Ti/Zr isomorphism in wadeite: The crystal structure of the titanium-dominant K2(Ti0.55Zr0.45)Si3O9 member of the series

The crystal structure of the titanium-rich mineral wadeite K2(Ti0.55Zr0.45)Si3O9 from rischorrites of the Khibiny Alkaline Massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0459): a = 6.8611(6) A and c = 10.0611(9) A; space group P63/m, Z = 6, D x = 3.03 g/cm3. It is shown that the unit-cell parameters and volume of the mineral of mixed (Ti/Zr) composition are naturally intermediate between those of the terminal members of the isomorphous wadeite-based K2ZrSi3O9–K2(Ti0.55Zr0.45)Si3O9–K2TiSi3O9 series. The expected correlation is due to the ionic radii of Zr4+ and Ti4+ which determine the lengths of Zr/Ti–O bonds in octahedra. The data of field observations and microscopic studies show that the Ti-dominant wadeite is formed on the basis of primary zirconium mineral in the course of a late imposed process under unique geochemical conditions.