Origin, mineralogy, and chemistry of manganiferous and ferruginous precipitates found in sand and gravel deposits in Finland

Abstract

Different types of manganiferous and ferruginous precipitates found in glaciofluvial sediments are described. Mn is enriched into places where oxidizing conditions prevail, probably through chemical processes alone. In moist oxidates the enrichment of Mn originally coprecipitated with Fe continues through the partial dissolution of the precipitated Mn-compounds in interparticle fluids. The only Mn-rich mineral identified through X-ray diffraction of manganiferous cements was birnessite. It precipitates from groundwater instead of pure manganese oxides because of the foreign cations present. Of these, Ca seems to be the most important, while the amount of other cations: Fe, Al, Mg, K, and Na, varies, and is dependent on ithe composition of percolating waters and the state of crystallization. Goethite and lepidocrocite were observed in ferruginous precipitates. Upon heating, birnessite recrystallizes as hausmannite and spinel when few foreign ions are present, and also as bixbyite if the sample contains enough iron. Ferruginous precipitates recrystallize as hematite upon heating. The precipitation of Mn-rich compounds from groundwater in which more Fe than Mn is present is tentatively attributed to the lower stability of Mn-complexes — with organic and -HCO3ligands usually — which within higher pH—Eh decompose in sand and gravel before Fe-complexes do. This leads, under suitable conditions, to the oxidation of Mn2+ in groundwater discharges and in well-aerated sediments. Fe precipitates after Mn because it is complex-bound and does not precipitate before the complex is destroyed, e.g., by organisms.