In this paper we review the distribution, composition and relationships among the four TiO2 minerals [anatase, brookite, rutile and TiO2 (B)] and discuss the possible importance of a fifth phase, TiO2 II, which has a stability field at high pressure. We also consider the geological application of kinetic data for the anatase to rutile transformation. Previous experimental studies of this reaction were carried out between 610 degrees and 1000 degrees C. Extrapolation using reported rate laws over geologic time results in predicted and natural distribution patterns that are broadly consistent. Experimental data reported here show enhanced anatase-rutile transformation rates if the anatase is very finely crystalline. We attribute the rapid transformation in these materials to a lower activation energy (similar to 60 kcal mol(-1) compared to 80-148 kcal mol(-1) reported previously). Rapid coarsening accompanies early, rapid transformation. Declining coarsening rates correlate strongly with declining transformation rates and extrapolate to those calculated from published data for coarsely crystalline material. The first-formed crystals of rutile have average volumes approximately eight times greater than those of coexisting anatase, suggesting that anatase must achieve a critical particle size before it transforms to rutile. Dopants can affect both coarsening and transformation rates. Y ( 1%) retards both coarsening of anatase and its transformation to rutile. However, Cr and Ta have very little effect on coarsening rates but significantly reduce transformation rates. Cr in anatase lowers both the transformation activation energy (similar to 47 kcal mol(-1)) and the frequency factor (similar to 10(11) hr(-1)). Together these result in comparable transformation rates at temperatures similar to 50 degrees C higher than in pure anatase; Ta exerts a similar effect. The distribution of the impurities (within particles vs. on the surface) may largely explain differences between Y and Ta or Cr. Results suggest that quantification of the dependence of the transformation kinetics on impurities may allow the interpretation of anatase-rutile distribution patterns to provide time-temperature information in low-grade metasediments.
