CRYSTAL-STRUCTURES OF THE BODY-CENTERED TETRAGONAL TECTOSILICATES - K1.14MG0.57SI1.43O4, K1.10ZN0.55SI1.45O4, AND K1.11FE(1.11)3SI0.89O4

The title compounds are isostructural stable phases that occur on bina ry joins of mineralogic interest: SiO2-K2MgO2, SiO2-K2ZnO2, SiO2-KFeO2 ; a corresponding phase occurs on the SiO2-KAlO2 join. The structure h as I4mm symmetry, with cell dimensions a = 8.957 angstrom, c = 5.281 a ngstrom (Mg phase). The eight Si plus Mg (or Zn, Fe) atoms per cell ar e disordered over a single set of equivalent tetrahedrally coordinated sites. The tetrahedra form four-membered rings further interconnected into six- and eight-membered rings with two types of intervening larg e cavity sites: K2 sites between facing four-membered rings are full, but K1 sites in large, square channels show a smeared out partial occu pancy probably caused by K-K repulsion. The body-centered tetragonal t ectosilicate (BCTT) structure is structurally related to other major t ectosilicates, though no natural occurrences are yet known.