STRUCTURAL ENVIRONMENT OF NICKEL IN SILICATE GLASS MELT SYSTEMS .2. GEOCHEMICAL IMPLICATIONS

The two Ni coordination states put in evidence in Part 1 indicate distinct connections with the silicate glass structure. If low field-strength cations are available for charge compensation, Ni occurs mainly as four-coordinated Ni([4]Ni) and belongs to the silicate framework. The higher the field strength of the charge-compensating cation, the lower the relative amount of Ni present in this coordination state. The Ni-O-Si (alpha) angle keeps a value close to 130-degrees whatever the associated cation. This intertetrahedral angle is smaller than that measured in alkali aluminosilicate glasses, which indicates a smaller repulsion between [4]Ni and Si than between Al and Si. Five-coordinated nickel ([5]Ni) is present in glasses containing higher field strength cations (Ca, Na, Mg). Bond length-bond strength considerations show that [5]Ni may only bond to oxygens belonging to the tetrahedral network (including bridging oxygens). [5]Ni inherits the dynamic structure of silicate melts and appears to be the consequence of the fast oxygen motion due to the viscous flow, a structural property which is quenched in the glass. Both Ni coordination states are observed in all silicate glasses, and their proportion only depends on glass composition. These structural data provide a rationale for understanding Ni behavior in natural magmas because they are independent of the Ni content of the glass over more than two orders of magnitude (0.1-27 wt% NiO). Crystal Field Stabilization Energy (CFSE) has been calculated for [5]Ni and [4]Ni in glasses; by considering the Ni-site distribution observed in the compositions studied, the average CFSE varies between 45 and 65 kJ . mol-1. There is thus a strong preference of Ni for the octahedral sites of all rock-forming minerals which explains, at least in part, the strong enrichment of Ni in olivines crystallizing from a basaltic melt.