The mineralogy and chemistry of manganese in some Australian soils

The mineralogical and chemical compositions of 28 manganese nodules from Australian soils were studied. Specific manganese minerals were identified in 26 of these after extraction and concentration from the associated soil minerals; 10 were lithiophorite Li2A18 (Mn2+, Co, Ni)2, Mn4+10O35.14H2O; 10 were birnessite, (Ca, Mg, Na2, K2)x Mn4+Mn2+(O, OH)2; 3 were hollandite, Ba(Mn4+, Fe3+)8O16; 1 was todorokite (Mn2+, Mg, Ca)Mn4+6O13.3-4H2O; 1 was pyrolusite MnO2; 1 contained both lithiophorite and hollandite. With the possible exception of pyrolusite, all samples were found to contain varying amounts of Ni, Co, Mg, Ba, Al, K, Na, Fe, and Ca. Although there was overlap in the chemistry of the birnessite and lithiophorite groups, the mean concentrations of aluminium, iron, and lithium were lower and the mean concentration of calcium and magnesium higher in the former group. Pure manganese oxides and hydroxides appear to be quite rare as secondary soil minerals. Lithiophorite occurred mainly in neutral to acid subsurface soils, whereas birnessite, although found in both acid and alkaline soils, was more common in alkaline surface horizons. The average crystallite size of the manganese mineral aggregates was quite small, being of the order of 0.02 µm for birnessites and 0.1 µm for the lithiophorites.