In the system Na2O-Al2O3-SiO2-H2O-TiO2, the behavior of Ti-containing structural complexes has been determined in H2O-saturated silicate melts and in coexisting silicate-saturated aqueous fluids as well as in silicate-rich supercritical fluids to 900 °C and 2225 MPa. Titanium speciation in aqueous fluids in the system TiO2-H2O was also characterized. All measurements were carried out in situ at the desired temperature and pressure using confocal microRaman and microFTIR spectroscopy. The experiments were carried out in an Ir-gasketed hydrothermal diamond-anvil cell (HDAC) with K-type thermocouples for temperature measurement and the Raman shift of 13C synthetic diamond to monitor pressure.
In the system Na2O-Al2O3-SiO2-H2O-TiO2, four or five O atoms surround a central Ti4+ cation in melts, fluids, and supercritical fluids. In this environment, the titanium solubility mechanism is the same for melt, fluid, and single-phase liquid and can be described with the equation, 4Q1Si(Na)4H2O + TiO2 ↔ 4Q0Si(HNa) + Q0Ti(Na). Subscripts denote the nature of central cation, superscripts, the number of bridging oxygens, and the symbol(s) in parentheses the type(s) of cation(s) that form bonds with nonbridging oxygen in the Q-species. The ΔH of this reaction is on the order several tens of kJ/mol. Because of the structural similarity of Ti-complexes in hydrous silicate melts and silicate-rich aqueous fluid, the fluid/melt partition coefficients also resemble one another. The partition coefficient is between 0.1 and 1 and is positively correlated with temperature, pressure, and Al/(Al+Si) of the silicate melt or aqueous fluid. In the chemically simpler system TiO2-H2O, titanium in aqueous fluid occupies the central position in oxygen polyhedra surrounded by approximately six O atoms. Here, the enthalpy change for the solution reaction is between 50 and 60 kJ/mol absent pressure corrections for volume differences between TiO2 in rutile and in aqueous solution. The Ti concentration in aqueous fluid is on the order of fractions to a few tens of parts per million. This solubility is orders of magnitude lower than in Ti solubility in silicate-saturated aqueous fluid in the system Na2O-Al2O3-SiO2-H2O-TiO2 at similar pressures and temperatures.
In natural environments such as high-grade metamorphic terranes and subduction zones, aqueous fluids are silicate-saturated. Such fluids are more efficient solvent refractory oxides, perhaps by 2–3 orders of magnitude for an oxide such as TiO2, than inferred from their solubility in pure H2O fluids.