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[valere]

Tantara, RDC, is mostly known for its dioptase speciments. The Congolese fanatics also know that locality for its pseudomorphs. In the heydays of the mine, before WWII, plancheite after cuprite have been found, also dioptase after calcite, and plancheite after dioptase after calcite. The latest having been studied again in the Nov-Dec "2005" of MR. Less known but also significant, the mine gave blue quartz (inclusions), the upper layers gave limonite after pyrite pseudomorphs (one was found in "2011") and a few old pieces show a beautiful combination between cobaltoan calcite and dioptase.

Both in historic finds and "2011" finds dioptase is clearly a step in the process (evidences of this are present on speciments).

The big hit in the first half of "2011", following a very small find in "2010" gave a surprising new find of pseudomorphs, with crystals of a new style. A few samples showed again the typical scalenoedral calcite shape (pseudomorphs and casts) known in the historic samples. However, the vast majority show a new style of pseudomorphs, admitted as after calcite by many. In reality the situation is not so simple: I have seen no sample of the first mineral (before the pseudomorphic process), so there is a missing link! Although no typical full dioptase shaped pseudomorph is known to me, the first preliminary study by a friend specialized in crystallography, of the crystal faces, may suggest dioptase faces, so dioptase can't be ruled out with the data actually in my possession (on samples illustrated below, details from J Scovil pictures, copyrighted).

This discussion has "2" purposes: to look after samples that could give an answer, and to have the opinions of experts who already worked on these "2011" samples. Many "2011" pseudomorphs are A(after B, after B after C) after calcite, can the statement: A(after B, after B after C) after dioptase be more than a supposition with the illustrated pseudomorphs?

All the best

[valere]

More pictures

[valere]

The last one...

[Pierre Joubert]

Hi Valere. Fantastic topic. Hope to see a lot more pictures and info. We have a very good friend working in that area at the moment, managing a new copper mine. Stay well.

[Jordi Fabre]

Yes, a very interesting topic. After the apparition of these pseudo I did some checkings with several different persons. All they said that the pseudos are just after Calcite, sometimes covered by Dioptase. Of course this is not a scientific matter, is just a comment and unfortunately I can't say the names of the people whom I asked it because I'm not sure if they want his name publicly.

Anyway, here we have several "Peters" ;-) with great knowledge about crystallography, hopefully they participate and we can start a serious discussion about the topic.

[valere]

Thanks Pierre and Jordi,
This is a very hard topic, there are not many study of this locality, probably because of its poor industrial interest and history, when compared to the major (but completely different) mines of the area.
Everything goes for calcite, but with a lack of scientific arguments for the (2011) crystals (my big problem), this is probably why many experts are cautious about making statements about this. This is when I handled these than I regret to not have studied crystallography!
I join here a picture showing tiny octaedral pseudomorphs (cuprite), (2-3) mm on edge maximum, These can be seen on the border between oxyde matrix and pseudomorph area. Only on one place on all the material I have, these can be seen.
I will present some speciments in Munich, I will glady discuss about them there too.
If anybody has interesting exemples from this find, pictures are wellcome, A batch of these landed in the US last year in Springfield, so I have not seen them all and significant pieces might have escapped!
Cheers
Valère

[valere]

Another pic of a very good example (size: 9 cm)
Val

[Pete Richards]

[Pete Richards]

The question of a two-stage replacement (calcite->dioptase->shattuckite) as opposed to the two minerals replacing calcite independently could probably be resolved by study of thin sections with a petrographic microscope. With this approach one can look at micro-textures that indicate which mineral is replacing the other one, or that might indicate that both dioptase and shattuckite replaced calcite in different parts of the specimen, without interacting with each other.

Since dioptase is itself a rhombohedral mineral, one might consider whether the original mineral was dioptase and the only pseudomorphs are the shattuckite. However, dioptase crystals "always" have prisms (based on my experience and Goldschmidt's drawings), so the habit of these crystals is wrong for primary dioptase.

Pete Richards

[valere]

Many thanks for these interesting words Pete
More data to go with your suggestions:
- absolutely no pseudomorph show any prismatic habitus on all the samples I have, which thus can be an argument against dioptase (on the contrary, the real direct dioptase crystalisations from this deposit do show prismatic faces and luster)
- the crystals that show double terminaisons are clearly curved in between, nothing prismatic
- the replacement of dioptase pseudomorphs by shattuckite can be seen with naked eyes on at least 5 pieces where the crystals are partly replaced by shattuckite (this can be seen either on broken shattuckite pseudomorph crystals where the interior is still partly dioptase, or on complete dioptase crystals partly "eaten" by the shattuckite currently replacing by "spherical " expansion (the same way malachite "eats" azurite). The same process can be seen on pseudomorphs of the same crystal habit where malachite is being replaced by shattuckite, I will post a picture of this when at home.
- up to now (within my limited knoweledge), no crystillized smithsonite have been described from there (although, in Congo, everything is possible, but Tantara is less polymetallic than most congolese mines)
Best regards
Valère

[valere]

Here is an other pic to show the diversity of this find, with novelties not found in the historic times
Enjoy!

[valere]

Another one, malachite replacing shattuckite
Far from boring... often unique pieces!

[Jordi Fabre]

[valere]

Yes Jordi,
I admit the pictures can be confusing, especially the last one that shows a partial view of pristine crystals. On this sample, and on others, some broken pseudomorphs clearly show deep and complete replacements by malachite, not only a superficial coating!
I will see you with pleasure in Munich where I will exibit some of these in a collector's case, an opportunity to go further on the discussion with some real specimens. That will be a great opportunity since most of the African mineral specialists should be there, and other samples might appear in the collectors cases, from peoples that are a step further in the studies!
However, one single specimen (in the ones I have) shows malachite of the same shape, that are "empty crystals", this one might be the result of a superficial coating...
See you in Munich!
Valère

[Nico78]

Hello Valere!
And hello to all!

It's my first post in this board! And I'm glad to join your community!
I read the forum since a long time, but just subscribe now!
I'm a french young geology engineer working in new Zealand in this day.
In fact i can't stay without saying anything about this subject!

A big thanks to share with us this interesting subject!
In fact, I'm passionate of secondary deposit copper mineral and especially from Katanga!
I have had the chance few years ago to got some pieces from an old collection belonged by a French geologist present in the 70's/80's during the exploration.
Some of them show in fact some pseudo-morph of shattuckite after calcite with dioptase association in the same pieces.
I will study more and bring more details/pictures when i will go back to home during December!
(for the moment i work in New Zealand, in fact a little bit too far of my collection in Paris...)

[valere]

Many thanks for entering the discussion! I will be very interested to know more about your specimens, especially the pseudomorphs!
Moreover, a publication is needed (a tough and long work), and contributing specimens could be very welcome!
Looking forward hearing from you!
All the best, Valère, Belgium

A little surprise this year at the Munich show (although a bit on the side of the question): a pseudomorph (complete replacement) of malachite after cuprite with matrix, Tantara RDC, with matrix, the crystal is about "2.5"cm. The only such a piece I have ever seen

[Nico78]

Nice!
I hope being for the end of year celebrations in France!
if it is the case, i will take some pictures of my specimens! and why not a meeting to share our hobby somewhere between Paris and Belgium?