THE SOLUBILITY AND OXIDATION-STATE OF NICKEL IN SILICATE MELT AT LOW-OXYGEN FUGACITIES - RESULTS USING A MECHANICALLY ASSISTED EQUILIBRATION TECHNIQUE

The solubility of Ni in a silicate melt has been measured using a new, mechanically assisted equilibration technique over a wide range of co ntrolled f(O2) values. The melt composition corresponds to the 1 atm e utectic in the system CaAl2Si2O8-CaMgSi2O6 + 10 wt% CaO. The experimen ts were performed at 1300-degrees-C and over an f(O2) range of 10(8.5) to 10(-13.75), and over a temperature range of 1270 to 1390-degrees-C at a constant gas mixing ratio (CO2/CO = 1:1). The experiment consist s of a sample of melt contained within a crucible of Ni metal and held in a 1 atm gas mixing furnace. A Ni spindle is entered into the sampl e from above and continuously rotated at a constant angular velocity u sing a viscometer head. The stirring of the sample serves to accelerat e the approach to equilibrium between the liquid sample and the metal crucible (and spindle). This arrangement allows relatively rapid equil ibration of Ni content following changes to higher or lower f(O2) valu es. Samples of the melt may be taken at any time for analysis, and thu s the equilibrium solubility of Ni in the silicate melt may be determi ned from unambiguous experimental reversals. The Ni contents of sample s, analysed both by INAA and by ICP-AES, range from 25 to 5300 ppm. Th e data presented in this paper indicate that the oxidation state of Ni in the investigated melt is Ni2+ over the entire range of f(O2) inves tigated. This conclusion contrasts with recent reports in the literatu re of an inflection in the f(O2) dependence of Ni solubility, which ha s been interpreted as solution of neutral Ni at low f(O2) (MORSE et al ., 1991; COLSON, 1992; EHLERS et al., 1992). We also present data for the temperature dependence of Ni solubility in the investigated melt. The solubility decreases with increasing temperature at constant f(O2) . The present results are in good agreement with the metal-loop-equili bration experiments reported by HOLZHEID et al. (1994).