Examination of the Thermal Transformation of Chrysotile by Using Dispersion Staining and Conventional X-ray Diffraction Techniques

Chrysotile has been used industrially as a component of refractory products, and in friction products, such as brake linings. Examining the decomposition or transformation of chrysotile as a function of time and temperature will help clarify the characteristics of particulates released during processes such as automotive braking. Previous studies have reported that the thermal treatment of chrysotile alters both its surface and structure, resulting in deviations from its natural properties, possibly reducing its biological activity (Langer, 2003: Reg Tox Pharm, v38, p71). In past studies, the nonequilibrium thermal decomposition of chrysotile has been investigated by using static dehydration, X-ray diffraction, differential thermal analysis, and thermogravimetric analysis. These studies suggest that the thermal transformation of chrysotile follows a two step sequence of dehydroxylation/dehydration and recrystallization where (1) chrysotile yields forsterite + silica + water with (2) forsterite + silica later forming enstatite (Ball and Taylor, 1963: Mineral. Mag. v33, p467, Brindley and Hayami, 1965: Mineral. Mag. v35, p189). In this research the decomposition of chrysotile from Thetford Quebec was studied. Samples were heated isothermally in air at temperatures from 200° C to 1000° C. After heating for up to 24 hours the refractive indices of remaining chrysotile fibers were measured by using dispersion staining. In addition, reaction products were identified by optical methods, electron probe microanalysis, and X-ray diffraction performed after the termination of the experiment. Preliminary results show that there is no change in optical properties of chrysotile heated to 400° C for 24 hours. From 400° C to 575° C for 24 hours, the index of refraction increases parallel to the length of the fiber from 1.552 to 1.560. From 400° C to 575° C for 24 hours, the index of refraction perpendicular to the length of the fiber varies irregularly from 1.538-1.548. The variability of the refractive indices, both parallel and perpendicular, observed in unheated chrysotile reduces from approximately ± 0.04 to ± 0.01 when heated to 575° C for 24 hours. Chrysotile heated to 575° C for 24 hours shows an overall loss of X-ray intensity for the main chrysotile peaks and possible growth of forsterite. At 800° C, although the fibrous morphology remains, X-ray diffraction analysis of the run products shows no remaining chrysotile and the index of refraction parallel to the length of the fiber is approximately 1.594. Initial data show that the reaction rates associated with the dehydroxylation/dehydration and recrystallization of chrysotile are dependent upon the texture of individual fibers.