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

Abstract

The crystal structure of the titanium-rich mineral wadeite K₂(Ti_0.55Zr_0.45)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) Å and c = 10.0611(9) Å; 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(Ti_0.55Zr_0.45)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.