Phase transformation and microstructural evolution of black tourmaline mineral powders during heating and cooling processes

Abstract

The microstructural evolution of black tourmaline was studied at various temperatures from ambient one to the melting point through novel combined characterizations of metallographic microscopy and X-ray diffractometry established for high temperature measurements. The grain size was found to decrease gradually with the increase in the temperature, and it decreased rapidly at 950 °C. Tourmaline began to melt at 1050 °C and melted completely at 1200 °C. As the system was cooled down to 950 °C, the tourmaline phase precipitated in the molten body, and its grain size increased gradually. However, as the temperature went below 950 °C till to room temperature, more tourmaline phases formed whereas the grain size decreased, which could be attributed to the competitive growth between the tourmaline and cordierite phases. Moreover, the previously remaining melt was rapidly solidified due to the retention of solid state, and a portion of the tourmaline nuclei formed by the heterogeneous nuclei grew up when the temperature decreased from 1200 to 1050 °C. The average grain size of the tourmaline phase increased to the maximum when the temperature decreased to 950 °C, and it continued to decrease with a further decrease in the temperature to 25 °C, indicating that the tourmaline grain refinement was related to the pinning effect of the second relative grain boundary.