Experiments on Distribution of Divalent Metal Cations between Olivine Solid Solutions and Aqueous Chloride Solutions at 700 °C and 100 MPa

Experiments on divalent cation distribution between Mg–Fe olivine solid solutions and 1 mol/L (Ni²⁺, Mg²⁺, Co²⁺, Zn²⁺, Fe²⁺, Mn²⁺)Cl₂ aqueous solutions were carried out at 700 °C and 100 MPa. Regardless of the composition of the Mg–Fe olivine solid solutions, the tendency of divalent cation distribution into the olivine solid solutions against the aqueous chloride solutions was in the order of Ni²⁺ > Mg²⁺ > Co²⁺ > Mn²⁺ > Fe²⁺ > Zn²⁺. Excluding Zn²⁺, this order corresponds to the order of the sixfold coordinated ionic radius in olivine. This indicates that a smaller ionic radius allows the cation to more easily enter the olivine solid solutions. However, Zn²⁺ showed a negative distribution anomaly and did not easily enter the sixfold coordinated site of the olivine solid solutions, regardless of the ionic radius. This is because Zn²⁺ has a strong preference for fourfold coordinated sites. The Mg–Fe olivine solid solutions with more fayalite endmembers have smaller differences in the distribution coefficient of cations. This means that the gradient of the PC–IR curve becomes smaller as the content of fayalite endmembers increases. This trend indicates that fayalite consisting of a large cation (Fe²⁺) tends to exchange other cations more easily than forsterite consisting of a small cation (Mg²⁺).