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Rounded yellow-green crystals in calcite
  
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John Betts




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PostPosted: Mar 27, 2020 15:42    Post subject: Rounded yellow-green crystals in calcite  

I acquired an unlabeled specimen from a collection assembled around 1985-1995 that contained this specimen of translucent gray calcite with rounded yellow to yellow-green "blebs" of an unidentified mineral with an approximate hardness of <8>6.

The collector though he got it from Tanzania, but that cannot be counted on, given the time elapsed.

Any ideas that the yellow mineral might be?



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John Betts




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PostPosted: Mar 27, 2020 15:43    Post subject: Re: Rounded yellow-green crystals in calcite  

Another photo.


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Jordi Fabre
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PostPosted: Mar 27, 2020 17:28    Post subject: Re: Rounded yellow-green crystals in calcite  

Seems to be Prehnite on Calcite from Merelani


Prehnite - Merelani Hills_Tanzania.jpg
 Mineral: Prehnite with Calcite
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Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania
 Dimensions: Specimen size: 4.7 × 4.7 × 4.2 cm
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Peter Megaw
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PostPosted: Mar 27, 2020 19:12    Post subject: Re: Rounded yellow-green crystals in calcite  

I was going to suggest a poorly formed calc silicate in a skarn. A needle will tell!
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Pete Richards
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PostPosted: Mar 27, 2020 19:38    Post subject: Re: Rounded yellow-green crystals in calcite  

I agree with Peter. Could even be a piece of a carbonatite. Suggestive somewhat of Crestmore, CA, or Oka, Quebec, or some places in upstate New York or the Grenville Province in Ontario/Quebec. Well-cooked limestone.
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John Betts




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PostPosted: Mar 27, 2020 19:39    Post subject: Re: Rounded yellow-green crystals in calcite  

Jordi Fabre wrote:
Seems to be Prehnite on Calcite from Merelani


Thanks Jordi. That was actually my first thought, except there is no sign of bladed structure.

They look like the "melted" appearance found in spinel crystals from Mogok or Vietnam.

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PostPosted: Mar 27, 2020 19:41    Post subject: Re: Rounded yellow-green crystals in calcite  

I should have mentioned, there is no reaction to HCL which rules out most carbonates.
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Bob Carnein




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PostPosted: Mar 28, 2020 10:37    Post subject: Re: Rounded yellow-green crystals in calcite  

The color looks right for Merelani prehnite, but the mode of occurrence and general appearance remind me of an iron-poor member of the humite series. I can't say I've ever seen a member of that series that's such a bright yellow color.
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lluis




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PostPosted: Mar 28, 2020 12:18    Post subject: Re: Rounded yellow-green crystals in calcite  

Hi, all

As Bob said, it looks to me not as a prehnite from Merelani... When I saw it first, looked me more as a chondrodite or any related silicate. Yes,we all are thinking in silicates, but which one?...

An analysis would be magnificent!

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