Experimental studies on decomposion of olivine and crystallization of chrysotile under hydrothermal conditions

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Some hydrothermal experiments, especially rate studies, were made on hydration of olivines in pure water and also in dilute sodium hydroxide solution. Oxide mixtures were also used some purposes. The results are as follows. 1. Several natural olivines held at 250°C to 350°C and 1000kg/cm2 for 30 days to 80 days in pure water were not hydrated except in the case of iron-poor specimen. In dilute sodium hydroxide solution, however, all specimens used in experiments were changed to chrysotile+magnetite+brucite+amorphous substance at about 330°C and 300kg/cm2. 2. At about 450°C, forsterite+quartz( serpentine composition) in pure water changed to talc+forsterite in one day and the latter mineral assemblage remained stable even after 30 days. Change to chrysotile as expected from the phase diagram of MgO-SiO2-H2O by Bowen and Tuttle has not been observed in the present experiment. 3 Ratlstudies for hydration of synthetic forsterite were carried out in dilute sodium hydroxide solution at 250-330°C and 20-300kg/cm2. It is indicated that the decomposition velocity of forsterite is proportional to concentration of sodium hydroxide solution. It is also indicated that crystallization velocity of chrysotile is smaller than the decomposition velocity of forsterite. Hence it is estimated that there is some kind of amorphous substance which coexists with chrysotile and brucite. 4. Rate studies on crystallization of chrysotile from the oxide mixture (2MgO+SiO2) in pure water at 250°C and 300°C and 50kg/cm2 were made. Solid phase which was amorphous or poorly crystallized was washed with 10% NH4C1 solution, and free Mg ions in the solution was analyzed by means of chelatometric titration. The composition of amorphous substance after 7 hours was almost the same as that of serpentine. It is concluded that the amorphous substrance is formed at first by the hydration process of forsterite, followed by the crystallization of chrysotile from it.