Structural changes of magnesium hydroxide and its decomposition product, magnesia, with heat treatments were examined by means of powder X-ray diffraction, and the sintering behaviour of the compacts of magnesium hydroxide was also studied.
Magnesium hydroxide began to decompose by 2hrs soaking at 300°C. During the decomposition, the crystallite size of magnesium hydroxide initially decreased, then increased to a certain maximum and then decreased rapidly, with raising temperature or prolonging soaking time. Its lattice parameter, c, also changed similarly, i.e. increased initially to a certain maximum, and then decreased rapidly. These changes could be explained clearly by ionic diffusion processes proposed by Taylor et al. From the results thus obtained, a detailed mechanism of the decomposition process was inferred.
Magnesia formed by the decomposition of magnesium hydroxide at low temperatures showed a larger lattice parameter than that of the usual magnesia. With raising temperature, the parameter decreased gradually and approached to 4.211A at about 1400°C. Above that temperature, no further change was observed. Magnesia formed at low temperatures also showed a distorted cubic lattice, whose [111] direction was somewhat contracted compared with any other directions. The distortion was not released, even when the magnesia was fired to considerably high temperatures.
Small tablets were prepared by compressing magnesium hydroxide, and fired according to various schedules. Bulk density of the compacts decreased rapidly at about 500°-700°C, due to the decomposition of the hydroxide, but little change was observed in the temperature range 700°-900°C. Above 900°-1000°C, sintering of the compacts progressed gradually with firing temperatures. From the results obtained, it is inferred that the sintering behaviour of the compacts consisting of magnesium hydroxide is not governed by the structural factors of magnesia formed, except during the period of the decomposition of the hydroxide, but is rather greatly governed by other external factors such as firing temperature, soaking time etc.

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In previous papers, the author reported about the formation and the color development of titanium-tin single spinels, and titanium-aluminium single spinels containing Co²⁺ and Ni²⁺ ions. For the purpose of researching into the formation and the color development of titanium-chrome single spinels, the gradual substitution according to the following formula was carried out for each titanium spinel.
Mg²⁺(Zn²⁺)+Ti⁴⁺_??_2Cr³⁺
Thus, MgO-TiO₂-Cr₂O₃, ZnO-TiO₂-Cr₂O₃, CoO-MgO-TiO₂-Cr₂O₃, CoO-ZnO-TiO₂-Cr₂O₃, NiO-MgO-TiO₂-Cr₂O₃, NiO-ZnO-TiO₂-Cr₂O₃, CoO-NiO-MgO-TiO₂-Cr₂O₃ and CoO-NiO-ZnO-TiO₂-Cr₂O₃ systems were investigated. Each oxide mixture was calcined at 1350°C for 1 hour. Reflectance between 400-760mμ was recorded by a spectrophotometer to pursue the displacement of absorption, and to represent the results by C. I. E. color specification. X-ray analysis was also carried out to observe the spinel formation and to calculate the lattice constant. The results were summarized as follows.
1. MgO-TiO₂-Cr₂O₃ and ZnO-TiO₂-Cr₂O₃ systems.
Samples with the composition of (2-x)MgO⋅(1-x)TiO₂⋅xCr₂O₃ and (2-x)ZoO⋅(1-x)TiO₂⋅xCr₂O₃ were prepared to observe the displacement of the absorption of Cr³⁺ ions, and only single spinels were obtained. The lattice constant of these spinels changed linearly, and the absorption of Cr³⁺ ions shifted towards the violet region with increasing amount of Cr³⁺ ions.
2. As shown in Table 1-6, single spinels were obtained except No. 37 (NiO⋅MgO⋅TiO2).
3. CoO-MgO-TiO₂-Cr₂O₃ system.
Samples were prepared according to 0.2CoO⋅(1.8-x)MgO⋅(1-x)TiO₂⋅xCr₂O₃, 0.5CoO⋅(1.5-x)MgO⋅(1-x)TiO₂⋅xCr₂O₃ and CoO⋅(1-x)MgO⋅(1-x)TiO₂⋅xCr₂O₃. As Co²⁺ ions have tetrahedral preference and Ti⁴⁺ and Cr³⁺ ions octahedral preference whereas Mg²⁺ ions have no preference, the cation distributions in these spinels are thought to be as follows. Co_0.2Mg_0.8[Mg_(1-x)Ti_(1-x)Cr_2x]O₄, Co_0.5Mg_0.5[Mg_(1-x)Ti_(1-x)Cr_2x]O₄, Co[Mg_(1-x)Ti_(1-x)Cr_2x]⋅O₄ (the bracket represents octahedral interstices). When x=0, clear hues ranging from greenish blue (0.2CoO⋅1.8MgO⋅TiO₂) through bluish green (0.5CoO⋅1.5MgO⋅TiO₂) to deep green (CoO⋅MgO⋅TiO₂), resulting from the tetrahedral Co²⁺ ions, developed. When 0<x<1, green, and when x=1, bluish green, resulting from the tetrahedral Co²⁺ and octahedral Cr³⁺ ions, developed. In this system, the coordination number of Co²⁺ ions was continually 4, and the displacement of the absorption of tetrahedral Co²⁺ ions towards the violet region was observed distinctly, but that of Cr³⁺ ions was not observed, owing to the overlapping of the absorptions of Co²⁺ and

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In order to search after the condition for the utilization of alunites as ceramic raw materials, changes of their characteristics by calcination and by various treatments were investigated.
For the use as ceramic raw materials, alunites must be calcined above the volatilization temperature of SO₃, up to the temperature where the internal surface area reaches the maximum. By wet ball-milling and filter-pressing on the calcined material, almost all alkalis and SO₃ were extracted and the property as ceramic raw materials was found very satisfactory.

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