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One of the most acceptable mechanism of spinel formation on sapphire in MgO-Al2O3 solid-state reaction is proposed on the basis of the crystal structure and other data. Although the mechanism of the sapphire-grown spinel formation has been explained in terms of topotaxy, the details have not been proposed nor discussed. The proposition about the reaction processes is shown as follows: (1) A small amount of MgO disolves into sapphire. (2) The oxygen stacking sequence is reconstructed from hcp to ccp by the partial dislocation mechanism. (3) This reconstruction accompanies with the synchro-shear movement of metal ions. (4) Mg2+ and Al3+ ions exchange their positions by inter-diffusion through the spinel lattice, and the total number of metal ions in the sapphire side increases. (5) The metal ions move to their proper positions of spinel structure, and then, the disordered spinel is formed. Displacement vectors of metal ions for this transformation were estimated. (6) The disordered spinel changes into that having the ordered structure. The validity of this proposition is discussed. [Received February 14, 1974]

The Mechanism of Formation of Spinel on Sapphire in MgO-Al<sub>2</sub>O<sub>3</sub> Solid-State Reaction