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 Post subject: prehenite?
PostPosted: Wed Jul 08, 2020 6:44 am 
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Found it in the cleanup. I'd forgotten all about it. The original find was five years ago. Slag heap from a tanzanite mine. Do any of you have any idea what this mineral is? My first impression was prehenite. But I'm not sure.


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 Post subject: Re: prehenite?
PostPosted: Wed Jul 08, 2020 12:02 pm 
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Prehnite does not form nice prismatoc crystals.
(You also shouldn't find slag at a tanzanite mine--that shjnt stuff should be graphite, right?)
It's a very cool stone, nothing leaps out to me as a slam dunk on ID. It would need testing.

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 Post subject: Re: prehenite?
PostPosted: Wed Jul 08, 2020 3:34 pm 
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Give it a sniff. Looks kinda like sulfur.


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 Post subject: Re: prehenite?
PostPosted: Wed Jul 08, 2020 4:18 pm 
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Zoisite does come in yellow in the same deposits as Tanzanite. Scapolite is also found in association with Tanzanite. I would think these are the most likely answers. But without being able to examine and test it in person, this is speculative at best.

The more orange to green minerals could be grossular garnets (Tsavorite). These can go through retrograde metamorphism into Tanzanite.


Although Sulfur is associated in the Tanzanite bearing rocks it is usually absorbed into pyrites.


Some basic info here:

https://saltworkconsultants.com/tsavori ... tanzanite/


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 Post subject: Re: prehenite?
PostPosted: Wed Jul 08, 2020 8:13 pm 
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Makes sense, Steve. I'd still sniff it cause that's just the way I roll. :wink:


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 Post subject: Re: prehenite?
PostPosted: Thu Jul 09, 2020 4:17 am 
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The crystal sits on graphite matrix. I can definitely rule out sulfur. I was holding a lighter on it. No change in physical state.


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 Post subject: Re: prehenite?
PostPosted: Thu Jul 09, 2020 12:46 pm 
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Have you checked it for fluorescence? Scapolite fluoresces although I'm not aware of an association with galena....Steve? Weigh in?


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 Post subject: Re: prehenite?
PostPosted: Mon Jul 13, 2020 5:13 am 
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No fluorescence neither short-wave nor long-wave UV light.


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 Post subject: Re: prehenite?
PostPosted: Mon Jul 13, 2020 11:04 am 
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All we need is a Raman and all will be known....


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 Post subject: Re: prehenite?
PostPosted: Mon Jul 13, 2020 1:18 pm 
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Right. Have you heard back on the tourmaline?
Maybe you could find someone at DGemG to test your unknown for you.


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 Post subject: Re: prehenite?
PostPosted: Mon Jul 13, 2020 3:14 pm 
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The color reminds me of arsenides. I would be careful with it.


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 Post subject: Re: prehenite?
PostPosted: Tue Jul 14, 2020 7:43 am 
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Place the sample with a few aluminium beads in a test tube and add sodium or potassium hydroxide solution. The test tube is fitted with a loose wad of cotton wool as a splash filter. The test tube is closed with blotting paper or household paper soaked in silver nitrate. Even traces of hydrogen arsenic will show a brown colouring of the paper after some time. Hydrogen arsenic is very toxic. Try to do it only outdoors and with very small amounts!

Or the harmless variant. Digestion with potassium hydrogen sulfate. A little sodium nitrite is added to the solution. A red-brown precipitate forms with silver nitrate solution.

In both cases the mineral is used up.


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 Post subject: Re: prehenite?
PostPosted: Tue Jul 14, 2020 12:34 pm 
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Yeah, OK....
Where did that come from?


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 Post subject: Re: prehenite?
PostPosted: Tue Jul 14, 2020 3:41 pm 
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I see problems of health risks, and destruction of the sample.

There are much better solutions available.


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 Post subject: Re: prehenite?
PostPosted: Wed Jul 15, 2020 2:46 pm 
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I agree. I just suggested to be careful with it until you know what it is. Clearly you need a Raman or someone with a Raman to give you a quick ID.

Don't know why it gives me heebee geebees. But it does.


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