In an earlier paper (1) it was pointed out that quartz or amorphous silica can be converted to cristobalite when heated for ten to thirty minutes with alkali fluxes at temperatures in the vicinity of 1150° C. A further investigation of several commercially prepared, finely divided amorphous silicas disclosed that crystallization of these substances could be brought about during ignition at temperatures between 1070° C. and 1100° C., provided CaO or CaCO3 were present in amounts of five to ten per cent by weight. Before an opportunity to present the results had been realized, a paper on the crystallization of silicic acid by Schulman, et al. (2), appeared in a recent issue of the American Mineralogist, The authors show that quartz and cristobalite can form from silicic acid heated at 1150° C. for three hours in the presence of calcium, added as CaCO3 or CaSiO3. The present writer finds his results to be consistent with the observations of Schulman and his co-workers. Additional points revealed in the writer's investigation but not covered by Schulman's paper are discussed below.

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