New Data on the Isomorphism in Eudialyte-Group Minerals. V: Crystal Structure of an Intermediate Member of the Manganoeudialyte‒Ilyukhinite Isomorphous Series

The crystal structure of a high-symmetry representative of the eudialyte group from the Norra Karr massif (Sweden) has been investigated by X-ray diffraction analysis and IR spectroscopy. The trigonal unit-cell parameters are found to be a = 14.1767(4) A, c = 30.286(1) A, and V = 5271.3 (3) A3; sp. gr. R$$\overline 3 $$m. The crystal structure is refined to the final reliability factor R = 6.4% in the anisotropic approximation of atomic displacements using 1071 reflections with F > 3σ(F). The idealized formula of the mineral (Z = 3) is [(Na,Н3О)13.5REE1.5]Ca6Mn2Zr3[Si25.5O73.5]Cl · H2O. The problems of hydrolysis and central symmetry of minerals with eudialyte-type structure are discussed.