Experimental syntheses of clay minerals from obsidian were performed in 0.001N and 0.001N HNO
3 solutions at 150°C and 200°C for 1 to 60 days. Formation and transformation processes of the clay minerals during the reactions were examined by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, infrared absorption spectroscopy, and X-ray photoelectron spectroscopy. At 200°C reaction, a small amount of boehmite was formed at the early stage, and spherical kaolinite appeared and grew successively as reaction proceeded in 0.01N HNO
3 solution. On the other hand, a flaky smectite was mainly formed in 0.001N HNO
3 solution. At 150°C reaction, boehmite, allophane, spherical halloysite, and platy kaolinite were produced in both 0.01N and 0.001N HNO
3 solutions. Based on a stability diagram for the system of Na
2O-Al
2O
3-SiO
2-H
2O, it was found that the spherical and platy kaolinites and smectite were formed as a stable phase, whereas allophane and halloysite appeared as a metastable phase. In this paper, we discussed the following problems: 1) formation conditions of kaolinite and smectite, 2) formation conditions of spherical and platy kaolinites, and 3) formation conditions of kaolinite and halloysite.
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