The formation and growth of MgAl2O4 spinel crystals on a single-crystal MgO substrate submerged in a 40 pct CaO, 40 pct SiO2, and 20 pct Al2O3 slag were directly observed using high-temperature microscopy. This showed that the crystals initially form on the MgO surface, but may break off and be carried out into the liquid slag. Still pictures extracted from digitally recorded images were used to measure the size of these crystals at 1420 ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ°C, 1440 ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ°C, and 1460 ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ°C as a function of time. Growth of the crystals was found to follow the parabolic rate law, with rates increasing with temperature. An activation energy of 564 kJ mol1 was estimated from the experimental data. This was found to be comparable with previously published results from different types of experiments on spinel formation.