Microscopic and microchemical study of iron sulphide weathering in a chronosequence of technogenic and natural soils

Abstract Sulphide alterations are among the most important mineralogical aspects of the functioning of soils developed from sulphide-bearing parent materials. In the present study, the processes of iron sulphide weathering were investigated in a system including sulphide crystals and secondary minerals adjacent to oxidised sulphides. Light microscope observations and scanning electron microscope–energy dispersive spectrometry (SEM–EDS) analyses were used in order to examine evidences of iron sulphide decay at a microscopic and submicroscopic scales, as well as to determine the modes and effects of sulphide weathering on an example of a chronosequence of technogenic soils (Technosols) and naturally developed soils (Leptosols and Cambisols) in Poland. The study covered (1) several years old Technosols developed on the dump of the abandoned hard coal mine in Trzebinia town, (2) several dozen of years old Technosols developed on mine spoils of the abandoned pyrite mine in Rudki village, (3) 100–200 years old Technosols developed on dumps of the abandoned pyrite mine in Wieściszowice village, as well as (4) Leptosols and Cambisols developed on natural outcrops of pyrite-bearing schists in the vicinity of the mine in Wieściszowice. Chemical weathering was the most important process involved in the alteration of iron sulphides in the soils studied. The process is expressed by (1) the oxidation of sulphides resulting in a subsequent crystallisation of iron oxides and sulphates from Fe and sulphate ions released to the soil solution, as well as by (2) the development of pseudomorphs after sulphides due to the gradual in-situ transformation of sulphides into secondary minerals (iron oxides mainly). Porous (poorly crystalline) iron oxides predominated across the products of iron sulphide oxidation in “young” technogenic soils from Rudki, in contrast to “old” technogenic and natural soils from Wieściszowice, where massive (well crystallised) iron oxides prevailed. This suggests that the degree of crystallinity of iron oxides increases along with the age of soils. Microstratification was a typical feature of secondary iron oxides occurring in “old” soils from Wieściszowice. The EDS analyses performed along the lines perpendicular to stratified oxides showed that the alternating microlayers differed from each other in terms of contents of oxygen (O-poor layers were alternated with layers rich in this element). This suggests that temporal changes of oxygen concentration in soils may affect the process of iron sulphide transformation. The content of most trace elements (including heavy metals such as Cu, Zn, and Pb) in the examined iron sulphides, analysed by EDS, was low. Furthermore, amounts of these elements in secondary oxide rims were reported to be slightly elevated in comparison with the adjacent sulphides. This suggests that the rims did not constitute places of concentration of high amounts of trace elements in the soils investigated.