49 An experimental approach for the quantitative definition of asbestos-like morphology in naturally occurring mineral fibres

Hundreds of naturally occurring asbestos and asbestos-like minerals (NOA) fall under the NIOSH definition of elongated mineral particles (EMPs, 2011). When natural or anthropogenic processes promote fibre dispersion, their occurrence in rock and soil poses environmental and health risks. Evaluating the hazard of NOA-bearing outcrops requires the predictive evaluation of the presence of potentially harmful mineral fibres, commencing with a quantitative definition of their asbestos-like morphology. This study introduces the “Fibrosity parameter (Fp)” to express quantitatively the asbestos-like morphology of potentially hazardous mineral fibres in natural sites. Fp evaluates whether a fibrous mineral retains its fibrous behaviour under mechanical stress. Antigorite and tremolite samples with fibrous to lamellar/prismatic habits at hand-scale were subjected to ball milling, and morphometric parameters were determined using light and electron microscopy coupled automated image analysis. Results were compared with asbestos (UICC crocidolite, long amosite), and non-asbestos standards (MMVF, talc). Two quantitative parameters discriminate asbestos-like from non-asbestos morphologies: the percentage of EMPs at max grinding time (EMPmax%), and the variation of EMP% with milling time, ie the slope, S = δEMP% / δ milling time. Four scenarios describe mineral behaviour: (i) asbestos and asbestos-like (EMPmax% >50%, S < −1), asbestos-like fibres (EMPmax% >50%, S > −1), non asbestos-like fibres (EMPmax% <50%, S > −1), and non-asbestos (EMPmax% <50%, S < −1). The Fp approach provides a reproducible methodology for characterizing fibrous minerals based on their morphological persistence under stress. By quantifying the variation of EMP% with milling, Fp supports assessing the asbestos-like morphology of a given NOA, and enables an evidence-based, standardized evaluation of the NOA hazard in rock and soil.