In abandoned mines in the Harz Mountains, peculiar corrosion phenomena can be observed on steel structures (pipes, steel struts, rails) whose cm-sized structures are reminiscent of closed clam shells. They have an internal cavity filled with an aqueous solution and do not contain any macroscopic organisms. The only known scientific description of these ferruginous shelled structures (FSS) comes from a lead-zinc mine in the Pyrenees (Fernández-Remolar et al., 2015).
The samples were investigated by electron microprobe and Raman spectroscopy. The shells can be subdivided into three layers, an inner layer of spherical crystallised goethite, a central layer of laminated magnetite followed by a layer of goethite and lepidochrochite. The outer two layers (approximately 1 mm thick) are composed of numerous lamellar of approximately 5-10 µm thickness. The spherical crystallisations of the inner layer grow into the aqueous filling. Areas containing magnetite correlate with decreased organic carbon. The highest organic carbon and sulphur in the innermost layer, indicate microbial activity such as a sulphur metabolism.
Microbial formation of the FSS, as described in the literature, could explain the preferred direction of growth direction of the crystallisations and the zoning. The organisms could derive energy from iron oxidation, leading to biomineralisation. The presence of magnetite can be attributed to the biological reduction of iron(III) oxides, as it occurs only in the central layer, where it is isolated from atmospheric oxygen and the aqueous filling.
Fernández-Remolar DC et al. 2015, Formation of iron-rich shelled structures by microbial communities. J Geophy Res, Biogeosci., 10.1002/2014JG002745.