A thermodynamic model for titanium and ferric iron solution in biotite

Recent crystallographic data indicate that in biotite Ti orders preferentially onto the M2 octahedral site rather than onto the M1 site as assumed in previous solution models for K(2)O-FeO-MgO-Al(2)O(3)-SiO(2)-H(2)O-TiO(2)-O(2) (KFMASHTO) biotite. In view of these data, we reformulate and reparameterize former biotite solution models. Our reparameterization takes into account Fe-Mg order-disorder and ferric iron contents of natural biotite as well as both natural and experimental observations on biotite Ti-content over a wide range of physicochemical conditions. In comparison with previous biotite models, the new model reproduces the Ti-content and stability field of biotite as constrained by experiments with significantly better accuracy. The predictive power of the model is tested by comparison with petrologically well-characterized natural samples of SiO(2)-saturated and SiO(2)-undersaturated rocks that were not used in the parameterization. In all these tests, the reformulated model performs well.