Crystal structure of cation-deficient calciohilairite and possible mechanisms of decationization in mixed-framework minerals

The crystal structure of cation-deficient calciohilairite from the Lovozero massif (the Kola Peninsula) was established (Siemens P4 diffractometer, MoKα radiation, 409 independent reflections with | F| > 4σ(F), anisotropic refinement, R(F) = 0.037). Like other representatives of the hilairite structure type, calcio-hilairite is described by the space group R32 (a = 10.498(2) Å, c = 7.975(2) Å, Z = 3), whereas its unit-cell parameter c is reduced by a factor of two. Two positions in the cavities of the mixed zirconium-silicon-oxygen framework are occupied by Ca and Na cations in the ratio of 1: 1 (partly occupied A(1) position) and oxonium cations (H3O) + and H2O molecules in the ratio of 1: 2 (A(2) position). Different types of isomorphous replacement accompanying the formation of cation-deficient mixed-framework structures (lovozerite, vinogradovite-lintisite, labuntsovite-nenadkevichite, eudialyte, etc.) are considered. Based on the X-ray diffraction data, the following scheme of isomorphism in the structure of cation-deficient calciohilairite is suggested: 2Na+ + H2O ⇄ 0.5Ca2+ + 1.5h□ + (H3O)+, where □ is a vacancy.