Genesis and characterization of manganiferous soils in the Eastern Piedmont, USA

Manganese is normally a trace element in soils, but it is important due to its effects on soil chemistry and morphology. There are rare, Mn-rich soils in the Eastern Piedmont of the USA that have not been previously studied. The morphology, mineralogy, and chemical and physical properties of these manganiferous soils were documented in order to better understand their pedogenesis. The soils were found to contain as much as 169 g kg−1 Fe and 140 g kg−1 Mn as oxides (dithionite–citrate–bicarbonate extractable). These high levels of Mn oxides impart an extremely dark color to the soil material (moist Munsell value and chroma commonly < 2/1). Manganiferous soil materials have unusual physical properties such as high particle density (up to 3.25 × 10−6 Mg m−3 on bulk soil), low bulk density (as low as 0.39 × 10−6 Mg m−3), and extremely high porosity (maximum calculated 88%). X-Ray Diffraction and Fourier Transform Infrared spectra show that the dominant Fe oxide and Mn oxide minerals in the soils are hematite and lithiophorite, respectively. Samples of the parent marble were dissolved in acid for Fe and Mn analysis and collection of non-carbonate residues (NCR). Based on the quantities of Fe and Mn in the bedrock, the mineralogy of the NCR, and micromorphological observations, it appears that the black, porous, Mn-rich material is derived directly from the dissolution of marble bedrock and accumulation of silicate residues plus Mn and Fe from within the rock. These soils, which appear to have a similar origin to Mn wad deposits, offer a unique opportunity for future studies of soil Mn due to the naturally high concentration of this metal.

► Rare soils with highest known concentration of Mn in the USA, up to 140 g kg−1.
► Black color, highest Mn and Fe, low bulk density, high particle density in subsoil.
► High-Mn residuum is non-carbonate residue left after dissolution of marble bedrock.
► Calcite/dolomite marble under manganiferous soils is source of all Mn and Fe.