The reaction between hydrogrossular and hydrogen chloride (HCl) gas was performed in a fixed bed reactor above 400 °C. The hydrogrossular was efficiently reacted with HCl gas in the temperature range of 400 to 950 °C and transformed into Ca 12 Al 9.9 Si 4.05 O 32 Cl 5.9 (calcium aluminum chlorosilicate). The crystal structure of the product phase Ca 12 Al 9.9 Si 4.05 O 32 Cl 5.9 was cubic, space group I43d, with unit cell a ) 12.0173(1) Å, and was similar to that of wadalite. The structure consists of a framework of (Al,Si)O 4 tetrahedra. A large cavity in the framework accommodates Ca-Cl-Ca linearly coordinated atoms.

A vacuum-pressure slip-casting technique and hot isostatic pressing (HIPing) were used to prepare high-strength and translucent alumina ceramics. A low-viscosity and highsolids-content slurry (46 vol% solids) was prepared, and a dense green compact was formed. The samples were sintered and subjected to capsule-free HIPing. Extremely highdensity (99.9%) and fine-grained (0.7 to 15 p m in diameter) alumina ceramics were obtained. The HIPed samples showed high bend strength and translucency with in-line transmittance of 30% to 46% (1 mm thick). [

Abstract--Spherical kaolinite has been synthesized for the first time from noncrystalline aluminosilicate material in hydrothermal experiments conducted between 150 ~ and 250~ and under autogenous vapor pressure. Sphernles, whose mean diameters depended on growth conditions (0.1-0.6 urn), were formed surrounding the noncrystalline aluminosilicate in all products of 150 ~ and 200*(2 runs and coexisted with platy or lath-shaped kaolinite in the products of 250~ runs. The estimated percentages of spherules in the products increased from about 1% in the 150~ days product to about 74% in 200~ days product, and decreased from about 21% in 250~ days product to 0% in 250~ days product. Lattice images by high-resolution electron microscol~e indicated that the spherules consisted of nearly concentric stackings of layers with a unit spacing of 7A, which were sectored by radiating boundaries. The mean chemical composition of the spherules (A1203/SiO 2 = 0.58) analyzed by the analytical electron microscope is similar to that of kaolinite (Al~O3/SiOz = 0.5). Even in the case of the product abundant in spherule (200~ days), X-ray powder diffraction patterns of the wetted products, e.g., of the 200~ day run, showed the 7.14-~ (001) reflection of kaolinite. The 020 reflection was broad, indicating the existence of abundant (001) layer displacements. The b axis (8.94 ,~) were within the kaolinite range (8.93-8.94 ~,). No infrared absorption peaks were observed at 3550 cm -~ which would correspond to halloysite. The differential thermal analysis slope ratios of the endothermic peak at about 550~ (1.4-2.3) were in the kaolinite range (0.78-2.39).

Abstract--Kaolinite was hydrothermally synthesized from two kinds of silica-alumina gels to examine the effect of the structure of the starting material. Two kinds of gels were prepared by precipitation at different pH conditions (pH = 9.6 and 4.2) from solutions containing water glass and aluminum sulfate. Na ions in the gels were removed with a resin before the hydrothermal treatment, but a slight amount of sulfate ions was still present in the gels. The nuclear magnetic resonance spectra of the starting gels suggested that the gel prepared at pH 9.6 consists of networks with alternating SiO4-and A104-tetrahedra (partially A106-octahedra), whereas the gel prepared at pH 4.2 consists of a sheet structure related to allophane. After the hydrothermal treatment at 220"C for 9 days, kaolinite particles with spherical shape were obtained from the former gel, and platy kaolinite was crystallized from the latter one. The difference in morphology of synthetic kaolinite was attributable to the structures of the starting gels, and the pH values in the hydrothermal reactions were not very significant to the morphology.