Kinetic analysis and in-situ IR/Raman spectroscopy of lizardite and chrysotile dehydroxilation

Low pressure dehydroxylation of serpentine phases is important in processes such as the inertialisation of chrysotile asbestos or the CO2 sequestration in ultramafic rocks. Although research on the mechanism of this process goes back to the forties of last century, the dehydroxylation mechanism is still not fully elucidated. In this study with used non-isothermal thermogravimetric (TGA) analyses, in-situ IRand Raman spectroscopy as well as in-situ Xray diffraction to monitor the dehydroxylation process. Reaction-progress (α) resolved apparent activation energies (Ea) were obtained by model-free treatment of TGA runs (Ozawa-Flynn-Wall isoconversional method). The evolution of Ea vs. α is distinct for both serpentine phases: in lizardite the curve is convex, whereas in chrysotile it is concave. The initial increase in Ea observed for both phases is due to competing parallel reactions involving a) two symmetrically different OH-sites and b) several possible parallel dehydroxylation reaction steps (H and OH debonding, recombination of H and OH before debonding etc.). The simultaneous decrease of the IR absorbance for all OH-bands indicates, that dehydroxylation affects both OH-groups at the same time, but at different rates. The contribution of the reaction step/OH-group with the higher activation energy is becoming more important with increasing α. Activation energies calculated by DFT for similar dehydroxylation steps in pyrophyllite (Molina-Montes et al., 2008) compare well with the range of present values. In chrysotile, the situation is more complicated owing to the non-translational symmetry of the structure. Each TO-layer has its proper curvature and is , therefore, structurally unique. The outer layers with larger curvature are less stable than the inner layer. The latter dehydrate, therefore, at higher temperature.