Some Iron mineral or possibly a pollutant?

I am not a microbiologist, but have you considered that could be bacteria?...In any case, a XRF analysis could be useful to track the elemental composition of those dark areas

Hi

I think this is pyrite that transformed to iron oxides after oxidation

Thanks everyone.

I like the food waste theory, and if I get a chance I'll throw some forams under the Raman and check for the pyrite/Fe-oxides too.

Using EDX analysis with the SEM the black material corresponds to carbon-enriched areas, relative to the surrounding calcitic tests, which coat the insides of chamber walls. An image of a broken Ammonia is attached and shows the coating on the inner regions of the tests and pores.