The activities of NiO, CoO and FeO in silicate melts

Abstract

Solubilities of NiO, Coo and FeO in silicate melts have been determined experimentally as a function of temperature, oxidation state and melt composition. The results -show that Fe, Ni and Co are dissolved as divalent cations in silicate melts at oxygen fugacities varying from IW + 1.5 to IW − 3. The well defined valence state of these elements in the melt allows the calculation of activity coefficients. The activity coefficients of FeO, NiO and Coo in the silicate melt, calculated by assuming oxides as melt components, were found to be independent of oxygen fugacity and of temperature within a temperature range of 1300°C to 1600°C. Earlier reports on temperature-dependent NiO activity coefficients were based on a standard state for NiO in the melt of solid NiO. When the more appropriate liquid standard state is used, the temperature dependence disappears. The activity coefficients were not affected by variations in FeO (from 0 to 12 wt.%) and MgO (from 4 to 30 wt.%) contents, except for a small increase of γcoo and γ“Feo” at MgO-contents above 20 wt.%. The average activity coefficients obtained are γ Nio = 2.70 ± 0.52 (77 experiments), γ coo = 1.51 ± 0.28 (76 exp.) and γ“Feo” = 1.70 ± 0.22 (57 exp.) relative to the respective pure liquid oxide standard states and simple'oxide mole fractions. From the activities of NiO and Coo and additional thermodynamic data a distribution coefficient K D Ni/Co (Ni/Coolivine/Ni/Co-melt) is calculated. The K D Ni/Co is slightly temperature-dependent, increasing from 1.13 ± 0.28 at 1100°C to 1.34 + 0.33 at 16000C. The difference between the activity coefficients of NiO and Coo in silicate liquids is responsible for the preferred partitioning of Ni into olivine, i.e., the larger D olivine/melt Ni (NiO olivine/NiO melt) compared to D olvine/mel Cot is, contrary to general belief, not the result of preferred acceptance of Ni into the olivine structure, but of stronger rejection of NiO compared to Coo from the melt. Simple considerations based on crystal field stabilization energy (CFSE) can rationalize but not explain quantitatively the high NiO activity coefficient in silicate melts compared to Coo and FeO and the nearly ideal mixing of Ni-olivine and Co-olivine with Mg-olivine. It is suggested that the high degree of preference of Ni for octahedral coordination rather than the tetrahedral sites at high temperatures in the melt is responsible for the unusual behaviour of NiO.