Redetermination of synthetic warwickite, Mg₃TiO ₂(BO₃)₂

Single crystals of warwickite, trimagnesium titanium(IV) dioxide bis-(borate), Mg₃TiO₂(BO₃)₂, were prepared by slow cooling of the melt. The title compound is isotypic with Co₃TiO₂(BO₃)₂. In contrast to the previous refinement of warwickite [Moore & Araki (1974). Am. Mineral. 59, 985-1004], that reported only isotropic atomic displacement parameters for all atoms, anisotropic displacement parameters of all atoms were refined during the current redetermination. All atoms are situated on special positions (site symmetry.m.). One of the two Mg sites is statistically disordered with Ti atoms (ratio 1:1), while the other is fully occupied by Mg atoms. The occupancy ratio of the Mg and Ti atoms is similar to that reported in the previous study. Metal atoms (M) at the Ti/Mg and Mg sites are coordinated by six O atoms in form of distorted octa-hedra. Four edge-sharing MO₆ octa-hedra form M 4O ₁₈ units, which are connected by common corners into layers parallel to (010). Adjacent layers are linked along [010] into a framework structure by sharing common edges. The B atoms are located in the triangular prismatic tunnels of the framework.