Titanium complexation with fluoride and chloride in supercritical aqueous fluids

The high field strength elements (HFSE, i.e. Ti, Zr, Hf, Nb, Ta, W, Th) reside in minerals that are traditionally considered to be nominally insoluble in aqueous fluids, but in recent years experimental and field-based evidence to the contrary has been presented. Here, first principles molecular dynamics simulations are used to demonstrate that at low concentrations both fluoride and chloride form complexes with the lightest HFSE titanium in supercritical fluids under subduction zone pressure-temperature conditions (1000 K and fluid densities from 0.6 – 1.5 g cm-3). Simulations provide insight into the structure and dynamics of the complexes and their evolution with density. Under all studied conditions, fluoride and chloride form aqueous complexes with titanium, i.e. hydrated TiF and TiCl. With increasing density, an increase in the number of first hydration shell water molecules is observed for both TiF and TiCl complexes. This increase is accompanied by (a) (re-)organisation of the first hydration shell atoms into a different geometry (b) an increase in the Ti-O bond lengths in Ti-F complexes but a decrease in Ti-Cl complexes and (c) an increase in Ti-Cl and Ti-F bond distance. The net effect is an overall volume expansion of the first hydration shell by ~5.7 Å3 for aqueous TiF and TiCl complexes with increasing density from 0.6 – 1.5 g cm-3. Hydration enthalpy calculations show that complexation reactions of the type observed here will increase the solubility of Ti, qualitatively consistent with recent experimental data showing enhanced rutile solubility in the presence of Cland F-.