NUCLEATION AT CORDIERITE GLASS SURFACES - KINETIC ASPECTS

The surface nucleation of high-quartz solid solution crystals at fract ured surfaces of glasses of the stoichiometric cordierite composition (2MgO . 2Al(2)O(3) . 5SiO(2)) was studied by optical microscopy. Parti cular attention was focused on the nucleation kinetics. A constant nuc leation density, N approximate to 10(-4) mu m(-2), was found not to be significantly influenced by the time and the temperature of nucleatio n treatment. Even a very fast heating of samples employing heating rat es up to 1200 K/min does not lower N substantially. However, for small average crystal diameters (<20 mu m) a distribution of crystal size i n the same order of magnitude is detectable indicating a simultaneous appearance of both measurable nucleation rates and growth velocities. It can be concluded that the surface nucleation of mu-cordierite occur s during the thermal treatment from a limited number of preferred nucl eation sites; these sites are ''used up'' rapidly enough to cause a st rong saturation effect of nucleation, but slow enough to cause a cryst al size distribution at the same time. The surface nucleation rate, I- s, was calculated from the observed distribution of crystal sizes. I-s progressively increases with rising temperature similar to the crysta l growth velocity indicating a broad temperature range of essential nu cleation activity. The latter must be regarded as the main obstacle to measure or to control surface nucleation density by means of two-step nucleation and growth treatments and must therefore be claimed to be mainly responsible for the observed constancy of N.