An unusual occurrence of asbestiform sepiolite, filling veins in the antigorite serpentinites of the Voltri Unit exposed in a borrow pit (now reclaimed) in the Deiva forest, near Sassello, NW Italy, was investigated with an in-depth analytical approach aimed at studying its crystal-chemistry and structure and evaluating its possible hazards for human health. Optical microscopy and scanning electron microscopy (energy-dispersive spectroscopy mode) proved that these sepiolite fibres, apparently up to several cm long, are made up of bundles of thinner fibrils (or laths: average length > 100 µm; thickness ≈ 80 nm), with a composition consistent to that reported in the literature. The dehydration process was monitored through thermo-gravimetric analyses and Fourier-transform infrared spectroscopy, performed at increasing T; the latter, in particular, showed the presence of moderate amounts of aliphatic hydrocarbons – not yet identified thoroughly – associated with the sample. The crystal structure refinement with the Rietveld method showed no relevant difference from the literature models, although a peculiar distribution of zeolitic H2O molecules was observed. The geological context suggests that the Sassello sepiolite precipitated from hydrothermal fluids, which were saturated in Mg and silica by the interaction of the host serpentinites. The same setting favoured formation of abiotic hydrocarbons, by means of the Fischer–Tropsch reaction. The extremely long and flexible fibrils (length/width aspect ratio >> 3) of this sepiolite specimen could represent a serious hazard for human health if air dispersed and inhaled; also, its atypical association with hydrocarbons (only reported once previously) might even favour further fragmentation in thinner units.

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 outcrops of serpentinite rocks. Microscopic and Fourier transform infrared analyses proved that these fibres, apparently up to several cm long, are formed by bundles of thinner fibrils (average length: 150 μm) potentially dispersible in the environment. When observed using transmission electron microscopy these fibrils show a rhomboidal to parallelogram cross section (<1 μm), of which surfaces are covered mostly by an aliphatic hydrocarbon film – an association not reported in the 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 a Fischer-Tropsch-type reaction. The presence of hydrocarbons has serious implications for the sepiolite habit, as the organic wrap interacts with the fibril’s surface reducing the amount of adsorbed water and favouring the fragmentation of thicker units into thinner ones, due to an ‘opening’ process implying separation along z and cleavage on (110). This defibrillation mechanism, coupled with the extraordinary length, further increases the aspect ratio of these fibrils (length/width ≫3) thus amplifying their potential danger for human health when dispersed in air and inhaled.