Anglesite (PbSO4) epitactic overgrowths and substrate-induced twinning on anhydrite(CaSO4) cleavage surfaces

Abstract

The dissolution–precipitation reactions occurring on mineral surfaces are considered an effective means to remove high amounts of dissolved toxic metals from fluid phases by precipitation of an ewsolidin whose structures the pollutant is immobilized. However, for the sereactionstobeefficient acontinuous communication between the fluid and mineral phasesis required. This communication can be significantly hindered when the new solid phase forms a continuous, homogeneous layer on the mineral substrate. The formation of such a layer is more likely to occur when epitactic relationships exist between the newly for medphase and the mineral substrate. Dissolved lead can be removed by interaction of the Pb-bearing aqueous solution with anhydrite (CaSO4) leading to the formation of anglesite (PbSO4). Here, the relationship between anglesite crystals and ananhydrite substrate is investigated. The three main anhydrite cleavage surfaces, (100),(010), and (001) are considered. Ahigh density of riented anglesite crystals growing with their (210) plane parallel to the substrate was observed on (001) Anh. The density of oriented an glesite was, however, significantly lower on (100) Anh and almost negligible on (010) Anh. Anglesite grew with its (001) plane parallel to the substrate on (100) Anh, but no evident epitactic relationship was found with (010) Anh. Differences in anglesite crystal density are explaine don the ground sof structural similar ities and the goodness of anglesite–anhydrite matching through the interface. Both on anhydrite (100) and (001) anglesite crystals showat least two specific orientations, which relate to each other through symmetry operators presentin the substrate. The coalescence of differently oriented anglesite crystals leads to the formation of twins. This twinning phenomen on is designated as substrate-induced twinning, since the twinning lawis determined by the substrate symmetry. In the particular case of anglesite growing on anhydrite (100) two different orientations have been observed, related eacho ther through (210) Ang which actas twin plane.