Mineralogy drives physical and chemical properties in the manganiferous soils and concretions of Graskop, South Africa

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. Mineralogy drives physical and chemical properties in the manganiferous soils and concretions of Graskop, South Africa. Geoderma. 430:116305. https://doi.org/10.1016/j.geoderma.2022.116305.
DOI: https://doi.org/10.1016/j.geoderma.2022.116305

Interpretive Summary: Manganese-oxide minerals are common in soils and aquatic systems where they alter contaminant and nutrient properties and mobility. However, these minerals are difficult to identify because of their low concentration and lack of definite structure, limiting our ability to quantify their environmental impact. We characterized soils and nodules with naturally elevated manganese-oxides on the macro and micro scales to determine the phases present and how this will alter soil properties. We found multiple manganese and iron minerals in the soils and nodules at the bulk and micro scales and proposed novel mechanisms for their stability and formation in these acidic soils and within the nodules. These findings provide insight into the metal-oxide minerals present in soils and their impact on fundamental properties such as pH, soil organic matter content, and nutrient availability. This information aids scientists in understanding contaminant and nutrient mobility in agricultural and industrial systems and how manganese-oxides in soils influence management goals.

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.