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

Authors: Fischel, Matthew; CLARKE, CATHY - Stellenbosch University; SPARKS, DONALD - University Of Delaware

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2022
Publication Date: 1/3/2023
Citation: Fischel, M.H., Clarke, C.E., Sparks, D.L. 2023. Synchrotron resolved microscale and bulk mineralogy in manganese-rich soils and associated pedogenic concretions. Geoderma. 430: Article e116305. https://doi.org/10.1016/j.geoderma.2022.116305.
DOI: https://doi.org/10.1016/j.geoderma.2022.116305

Interpretive Summary: 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.

Technical Abstract: Manganese-oxides are one of nature’s strongest sorbents and oxidants. These minerals are ubiquitous as crusts, coatings, and nodules in soils and aquatic systems where they can alter contaminant and nutrient fate and transport. Because manganese-oxides often occur in trace amounts and are amorphous, not much is known about their influence on the physical and chemical properties of soils or how they persist in diverse environments. We collected soils enriched in pedogenic manganese-oxides from Graskop, South Africa to determine the mineralogy and properties of naturally occurring manganese phases. Soils representing a range of manganese concentrations and nodule abundance are characterized. The complex mineralogic gradient inside concretions is elucidated with synchrotron µX-ray diffraction paired with µX-ray fluorescence. These spectroscopic techniques are combined with sequential extractions, surface area measurements, point of zero charge, particle size fractionation, cation exchange capacity, and scanning electron microscopy to characterize the soils and mineral phases. This analysis determined the high manganese content of these soils, ranging from 52 to 144 g kg-1. The nodules are enriched in manganese at the expense of the solum and contain up to 130 g kg-1. Synchrotron X-ray fluorescence mapping shows critical insight into the mechanisms stabilizing manganese and iron in these acidic yet carbonate rich soils. Manganese and calcium are found in a consistent ratio in the solum and nodules, and calcium can be important for manganese persistence and nodule aggregation/flocculation. Similarly, silicon and iron distribution are strongly correlated, and silica enhances iron stability through alteration of crystalline structure and cementation of mineral surfaces. These interactions help stabilize manganese and iron in extensively weathered acidic soils where aluminum is highly active. The mineralogy of the soils reinforces this weathering and aluminum integration that transforms birnessite into lithiophorite and then gibbsite. Additionally, the µX-ray diffraction elucidates the mineralogical gradient across a concretion transect for birnessite and dolomite in the outer regions and lithiophorite, spinel, and periclase within the center. It also indicates mineral phases obscured in the bulk X-ray diffraction like akhtenskite and albandite. These results provide evidence to quantify the chemical and physical properties of pedogenic manganese-oxides. We can begin understanding their importance in geochemical nutrient and contaminant cycling and the mechanisms that control their stability and reactivity in soils. Harnessing the unique properties of pedogenic metal-oxides is essential for soil remediation and carbon sequestration alike.