Indirect Dissolution of Sapphire into Silicate Melts

The forced‐convective dissolution of sapphire into CaO‐MgO‐Al 2 O 3 ‐SiO 2 melts was investigated at 1450° and 1550°C. Spinel, MgAl 2 O 4 , formed on the sapphire under all conditions although the morphology of the spinel, which varied from individual particles to a complete layer, depended on the MgO content of the melt, the rate of rotation of the specimens in the melt, and the temperature. The thermodynamic and kinetic conditions required for formation of spinel are considered, and the factors that are important in the development of a coherent spinel layer on sapphire are discussed. Although spinel inhibits the dissolution of sapphire into silicate melts, it does not stop the dissolution. Because the dissolution process occurs in an indirect manner when a complete layer of spinel forms, this type of dissolution process is designated as indirect dissolution.