9.5 According to Alper et al. [J. Am. Ceram. Soc., 45 (6) 263-66 (1962)], Al2O3 is soluble in MgO to the extent of 3% by weight at 1700C, 7% at 1800C, 12% at 1900C, and 0% at 1500C. They observed crystallization of spinel crystals from the solid solution region on slow cooling. Fast quenching retained the solid solution as a single phase at room temperature. The exsolved spinel appeared uniformly without regard to grain boundaries within the periclase grains but on specific planes.

(a) Is the nucleation of spinel homogeneous or heterogeneous within the periclase grains?

(b) Account for the appearance of spinel crystals along specific planes of periclase crystals. Predict the shape of the rate of crystallization versus temperature for nucleated periclase solid solution containing 5% Al2O3 over the temperature range 0C to 1850C.