Studies on synthesis of allophane and imogolite are briefly reviewed.
Imogolite is synthesized by heating partially neutralized and slightly re-acidified solutions containing monomeric silicic acid and aluminum salts. The optimum Si/Al molar ratio is slightly greater than 0.5. Allophane is synthesized by heating neutralized solutions which have Si/Al molar ratio of 0.5 to 4. In the light of recent studies on the reactions between monomeric silicic acid and aluminumions, imogoliteis considered to from by the growth of ionic “proto-imogolite” Allophane is considered to form by reorganization of the structure of precipitated noncrystalline aluminum silic ates.
The structure and properties of synthetic allophane and imogolite are considered to be essentially the same as those of natural counterparts except that the diameter of the tubular structural unit of the synthetic imogolite is greater than that of naturalimogolite.

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Studies on the synthesis of kaolinite are reviewed. They were grouped into four categories: 1) studies on the phase diagrams or activity diagrams including kaolinite, 2) studies on procedures to synthesize kaolinite at room temperature, 3) studies on mechanisms of transformation from rock forming minerals or volcanic glasses to kaolinite, and 4) studies on partial equilibrium reaction or reaction kinetics of kaoli-nite precipitation. Recent results on the synthesis of a large quantity of kaolinite are introduced, and the morphology of the synthesized kaolinite and its reaction kinetics are discussed.

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Synthesis of tri-octahedral smectites such as hectorite, Mg-smectite, saponite, etc., was investigated for the purpose of industrial uses. They were synthesized according to the following procedure;(1) preparation of homogeneous Mg-Si precipitate from acidic Mg-Si solution, (2) washing and filtration of the Mg-Si precipitate to remove the secondary product, (3) slurry preparation from Mg-Si precipitate, NaOH, LiOH, HF, etc., (4) hydrothermal formation of smectite, and (5) drying and powdering of the product. X-ray powder diffraction patterns of the Mg-Si precipitate produced at room temperature show that this precipitate has low crystalline smectite-like structure
(Fig.1). This fact may be allow the formation of tri-octahedral smectites under relatively mild autoclaving conditions. The effects of temperature, time and concentration of Na, Li, Mg, Al and F, were examined.
Synthetic smectites were evaluated. Synthetic smectites were evaluated in comparison with Laponite, synthetic hectorite, and a purified bentonite product by methylene blue adsorption (MB), transparency (Trans), Hunter whiteness (WH), apparrent viscosity (AV), plastic viscosity (PV), yield value (YV) and gel strength (GS). The properties of representative synthetic smectites are as follows: hectorite (Si-Mg-Li-Na=8-5.4-0.6-0.7, 200°C, 2hr); MB=92m.equi./100g, Trans=89%(1% solu.), HW=96.4, AV=19cp (2.5% solu.), PV=10cp (2.5% solu.), YV=17 lb/100ft² (2.5% solu.), GS=13 1b/100ft² (2.5 % solu., after 10min.), hectorite (Si-Mg-Li-Na-F=8-5.4-0.6-0.7-3, 150°C 2hr); MB=96m. equi./100g, Trans=98%, HW=96.4, AV=34cp, PV=6cp, YV=55 lb/100ft ², GS=104 1b/100ft ², Mg-smectite (Si-Mg-Na-F=8-6-0.7-3, 200°C, 3 hr); MB=96m. equi./100g, AV=23cp, PV=6cp, YV=33 lb/100ft ², GS=88 lb/ 100ft ², saponite (Si-Al-Mg-Na-F=7.4-0.6-6-0.7-2, 300°C, 3hr); MB=104m. equi./100g, AV=17cp, PV=7cp, YV=18 1b/100ft ², GS=40 1b/100ft ².

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This paper presents a review on the syntheses of interstratified clay minerals, especially on interstratified mineral of mica/smectite. Concerning the syntheses of interstratified mineral of mica/smectite, three different methods from the stand point of view of starting materials have been reported. They are 1) formation of mixed-layer mineral from mica; 2) formation of mixed-layer mineral from smectite, and 3) formation of mixed-layer mineral from artificial gels or minerals other than mica and smectite. When mica is used as a starting material, the extraction of K ions from interlayers is necessary, and various techniques are employed to extract the interlayer K ions such as penetration of cations into vacant sites in octahedral layers; the oxidation of Fe²⁺ in octahedral layers, and the extraction of K ions from mica or artificially distorted mica by treatment with chemicals. When smectite is used as a starting material, interstratified mineral of mica/smectite is synthesized in the presence of Kions and/or other cations under hydrothermal conditions. When artificial gels or other minerals are used as a starting material, interstratified mineral of mica/smectite is synthesized under the same conditions. Interstratified mineral of chlorite/smectite has been synthesized from smectite under hydrothermal conditions, and from chlorite by oxidation of Fe²⁺ in octahedral layers. Synthesis of interstratified mineral of kaolinite/smectite from Al (OH) ₃^- montmorillonite complex under hydrothermal conditionhas been reported.

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