Aluminum solubility in rutile (TiO2)

The solubility of aluminum in rutile has been studied in the systems Al2O3–TiO2–H2O and Al2O3–SiO2–TiO2–H2O at 700–1200 °C and 0.075–3.3 GPa. Electron probe microanalysis (EPMA) measurements of rutile crystals grown in equilibrium with corundum show that the concentration of Al increases with increasing temperature, pressure, and oxygen fugacity. Solubility is enhanced by the addition of Nb and reduced by the addition of trivalent cations. These results are consistent with the substitution of Al3+ for Ti4+ on normal cation sites and the simultaneous incorporation of positively charged hydrous defects for charge compensation. Parameters for a thermodynamic model of Al substitution in rutile are calculated and the merits of using the model as a geochemical tool are discussed.