Man-made mineral fibers (MMMF) (ceramic fibers), which were used as insulation material in industrial furnaces at temperatures ranging between 1, OOO and 1, 240"C, were investigated. New phases with demonstrated biologic activity were detected. Formation of cristobalite and mullite phases could be identified by polarised light microscopy, (with the help of color immersion methods for phase contrast), as well as X-ray diffraction and electron microscopy.Experimental investigations on MMMF (vitreous and ceramic fibers) demonstrate that recrystallization begins to occur at temperatures as low as 150°C at relatively low water vapor pressures, e.g., 5 bars (Fig. 1).Quantitative electron microscopic investigation of the suspension of a sample treated with water vapor at a temperature of 150°C over a period of 20 days showed the following results (enlargement: 10,OOO-fold): ca 40 * lo6 F/mg of all lengths, and 6 -lo6 F/mg of lengths > 5 pm.The elemental composition of silicon (Si) and aluminum (A), as well as the unstable diffraction pattern in the electron beam, all point to the formation of fibers of zeolite composition. The fiber-size distribution of the new crystalline product shows them to possess dimensions of L 2 5 pm and D 2 0.5 pm (Fig. 1). These sizes are important to consider in cytotoxicity, fibrogenicity, and oncogenic effects.At temperatures of 2 200°C, chrysotile fibers appear to form. After a reaction time of 10 days at 30O"C, chrysotile fibers with the following size distributions were observed: 1.5 * lo6 F/mg of all lengths, and 0.5 * lo6 F/mg of lengths > 5 pm.