Hi all,
I am hoping to get some help with identifying the particles in the attached images. All photos were taking in polarized light, scales should be on the images.
A bit of context: the thin sections are of a speleothem sample from the south coast of South Africa. The cave is a sea cave in a granite host rock. The source carbonates for the speleothem are calcretes and aeolianites on top of the granite.
Most of the sample is Calcite with a microcrystalline to micritic fabric. Besides the particles in question, there is a lot of fine sand-sized quartz in the sample. I am suspecting that the cave was partly open when this speleothem formed (ages are Holocene 7.8 ka- 0.7 ka BP). The sand may have been washed in by high waves from the beach and the particles could originate from within the cave, host rock, or from the beach/shoreline environments. The color made me think it was maybe something organic, but It doesn't have any cell structures as wood or other plant materials would have. It also doesn't seem to be bone. I would be grateful for any help.
Kerstin
Dear Kerstin,
your micrographs are in XPL, right? Phosphate is an option, but should be isotropic in XPL (unless is recrystallized phosphate)
You can check Pas Damien's suggestion using BL to verify if your 'object' shows autofluorescence. If it doesn't, it might be a textural pedofeature; is there any evidence of clay \ silt migration at the site?
Best,
Diego
Thanks a lot Diego, with migration, do you mean any kind of sediment transport? This cave is very close to a beach and I think it was open, I don't think there is much clay there since it is a high energy environment, but I have quite a lot of fine-sand sized rounded quartz grains embedded in the carbonate. The caves in this region are usually very small (max of several 10s m long) cut into the coastal cliffs along fault lines. I have not been to this cave, so I am not sure, but the back of the site could have some clay deposits in it.
Kenneth, I thought at first these might be bone. I sent the images to a colleague who is a specialist in micromorphology of archaeological cave deposits and he thinks these are not bones since they don't seem to have osteon structures. Please let me know if you disagree.
Dear Kerstin,
I mean clay translocation as it happens in soils or caves, due to leaching, water circulation through voids etc.
There are many papers and handbooks on clay migration in the soil science literature. Re micromorphology you can check:
Stoops, Marcelino and Mees (eds.) 2010. Interpretation of Micromorphological Features of Soils and Regoliths. Elsevier
or even:
Nicosia and Stoops (eds.) 2017. Archaeological soil and sediment micromorphology. Wiley Blackwell.
And the fundamental handbook by Stoops 2003, which I guess you already know.
Best,
Diego
Daar Kerst in,
Not sure about the clay. The extinction pattern does not seem to match. I woud hazard they may be Ca, Fe(III) phosphates. They are not always fluorescent, so you may need to check wit SEM-EDX, micro-XRF or similar techniques.
All the best,
Hans
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