hi!

Almost every collector has seen the famous copper after aragonite crystals that several mines at Corocoro, Bolivia have produced for many years, some of them very nice specimens ( https://www.mindat.org/photosearch.php?frm_id=mls&cform_is_valid=1&minname=copper&cf_mls_page=1&region=corocoro&text=pseudomorph ).

But does anybody know if there is scientific evidence that the former species was really aragonite? As is well known, there are a lot of minerals that show a hexagonal prism habit, not to mention chalcocite, which for its chemical composition is very close to copper itself.

[Jordi Fabre]

Few more here -> https://www.topminerals.info/index.php?searchterms=copper+corocoro&searchauthor=-&level=search

[lluis]

Hi, Carles, List

Well, seems that the proof is that there are some crystals that are pure aragonite in a half andf pure copper in the other.
Then, no guess

https://www.science-frontiers.com/sf123/sf123p11.htm
(link normalized by FMF)

That send to an article in Rocks and Minerals of 1988
Hyrsl, Jaroslav, and Petrov, Alfred; "Pseudomorphs from Bolivia," Rocks and Minerals, 73:110, November/December 1998.)

Alfredo sure could apport more info, being one of tyhe authors.

With best wishes

Lluís

[Carles Millan]

[lluis]

Hi, Carles, list

I think to remember have seen one of those partial pseudo crystal offered in the net, but was in a time in which I had not very much time and I let it pass, thinking that I will see more.
Evidently, no luck :-(
It is a mist, but I think that would be around the time when the first pseudos have been seen, say more that 20 years ago...

With best wishes

Lluís

[Peter Megaw]

I would guess Alfredo might have a comment on this?

[arturo shaw]

Frank de Wit (who digged some of those crystals with Alfredo) gave me some and told me that the cooper is growing from the inside the aragonite. I haven't asked him more but he mentioned a paper on that. Alfredo certainly knows a lot about the subject but he's traveling to Kyoto now, not sure if he has worst or better access to the Internet (better I guess when he is not flying)

Cheers

Arturo

[Carles Millan]

[str4hler]

Mail me on str4hler @ gmail . com Carles, and I will reply you the articles that I have on these beauties. I don't collect much minerals, but the pseudomorphs that I collected at Corocoro are the only ones that are coming with me in my grave ;) Some pictures etc are here http : // www . mineraltravel . com / photos / bolivia /

Cheers, Frank

[alfredo]

My observations are that most of these are only partially pseudomorphed, and many have only small amounts of copper in them. The completely replaced ones (which you can feel by the density) are rather uncommon, so be happy if you have one.

I'd not previously questioned the assumption that the original material was aragonite, but I suppose it's wise to question everything! Sometimes Nature surprises us. Nevertheless, considering their geological environment, poorly-consolidated Miocene mudstones and sandstones with gypsum - not too different from the famous aragonite-bearing marls of Spain - I suppose aragonite is the most likely precursor. I've been told (but not seen the PXRD myself) however that no aragonite is left anymore, and that these are now calcite paramorphs after aragonite. So did the copper replace aragonite directly, or the later calcite paramorphs? The true labelling would depend on the timing of the replacements.

Cheers,
Alfredo

[John S. White]

I'll add my two cents to this discussion. I have seen a number of these "pseudomorphs" where the copper is but a thin coating over aragonite. The core is still aragonite, so one might even consider such as epimorphs rather than pseudomorphs.

[alfredo]

John, you're right of course - a lot of the "pseudomorphs" on the market are really just a surface alteration, but, as Frank alluded, one sometimes finds the opposite case - aragonite crystals with copper on the inside and only carbonate visible on the surface, and then non-destructively identifiable only by their density, or with an electronic metal detector. Werner Lieber published a paper in Lapis, in the late 1990s I think, where he showed several specimens he had sawn open. The pattern of internal replacement can be quite mystifying.

Rarely there are also cuprites after copper after aragonite (or should that be "cuprite after copper after calcite after aragonite"?), and just two known specimens of malachite after cuprite after copper (after calcite?) after aragonite. When we first found those two green ones, I joked with Jaroslav Hyrsl that we should soak them in weak phosphoric acid for a while and see whether they turned into libethenite after malachite after cuprite after copper after calcite after aragonite, and then just open a shop named "Pseudomorphs Made to Order". (Sorry, John.... just my sick sense of humor.)

At the risk of boring you all with ever more details, there are also specimens in which the carbonate is partially replaced by gypsum, or (very rare) where the copper is partly replaced by native silver, or where copper wires are growing out of the surface of the crystal! (these latter having the "extruded toothpaste" shape common to many native silvers)

Changing climate (and water tables) are involved in these changes, the crystals being sometimes exposed to fresh water, sometimes very saline water, either oxygen-rich or anaerobic, sometimes dry air...

[arturo shaw]

In wikipedia (german) I've found two references:

↑ J. Hyrsl, A. Petrov: Eine der größten Kupferlagerstätten der Welt: Corocoro in Bolivien. In: Mineralien-Welt 8, 6. Bode-Verlag, Haltern 1997, S. 30 bis 35.
↑ G.C. Amstutz: Kupfer nach Aragonit. In: Lapis Pseudomorphosen-Heft. Lapis 6, 11. Weise-Verlag, München 1981.

Cheers

Arturo

(Alfredo, no way you could bore anybody, the more information, the best... :-) Thank you)

[Carles Millan]

Alfredo, a lot of thanks for your accurate and clarifying posts, even for your amusing comments that have made me laugh.

The articles kindly supplied by Frank (str4hler) are superb and deserve being well kept for further reading. Thank you Frank.

Thanks also to all.

[Carles Curto]

I just received notices from a colleague that works with the Museum of Oruro in a program of cooperation. He explains me that crystals of aragonite with the same characteristics of pseudomorphic copper exist in the zone. At the same time there are not completely pseudomorphized crystals and twinned crystals of aragonite coating pseudomorphic (after aragonite) copper. At the Museum of Barcelona we’ll receive some samples as example of the different cases. We are expecting.

[Montanpark]

Dear all,
very interesting thread as those pseudomorphs have interested me since a long time. I remember that i saw the first one during my student times at the Aachen University mineral collection where a quite large totally (at least visually) copper replaced "aragonite" was on display in the pseudomorph case (and it seemed to be a quite old one).

Almost 2 decades later i was lucky to acquire quite a lot of those and there were different stages of replacement from fully replaced to only skeletal like replacement with lot of carbonate left. I intentionally say carbonate as i have never tested if it was aragonite or calcite paramorphosing twinned aragonite. I performed some tests by carefully etching some specimens and very interesting structures were revealed, one of them Frank de Wit has seen recently when visiting me, you may remember that small specimen, Frank.

Last year when i visited the Krantz store in Bonn i found a few - 2 or 3 - samples labeled copper after aragonite and i purchased them - to see later (they were dusted from decades) that there was obviously no copper, just carbonate. If anybody wants to perform analyses on a sample i´ll be glad to send one of those, as i won´t have the time for quite a while now. Just leave me a message.

And Alfredo, ya know i like your sense of humour ;-)

Cheers
Roger

[Carles Curto]

I add a PDF page of Kempf, O., Tawackoli, S., Para, W.H. 2003. Minerales de Bolivia. Minera San Cristobal, S.A. 121+16 pp. La Paz.

[Carles Curto]

And more (from: Salomón Alivas, V., Ahlfeld, F. 1998. Los minerales de Bolivia y sus parajes. Tomo I. 326 pp. Santa Cruz de la Sierra - Bolivia.

Textual, pg. 295: (Aragonito) "Bien conocido son los cristales de aragonita en las arcillas rojas de los "Ramos" de Corocoro, principalmente en las minas de San Agustín y Malcocoya. Tienen hasta 3 cm. de diámetro y se presentan en maclas cíclicas de tres individuos, de contornos hexagonales.
A veces están parcialmente o enteramente substituídos por cobre nativo".

And, more or less translated to english: The crystals of aragonite are well known in red clays at "Los Ramos" in Corocoro, mainly in San Agustín and Malcocoya mines. They reach 3 cm on diameter and form cyclic twins (of hexagonal contour) of three individuals.
In some cases they are partially or completely substituted by native copper.