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PhotosBayldonite - Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia

31st Oct 2014 16:08 UTCRichard Gunter Expert

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The massive sulphide matrix of the Bayldonite crystals has a phase that gives a bright red internal reflection, almost like Proustite. It can be seen in close up on the left side of the enclosed photo. Does anyone know what this phase might be?

31st Oct 2014 16:22 UTCReiner Mielke Expert

Why not cuprite?

31st Oct 2014 16:29 UTCRichard Gunter Expert

Hi Reiner:


The grains in question are locked in the massive sulphide and do not look like an oxidation product. This sample has very sharp boundaries between the oxidation veinlets and the massive sulphides. I have an unknown green-blue phase as an alteration product on the other side of the sample but the crystals are so small and so tightly intergrown on the sulphides I have not determined what they are.

31st Oct 2014 16:42 UTCReiner Mielke Expert

Maybe the sulphides are also secondary.

31st Oct 2014 16:52 UTCRichard Gunter Expert

Hi Reiner:


That is possible though from the literature it seems as if the secondary phases at Tsumeb were always oxidized. I have not seen any specimens of Bayldonite crystals directly on sulphides; they always occur with other oxidized phases.

31st Oct 2014 21:01 UTCMalcolm Southwood 🌟 Expert

Hi Richard,

Are you certain that it's bayldonite?

mal

31st Oct 2014 21:46 UTCRichard Gunter Expert

Hi Malcolm:


It was sold to me as Bayldonite, Tsumeb, S.W.A. and the green, transparent trillings match the Bayldonite crystals from the Mindat.org Tsumeb thread so I am fairly certain the designation is correct. It was purchased in 1985 as a thumbnail with no previous tags so I don't know how old the sample might be. The Bayldonite crystals are far better than my camera can record.

4th Nov 2014 17:16 UTCRichard Gunter Expert

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Hi Malcolm:


Attached is another photograph of the Bayldonite with the light almost parallel to the sample face to give a better idea of the shape of the Bayldonite crystals. There are many smaller prismatic green crystals associated with the Bayldonite that I have assumed are Olivenite-Adamite. The second photo is the back of the sample with deep blue crystals of what I think are deep blue Keyite crystallizing directly on the sulphides on the lower left side of the photograph.


Do you think my identifications are correct?

4th Nov 2014 17:31 UTCReiner Mielke Expert

What exactly is the matrix? Massive sulphides is rather vague.

4th Nov 2014 17:48 UTCRichard Gunter Expert

Hi Reiner:


That was the original purpose for the thread as I was investigating the sulphides. The bulk of the sulphide appears to be Tennantite as there is no cleavage and it is not the reddish colour from Germanite. I still have no idea what the sulphide grains are with the bright red internal reflections. The sulphides do not crystallize on the fracture surfaces.


I did not receive any detailed locality within the mine when I purchased the sample.

4th Nov 2014 18:30 UTCReiner Mielke Expert

I think you need to get some EDS done on this specimen.

4th Nov 2014 18:41 UTCRichard Gunter Expert

I thought there might be someone with enough experience to have answers but it seems this sample will join the list of those needing analyses.

4th Nov 2014 20:00 UTCBrent Thorne Expert

Hello Richard,


If the green crystals, on the front of your specimen, are truly bayldonite, then the combination with cuprian adamite, and keyite is something I have not seen before. I have seen red glassy masses in tennantite, and I had always thought that it was cuprite.


It will be interesting to see what it turns out to be.


Brent

4th Nov 2014 21:13 UTCRichard Gunter Expert

Hi Brent:


The red internal reflections on the sulphides are more metallic than those from my Cuprite samples and much closer to Proustite in colour. The sulphide grains are intergrown with each other rather than the red phase being an alteration grain.


The Bayldonite crystals are not prismatic as are the Olivenite-Adamite crystals but have definite twin planes in the form of a trilling. Bayldonite is the only green arsenide that twins in that manner. My Duftite is a very different crystal shape, closer to a barrel shape. As the Mineralogical Record Special Tsumeb issue says, these green arsenides look very much alike and have often been misidentified with each other, but in this case the two phases of crystals are very distinct.


It will be interesting to see what they are.

4th Nov 2014 22:18 UTCRichard Gunter Expert

Hi Brent:


One other note; the Keyite and Bayldonite do not occur together. The Keyite and Olivenite-Adamite occurs on one side of the sample and the Bayldonite and Olivenite-Adamite occur on the other side. It is possible the two sides may have been separate vugs.

5th Nov 2014 04:57 UTCMalcolm Southwood 🌟 Expert

Richard,

My apologies - I only just picked up on this thread again. Even when you have the specimen in hand and with a good microscope to help you, visual identification of the "Tsumeb greens" is often problematic; to attempt it from photographs like these is downright dangerous!

My first impression from the lower of the two "new" photos you posted was arsentsumebite, but something in the adamite-olivenite seires (not to forget zincolivenite) is certainly a possibility. I'm less convinced about the bayldonite, but certainly wouldn't rule it out from what I can (or can't) see in the photos.

At this stage I see nothing to convince me that you have keyite; to my eye the blue in your photo could simply be a tarnish on the sulphide. However, if your green mineral (on one side of the specimen at least) is something in that adamite-olivenite series then its well worth having a scan under reasonably high magnification to see if you can see the characteristic discreet blue crystals.


Brent - keyite with olivenite and bayldonite does occur, and is documented by Williams (1990): "The early bayldonite collected there was invariably associated with olivenite. Subsequently large crystals together with cuproadamite, keyite, and schultenite were unearthed."

At the end of the day, Reiner is quite right; getting some analytical work done is always a good idea with those "Tsumeb greens"!

Hope this helps,

mal


Ref: Williams, P.A. (1990). Oxide Zone Geochemistry. Ellis Horwood. London

5th Nov 2014 09:21 UTCUwe Kolitsch Manager

Fahlore can show red internal reflections.

5th Nov 2014 15:28 UTCRichard Gunter Expert

Hi Uwe:


I was thinking that was a possibility; red internal reflections are not mentioned in conjunction with Tennantite in any of the Tsumeb literature. It might have been missed.


Malcolm:


The blue crystals of Keyite are there but are so small that my camera cannot reach them. It took many years after I acquired the sample to find them. I originally though the blue was a tarnish as well because the Keyite crystals grow so close to the sulphide matrix. Two of them are thick enough that the blue interior colour can be seen under the microscope.


I thank you all for your input. I sounds like the "Tsumeb greens" always need analyses.

5th Nov 2014 19:45 UTCMalcolm Southwood 🌟 Expert

That's good Richard,

Sounds like a very interesting piece of rock to have!


On the issue of red internal reflection in tennantite - I would expect to pick this up with a reflected light microscope (and particularly with oil immersion), but I'd be surprised if they are obvious with a stereo microscope (which I assume you're using). Also worth analysis I guess.

mal

5th Nov 2014 21:24 UTCRichard Gunter Expert

Hi Malcolm:


Very low Fe Tennantite can have reddish internal reflections that are visible with a binocular microscope but it is a rare texture. I have a sample of reddish reflecting Tennantite from the Kidd Creek Mine. I compared the two samples after Uwe suggested it and the colour of the Kidd Creek Tennantite is much closer to the Tsumeb sample than Cuprite or other phases. I would have expected all of the Tennantite grains on the Tsumeb sample to be reddish reflective as that is the case from the Kidd Creek Tennantite where all of the Tennantite grains on a cabinet sized sample have the same reflection. Only two grains on the Tsumeb sample have that reddish internal reflection so there may be more going on.


Yes this is an interesting sample and I thank you for your comments.

7th Nov 2014 16:53 UTCRichard Gunter Expert

Hi Malcolm:


A search of the literature for Tennantite chemistry indicates that the red reflecting Tennantite for Kidd Creek (Hannington (1999)) and Tsumeb (Dana) are very similar. The red reflecting Tennantite from Kidd Creek has 0.67 wt % Fe and 7.19 wt % Zn and one of the reported Tsumeb analyses in Dana is similar. There may be a zone within the Tennantite chemistry that is not opaque.

6th May 2015 02:23 UTCLeor Goldberg

Sphalerite?

6th May 2015 17:17 UTCRichard Gunter Expert

Hi Leor:


This is not Sphalerite as the red reflections are very different, much closer to the internal reflectance seen in Pyrargyrite. The red-reflecting Kidd Creek Tennantite has been measured by microprobe so its chemistry is known. Interestingly the Sphalerite in the Kidd Creek paragenesis is almost Fe free and is a very light yellow to colourless. I have not seen that type of Sphalerite reported from Tsumeb.

6th May 2015 17:28 UTCAlfredo Petrov Manager

If indeed the red matrix mineral is not cuprite or sphalerite, then low-Fe tennantite would certainly fit. Both tennantite and tetrahedrite are somewhat translucent red when the Fe content is very low.
 
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