Effect of Ti substitution in biotite-1M crystal chemistry

Crystal structure refinements were performed on eight Ti-rich biotite-IM crystals from lamproitic rocks (Murcia and Albacete provinces, Spain), a monzonitic-alkali syenite pluton (Norway), an alkaline gabbro-peralkaline syenite association (Quebec, Canada), and a quartz diorite (Calabria, Italy). The refinements, carried out in space group C2/m, gave R values between 0.019 and 0.030. The crystals cover a Ti range from 0.29 to 1.05 atoms pfu (O + OH + F = 24) and a ratio of Mg/octahedral occupancy between 0.4 and 0.7. The structure refinements show the following: (1) In all samples the octahedral trans M1 site is larger and flatter than the cis M2 sites because of the preferential ordering of Fe plus Ti with respect to Mg. (2) The octahedral M1 site distortion increases as Ti increases and decreases as Mg/octahedral occupancy decreases. (3) For samples with Mg/octahedral occupancy >0.5, the refined occupancies of the M1 sites show that Ti⁴⁺ substitution is balanced by octahedral vacancies; M2 sites have features similar to those in Mg end-members because of the preferential ordering of Mg. (4) Octahedral sheet thickness in (Ti,Mg)-rich biotite is smaller than that in Fe-rich biotite.