The greatest heat evaluation was observed for andalusite followed by kyanite and sillimanite. Sillimanite is the most stable form of these minerals. All of them become instable on heating to a white heat. They are then transformed into a new silicate, 3Al2O3.2SiO2 which is called mullite, and a glassy melt.
Mullite is the most stable aluminum silicate at high temperature.
It is of industrial importance because it occurs in ceramic bodies and is formed on solidification of melts of a suitable composition containing alumina and silica.
The temperature differ at which the three minerals change into mullite
1. Kyanite - 1380.c
2. Andalusite - 1400.c
3. Sillimite - 1550.c
The transformation takes place over large temperature interval and in all cases a silica-rich glass is formed together with mullite.
To be contd………..
