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Calcite????
    
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Pierre Joubert




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PostPosted: Oct 24, 2012 12:52    Post subject: Calcite????  

I feel rather embarrassed to ask this question, as it reveals my limited knowledge of minerals, but here goes: Can anyone please tell me what this white mineral on the amethyst crystal is. If it is calcite then it has a very unusual appearance for Brandberg. I do not want to test it with acid or remove pieces as it is a nice specimen. Any thoughts would be welcome. I have not cleaned the specimen yet as I need to know what the white mineral is.


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Brandberg, Namibia
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The same specimen.
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Riccardo Modanesi




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PostPosted: Oct 24, 2012 14:20    Post subject: Re: Calcite????  

Hi Pierre!
Have you ever tried with chloridric acid? It might be chalcite or aragonite! If it doesn't react, it's even more interesting because you need other proofs.
Greetings from Italy by Riccardo.

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Hi! I'm a collector of minerals since 1973 and a gemmologist. On Summer I always visit mines and quarries all over Europe looking for minerals! Ok, there is time to tell you much much more! Greetings from Italy by Riccardo.
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PostPosted: Oct 24, 2012 14:30    Post subject: Re: Calcite????  

I would think that a very small dab of HCl is certainly not going to harm the specimen, and then you'd know--if it was one of the carbonates. Opal or chalcedony could be another possibility--just the hardness, a tiny attempt to scratch with a needle or knife tip, would tell you that too. You might check if it fluoresces green, with shortwave UV--much chalcedony or opal "that looks like this" does so, and calcite, rarely green.
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Pierre Joubert




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PostPosted: Oct 25, 2012 02:09    Post subject: Re: Calcite????  

Thank you Riccardo and Pete. I will attempt to take a few photos with a microscope camera and do a few tests and share them with you. I have handled thousands of specimens from Brandberg and have never seen something like this. This pocket of amethyst also came out from a new area, as I have not seen similar amethyst from that area yet.
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Pierre Joubert




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PostPosted: Oct 25, 2012 07:23    Post subject: Re: Calcite????  

I tested the white mineral with pool acid. It fizzes. I took a few more photos with both a camera and a (rather inferior) microscope camera. I have a feeling that there are more than one mineral present, perhaps calcite and something else?


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James
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PostPosted: Oct 25, 2012 08:29    Post subject: Re: Calcite????  

I think we are talking about calcite

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Pierre Joubert




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PostPosted: Oct 25, 2012 08:40    Post subject: Re: Calcite????  

James wrote:
I think we are talking about calcite

James


Can you please elaborate? What makes you come to that conclusion?

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Tobi




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PostPosted: Oct 25, 2012 09:21    Post subject: Re: Calcite????  

Hi Pierre,

i agree with James: I know such white crustifications of calcite from Brandberg specimens, i've seen them several times at the back of amethyst specimens like yours.

Cheers!
Tobi
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Pierre Joubert




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PostPosted: Oct 25, 2012 09:33    Post subject: Re: Calcite????  

Tobi wrote:
Hi Pierre,

i agree with James: I know such white crustifications of calcite from Brandberg specimens, i've seen them several times at the back of amethyst specimens like yours.

Cheers!
Tobi


I know calcite has probably the biggest variety of crystal systems, so I would not be surprised.

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Tobi




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PostPosted: Oct 25, 2012 10:14    Post subject: Re: Calcite????  

Though there's an enormous variety in shapes of calcite, some are distinctive if they appear in a certain locality. And this is, for sure, a "typical" calcite to appear on such an amethyst crystal from Brandberg area ...
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PostPosted: Oct 25, 2012 12:31    Post subject: Re: Calcite????  

Tobi wrote:
And this is, for sure, a "typical" calcite to appear on such an amethyst crystal from Brandberg area ...


Tobi, thanks for your comments, but on this I beg to differ. I have a number of Brandberg quartz crystal specimens with an assortment of crystal systems, but this type, I have never seen before. (I have handled thousands of Brandberg specimens) :-)

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mihailovici79




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PostPosted: Oct 25, 2012 13:09    Post subject: Re: Calcite????  

I think it is chalcedony, like this:

Best regards

Mihail



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PostPosted: Oct 25, 2012 13:42    Post subject: Re: Calcite????  

mihailovici79 wrote:
I think it is chalcedony, like this:

Best regards

Mihail

not likely if it fizzes.
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PostPosted: Oct 25, 2012 14:14    Post subject: Re: Calcite????  

GneissWare wrote:
not likely if it fizzes.

Ah , sorry, my fault. You`re right
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PostPosted: Oct 25, 2012 15:36    Post subject: Re: Calcite????  

It is Calcite for sure
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Pierre Joubert




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PostPosted: Oct 26, 2012 03:50    Post subject: Re: Calcite????  

Jordi Fabre wrote:
It is Calcite for sure


Thanks Jordi. To clean the quartz crystal is nearly impossible now as I cannot use oxalic or pool acid. I will have to leave the specimen just as it is. The main feature is the unusual calcite.

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