Experimental investigation of the simultaneous partitioning of divalent cations between löllingite or safflorite and 2 mol/L aqueous chloride solutions under supercritical conditions

抄録

In order to elucidate partition behavior of divalent cations between minerals and aqueous chloride solutions under supercritical conditions of fluid phase, we conducted experiments of the simultaneous partitioning of Ni²⁺, Mg²⁺, Co²⁺, Zn²⁺, Fe²⁺, and Mn²⁺ between löllingite (FeAs₂) or safflorite (CoAs₂) and 2 mol/L aqueous chloride solutions under the conditions of 500 and 600 °C, 100 MPa. Natural löllingite and safflorite were used as starting materials. The bulk partition coefficient (K_PB) for the cation partition reactions can be expressed as follows:

 

where x_MeAs2 indicates the molar fraction of each end–member in the solid phase. m_Meaq is the total molar concentration of Me–bearing aqueous species in aqueous chloride solution. For the experiments using löllingite, the order of partition coefficients of divalent cations was:

 Co²⁺ > Fe²⁺ > Ni²⁺ >> Mg²⁺ = Zn²⁺ ≥ Mn²⁺,

ranging from −4 to 2.3 in logarithm. The order of partition coefficients for safflorite was:

 Co²⁺ > Ni²⁺ >> Fe²⁺ > Mg²⁺ > Zn²⁺ ≥ Mn²⁺,

ranging from −6 to 0.6 in logarithm. The partition coefficient–ionic radius (PC–IR) curves of löllingite and safflorite showed almost no difference between 500 and 600 °C. The PC–IR curves of löllingite and safflorite have a steeper slope than multiple oxide minerals. Mg²⁺, Co²⁺, Fe²⁺, and Mn²⁺ exhibit partition behavior according to ionic radius. On the other hand, Ni²⁺ shows a positive partition anomaly in both löllingite and safflorite, whereas Zn²⁺ shows a large negative partition anomaly that increases with increasing As concentration in the order of pyrite < arsenopyrite < löllingite. Minerals with high ionic bond properties (e.g., ilmenite, magnetite) that have the same 6–fold coordinated sites have a wider PC–IR curve than sulfide, arsenic sulfide, and arsenide minerals with high covalent bond properties. This observation suggests that the selectivity of cations is strong in sulfide, arsenic sulfide, and arsenide minerals with covalent bonds compared with that of multiple oxide minerals with ionic bonds. However, because the electronegativity of arsenic is slightly smaller than that of sulfur, the width of the PC–IR curve seems to slightly narrow in the order of sulfide minerals (pyrite) > arsenic sulfide minerals (arsenopyrite) > arsenide minerals (löllingite).