The Libiola Fe-Cu-sulphide mine, near Sestri Levante (eastern Liguria), represents one of the most extensively exploited sulphide deposits in Italy. In this area, active Acid Rock Drainage (ARD) processes are evident. The major resulting mineral phases are Fe oxides and oxyhydroxides, occurring in varicoloured crusts on the surface of waste rocks and in unconsolidated muds.
In this study, the Fe assemblages of the waste rock were investigated by microchemical (SEM), structural (XRD), microstructural (TEM) and spectroscopic (DRS, IR, μ-Raman) techniques, in order to determine the phase composition, the textural relations among the minerals and their genetic evolution. They are characterized by intimate intergrowths of hematite and goethite with minor quartz and lepidocrocite; in some samples, the presence of very minor schwertmannite was detected.
TEM and HR-TEM observations revealed that hematite is present within pseudo-elliptical bodies as pseudo-hexagonal to subrounded nanocrystalline lamellae (from 18.9 to 26.5 nm in diameter), whereas goethite occurs either as parallel intergrowths of acicular crystals (from 10 to 16.3 μm in length) or as sheaf-like assemblages. On the basis of the present data, the studied Fe oxide and oxyhydroxide assemblages are found to represent distinct spatial and temporal stages of a nano-scale evolution process.