The reaction of magnesium hydroxide with soluble silica under hydrothermal conditions below the critical temperature.

概要

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Mg(OH)2 powder packed into a basket was placed apart from solid SiO2 in an autoclave so that the SiO2 might react with the solid Mg(OH)2 in the state of soluble silica. At given temperatures of 250–360 °C, the Mg(OH)2 changed into talc(Mg3Si4O10(OH)2) through serpentine(Mg3Si2O5(OH)4): the reaction was a first-order consecutive-irreversible reaction which can be represented as: Mg(OH)2\overset+SiO2→ Mg3Si2O5(OH)4\overset+SiO2→ Mg3Si4O10(OH)2. The reaction rate of serpentine into talc as a single reaction was also first-order. The formation of serpentine was much faster than that of talc. The reaction rate depended on the silica concentration in the hydrothermal solution rather than on the reaction temperature. In the course of the reaction, the crystallite size of the Mg(OH)2 and the serpentine was hardly changed. In addition, the crystallite size of Mg(OH)2 did not affect that of the resulting serpentine. From these results, the rate-limiting step is presumed to be the formation of the crystal nuclei of the serpentine or talc.
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