Hull Geological Society
Mike Horne FGS
This is unfinished work that has not been edited or peer reviewed by the Society.
Manganese coatings on Chalk
A while ago I collected a sample of Chalk and unintentionally left it in a damp sealed plastic bag. I found it a few years later and observed that a black coating has appeared on the surface of that sample. I assumed that this was caused by bacteria. Then I wondered if the black and grey coatings on that develop on Chalk and limestone scree fragments have a similar origin. These coatings on limestones are generally thought to be a thin layer of manganese and are sometimes dendritic.
To investigate further I collected some pieces of Chalk screen from a disused quarry and asked my friend Bob Knight to analyse them using ICP-ES. I prepared four samples - two from the inside of two clasts and two crudely from the outer part of those clasts.
The results showed that the outer layers were enhanced in manganese. Unfortunately the full geochemical results are no longer available due t the University of Hull IT department turning off my access to the file without notice in September 2021.
So I was wrong in doubting the general wisdom. However it does raise some further questions. Where does the manganese coating come from? Is there a bacterial process that draws it from the rock to the surface?
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