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, MoK-alpha radiation, 409 independent reflections with |F| > 4sigma(F), anisotropic refinement, R(F) = 0.037). Like other representatives of the hilairite structure type, calciohilairite 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+ + 1.5* + (H3O)+, where * is a vacancy.