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Thin section staining

Posted by Abrag  
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?
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??
Alfred L. Ostrander December 07, 2017 01:48PM
Absurd? Carbonate staining by various methods has long been known in petrology.
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.
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.
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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