Morphological, chemical and mineralogical properties of some manganese-rich oxisols derived from dolomite in Mpumalanga province, South Africa

We investigated the morphological, chemical and mineralogical properties of unusual black oxisols, rich in Mn oxides derived from intense weathering of dolomite, near Graskop in Mpumalanga Province, South Africa. Soil mapping of a small catchment was carried out and seven horizons from two profiles were sampled for detailed analysis. A consistent pattern was evident in which black, lower subsurface horizons with weak nodule development grade upwards into redder or browner horizons containing abundant pisolitic, occasionally magnetic, nodules. Surface horizons are humus-rich (> 3% C) and acidic (50–60% acid saturation of effective CEC). Subsurface horizons have low ECEC (< 6 mmolc kg− 1), net positive charge, negligible acid saturation, and CEC in pH 7 NH4OAc between 8 and 14 mmolc kg− 1. The clay fraction is dominated by lithiophorite (Al-interlayered Mn oxide) associated with gibbsite and smaller amounts of kaolinite, chlorite, quartz, mica and iron oxides (hematite, goethite and maghemite). Despite a more than 3 times enrichment of Mn in the nodules, relative to the > 2 mm fraction, only very weak lithiophorite peaks could be identified in x-ray powder diffractograms of the nodular material. Aluminium, in the form of gibbsite, was slightly enriched in the nodules but not Fe. It is proposed that the nodules have formed via the reprecipitation of Mn, reductively dissolved from the fine earth fraction during periods of low redox potential. We envisage that this has given rise to progressive sequestration of Mn in hard nodules and removal of the black matrix colour, unveiling the colours of iron oxides and humus. The presence of Al in lithiophorite, a higher point of zero charge than for other Mn oxides and redox inertia associated with the exceptionally large concentrations of Mn (ranging from about 10 to 18 % MnO2 in the whole soil including nodules), are all thought to contribute to the accumulation and persistence of Mn in these soils. Some trace metals showed marked enrichment in the nodules (up to 800, 500 and 1000 mg kg− 1 of Co, Cu, and Ni, respectively).