THERMOGRAVIMETRIC STUDY OF THE DEHYDRATION KINETICS OF TALC

The dehydration kinetics of nearly pure talc, (Mg0.99Fe0.01)3Si4O10(OH )2, and of pure synthetic talc to enstatite and SiO2 was studied as a function of temperature between 777 and 977-degrees-C and of grain siz e by thermogravimetry experiments. In the grain-size range of 15-1 mum , the rate of dehydration of talc increased with decreasing grain size , but further decrease of grain size did not significantly affect the dehydration rate. This may be caused by (1) clustering of the fine-gra ined particles as a result of surface 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 kineti c data can be adequately modeled by a second-order phenomenological ra te law. The rate constant (k) for the size fraction of 10-15 mum of th e natural talc follows an Arrhenian relation, k = 1.98 x 10(14) exp(-Q /RT)/min, where the activation energy Q = 372 +/- 7 (sigma) kJ/mol. Co mpared with talc of the same grain size, the synthetic talc was found to have a significantly faster dehydration rate. TEM images showed top otactic growth of enstatite on talc, with a concomitant formation of t ridymite.