The growth of spinel crystals on single crystal MgO in a CaO-Al2O3-SiO2 slag has been investigated over the temperature range 1420°C to 1460°C using high temperature microscopy. Key findings of the investigation were that the spinel crystal that grew on the MgO had a well developed faceted morphology and displayed an octahedral form of a cubic crystal bounded by (111) planes. After a period of time the crystal detached from the MgO, floated off into the slag and dissolved. From kinetic analysis it was found that the spinel crystal growth rate was consistent with the parabolic rate law. Arrhenius constants were evaluated to establish the temperature dependence of the parabolic growth rate constant. The activation energy for growth of the spinel layer was found to be 564 kJmol.-1.