The speciation of Ni and Co in silicate melts from optical absorption spectra to 1500°C

The optical absorption spectra of Co2+ and Ni2+ in haplogranitic glass and melt were measured from room temperature to 1500°C. For Co2+, a continuous decrease of the intensity of all bands with increasing temperature is observed both below and above the glass transition. This is attributed to a decrease of extinction coefficients with temperature. Co2+ is in a distorted tetrahedral environment both in the melt and in the quenched glass. The decrease of extinction coefficients with temperature may be related to a reduction of tetrahedral distortion. At low temperatures, a small fraction of Co2+ may be octahedrally coordinated, but spectroscopic evidence is not conclusive. Ni2+ is mostly in a distorted octahedral environment in the glass. Up to the glass transition temperature, the extinction coefficients of the bands of octahedral Ni2+ decrease slightly; however, beyond the glass transition, the intensity of the octahedral bands decrease rapidly and above 1300°C, the optical spectra are dominated by a band due to tetrahedral Ni2+, indicating a change of coordination with temperature. The equilibrium constant for the reaction Nioctahedral2+Nitetrahedral2+ is estimated to be lnK=8.42–11.13 K/T, corresponding to a reaction enthalpy of 92 kJ/mol. Preliminary measurements suggest that the speciation of Ni2+ and Co2+ in basaltic systems is similar to that observed in haplogranite. Our results imply that the partitioning of Ni2+ between minerals and silicate melts should depend more strongly on temperature than for Co2+ and most other trace elements.