This paper describes the effect of chemical composition on the crystallization rate of minerals in an Al₂O₃-SiO₂-H₂O system. Eleven Al₂O₃-SiO₂ gels different in Al/(Al+Si) ratio were reacted with water at 473 K for 2, 10, 49 and 280 days. The gels and the products were analyzed with X-ray diffractometry (XRD) and thermal gravimetry (TG). The weight ratios of minerals were determined from X-ray diffraction intensities with an internal standard method. Boehmite crystallized from a pure alumina gel in 2 days, and kaolinite began to crystallize in 10 days from the gels with the Al/(Al+Si) ratios of 0.3 to 0.6. Pseudoboehmite, which has the structure close to boehmite but has poor crystallinity, was formed from the gels with the Al/(Al+Si) ratios of 0.7 to 0.9. Kaolinite and boehmite crystallized fast when the Al/(Al+Si) ratios of the gels were similar to those of kaolinite and boehmite, respectively. On the other hand, kaolinite and boehmite formed extremely slowly from the gels with intermediate compositions between kaolinite and boehmite. The crystallization rates of minerals in an Al₂O₃-SiO₂-H₂O system depend on the composition of starting materials.

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Activation energies of H₂O and H₂ diffusions in silica glass were evaluated using a quantum chemical method. In order to mimic diffusion process, an interaction of a molecule with a ring cluster (Si_nO_n(OH)_2n: n=4–8) was considered. The molecule was forced to go through the ring, and heats of formation along the pass were calculated using the MNDO-PM3 semi-empirical molecular orbital method. The obtained activation energies for H₂O and H₂ molecules with 6-membered ring are most close with those of experimental studies for silica glass. Thus the present result suggests that “bottleneck” of diffusion process is due to the passage of the molecules through 6-membered rings in the glass structure. This model can be applied to diffusion of other gas molecules in framework silicate crystals and glasses.

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The local structures around Ga³⁺ in the tetrahedral sites at the compositions x=1.0 and 1.6 and around Fe³⁺ in the octahedral sites at the compositions x=3.8 and 4.6 in the Y₃Fe_5−xGa_xO₁₂ garnet structure were investigated by EXAFS method in order to evaluate the short-range correlation of vibration for each backscattering atom situated in the first-, second- and third-nearest neighbor positions with respect to Ga³⁺ and Fe³⁺. The Ga³⁺ atoms in the solid solution occupy only the tetrahedral site in the composition range 0.0<x≤1.6 while the Fe³⁺ atoms occupy only the octahedral site in the range 3.8≤x<5.0. The distances of Ga-O for the tetrahedral sites in Y₃Fe₂(Fe_2.0Ga_1.0)O₁₂ (1.855(6)Å) and Y₃Fe₂(Fe_1.4Ga_1.6)O₁₂ (1.843(6)Å) are in agreement with that in the Ga end-member and empirical distance calculated from the effective ionic radii. The distances of Fe-O for the octahedral sites in Y₃(Fe_1.2Ga_0.8)Ga₃O₁₂ (2.026(5)Å) and Y₃(Fe_0.4Ga_1.6)Ga₃O₁₂ (2.035(5)Å) are in agreement with the empirical distance but are slightly longer than that in the Fe end-member. Because no anharmonic contribution to the Debye-Waller factor appeares at room temperature, we evaluated the harmonic effective pair potential V(u)=1⁄2αu² for Ga-O and Fe-O bonds from the Debye-Waller factors. The derived potential values, α, for Ga-O and Fe-O bonds were 18.0(9)×10⁻¹² and 9.9(9)×10⁻¹²erg/Ų, respectively. The values of the EXAFS Debye-Waller factors for Y with respect to Ga are small due to the short-range correlation of vibration. The largely correlated displacement between the Ga atom and second-nearest Y atoms should be attributed to the cation-cation interaction or the repulsive force between the cations across the shared edges. It is concluded that the cation-cation interaction in garnet structures, especially between the tetra- and dodecahedral sites, plays an important role in the stability of the crystal structure.

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