Crystal chemistry of Ti3+-Ti4+-bearing synthetic diopsides

A series of Ti-doped diopsides were synthesised by flux growth in a boron-rich melt under low fO2 conditions. Following chemical characterisation by EMPA and nuclear reaction microanalysis, their crystal structures were refined by single crystal X-ray diffraction. The diopside samples were doped with increasing amounts of Ti, resulting in a coupled increase in Na and B. The tetrahedral Si is substituted by Ti4+ and B, whereas Mg is substituted by Ti3+ and Ti4+ in the M1 octahedron with simultaneous charge compensation by coupled substitution of Na for Ca in the M2 octahedron. The occurrences of Ti4+ in the tetrahedral site and Ti3+ in the M1 octahedron are in agreement with optical absorption studies performed on the same samples. The diopside framework is strongly influenced by the incorporation of Ti4+ and Ti3+, with the M1 and M2 sites becoming more distorted as the substitutions increase, accompanied by a decreasing distortion of the T-site and the T-chain and an increase of the c lattice parameter.