Fractionation of hydrogen isotopes caused by the difference of the crystal structure of hydrous minerals.

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The purpose of this study was to present results for D/H fractionation in the crystal water-saturated solution-water vapor system of hydrous minerals. The D/H fractionation factors between the crystal water in crystalline hydrates and the saturated solution of transition metal dichloride, α_<CW-st.sol>, were experimentally determined under equilibrium condition. The empirical relationship between lnα_<C.W.-st.sol> and 1/T^2 on the crystal water of cobalt dichloride hexahydrate in the temperature range from 10 to 43℃ was obtained as : 10^3ln α_<CW-st.sol> = -2.46(10^6/T^2) + 17.6. The temperature dependence of In α_<C.W.-st.sol> is very small in comparison with that of the D/H fractionation factor between pure water and its vapor. The crystal water of cobalt dichloride dehydrate exhibits larger depletion of deuterium than that of cobalt dichloride hexahydrate. The value of α_<CW-st.sol> in cobalt dichloride dehydrate shoves similar to that of copper dichloride dehydrate at 25℃. The D/H fractionation factor between the crystal water in crystalline hydrate and saturated solution strongly depends on the structure of crystalline hydrate rather than the chemical species of cation and/or the crystallized temperature.In order to ensure that the vapor pressure of H_2O and D_2O is accurately and precisely obtained using the capacitance manometer, the apparatus of the vapor pressure measurement was made by Pyrex glass tube and was produced a vacuum. The dissociation pressure far CoCl_26H_2O and CoCl_26D_2O and the vapor pressure in equilibrium with the saturated solution of CoCl_2 in H_2O and D_2O were measured. It was found that the dissociation pressure of crystalline hydrate in H_2O is higher than that in D_2O and is slightly different from the vapor pressure in equilibrium with its saturated solution.