Forms of iron in parent rocks of soils developed from Lower Triassic (Buntsandstein) deposits in the NE part of the Holy Cross Mountains (Poland)

The aim of the study was to characterize pedogenic iron forms in the sediments of the Lower Triassic (Buntsandstein) in the north-eastern part of the Holy Cross Mountains. These are sediments of continental origin and unique features as the so-called ‘red beds’. Three main types of rocks were studied – sandstones, mudstones and claystones. Chemical forms of iron: Fe t (XRF), Fe HCl (6M HCl), Fe d (dithionite-citrate-bicarbonate, DCB), Fe o (ammonium oxalate), Fe p (sodium pyrophosphate) were determined. The XRD method revealed the occurrence of goethite in sandstones and hematite in mudstones and claystones. Differential XRD (DXRD) analysis did not show the presence of amorphous iron minerals. The average Fe t content in sandstones was 83 g·kg −1 , in mudstones 47 g·kg −1 , in claystones 55 g·kg −1 , while there were lower concentrations of Fe d (66 g·kg −1 , 13 g·kg −1 , 26 g·kg −1 , respectively). Fe o form content was in the range of 0.3–4 g·kg–1, while there were only traces of Fe p . Only Fe t and Fe HCl occurred in good correlation. In the pedogenic interpretation of the studied rocks, mutual relations between chemical forms of iron were used and their reference to the identified iron minerals. Siltstones and claystones have high resistance to pedogenic factors, which is indicated by low release rates Fe d :Fe t and very low amorphization index Fe o :Fe d . The reason is the presence of well crystalline hematite in these rocks. In sandstones, resistance is conditioned by the occurrence of goethite in the matrix. Chemical destruction of iron-carbonate cement promotes the release and amorphization of iron in soils developed from sandstones. The presented results indicate that the specificity of Lower Triassic rocks is important for the proper classification and assessment of properties of soils with a characteristic red colour.