Abstract
Dehydration of boehmite or gibbsite under hydrothermal conditions gives corundum which consists of separated single crystals of uniform size and habit. The present authors have found that the crystal habit and the reaction velocity vary with hydrothermal conditions and are intimately related to each other.Addition of impurity ions to the hydrothermal solution had remarkable effects on the habit. In our experiments we observed some of c {0001}, a {1120}, n {1123} and r {0112} (taking R3C hexagonal axis after International Tables for X-ray Crystallography), case by case and were able to develop any of them particularly by addition of certain impurity ions. Some of metallic ions were solid-soluble in corundum and gave colored crystals. Effects of some metallic ions such as copper and nickel depended on co-existing anions such as SO4--, NO3-, which seem to affect the valence of solid-soluble metallic ions.
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