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Thermogravimetric study of the dehydration kinetics of talc

The dehydration kinetics of nearly pure talc, (MgorrFeoo,).Si4oro(OH)r, and of pure synthetic talc to enstatite and SiO, was studied as a function of temperature between 777 and 917 oC and of grain size by thermogravimetry experiments. In the grain-size range of I 5I pm, the rate of dehydration of talc increased with decreasing grain size, but further decrease ofgrain size did not significantly affect the dehydration rate. This may be caused by (l) clustering ofthe fine-grained particles as a result ofsurface charging, (2) a tradeoff between the positive effect of larger surface area and the negative effect of nucleating a larger number of product phases, or (3) both. The kinetic data can be adequately modeled by a second-order phenomenological rate law. The rate constant (ft) for the size fraction of 10-15 pm of the natural talc follows an Arrhenian relation, k: 1.98 x l}toexp(-Q/ RQ/min, where the activation energy Q: 372 + 7 (o) kJ/mol. Compared with talc of the same grain size, the synthetic talc was found to have a significantly faster dehydration rate. TEM images showed topotactic growth of enstatite on talc, with a concomitant formation of tridymite.