New Data on the Isomorphism in Eudialyte-Group Minerals. III: Modular Structure of K Analog of Centrosymmetric Labyrinthite

Abstract

The crystal structure of a new modular eudialyte-group mineral from the Khibiny massif—potassium-rich analog of labyrinthite—has been investigated using X-ray diffraction. The trigonal-cell parameters are found to be a = 14.2167(6) Å, c = 60.798(3) Å, and V = 10641.8(1) ų; sp. gr. R\(\bar {3}\)m. The crystal structure is refined to the final reliability factor R = 5.0% in the anisotropic approximation of atomic displacements using 1321F > 3σ(F). The idealized mineral formula (Z = 3) is Na₃₀K₅Fe₃Ca₁₂Zr₆Si₅₂O₁₄₄(OH,O,H₂O)₆Cl₃. The problems of the central symmetry of modular representatives of the group and the isomorphism at М2 sites of alluaivite module are discussed.