Hydrothermal experiments in the system MgO–SiO₂–H₂O were carried out with the use of natural serpentinite as starting materials without crushing. The serpentinite specimen was prepared as a prismatic form of about 4 mm × 4 mm × 20 mm in size. A serpentinite prism was put into a silver capsule together with fine quartz powder and distilled water. Both sides of the capsule were sealed by welding. All experimental runs were made at 1,000 bars in pressure and at 360°C, 450°C, 495°C and 540°C in temperature. The duration times for maintaining constant temperature were from 16 to 1,000 hours. At temperatures above 450°C, in most experiments many white fibrous diopside crystals grew on surface of the serpentinite specimen. A talc layer was formed at the marginal part of serpentinite specimen, its thickness varied from 0 to 50 μm through all experiments. At temperatures below 360°C, serpentine minerals were preserved in the inner part of the specimen, but at temperatures above 450°C, olivine crystals were formed. These reaction processes were discussed.

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Hydrothermal reactions between natural serpentinite and quartz powder was reported in our previous paper. A thin talc layer was formed over the whole surface of serpentinite due to the reaction with quartz and H₂O. In most treatment at temperatures above 450°C, many fibrous diopside crystals grew on the surface of the serpentinite prism toward the outside. This paper gives some considerations on the migration mechanism of chemical elements responsible for these hydrothermal reactions.
The relation between average thickness and experimental conditions shows that the chemical components were transported by mean of diffusion process, and an activation energy for diffusion is 21.5 kcal/mol. Diffusion coefficient in the 5 μm thick talc layer is 10^−6.5 cm²/sec, and that in the 10 μm thick layer is 10^−5.5 cm²/sec.
The migration process of a chemical component is closely related to its solubility in hydrothermal solutions. The solubilities of MgO and CaO are related to pH of the solution. The pH values of the solution through which CaO will migrate but MgO will not, should be in a restricted condition.
During the formation of diopside crystals, CaO must have been supplied from inside of the serpentinite by H₂O solution. The rate of movement of the solution was calculated by diffusion process. The diffusion coefficient of solution in serpentinite was estimated as 4.5×10⁻⁴ cm²/sec. It may be concluded that the solution moved very easily through solid serpentinite.

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The cristobalite rocks, which are mainly composed of opal-CT, are abundant at Higashidorimura, Aomori Prefecture, Japan. The SiO₂ content is about 90 wt%. The cristobalite rocks have been expected to be a ceramic raw material as a source of silica. In this study, type A zeolite was synthesized from the cristobalite rocks in the presence of sodium aluminate, sodium hydroxide and water at the low temperature of 40°C and atmospheric pressure. The rate of the formation of the type A zeolite was governed by the concentration of sodium hydroxide, and the SiO₂/Al₂O₃ molar ratio.
When the SiO₂/Al₂O₃ molar ratio was 2, the Na₂O/SiO₂ molar ratio 2, water 50 ml per 1 g of the cristobalite rock, the type A zeolite with high crystallinity was obtained at 40°C for 216 hours and at 50°C for 72 hours. The synthesized type A zeolite showed cubic crystal habit with a maximum crystal size of 1 μm, and the chemical formula is almost the same as the theoretical one (Na₂O·Al₂O₃·2SiO₂·4.5H₂O). Present results make the cristobalite rock attractive candidates for the preparation of zeolite.

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