Lithium borate (Li2B4O7) and sodium borate (Na2B4O7) mineralize spinel formation from stoichiometric MgO and Al2O3 between 1000 degrees and 1100 degrees C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution-precipitation mechanisms are operating, one dominating over the other under certain conditions. Na2B4O7-mineralized compositions show predominantly templated grain growth at 1000 degrees C, which changes to dissolution-precipitation at 1100 degrees C, whereas Li2B4O7-mineralized compositions show dissolution-precipitation from 1000 degrees C. Li2B4O7 is a stronger mineralizer as spinel formation is complete with 3 wt% Li2B4O7 at 1000 degrees C and with >= 1.5 wt% addition at 1100 degrees C, whereas Na2B4O7-mineralized compositions are found to retain some unreacted corundum even at 1100 degrees C.