In my petrographic analyses of (mostly) carbonate thin sections I consistently see a black, opaque (or vary slightly transparent - red/orange) mineral either disseminated throughout the sample or as discrete aggregates of crystals. This usually accompanied by a brown-orange mineral. The black mineral is not reflective and is therefore probably not pyrite. I had thought that the granular/framboidal texture suggested potentially hematite (?). The orange mineral may a product of the oxidisation of the black mineral, and might be siderite considering the carbonate setting.
I presume these minerals are secondary, and I am curious about where they are coming from and what implications they might have for diagenetic/recrystallisation processes. Am keen to hear any suggestions! Or if anyone knows of any papers dealing with something similar, I'd love to hear about them.
Dear Marissa,
In my opinion black opaque mineral grains belong to early diagenetic pyrite. From the photos I can see the crystal cross sections that belong to {100} cube and {110} octaedron and I did not observe any shapes that would resemble {101} crystals of siderite. Brownish-redish minerals are filling in the fractures and porosity of the rock. Fractures are a good way to get oxygenated water into the rock body, causing oxydation of sulphide minerals - in this case oxydation of pyrite to limonite (?).
Do you know geological context of the rock? Environment of sedimentation, thermal history, …
Best regards,
A
Thanks Ales!
Yes, I think the brownish mineral is probably limonite! The rocks are mostly carbonates (early Cambrian) from the Flinders Ranges in South Australia (shallow stromatolite accumulations through to reef facies and also carbonate platform and slope facies). I see these brown and black minerals in pretty much every thin section we have made!
The rocks are are often rich in shelly fossils and archaeocyaths. The ranges are extensively folded, but have not been heated to a high grade (in most places). Though there is sometimes evidence for recrystallisation of micrite to calcite spar. And there are a few areas that have been extensively hydrothermally metamorphosed (but not proximal to our sections). As for the black mineral, yes pyrite is possible. I think though, that it has probably been oxidised and is now difficult to ID.
Thanks so much for your answer, I really appreciate it!
Marissa
I agree with Ales, what you have is pseudomorphed pyrite altered to lemonite/hematite
Pyrite is very common in that kind of environments (carbonate & clastic), where you have enough organic matter to locally establish suitable Eh conditions for sulphide crystalization. However, formation of pyrite is complicated and not well resolved.
If you are interested in simple further reading about pyrite formation from geobiological point of view I recommend following material:
Juergen Schieber 2011: Iron sulphider formation. Encyclopedia of Geobiology, Springer Verlag, J. Reitner & V. Thiel (Eds.), p. 486-502.
If you have further questions feel free to ask.
Regards,
Aleš
Thanks Ales! I've got myself a copy of that reference now, it is most helpful! I am also quite interested in the production of minerals like limonite etc. I presume oxygenated waters must have moved through the rock at some point and reacted with the pyrite to produce them. Thanks again for your help!
Cheers,
Marissa