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Formula: (Na,Ca,K)0.6(Mn4+,Mn3+)2O4·1.5H2O
Multiple oxide, birnessite group, manganese-bearing mineral
Crystal System: Monoclinic
Specific gravity: 3 measured, 3.4 calculated
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
Environments
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
manganese-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).
Localities
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 manganese-bearing dolomite
(Dana),
in small discontinuous lenses or beds in metamorphosed sedimentary rocks, chiefly
mica schist
(AM 45.871-875); another associated mineral is 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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