COMPOSITIONAL DEPENDENCE OF THE ACTIVITY OF NICKEL IN SILICATE MELTS

The effect of melt composition on the solubility of Ni in haplobasalti c melts was investigated at 1 atm via additions of SiO2 (Quartz), Mg2S iO4 (Forsterite), and Na2SiO3 (Na-metasilicate) to the 1 atm diopside- anorthite eutectic melt composition. The experimental method applied w as the mechanically assisted equilibration technique of Din well et al . (1994). Quenched glass samples were analyzed for major elements and Ni by the electron microprobe, and Ni was also determined in all sampl es by ICP-AES techniques. The Ni solubility and hence activity of NiO is not influenced by the addition of either SiO2 or Mg2SiO4 to the 1 a tm anorthite-diopside eutectic composition. In contrast, addition of N a2SiO3 leads to a decrease in the Ni solubility up to 20 wt% of Na2SiO 3 in the melt. Further addition results in a reversal of this trend, i .e., an increase of the Ni solubility up to about 45 wt% Na2SiO3. This observation is likely due to an exchange reaction of 2 Na for each Ca and/or Eng atom stabilizing aluminate tetrahedra in the melt structur e. After all aluminate tetrahedra are stabilized by Na atoms, further addition of Na-metasilicate results in a network-modifying role of Na in the melt structure, creating new coordination possibilities for Ni. Activity coefficients for NiO in these melts have been calculated and compared with the literature. The present work leads to the conclusio n that melt composition plays a minor role in determining the melt-oli vine Ni partitioning. Other factors, amongst them temperature, are exp ected to play a much larger role. Copyright (C) 1997 Elsevier Science Ltd.