Asbestiform sepiolite wrapped by aliphatic hydrocarbons from Perletoa, Aosta Valley (Western Alps, Italy): characterization, genesis and possible hazards

An atypical asbestiform sepiolite was found in the Gressoney Valley (Italian Western Alps), with exceptionally long fibres coated by an aliphatic hydrocarbons sheath – an association never reported before in literature. This sepiolite was characterized with a multi-analytical approach with the aim to infer its genesis, the role of its organic coating and its potential noxiousness for human health. Microscopic and FT-IR analyses proved that these fibres, apparently up to several cm long, are formed by bundles of thinner fibrils (average length: 150 m) potentially dispersible in the environment. When observed with TEM, these fibrils show a rhomboidal to parallelogram cross section (< 1 m) with surfaces covered at most by a thin film of aliphatic hydrocarbons. The sepiolite fibrils and their organic coating originated in sequential steps from precipitation of Si/Mg rich hydrothermal fluids, resulting from serpentinization of olivine and clinopyroxene and Fischer-Tropsch-type reaction in the Combin Zone (Aosta Valley). Presence of a hydrocarbons sheath implies serious consequences on the sepiolite habit: the organic matter, in fact, interacts with the fibrils surface reducing the amount of adsorbed water and favouring fragmentation of thicker units into thinner ones, due to an ‘opening’ process implying separation along z and cleavage on (110). Similar mechanisms were observed in several sections of amphibole asbestos and fibrous serpentine wrapped by organic matter. This progressive ‘defibrillation’ of the Perletoa sepiolite, triggered by the hydrocarbons sheath, not only causes a significant increase in the interfibrillar (open) porosity but also affects the fibre morphology enhancing its aspect ratio (length vs. thickness) from ‘high’ to ‘very high’. The thinner and exceptionally long fibrils (rods and/or laths), therefore, potentially become more dangerous for human health due to their carcinogenic potential if dispersed in air and breathed in high doses.