Manganese Micrograph Mystery!

What a treat for you this month, another set of micrographs and it's only the end of the first week of the month! Actually, I am posting this because I need a bit of help - here we have an image of dung deposits in plane (PPL) and cross polarised light (XPL). You can see in the upper image, the layered appearance associated with ruminant dung, but we also have a lot of black 'speckling'. This appears to be concentrated in bands throughout the deposit. When we look at the deposit under XPL it turns out that these speckles are 'stains' on the dung spherulites! The only thing I have seen like this before was called microbial/fungal or manganese staining, but I can't find the reference (or it may have been something that was mentioned to me in passing?). Anyone got any insights/references? It's probably not vital to the interpretation but it could help understand the post-depositional processes going on here, and it is quite fascinating in itself (well, maybe just if you're a micromorphologist...). When I get round to publishing this any help will will fully acknowledged!