THE CRYSTAL STRUCTURES OF MINERAL FIBRES

Since 2011, a research project on mineral fibres entitled “Sviluppo di un modello generale di interazioni tra fibre minerali e cellule biologiche”, has been conducted as part of the long term Italian Research Project of National Interest (PRIN) “Interazione fra minerali e biosfera: conseguenze per l'ambiente e la salute umana”. The research project is specifically focussed on mineral fibres and is aimed at understanding the biochemical reactions that make them cytoand geno-toxic and at the development of a general model capable to classify each mineral fibre based on its toxicity potential. The attempt to understand the complex biochemical mechanisms between the mineral fibres and the organic matter requires a basic systematic mineralogical-crystallographic study. Hence, eight mineral fibres samples have been selected for the study based on their socio-economic and industrial importance: three chrysotile of different origin (UICC, Canada; Balangero and Val Malenco, Italy), four amphibole asbestos species (amosite UICC, anthophyllite UICC, crocidolite UICC and tremolite from Val d'Ala) and the fibrous zeolite erionite (from Jersey, Nevada, USA). The surface reactivity and the chemical environment of iron within the crystal structure of these samples have been recently investigated (Pollastri et al. 2014; 2015). In order to complete the crystal structural characterization, X-ray powder and single crystal diffraction experiments have been conducted using both conventional and synchrotron radiation sources (Elettra, Trieste, Italy; SLS, Villigen, Switzerland), for the determination of impurities and the refinement of the crystalline structures. Concerning the chrysotile samples, because of the low to null effect of conventional grinding techniques on chrysotile fibre bundles, we have opted for a cryo-milling process in wet conditions; this procedure allowed to obtain perfectly powder samples. For chrysotile and erionite samples, diffraction patterns were collected with either resonant radiation at the absorption K-edge of Fe (≈7 keV, λ 1.7428 Å) and with wavelength off of the absorption edge (≈10 keV, λ 1.2408 Å) in order to investigate the crystal chemistry of iron within the fibres. The collected data were analysed both with the TOPAS and GSAS. The results of the structural refinements, in particular occupancy and position of the iron atoms within the crystal structures (a primary cause of toxicity as it generate active oxygen species, mobilization by chelators and iron-catalyzed reactions; Hardy and Aust, 1995), were compared to that obtained from the previously performed spectroscopic investigations. The obtained structures of chrysotile samples are similar to each other and quite similar to the only available structure of chrysotile present in literature (Falini et al., 2004). Also the structures of other samples were compared to the few data reported in literature to highlight differences between the fibrous and non fibrous forms of the minerals