Experiments on the stability of aqueous metal chloro complexes in supercritical hydrothermal solutions

为了获得金属在超临界水热溶液中水形态的信息，研究了在两相矿物几乎互不混溶的情况下，NaCl对矿物与氯水溶液离子交换平衡的影响。本研究采用CaWO_4-MeWO_4体系对Mg^<2+>、Sr^<2->、Fe^<2+>、Mn^<2+>、Co^<2+>和Ni^<2+>进行了水相态实验，采用CaTiO_3MeTiO_3体系对Zn^<2+>进行了水相态实验。实验条件主要为600℃，1kb。对于Mg^<2+>和Sr^<2+>，尽管NaCl的增加，但与两固相共存的氯水溶液的Ca/(Ca+Me)比保持不变。这表明在超临界水热溶液中，Mg^<2+>和Sr^<2->作为中性物质溶解，而不是作为高阶氯配合物溶解。另一方面，Fe^<2+>、Mn^<2+>、Co^<2+>、Ni^<2+>和Zn^<2+>的Ca/(Ca+Me)比值随着NaCl的增加而显著降低。这可归因于金属的高阶氯配合物的形成。在实验条件下假设金属以三氯配合物存在，三氯配合物的形成常数依次为Mn^<2+>>Co^<2+>>Fe^<2+>>Ni^<2+>。根据配体场理论，假设金属在超临界水热溶液中形成低自旋态的四面体氯配合物。对于Zn^<2+>，实验分别在500 ~ 700 ~ 1kb和600 ~ 0.5kb的温度下进行。结果表明，三氯络合物的形成常数随温度的升高而增大，随压力的增大而减小。这表明，过渡金属可以在较高温度和较低压力下从岩浆中有效浸出。

In order to obtain the information on the aqueous speciation of metals under supercritical hydrothermal solutions, the effect of NaCl on the ion exchange equilibria between minerals and aqueous chloride solution under the coexistence of two phase minerals almost immiscible to each other. In the present study, experiments on the aqueous speciation for Mg^<2+>, Sr^<2->, Fe^<2+>, Mn^<2+>, Co^<2+> and Ni^<2+> were carried out using the CaWO_4-MeWO_4, system and for Zn^<2+> using the CaTiO_3MeTiO_3 system. The experimental conditions are mainly 600ﾟC and 1kb.As for Mg^<2+> and Sr^<2+>, the Ca/(Ca+Me)ratios of the aqueous chloride solution coexisting with the two solid phases are constant in spite of the increase of NaCl. This indicates that under supercritical hydrothermal solutions, Mg^<2+> and Sr^<2-> dissolve as neutral species, but not as higher-order chloro complexes. On the other hand, for Fe^<2+>, Mn^<2+>, Co^<2+>, Ni^<2+>, and Zn^<2+> the Ca/(Ca+Me) ratios decreased significantly with the increases of NaCl. This can be attributed to the formation of the higher-order chloro complexes of the metals. On the assumption that the metals exist as tri-chloro complexes under the experimental conditions, the formation constants of the tri-chloro complexes are in the order of Mn^<2+>>Co^<2+>>Fe^<2+>>Ni^<2+>. Based on the ligand field theory, it is supposed that the metals form tetrahedral chloro complexes with low-spin state under supercritical hydrothermal solutions.As for Zn^<2+>, the experiments were performed at 500 to 700ﾟC,1kb and 600ﾟC,0.5kb. As a result, it is clarified that the formation constant of the tri-chloro complex incerases with the increase of temperature but decreases with the increase of pressure. This suggests that transition metals can be leached effectively from magmas under higher tempearature and lower pressures.