Synchrotron resolved microscale and bulk mineralogy in manganese-rich soils and associated pedogenic concretions

Abstract

Manganese-oxides are one of nature’s strongest sorbents and oxidants which often occur in trace amounts in soils as amorphous coatings and crusts. Thus, not much is known about their microscale mineralogy in soils and concretions. We collected soils enriched in pedogenic manganese-oxides and concretions from Graskop, South Africa to determine the mineralogy of naturally occurring manganese phases from soils with varying degrees of pedogenic alteration. Bulk X-ray diffraction (XRD) demonstrates the dominance of lithiophorite and the presence of todorokite in the less altered wad soil compared to the more pedogenically altered soils enriched in gibbsite and birnessite. The mineralogy inside concretions is elucidated with synchrotron µXRD paired with X-ray fluorescence (XRF). Synchrotron XRF mapping shows critical insight into the mechanisms stabilizing manganese and iron in these dolomite-derived yet acid soils. Manganese and calcium are found in consistent ratios in the solum and nodules, and calcium is important for manganese persistence and nodule aggregation/flocculation. Similarly, silicon and iron distribution are strongly correlated, and silica enhances iron stability by altering the crystalline structure and cementing mineral surfaces. The µXRD elucidates the mineralogical gradient across a concretion transect. With birnessite occurring in the outermost layer and todorokite, gibbsite, lithiophorite, and maghemite becoming more abundant in the concretion middle layers. µXRD also indicates mineral phases obscured in the bulk XRD like anatase and ramsdellite and minerals typically from metamorphic or hydrothermal origin including periclase, wüstite, manganosite, and spinel. These novel results quantify the mineralogy and nanoscale distribution of pedogenic manganese-oxides.