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

An atypical asbestiform sepiolite occurrence with exceptionally long fibres wrapped by a sheath of aliphatic hydrocarbons was found in the Gressoney Valley (Italian Western Alps) while monitoring asbestos presence in outcropping serpentinite rocks. 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), whose surfaces are mostly covered by an aliphatic hydrocarbons film – an association never reported in literature. The sepiolite fibrils and their organic coating probably originated in sequential steps from precipitation of Si/Mg rich hydrothermal fluids, resulting from serpentinization of olivine and clinopyroxene, and Fischer-Tropsch-type reaction. Presence of hydrocarbons implies serious consequences on the sepiolite habit, as the organic wrap 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). This defibrillation mechanism, coupled to the extraordinary length, further increases these fibrils aspect ratio (length/width >> 3) thus amplifying their potential danger for human health when air dispersed and breathed.