Asbestos may cause adverse effects, but relationship between mineralogy and texture of fibres versus toxicity is still lacking. Toxicological studies can be interpreted and compared only if quantitative features of fibres are determined. Here, riebeckitic ("crocidolite") amphibole fibres were analysed by XRPD, FTIR, SEM-EDS and EMP-WDS; only crystals with stochiometry Na-A square B(2)C(Fe2.52+Mg0.5)Fe-C(2)3+ (Si8O225W)-Si-T(OH)(2) are present in the starting material used for the experiments. Fibres deposited from solutions of 0.1, 1, 10, 25, 50, 75 and 100 mg/L were counted by image analysis using SEM images. At 0.1 and 1 mg/L the fibres are well separated, whereas between 1 and 10 mg/L they start to agglomerate. In-vitro tests performed on fibres deposited at the same mg/L concentrations show that the toxic potential follows a curvilinear increasing trend with a decreasing rate. Since the range of sizes of single fibres and their mineralogy are constant, this decreasing rate can be only attributed to the increasing amount of agglomerated fibres. Hence, single versus agglomerated fibre population is a factor that cannot be neglected in defining the final adverse effects of asbestos. The analytical protocol proposed here is valuable for any aero-dispersed dust, in polluted environments, as well as in the interpretation of experimental studies. (C) 2017 Elsevier B.V. All rights reserved.