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

Abstract

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.

Characterisation and possible hazard of an atypical asbestiform sepiolite associated with aliphatic hydrocarbons from Sassello, Ligurian Apennines, Italy

“…The minerals of the palygorskite–sepiolite group are sometimes known to form compact and soft cardboard-like felts (‘mountain leather’: Grim, 1968; Frost et al ., 1998; Imai and Otsuka, 2000), in which tangled fibres form an apparently continuous mat. Asbestiform occurrences, however, have also been reported (Giustetto and Compagnoni, 2011; Giustetto et al ., 2014). Observation under the stereo-microscope confirms that these long fibres are bundles of thinner fibrils, intricately intergrown and separable by sheer mechanic stress.…”

“…The chemical composition resulting from EDS spot analyses (Table 1) is consistent with previous data (Garcia-Romero and Suàrez, 2010; Suàrez and Garcia-Romero, 2011; Giustetto et al ., 2014). Silicon and Mg are the main components, together with moderate Fe substituting for Mg in octahedral coordination.…”

“…Later, fractional coordinates were refined for all atoms as well as occupancy factors for structural OH 2 and zeolitic H 2 O, at first in alternate cycles (to minimise correlations) and finally all together. Soft constraints were imposed on tetrahedral and octahedral bond lengths and angles and progressively reduced; only the lower weighting factor ( F = 10) yielding reasonable bond lengths was kept, in order to avoid unrealistic polyhedron distortions (Post et al ., 2007; Post and Heaney, 2008; Giustetto et al ., 2011 a ; Giustetto and Compagnoni, 2011; Giustetto et al ., 2014). Isotropic displacement parameters ( U iso ) for all atoms were adjusted using overall constraints for each chemical species.…”

“…All commercially sold sepiolites have fibres < 5 µm long (at times, achieved after preliminary grinding) – the threshold of non-carcinogenicity fixed by the International Agency for Research on Cancer, World Health Organization (IARC, 1997). However, sepiolites with unusually long fibres (> 10 µm, due to variability in the crystallinity degree and texture; Suárez and García-Romero, 2012; García-Romero and Suárez, 2013) have been seldom signalled for their carcinogenic potential (Pott et al ., 1990; Pott et al ., 1991; Bellman et al ., 1997; Giustetto et al ., 2014). This study describes an atypical asbestiform sepiolite, with extremely long fibres, associated with small amounts of aliphatic hydrocarbons.…”

“…Similar samples have been seldom reported in the outcrops of Northwestern Italy (e.g. Valle d'Aosta, Piemonte and Liguria: Belluso and Sandrone, 1989; Belluso et al ., 1995) – only once associated with hydrocarbons (Giustetto et al ., 2014).…”