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 M(2)Na-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) Å, c = 30.4726(3) Å, V = 5309.90(11) Å3. 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)2Na]Zr3Si2[Si24O72]O(OH)·2H2O with bivalent components, Mn2+ and Fe2+, dominating at the M(2) site. The regularities of isomorphism involving M(2)Na in EGMs and the problem of the existence of the M(2)Na-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 M(2)Na analogue of eudialyte remains questionable.