On the high-temperature structural behaviour of talc (Mg3Si4O10(OH)2) to 1600°C: Effect of mechanical deformation and strain

The high-temperature structural behaviour of talc, Mg3Si4O10(OH)2, deformed by compaction and shear was investigated for the first time by extending the temperature range to 1600 degrees C. The deformation was induced with low mechanical load through a specifically built planetary ball milling working in a controlled thermodynamic environment (25 degrees C and at a vacuum of 0.13 Pa). The mechanical energy transfer to the material was measured via the microstrain <epsilon 2 > 1/2. In our experimental set-up, amorphisation was not deliberately reached since we wanted to investigate the details of the evolution of the talc structure as a function of the microstrain. At the very early stage of milling (up to 1 h), no strain was accumulated in the talc structure which, however, presented lamination, layer flattening and texturing. Further milling induced a progressive reduction of the stacking layer coherence and an increase of the microstrain, in both cases as a non linear function of the deformation time. The thermo-structural behaviour of talc was investigated by TG-DTA in a helium atmosphere at 10 degrees C/min of heating rate. In the medium temperature range (400-1100 degrees C), the mechanical milling affected the dehydroxylation reaction by a significant anticipation of about 200-300 degrees C of the temperature range, which usually occurs between 750 and 1050 degrees C and gave rise to a clear exothermic reaction at about 840 degrees C, related to an increase of the recrystallisation kinetics of MgSiO3 (orthoenstatite). The mechanical deformation strongly influenced also the kinetics of the high-temperature endothermic reactions at about 1555 degrees C, involving cristobalite and MgSiO3 polymorphs melting and transformation. All thermal reactions linearly correlate to the microstrain <epsilon 2 > 1/2 accumulated in the talc structure.