Observation of manganese nodules and encrustations in various lakes of Finland

The limnic ferromanganese ore concretions in some Finnish lakes are described. Their chemical and mineral compositions have been measured as have their natural surroundings — the latter by means of physico-chemical in-situ analysis. The sources of the nodules' contents are discussed, and a theory based on the calculated precipitation fields of the important ore minerals is presented for the ore formation.