Formula: (Na,Ca,K)0.6(Mn4+,Mn3+)2O4·1.5H2O
Multiple oxide, birnessite group
Specific gravity: 3
Hardness: 1½
Streak: Brown to dark brown
Colour: Black; dark brown in transmitted light
Common impurities: Cl,Co,Cu,Fe,K,Ni,Mg,S,Si

Sedimentary environments
Hydrothermal environments

Birnessite is a major manganese-bearing mineral of many soils, a common alteration product of manganese-rich mineral deposits, a component of bacterially-precipitated manganese oxides, and an important constituent of marine manganese nodules (AM75: 477-489, Dana, Mindat, HOM); it also occurs as an alteration product of Mn-rich ore deposits (Dana). In marine nodules it is associated with manganese and iron oxides and calcium carbonate (HOM). It may be formed by the decomposition of buserite (AM 68.972-980).


At Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Quebec, Canada, birnessite pseudomorphs after serandite have been found (KL p148).

Birnessite has been found in a core from the Gulf of Mexico just south of the northwest tip of Cuba in sediments in 2,244 metres of water. Manganese oxides occur in microclusters in a zone 200 to 400 cm below the top of the core. They are most abundant at the 240 cm level, where the clusters contain mainly birnessite and todorokite; at the 300 cm level and below, the clusters are almost exclusively birnessite (AM 62.278-285).

At the type locality, Birness, Ellon, Aberdeenshire, Scotland, UK, birnessite occurs as concretions in an unconsolidated glacial sedimentary deposit, associated with quartz (Mindat).

At Cummington, MA, USA, birnessite occurs as an oxidation product of rhodonite, rhodochrosite, tephroite, spessartine, alleghanyite and manganoan dolomite (Dana), in small discontinuous lenses or beds in metamorphosed sedimentary rocks, chiefly mica schist (AM 45.871-875); it is associated with all these minerals, and also with cummingtonite (HOM ).

At Sterling Hill, Sussex couny, New Jersey, USA, birnessite has been identified as a secondary mineral associated with chalcophanite as a weathering product of franklinite-willemite ore (AM 45.871-875).

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