Technogenic soils developed on mine spoils containing iron sulphides: Mineral transformations as an indicator of pedogenesis

Abstract

Technogenic soils ( Technosols) containing iron sulphides, developed on mine spoils located in three abandoned industrial sites in Poland (the “Siersza” hard coal mine in Trzebinia, “Staszic” pyrite mine in Rudki, and pyrite mines in Wieściszowice), were investigated in order to identify the most important mineral transformations occurring in the soils. The X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometry (SEM-EDS) techniques, and selective extraction methods were applied in the examination of soil mineralogy. The soils studied represented weakly developed soil formations in which organic matter accumulation increasing with age was observed, along with intense weathering of parent materials containing iron sulphides. The degree of sulphide transformations is age-dependent. In young soils from Trzebinia and Rudki, poorly weathered iron sulphides occurred, whereas in old soils from Wieściszowice, iron sulphides were almost entirely transformed. The main products of sulphide weathering are iron oxides and sulphate minerals (gypsum and jarosite group minerals mainly). Inherited phyllosilicates and pedogenic clay minerals (smectite, vermiculite, kaolinite, and mixed-layer minerals) were present in the soils studied. The development of Al-hydroxy interlayers and the possibility of the formation of poorly crystalline Al phases were also documented. The processes seem to be pH-dependent and occur at pH ranging from 4.3 to 4.4. The development of iron oxides and sulphate minerals in the process of sulphide weathering, as well as phyllosilicate transformations (e.g. the development of swelling clay minerals at the expense of inherited mica and chlorite, and the formation of hydroxy interlayers in clay minerals) are the main mineralogical aspects of pedogenesis of technogenic soils developed on mine spoils containing iron sulphides. The WRB and Soil Taxonomy systems require more precise solutions concerning the classification of technogenic soils investigated. The thionic horizon (WRB) and sulfuric horizon (Soil Taxonomy) definitions, as well as the sulphidic material (WRB) and sulfidic material (Soil Taxonomy) definitions need to be supplemented. The paper suggests the introduction of the Sulfic Udorthents taxon in the Soil Taxonomy, as well as the introduction of “Thionic” suffix qualifier to the key to the Technosols in the WRB.