Micromorphology, chemistry, and mineralogy of bog iron ores from Poland

The influence of different land use history on the chemistry, mineralogy, and micromorphology of bog iron ores was studied in the vicinity of Warsaw, Poland. At three Holocene terraces of the Vistula River, eight profiles were chosen, which represent three different types of bog iron ore: (i) continuous hard layers (meadow, fallow at Wilanow), (ii) horizons with small ore fragments (arable land at Northern Praga), and (iii) soft bog iron ore (exploitation area at Brwinow). Chemical analyses of all bog iron horizons show a high content of Fed (up to 51%) with a Feo/Fed ratio from 0.19 to 0.58, indicating a mixture of crystalline iron and amorphous iron compounds. X-ray diffraction (XRD) analyses confirm the presence of ferrihydrite and goethite. The micromorphology of the bog ore at Wilanow shows a porous (vugh and channel) microstructure. The groundmass consists mainly of fine iron hydroxides. Compound coatings of radial distributed goethite needles and isotropic (“amorphous”) iron hydroxides line the pores. In many cases, several coatings of isotropic iron hydroxides, differing in colour intensity, fill the pores. Ploughing and draining at Northern Praga changed the continuous bog iron ore into small fragments embedded into an aggregate structure. It comprises of amorphous as well as crystalline iron compounds (large number of coatings of radial fibrous goethite in pores and channels). In some cases, a single pore can contain several generations of crystalline goethite. The higher crystallinity of the iron oxides fits very well to the lower Feo/Fed ratios compared to Wilanow. The differences can be attributed to the drainage, subsequent lower groundwater level, and better aeration, respectively, at Northern Praga. No hard bog iron ore is present at Brwinow because of exploitation, sporadically fragments of hard bog ore may be encountered in the topsoil. However, soft bog ore is still developing in subsoils. It shows a vughs microstructure. Besides goethite, manganese impregnations on the crystalline forms of goethite, and isotropic iron compounds, its groundmass also contains vivianite as well as siderite. The latter can be attributed to a more reductive environment.