Wednesday, February 3, 2010

Ooids and De-Dolomite

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Today in the lab I finally finished the thin-sections that I will be investigating as part of my Diploma thesis. I gave them their final touch with a mesh 1200 powder. There is a simple polarisation microscope in the lab allow a quick quality check. I took an ordinary digital camera and held it to the eye-piece. These are some selected results of that. The thin-sections were not yet cleaned, occasional dark spots are left-over polishing powder. No deeper interpretation, yet, I have still to begin with that.

 An Oolite in thin-section at low-magnification. Lots of ooids within sparitic cement.

 An ooid in high-magnification.

 
Blue-dyed epoxy has filled the open porosity. Here, rhomboidal dolomite has been dissolved. Undissolved examples directly above.

7 comments :

Callan Bentley said...

Beautiful!

Suvrat Kher said...

stain them and then click :)

Lost Geologist said...

Thanks for the comments!

Suvrat:

I am pondering what to stain! Might be pretty pointless to stain for aragonite in Jurassic limestones. I would like to stain for dolomite and either high- or low Mg calcite. I am not so sure what is the better alternative. The dolomite is actually quite rare. I only found rhomboids in 3 out of 48 thin-sections. I need to use a technique that I do not have to repeat once applied because I have time limited access to the lab here. Pondering...

Suvrat Kher said...

since I am not familiar with the history of these rocks I am guessing here...

if dolomite is rare maybe you can concentrate on calcite. Potassium ferrcyanide distinguishes non-ferroan calcite from ferroan calcite (stains blue). and titan yellow stain picks out Mg calcite.

Mg calcite if preserved at all will be likely restricted to either pristine shell material or early marine cements..or inside those lovely ooids :)...which is really a marine precipitate.

later meteoric cements will be low mg calcite.

if you think there is a lot of marine cement....then a stain of titan yellow for those samples (example..ooid facies) will be useful.

If you suspect from basic petrographic relationships and crystal habits that most of the rock mass has been altered then Ferricyanide stains to discriminate generations of diagenetic calcites (burial calcite likely to be ferroan) will be more useful.

exciting...keep me posted..!

Lost Geologist said...

Thanks for the additional hints Suvrat! I've discussed staining with my supervisor and we agree that I will first do the planed microfacies analysis and define microfacies types/associations and then pick one sample per MFT to stain.

I consulted a number of books from Tucker, Fl├╝gel and Scholle but none of them said if Titan yellow and Potassium ferrcyanide can be combined on the same thin-section- Do you have experience with that? If that is not a choice I probably will stain one half of the sections in Potassium ferrcyanide and another in titan yellow.

Suvrat Kher said...

I've combined alizarin red-S and ferricyanide but not combined titan yellow with any other stain.

half thin section for each stain sounds like a better idea.

curious to know if you have access to cathodoluminesence?

Lost Geologist said...

We have CL here in the Mineralogy department but I am not sure if I can get enough time to use it, though I thought about it. At the moment I see no immediatate benefite in CL that would be in a positive cost/result ratio to answer the questions of my thesis.

I will try to perform a simple microprobe analysis on 2 or 3 selected samples to determine the distribution of a certain contaminating element which I assume to be concentrated in the clay fraction or iron-minerals.