FMF - Friends of Minerals Forum, discussion and message board

[Manuel Mesa]

Today I would like to inform you about the presence of hoganite in the paragenesis of the Bou Nahas Mine (Morocco), and that I think this mineral will be talked about a lot and will be a point of discussions in the next minerals shows

In March 2017 a well-known spanish dealer from Zaragoza acquired some peculiar copper samples in his annual trip to Morocco. I had the opportunity to get a sample, and due to my great ignorance of what I had in hand, I sent it to analyze to Joy Desor in Germany.

These samples were P-XRD, Raman and SEM-EDS analysed and the blue crystals have turned out to be Hoganite, a very rare mineral.

Samples seem to be authentic and natural as specimens are covered by malachite in fibrous crystals

I attach some photos.

Enjoy!
Manuel Mesa

[Manuel Mesa]

And of course, I attach the analysis:

[Pete Modreski]

I had to look up hoganite to see what it was. So, it's copper acetate monohydrate. Given the unusual nature of the mineral and the abundance of it shown in these specimens (vs. the scarcity of it at previously reported localities, and the ease of crystallizing this artificially), it might be well to look for a detailed description of the exact mode of occurrence in this mine or mine dump before uncritically accepting this as a natural occurrence. The type locality occurrence (along with that of the related mineral, paceite) at Broken Hill, NSW was described by Hibbs et al. in Mineralogical Magazine in 2002:( https://pubs.geoscienceworld.org/minmag/article/66/3/459/140135/hoganite-and-paceite-two-new-acetate-minerals-from) as "isolated crystals embedded in ferruginous gossan from the Potosi Pit, Broken Hill, New South Wales, Australia... in the vicinity of a mass of decomposing leaf litter and carried goethite, hematite, quartz, linarite, malachite, azurite, cuprian smithsonite and cerussite. ... The minerals reported here are of natural origin, the acetate ion having been furnished in this case as a result of the decomposition of leaf litter and possibly mine timbers in the immediate vicinity of the recovered material."

[alfredo]

Hoganite crystals can also form as the accidental result of cleaning minerals with vinegar. Given the richness of these specimens, I suspect that's what might have happened here. Fibrous malachite on the hoganite could help prove that this was not due to vinegar treatment, but then the malachite should be analyzed too, to make sure it's not really brochantite or other non-carbonate.

(Reminds me of some colorless rod-like crystals that looked like achroite tourmalines on the famous Argentine hematite crystals. They turned out to be Pb citrate! ...the result of trying to clean off ugly crusts of anglesite with citric acid.)

[Manuel Mesa]

Pete Modreski, Alfredo

You're right, it's very strange. But keep in mind that the samples have been purchased directly from the miners in Morocco. They do not use acetic acid to clean the minerals. They use hydrochloric acid.

Where does the acetate ion come from? I do not know. But we have observed that some samples are presented with fibrous malachite crystals. I think it is incompatible with the use of acids. So both Joy Desor and us, we believe they are natural.

Thanks Peter

Manuel Mesa

[Cesar M. Salvan]

Some months ago, I received for analysis a sample of native copper from this locality covered by crystals similar to those in discussion here. The elemental analysis showed carbon and copper, but the Raman spectroscopy was inconclusive and pointed out to an hydroxy-carbonate.
As you can see in the SEM picture, some crystals are hollow and covered by a carbonation crust. We considered hoganite but we didn't take further actions with the sample, because we considered the crystals fake, fabricated or accidentally grown as a result of specimen's cleaning.

Acetate is formed in decaying organic matter and could be found as a result of decomposition of wood and other organic materials in old mines or organic-rich deposits.

Metallic acetates tend to grow beautiful crystals very easily (see the image I attached, it is a straightforward experiment). Also, acetate is quite unstable in a natural setting (free acetate is like candy in a primary school...favorite food for soil bacteria and, remember, divalent metal acetates are highly soluble in water); copper acetate moreover is carbonated slowly, forming 'verdigris' or even malachite.

To have a stable and significant amount of fresh acetate crystals, it is necessary a rich bacterial ecosystem and proximity to the supply. The natural acetates usually were found in small or anecdotal amounts in the vicinity of decaying or fermenting organic materials. This is because acetate, produced by bacterial fermentation, is readily consumed by other prokaryotes in bacterial consortium. This is why acetate minerals are quite rare, specially considering that acetate is the most abundant organic chemical on Earth surface. Another example: the calcium acetate has never been described as mineral, despite of its easy formation. All findings of calcium acetate turned out to be man-made.

So, months ago we discarded the sample as man-made, and, looking the beautiful pictures presented here, I think it is highly unlikely that fresh, well crystallized and abundant copper acetate could be formed in that setting without an abundant (and very recent) source of acetate. Also, acetic acid is a classic reagent to clean native copper....so....it is reasonable to consider the artificial origin.

In my opinion these specimens are man-made (intentional or accidental, by crystallization of acetate over specimens let immersed in solution for a few days, it is not my role to judge it). Of course, there are chances of a natural origin, but, in my opinion, it is unlikely. If a natural (considering natural the unintended formation in old mining remains) origin is demonstrated, these would be the best specimens of hoganite ever. As far as I know, aside of the observation of matrices and accompanying phases, the way to determine the natural/artificial origin of hoganite in a specimen-based approach, would be the determination of delta13C: if natural, carbon is of bacterial origin and its signature is different to those of industrial acetic acid or plant-derived vinegars. In my opinion, without compelling evidence (as unambiguous delta13C, together with the study of the exact context of the samples) I would think that the samples are artificial.

The samples shown here makes our samples make sense, as it seems that our sample is more "aged", with carbonated crystals. Our first impression with our sample was that someone tried to clean the copper in acetic acid solution and I think the work presented here confirms that. Whatever is the end of the story, the pictures and crystals are very beautiful.

[Enrique Llorens]

For lovers of Moroccan minerals, it is a pleasure to read and see a post of this highest level.
Thank you gentlemen Mesa and Salvan.

[Cesar M. Salvan]

To shed light on this topic and apply the Ockham's Razor to the Hoganite question, I show you the result of a very simple experiment:

Just prepare a solution with a piece of copper metal in diluted acetic acid (white vinegar for cleaning works great) to which hydrogen peroxide was added until abundant oxygen bubbles on the copper piece. Alternatively, you can dissolve copper sulfate or copper chloride in the white vinegar. I preferred the first method in order to get pure copper acetate.
Let the mixture stand for a couple of days, evaporating. You can immerse different pieces of natural samples, as native copper, rocks..whatever that will give a 'natural look' to the final 'hoganite', if you want to fool with newbies.

I present here several pictures with the beautiful results of this simple experiment. Compare the first picture of my "synthetic hoganite" with the previous picture I posted here of the sample we analyzed from Bou Nahas, and extract your own conclusions.

[Manuel Mesa]

Cesar, nice experiment !!

But I want to point out that some samples have fibrous malachite crystals, how did they form with the hoganite?

Manuel Mesa

[James Catmur]

[Manuel Mesa]

[alfredo]

The next thing we need to find out is whether the malachite is older or younger than the hoganite. If older, it could be that there was once more malachite and it has been partly etched away by acetic acid. If younger, it's possible that the malachite is a product of alteration of hoganite. Close up photos of the contact between the two species might help. Even if perhaps the hoganite was really the product of reaction with natural wood-derived acetic acid in the mine, the association with malachite is still problematic and needs some explaining.

[James Catmur]

I think we have to assume that they are not natural.

[Joan Rosell]

There is another important point... When I bought some of these "curious" specimens in the Munchen mineral show 2017, seller offered mixed samples of normal copper, carbonated ones and only few with hoganite crystals, but not asking more money for these special ones... For me that could be the fact to say that these specimens has not been synthezized specifically to be sold as "new" rare species from Oumjrane, probably by some chance.
Checking composition in mindat website, we observed that only few carbonates coincide, but no one with this color and crystal shape. Only one had similar color and shape, an organic compound, a hydrated copper acetate called hoganite. We proceed to identify crystals using Raman spectroscopy. Results were totally consistent with hoganite Raman parameters published by A. Musumeci and R. L. Frost (2007)[available online].
They are disposed on the copper and "limonite" matrix and accompanied by light green globular aggregates of a unidentified copper chloride (SEM-EDS) similar to atacamite (?). These chloride is partially covering hoganite crystals.

Frost, R.L., Musumeci, A. (2007): "A spectroscopic and thermoanalytical study of the mineral Hoganite". Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 67(1):48-57.
More info on:
https://rosellminerals.blogspot.com.es/2018/01/hoganite-rare-hydrated-copper-acetate.html

[Cesar M. Salvan]