My student found this feature in the Cretaceous clayey paleosol. The outer shell is rich in carbon and the inner part is composed of pure clay. As a clay scientist I have no experience with this kind of features and can only guess that it is of biological origin. If someone has experience any suggestion is highly appreciated.
It looks to me like soot coating clay particles. These are common on the Cretaceous- Tertiary boundary the result of the huge forest fires after the meteorites hit the Earth. It is a great finding!
Thanks for a great suggestion. Clayey paleosol (>99 % clay) is associated with Late Aptian-Early Albian regional emersion on the Adriatic carbonate platform. We consider this paleosol seasonally marshy soils (partly or wholly within the zone of water-table fluctuation) to permanently waterlogged soil . We also found glass shards in the fraction > 100 micrometer and kyanite as the dominant heavy mineral in the > 200 micrometer fraction. Very interesting Fe containing mineral phases are also present (pyrite, framboidal pyrite, magnetite,,). I attach a few SEM pictures more (clay feature closer look, glass shards).
Could it be the remains of a bacteria columny grown around a ferruginous clay agregate?
What does the microprobe tell you about the composition? It could be botryoidal hematite or something like that.
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