Micro-Raman investigation on Fe-bearing impurities in chrysotile fibres

Chrysotile, one of the six regulated asbestos minerals, is the most employed and commercialized one for industrial applications, thanks to its outstanding technological properties. As of today, asbestos is banned in more than sixty countries worldwide, but the mining and use of chrysotile is still permitted in many other countries. Carcinogenic to humans, asbestos is currently deeply examined in toxicological research aimed at understanding the complex multistep process of asbestos-related carcinogenesis. In the toxicity and pathogenicity of asbestos fibres, the biopersistence plays a key role. In addition, the release of toxic trace metals from the crystal structure during the dissolution of the fibres in the lungs is strictly related to the surface reactivity of the fibres and to the prompt ROS formation, inducing an acute cytotoxicity. Although chrysotile is characterized by a relatively low biodurability, adverse effects with early carcinogenic signs are to be considered.

In the characterization of chrysotile asbestos and its intergrown mineralogical species, micro-Raman spectroscopy is an effective tool for the identification of asbestos fibres and impurities even at trace level, whose occurrence is under the detection limit of other analytical techniques. An extensive micro-Raman characterization was carried out on chrysotile from different mining locations - i.e. the abandoned mine of Balangero (Turin, Italy) and the active Orenburg Minerals mine near Yasniy (Russia). In addition to the identification of the chrysotile structure, impurities of other fibrous or lamellar species consisting of balangeroite and antigorite were easily recognized by their characteristic Raman spectra. Within the fibres, traces of Cr as toxic metal were confirmed by the characteristic photoluminescence peaks of Cr³⁺ emissions, located at about 680 nm. Furthermore, micro-Raman investigations, combined with SEM-EDS analyses, enables the identification of micrometric crystals – even of a few microns - of several Fe compounds, whose eventual dissolution in the organic environment should be carefully considered. Besides magnetite (Fe₃O₄) as the most abundant Fe compound, other Fe oxides and oxyhydroxides were identified, including hematite (α-Fe₂O₃), lepidocrocite (γ-FeOOH) and akageneite (β-FeOOH). Further Fe-bearing compounds include Fe sulphides, also containing Ni as toxic metal. Mackinawite (Fe^IIS) was detected by micro-Raman analysis in both nanocrystalline and partially oxidized forms, suggesting the presence of both Fe(II) and Fe(III) species. Nanocrystalline mackinawite is characterized by an intense sharp peak at ~280 cm^-1 and a weak contribution at ~204 cm^-1, whereas partially oxidized one is distinguished by Raman modes at ~122, 168, 253, 309, 323 and ~355-360 cm^-1. Unlike Fe oxides, whose solubility in the organic environment in typically scarce, the rapid dissolution of Fe sulphides - such as mackinawite - may exhibit the so-called “Trojan-horse effect”, contributing to the acute toxicity of chrysotile. The presence of these micro-crystals should be considered in the release of metals in the reactivity of chrysotile fibres in the lungs. Micro-Raman spectroscopy was successfully proven as a quick and reliable technique to identify inorganic micro-crystals dispersed in chrysotile fibres.