Diaspore reaction, corundum

Similar to the procedure utilized by Rigby and Hutton, Lambertson [30] ground a 60% alumina-diaspore, fireclay brick and mixed this powder with sodium sulfate. The mixture was heated in a platinum crucible to 1300°C. X-ray and microscopic examination showed the decomposition of mullite to form glass and corundum. As the reaction proceeded, nephelite replaced the glass. 

In aqueous media oxides can spontaneously turn into hydroxides or oxohydroxides and vice versa. Let us explore the hydration-dehydration reactions for Al(III). Example 1 shows that corundum is the most stable oxide. Likewise hydrargillite has a more negative negative G° (and AGf) than the amorphous hydroxide and thus the former from is stable (not a big surprise), and diaspore is more stable than boehmite. In order to find which of the three forms (corundum, hydrargillite and diaspore) is thermodynamically stable, let us calculate AG" for the following reactions. 

Figure B. The PH2 0 equilibrium curve of the reaction 2 dias-pore = 1 corundum + 1 H2O, recalculated using the extracted thermodynamic data for diaspore. The experimental reversal brackets by Haas (1972) are displayed for comparison.

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