On the Isomorphism of Sodium at the M(2) Site in Eudialyte-Group Minerals: The Crystal Structure of Mn-Deficient Manganoeudialyte and the Problem of the Existence of the Na-M((2))-Dominant Analogue of Eudialyte

Sodium plays an important role in the crystal structures of eudialyte-group minerals given that it can occupy different crystallographic sites. Predominantly, it distributes between the N(1-5) sites situated in the large cavities of the heteropolyhedral framework. Rarely, Na occupies split sites of the M(2) microregion where it can predominate over other elements (predominantly Mn, Fe2+, and Fe3+). The crystal structure of the Mn-deficient manganoeudialyte from the Lovozero alkaline complex (Kola Peninsula, Russia) has been refined. The trigonal unit-cell parameters are: a = 14.1848(2) angstrom, c = 30.4726(3) angstrom, V = 5309.90(11) angstrom(3). The sample is a rare example of a high-sodium and high-calcium representative of the eudialyte group with Fe + Mn < 2 apfu. The idealized formula is Na14Ca6[(Mn,Fe)(2)Na]Zr3Si2[Si24O72]O(OH)center dot 2H(2)O with bivalent components, Mn2+ and Fe2+, dominating at the M(2) site. The regularities of isomorphism involving Na-M((2)) in EGMs and the problem of the existence of the Na-M((2))-dominant analogue of eudialyte are discussed. The new data obtained in this work confirm the previous conclusion that the complete isomorphism between Ca-deficient and Ca-rich members of the eudialyte group cannot be realized in frames of a single-space group (R3m, R-3m or R3). Thus, the existence of the Na-M((2)) analogue of eudialyte remains questionable.