Two amphibole asbestos, crocidolite and amosite, have been heated at 400 and 800 degreesC in order to examine the variations in some surface properties relevant to asbestos toxicity, such as iron mobility, free radical generation, and hydrophilicity. At 400 degreesC, only the surface is modified, while at 800 degreesC the crystalline structure partially collapses (X-ray diffraction) with loss in specific surface (Brunauer-Emmett-Teller). Two chelators, ferrozine and desferrioxamine, were employed to evaluate Fe(II) and Fe(III), respectively. More iron was extracted by deferoxamine from crocidolite than from amosite, but when compared per unit surface, the extent of iron removed from the two asbestos was the same. Ferrozine removed more iron from unheated amosite than from crocidolite. Upon heating, because of the oxidation of Fe(II) to Fe(III), less (400 degreesC) or no (800 degreesC) iron is removed by ferrozine. The amount of iron removed by deferoxamine is virtually unaffected by heating at 400 degreesC but dramatically decreases after the 800 degreesC treatment on both asbestos. Cyclic voltammetry evidences more redox active ions at low and neutral pH in the unheated crocidolite than in the 800 degreesC heated crocidolite. Both unheated asbestos generate HO. from H2O2 and carbon-centered radicals from the formate ion. The HO. yield is unaffected following the 400 degreesC treatment but suppressed by heating at 800 degreesC. Carbon-centered radicals are not generated by the heated fibers, but ascorbic acid restores radical activity on the 400 degreesC heated fibers but not on the 800 degreesC heated fibers. Water vapor is strongly and irreversibly adsorbed on 400 degreesC heated crocidolite but more weakly and only reversibly adsorbed on the 800 degreesC heated one, because of the stabilization of siloxane bridges and the embedding of surface-exposed cations. When crocidolite mixed with kaolin is heated at 800 degreesC, the above effects of the thermal treatment are further enhanced. In conclusion, at 800 degreesC amphibole asbestos lose most surface characteristics implied in their health effects, suggesting possible ways to inactivate asbestos in milder conditions than those currently employed, for example, with a plasma torch.