Trace element partitioning between wollastonite and silicate-carbonate melt

We have performed an experimental study of the influence of varying size and charge on cation partitioning between wollastonite and silicate-carbonate melt in the system CaCO₃-SiO₂. The experimental conditions (3 GPa, 1420°C) lie close to the wollastonite II tc/I tc phase boundary. Results for 1+, 2+, 3+ and 4+ partitioning show parabolic dependence of partition coefficients on ionic radius, which can be fitted to the elastic strain model of Blundy and Wood (1994), wherein partitioning is described using three parameters: site radius (r₀), site elasticity (apparent Young's Modulus) and the 'strain-free' partition coefficient (D₀) for an element with radius r₀. The apparent Young's Modulus of the Ca site in wollastonite, obtained from modelling the 2+ partitioning data, is 99 ± 3 GPa, similar to the bulk-crystal value for the polymorph wollastonite I tc. r₀ decreases with increasing charge on the substituent cation, while D₀ also shows an approximately parabolic dependence on charge, with a maximum for 2+ cations. Partition coefficients for divalent cations Zn, Co, Fe, Cd, Mn and Pb are lower than would be predicted from their ionic radii alone, indicating a preference for the melt. This may be a consequence either of cation-carbonate complexation in the melt, or of the more distorted nature of cation co-ordination environments in melts.