Secondary ferromanganese microconcretions in Proterozoic sandstones from Norway.

Ferromanganese microconcretions (nodules, crusts and coatings) were found as reddish dark brown spots in dark grey and black sandstones of the Brottum formation (Upper Proterozoic; the Sparagmite region of southern Norway). The microconcentrations have been precipitated in vugs from percolating ground-water containing Mn²⁺ and Fe²⁺ which were leached from biotite and chlorite of the host-rocks. The surface texture of the microconcretions may be smooth (Mn/Fe ratio approx 0.8), reticulated (Mn/Fe ratio approx 2) or made of crystal boxwork (Mn/Fe approx 4). XRD studies indicate the presence of mainly todorokite, birnessite and goethite. (Author's abstract)-E.v.P.