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Acid mine drainage processes occur when unaltered sulfide mineralisations bodies and sulfide-rich waste materials are continuously exposed to the weathering agents and acid production exceeds the buffering capacity of the of host rocks or the surrounding soils (Jambor & Blowes, 1994; Jambor 2003). This causes the generation of acidic sulfate waters, release of heavy metals and other potentially toxic elements, and formation of large quantities of secondary minerals. Precipitates of Fe3+, Al, and sometimes Mn, take place in a wide range of physico-chemical conditions, varying from strong acidic to slightly alkaline pH and T (Murad & Rojík, 2003) and provide a highly visible means of identifying mine-impacted waters. As a conseguence, a large amounts of ochreous- to reddish- and milky-white-materials are accumulated as unconsolidated muds, within the stream beds (stream sediments), or as thick (millimetric/centimetric) crusts, or as efflorescent blooms, on the surface of the waste-rock piles.
In Libiola Mine (Fe-Cu sulfide ore) there are numerous evidences that the AMD processes are active (Marescotti & Carbone 2003, Carbone et al., 2005) and acid sulfate waters (Dinelli & Tateo 2002; Marini et .,al 2003) are produced mostly by oxidation of pyrite and chalcopyrite processes that occur either within the underground excavations and within waste rock piles (Marescotti et al., 2007). This causes a production of large quantities of stream sediments that occur both as soft crusts inside the mine adits and as loose suspensions associated with overland flow of mine drainage.
The aim of this study is to describe the formation, the mineralogical properties, and the microstructural features of the nano and microphase Fe-oxyhydroxides and Fe, Al, and Cu hydroxysulfates present in mine adits and effluents in the Libiola mine area.
The samples, distinguished in ochre (10YR 5/6), milky-white (Gley 1 8/1), and greenish-blu (Gley 2 7/2) precipitates, have been investigated through diffraction powder analyses (XRPD) and transmission electron microscopy (TEM); moreover, for every sampling site, pH, Eh and temperature were measured on-site, whereas chemical determinations of sediments were carried out by ICP-MS analysis. Since these chemical precipitates are environmentally significant because they 1) add to the suspended sediment and bed load of receiving streams 2) decrease the effective life of treatment systems (wetlands and limestone drains) and 3) play a major role in the binding and transport of toxic elements, the complete mineralogical characterisation of these natural mineral assemblages is necessary to establish and to predict the fate of the toxic metals dissolved in the circulating solutions.
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