A comparative micromorphological and chemical study of “Raseneisenstein” (bog iron ore) and “Ortstein”

A comparative study of Raseneisenstein (bog iron ore, only oxidized type) and Ortstein was performed to develop archetypes of both hardened horizons. Therefore, we conducted chemical analyses on 15 samples of Raseneisenstein and 14 Ortstein samples and compared our results with data reported from literature. Micromorphological analyses were accomplished on six representative, undisturbed samples of each. Raseneisenstein showed a spongy microstructure composed almost exclusively of iron hydroxides, whereas Ortstein is characterised by a bridge microstructure between quartz grains. Distinct differences between the two materials were noted in the chemical composition and morphology of the coatings, as well as the type of organic matter. The only common feature is the microskeleton, which in both cases is composed of quartz. Chemical analyses indicated distinct differences between the hardened horizons as well. Raseneisenstein was characterised by a considerably higher content of redox-sensitive elements (Fe, Mn), phosphorus, as well as a higher pH and lower mobile Al content compared to Ortstein. In addition, Ortstein often showed a higher content of organically bound iron and aluminium. From additional observations regarding relief and groundwater level, we concluded that Ortstein formation is related to podzolization and Raseneisenstein to gleying processes. Archetypes were developed on basis of chemical and micromorphological evidences.