THE INFLUENCE OF WATER ON THE ENVIRONMENT OF TRANSITION-METALS IN SILICATE-GLASSES

The effect of water (= H2O + OH) on the environment of Co2+ and Ni2+ i n albite glass was investigated by electronic absorption spectroscopy. The visible spectra of Co2+-doped glasses change only slightly over t he range of water concentrations studied. Co2+ is in a distorted tetra hedral environment, producing a dark blue color in the glasses. Up to about 5 wt% water, Ni2+-doped glasses are brown and only minor variati ons in the spectra are seen. At 5.6 wt% water, however, the color of t hese glasses changes abruptly from brown to light green and a new type of absorption spectrum is observed. Three bands are observed in the v isible spectra of the brown glasses. Two bands near 20 500 cm(-1) can be assigned to Ni2+ in a distorted octahedral environment. A third ban d at 15 500 cm(-1) could either be due to the distorted octahedral sit e or to a small amount of tetrahedrally coordinated Ni2+. The spectra of the green glasses with 5.6 and 5.7 wt% H2O resemble closely spectra of aqueous NiCl2-solution containing the [Ni(H2O)(6)](2+) complex. Th e formation of such a hydration shell around transition metals in hydr ous silicate melts should strongly effect partitioning of these elemen ts between silicate melts, minerals, and a metal phase. Consideration of ligand field stabilization energies, suggests that hydrated Ni2+ is stabilized in the melt such that mineral-melt and metal-silicate melt partition coefficients decrease by one to two orders of magnitude rel ative to a dry melt at 1100 degrees C.