Micrograph of the Month: Ferrous phosphate (vivianite)

This month we have a lovely image of a particularly colourful mineral feature called vivianite. Vivianite is an amorphous hydrated ferrous (iron) phosphate, which is blue when oxidised. It has been linked to the decomposition of bone and/or human and animal waste in wet sediments, which fits with what we know about this context. Yes, we are still looking at those waterlogged deposits from medieval Riga! This is just one example of the many occurrences of vivianite in this layer. The small fragment of wood is also interesting. Unlike the waterlogged wood we looked at a couple of months ago, this fragment has been completely mineralised due to the presence of phosphate in the waterlogged sediment - at first glance I thought it was a bit of bone! You can also see that the pores spaces within the wood have been completely filled with fine sediments, which also indicates waterlogged conditions - the infilling of the voids occurs as fine sediments suspended in water are deposited within void spaces. All in all a very convincing set of features showing that these deposits have remained waterlogged sine their deposition. The combination of bone and wood fragments, along with human/animal waste and mineralisation of organic components, suggests we may have some sort of cesspit/midden in this part of the site.