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Tsumeb whites.
  
  Index -> What is it? - Where is it from?
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Heimo Hellwig




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PostPosted: Apr 07, 2017 01:42    Post subject: Tsumeb whites.  

Hello all. I just can't get to grips with the following 4 specimens. Any confirmation or correction to clarify will be appreciated. Thank you.


DSCF0272.JPG
 Mineral: Aragonite covered with Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 62mm x 46mm x 29mm
 Description:
Picture 1 of 3.
 Viewed:  2397 Time(s)

DSCF0272.JPG



DSCF0273.JPG
 Mineral: Aragonite covered with Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 62mm x 46mm x 29mm
 Description:
Picture 2 of 3.
 Viewed:  2400 Time(s)

DSCF0273.JPG



DSCF0274.JPG
 Mineral: Aragonite covered with Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 62mm x 46mm x 29mm
 Description:
Picture 3 of 3.
 Viewed:  2395 Time(s)

DSCF0274.JPG



DSCF0275.JPG
 Mineral: Dolomite with Calcite crystals.
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 53mm x 45mm x 27mm
 Description:
Picture 1 of 3.
 Viewed:  2403 Time(s)

DSCF0275.JPG



DSCF0276.JPG
 Mineral: Dolomite with Calcite crystals.
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 53mm x 45mm x 27mm
 Description:
Picture 2 of 3.
 Viewed:  2392 Time(s)

DSCF0276.JPG



DSCF0277.JPG
 Mineral: Dolomite with Calcite crystals.
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 53mm x 45mm x 27mm
 Description:
Picture 3 of 3.
 Viewed:  2387 Time(s)

DSCF0277.JPG



DSCF0278.JPG
 Mineral: Calcite on Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 50mm x 47mm x 26mm
 Description:
Picture 1 of 3.
 Viewed:  2399 Time(s)

DSCF0278.JPG



DSCF0279.JPG
 Mineral: Calcite on Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 50mm x 47mm x 26mm
 Description:
Picture 2 of 3.
 Viewed:  2401 Time(s)

DSCF0279.JPG



DSCF0280.JPG
 Mineral: Calcite on Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 50mm x 47mm x 26mm
 Description:
Picture 3 of 3.
 Viewed:  2399 Time(s)

DSCF0280.JPG



DSCF0281.JPG
 Mineral: Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 33mm x 40mm x 22mm
 Description:
Picture 1 of 3.
 Viewed:  2391 Time(s)

DSCF0281.JPG



DSCF0282.JPG
 Mineral: Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 33mm x 40mm x 22mm
 Description:
Picture 2 of 3.
 Viewed:  2406 Time(s)

DSCF0282.JPG



DSCF0283.JPG
 Mineral: Calcite
 Locality:
Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia
 Dimensions: 33mm x 40mm x 22mm
 Description:
Picture 3 of 3.
 Viewed:  2387 Time(s)

DSCF0283.JPG


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Heimo Hellwig




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PostPosted: May 04, 2017 10:18    Post subject: Re: Tsumeb whites.  

Since I did not receive any corrective comments and no confirmation either, with the vast expertise out there, is it fair to assume that my mineral descriptions of the above four specimens is correct?
I also realize that identifying specimens from pictures is a difficult exercise.
Thank you.
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Pete Richards
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PostPosted: May 04, 2017 10:36    Post subject: Re: Tsumeb whites.  

You probably did not receive any comments because telling those three minerals apart from photographs is very difficult if not impossible.

You can tell aragonite/calcite from dolomite based on the former's quick fizzing in dilute HCl, whereas dolomite fizzes gradually, especially if not a fine powder.

The areas of reflection in your images above show a rhombohedral strtucture, and therefore are almost certainly calcite or dolomite, but they could be either. They are not aragonite.

Figures 278-280 similarly show large rhombohedral crystals, for which the same comments are applicable.

One of the lumps in Figure 273 appears to have three faces (covered by "frosting") that may be related by a three-fold axis. If so, this is probably a calcite crystal, and unlikely to be an aragonite crystal.

Hopefully you can use these comments to do some tests and increase your confidence in what you have, though some uncertainty will remain.

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Heimo Hellwig




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PostPosted: May 04, 2017 11:07    Post subject: Re: Tsumeb whites.  

Hello Pete,
Thank you very much for your detailed explanations.
Unfortunately it is quite difficult for me to have my "problem" specimens analysed, since I'll have to most probably send them to South Africa.
Regards, Heimo
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Jim Robison




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PostPosted: May 04, 2017 21:16    Post subject: Re: Tsumeb whites.  

Tsumeb whites are always an enigma. As many pieces or aragonite (lead or nickel rich) tin my collection, the number of plain aragonite was very small. Calcite was the usual offender. Indeed, without the hex habit of the lead rich variety, there would have been almost no way short of acid test and analysis to be sure.

Dolomite is ubiquitous at Tsumeb and occurs in a very large variety of forms and shades. Not surprising since the principal host rock is dolostone. So a chip in vinegar, a low grade acid, is sometimes a good test. Even then, the material often has to be pulverized to get any reaction. And calcite and dolomite may both act this way.

If the question is the underlying mineral, then habit of that may help tell you what the coating is placed on, but dolomite on dolomite and calcite on calcite are always a possibility.

If I were to guess, based on past experience, which is in no way definitive, then dolomite is often the coating mineral. The mineralizing fluids at Tsumeb generally contained both calcium and magnesium, and the transitions between calcite and dolomite can be visually very subtle. But don't hang your hat on my guess. Acid test or analysis is the only sure way to be 'reasonably' sure.

Sorry
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Heimo Hellwig




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PostPosted: May 05, 2017 03:33    Post subject: Re: Tsumeb whites.  

Hello Jim,
Thank you very much also for your view/explanation.
To me ,as a novice, it just seems that the more you learn/find out about Tsumeb minerals, the more confusing it becomes.
Regards, Heimo
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Jim Robison




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PostPosted: May 05, 2017 12:34    Post subject: Re: Tsumeb whites.  

Helmo

Tsumeb is constantly full of surprises. A fascinating mine which at first seems simple, but after review turns out to be amazingly complex. But don't give up on it. Just enjoy the ride as you get acquainted. There are folks who have studied the mine for years and years who are still learning new things. In the meantime, enjoy your collection and don't hesitate to add to it if you can. Most of us Tsumeb addicts got into it because the mineral specimens are often lovely, and there is a huge variety of things to collect. Don't try to specialize too early in your exploration journey. Collect what you like and can afford, and have fun doing it. And keep up the questions. Folks like to help out where they can.
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