CRYSTALLIZATION BEHAVIOR IN THE SYSTEM MGO-AL2O3-SIO2

Several glasses were prepared in the MgO-Al2O3-SiO2 system with differ ent compositions within the formation zone for cordierite. The samples were examined after crystallization to investigate the influence of t he composition on the phases formed. Dilatometric measurements were pe rformed on the glasses and the results compared with those relative to stoichiometric cordierite. The nature of the excess component actuall y changes the stability ranges of cordierite IJ, and a, and the presen ce of sillimanite and forsterite is also revealed in samples containin g an excess of alumina and magnesia, respectively. For the glass with composition 2MgO . 2A(2)O(3) . 5SiO(2), corresponding to stoichiometri c cordierite, the crystallization kinetics have been studied by both t he non-isothermal method, based on differential thermal analysis and t he isothermal method, based on diffractometric measurements. For the c rystallization of the cordierite mu-phase, the calculated activation e nergy with non-isothermal method is E(a) = 85 +/- 5 kcal/mol, and the reaction order is n = 0.98 +/- 0.1, while for the cordierite alpha-pha se E(a) = 135 +/- 5 kcal/mol and n = 0.90 +/- 0.1. The values for Ea(( alpha)) and n((alpha)) determined by the isothermal method are 114 +/- 20 kcal/mol and 1.2 +/- 0.2, respectively. The value of n is consiste nt with surface growth of the crystallites.