Abstract Crocidolite is a fibrous mineral asbestiform which is widely used in industry. The fibers of this material have a high electron donor capability. This promotes electrostatic repulsion between the fibers and they are readily dispersed into the atmosphere. Airborne fibers are eventually inhaled into the lungs where they induce carcinoma and mesothelioma. Therefore much effort has been directed towards moderating the toxicity of crocidolite and related mineral asbestiforms. One detoxifying procedure has involved coating crocidolite fibers with an iron complex. In the present study, Mössbauer-effect spectroscopy has been used to monitor any crystal and chemical modifications that have occurred after this detoxification process has been applied. An analysis of the Mössbauer data has shown that the detoxification process 1) induces a change of valence at some of the bulk ferrous ion sites and 2) produces a ferric-base surface complex. There is therefore a concomitant modification of electron donor characteristics of the fibers. Experimental evidence has been presented which suggests that the coating applied in the detoxification process is a chemical complex of the form [Fe(H 2O) 6] 3+. The nature and location of this complex may help to inhibit surface Fenton-type reactions and may consequently moderate the toxicity of the fibers.