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

摘要

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°C. The deformation was induced with low mechanical load through a specifically built planetary ball milling working in a controlled thermodynamic environment (25°C and at a vacuum of 0.13 Pa). The mechanical energy transfer to the material was measured via the microstrain ϵ 21/2. 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°C/min of heating rate. In the medium temperature range (400-1100°C), the mechanical milling affected the dehydroxylation reaction by a significant anticipation of about 200-300°C of the temperature range, which usually occurs between 750 and 1050°C and gave rise to a clear exothermic reaction at about 840°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°C, involving cristobalite and MgSiO₃ polymorphs melting and transformation. All thermal reactions linearly correlate to the microstrain ϵ 21/2 accumulated in the talc structure.