Conversations with the Earth

Friday, February 10, 2017

And after all, it;s just a carbonate

1. Okay, today let's take a look of some polish-thin section using microscope.
2. Wow, this sample is colourful. Let's take some photograph of it.
3. Time to describe, what is inside. Carbonate? absolutely.... we can't missed from the birefringence color. 
4. Now, what else to describe. Habit? Fibrous, banding? I though fibrous is more correct.
5. What kind of carbonate is it? I think it's calcite, because usually calcite (CaCO3) has fibrous habit. Let's write calcite. (see explanation below this bullshit)

6. But, how about the red-brownish thin veinlet? I don't think it's homogenous. Let's coat it using carbon and identify using SEM (scanning electron microscope).
7. Vacuum is ready, let's find those spot with fibrous calcite.
8. Wow, cool... finally i found the spot. Lte's analyse using EDS (energy dispersive x-ray spectroscopy)
9. Dammmn, it's not even calcite. It's dolomite-ankerite (Ca,Mg(CO3)2). See, Ca-Mg are bright. Fe is not that bright. Now, we know, the red-brownish veinlet is siderite (FeCO3), and the fibrous is not calcite. It's dolomite-ankerite...

10. Yes, and then, what does it mean? Let's make some story...
11. Mg in dolomite. Hmmm...where does the magnesium come from? Let's search some literature. 
This finding is of significance as earlier reports of Phanerozoic radiaxial fibrous carbonates are exclusively of calcite mineralogy. Dolomite concretions described here formed beneath marine transgressive intervals within palustrine coal seams. This is of significance as seawater was arguably the main source of Mg2 + ions for dolomite formation. (literature somewhere )
12. Yes, correct... okay, don't forget, that most of sample were altered into dolomite. So, probably it is dolomitization (dolomitization: alteration of limestone into dolomite)
13. Woooow, I spend almost an hour only to describe one spot for one thin section............
14. Wooooooooow, I still have another samples to identify.....................
15. Wooooooooooooow, someone just learn mineralogy...... congratulatioooooon...
16. And after all, it's just a carbonate

explanation number 5: 
limestone does not always classified as calcite. Other form can be dolomite, ankerite, siderite, magnesite, or rhodochrosite. See ternary diagram below)

explanation point 9: 
the brighter the phase, means, the higher it's density, and the more abundant it's element. In this picture, only, Ca-Mg-O and minor Fe were detected. So, we can eliminate "rhodocrosite (MnCO3) and calcite (CaCO3). Now, see thin veinlet of Fe in the middle of fibrous. It is dark. It means, the thin veinlet should contain more Fe, and now, we understand, it is siderite)

suggested references:
-Microscopic description of veins

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