Effect of Trace A12O3 on Transformation of Quartz to Cristobalite

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Abstract
The effect of additions of 0.22, 0.44, 0.88, and 1.76% A12O3 (Si4+/A13+ ratio of 200:1, 100:1, 50:1, and 25:1) on the transformation of Brazilian quartz to cristobalite was studied at 1500°, 1530°, and 1570°C. The smaller percentages of A12O3 (0.22 and 0.44%) catalyzed the transformation of quartz and the formation of cristobalite considerably. The rates of transformation of quartz with 0.88 and 1.76% A12O3 were slower than with 0.22 or 0.44%, indicating a critical A13+/Si4+ ratio where the catalytic effect was found to be maximum. This appeared to occur at about 0.5% A12O3. The transformation rate of quartz indicated that the reaction was first order. Cristobalite, however, showed two different rates; the initial rapid growth was followed by a slower rate. The point of changeover was found to be at about 30 ± 5% cristobalite. The critical nature of the A13−/Si4+ ratio at about 0.01 (or A12O3/SiO2± 0.005) may have some bearing on the properties of silica refractories with more or less than 0.5% A12O3.

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