New Data on the Isomorphism in Eudialyte-Group Minerals. IV: Modular Structure of Titanosilicate with Replacement of Na by Mn in the Alluaivite Module

Abstract

A new modular eudialyte-group mineral—titanosilicate—from the Lovozero massif has been investigated using X-ray diffraction. The trigonal-cell parameters are found to be a = 14.069(4) Å, c = 60.63(1) Å, and V = 10393(1) ų; sp. gr. R\(\bar {3}\)m. The crystal structure is refined to final reliability factor R = 5.6% in the anisotropic approximation of atomic displacements using 1210F > 3σ(F). The idealized mineral formula (Z = 3) is Na₂₈Ca₁₂[Na_4.6][Mn_1.4]Ti₆[Si₅₂O₁₃₂(О,OH)₁₆](H₂O)_3.5Cl_1.5. The problems of the modular representatives of the group at the Zr–Ti isomorphism in the framework site and the Na–Mn isomorphism in the alluaivite module М2 sites are discussed.