In the context of the application of the TitaniQ geothermobarometer, especially to quartz veins or chemogenic quartzites. Also of interest are the minimum values of activity coefficient when using other geothermobarometers.
The figure shows an example of assessing the activity of TiO2 of work Huang et al. https://doi.org/10.1016/j.gca.2012.01.009
In the work of Bestmann and Pennacchioni very low Ti activities