Reaction‐expansion Measurements on Chromite and Allied Spinels*

The rate of formation of spinel in mixtures of magnesia with alumina or chromic oxide (Cr 2 O 3 ) was greatly increased by the addition of 2% of boric acid. When the three oxides were heated together in the molecular ratio (2MgO + A1 2 O 3 + Cr 2 O 3 ), reaction‐expansion set in at a temperature much below that observed with the binary mixtures. The product appeared to be a mixture of green magnesium chromite with a pink colored spinel whose lattice edge was greater than that of magnesium aluminate but smaller than that of the chromite. Test pieces made from unfired Cuban chromite began to shrink at 1300°C, and gave a final shrinkage of 2.2% after two hours at 1500°C. Microscopic examination showed that the serpentine had changed to a glass. The compensation of firing shrinkage by the addition of 20% of coarse Austrian magnesite or 10% of fine electrically fused magnesia was found to be due to the formation of a doubly refracting mineral resembling forsterite. With a 20% alumina addition, a firing expansion of 0.8% was obtained.