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A study was made of the mineral and chemical composition of a concentric precipitate formed around a ground-water discharge, where manganese had precipitated before iron. Birnessite was identified in the Mn-rich precipitate and goethite and lepidocrocite in the Ferich precipitate. Lepidocrocite is typically crystallized in hydromorphic soils where Fe is reduced. Ti and Cr are enriched more in the Fe-rich than in the Mn-rich precipitate, while the reverse is true for Ni and Cu. The K/Rb ratio is higher (790—3000) than in common rock types (160—300). This indicates the incorporation of K into the minerals formed, whereas Rb is withheld in the precipitate by adsorption. The total content of REE is increased in the precipitate studied but is lower than in oceanic Mn nodules. The La/Yb ratio is higher than usual in continental rock types, which indicates slower migration of the light REE (La — Sm) than of the heavy (Gd—-Lu). Compared with the other REE, Ce is enriched in many continental oxidates, though not in the young oxidates studied here. Probably because Ce is oxidized to valence 4 + , it is chemically and through adsorption bound more strongly to precipitates than the other REE with valence 3 + and enriched upon partial dissolution. This suggests that the trace element composition of Mn and Fe precipitates largely comes about through the partial dissolution of the original precipitate.

Case study of a manganese and iron precipitate in a groundwater discharge in Somero, southwestern Finland