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Thin section staining

Posted by Abrag  
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Abrag December 07, 2017 08:38AM
Hi All,

While examining a sandstone thin section slide, I came across blue-stained calcite cement. I performed spot EDS analysis on the cement and it gave Ca, C, O, and Mn peaks. Has anyone come across such absurd staining of calcite?
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Ben Grguric December 07, 2017 11:57AM
Yes, there a couple of methods designed to highlight the carbonate content (blue).
It could be a vegetable stain like H7066 powder but another one specifically for calcite is Rodger's Method which uses copper nitrate, distilled water and ammonium chloride solution. I haven't tried it but its possible the amount of Cu in the thin layer of stain is too small to detect with EDS??
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Alfred L. Ostrander December 07, 2017 01:48PM
Absurd? Carbonate staining by various methods has long been known in petrology.
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Paul Brandes December 07, 2017 07:55PM
Alfred L. Ostrander Wrote:
-------------------------------------------------------
> Absurd? Carbonate staining by various methods has
> long been known in petrology.

Agree; what is so absurd about blue staining in a thin section?
Sounds to me like it was stained using Alizarin Red S and Potassium Ferricyanide to check for an iron rich carbonate which, based on the deep blue colour you're seeing, tells me the sandstone is cemented by ferroan calcite.
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Paul Brandes December 08, 2017 12:06AM
One thing I hadn't thought of earlier..... the epoxy itself could be making the pore space between the sandstone grains blue, thus indicating porosity.

A photo of the slide in question might help.
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Ben Grguric December 08, 2017 12:51AM
You can request a thin section maker to use blue-dyed epoxy in order to highlight porosity e.g. in sandstones, but he said he analysed the cement and it gave EDS peaks consistent with a Mn-bearing calcite. Epoxy won't give you this sort of chemistry.
I would have thought if he had an iron-rich carbonate cement in the sandstone his EDS would have yielded an Fe peak.
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