1. Introduction

       According to the recommendation of the Commission on New Minerals, Nomenclature, and Classification of the International Mineralogical Association ^[1], the general formula of EGMs is N1₃N2₃N3₃N4₃N5₃M1₆M2_3–6M3M4Z₃(Si₂₄O₇₂)Ø_4–6X₂. In this formula, most symbols denote split sites (i.e., groups of closely spaced sites). The “rigid” part of the structures of eudialyte-type minerals (Figure 1) is a 3D quasi-framework consisting of the ^IVSi₃O₉, ^IVSi₉O₂₇, and ^VIM1₆O₂₄ rings (M1 = Ca, Mn²⁺, Fe²⁺, Na, Ln, Y, Sr; coordination numbers are denoted by Roman numerals) connected via M2O_4–7 polyhedra and ZO₆ octahedra (M2 = Fe²⁺, Fe³⁺, Mn²⁺, Mn³⁺, Mg, Zr, Ta, Na; Z = Zr, Ti, Nb) and containing additional M3 and M4 sites which are situated at the centers of two nonequivalent Si₉O₂₇ rings and can be occupied by ^IVSi, ^VINb, ^VITi, and ^VIW, as well as subordinate Al, Na, and other components whose charges vary from +1 to +6 ^[2][3] (Figure 1). In the structures of most EGMs, including eudialyte s.s., M1 cations can be disordered, but in some representatives of this mineral group they alternate in the ring of octahedra, which results in its transformation into the ring (M1.1₃M1.2₃O₂₄) and symmetry lowering from the space group R3m or R-3m to R3. In some samples, a splitting of the M1 site ^[4] or one of the M1.1/M1.2 sites ^[5] into two sub-sites located at short distance of ~0.2 Å from each other takes place. The M1–M4 sites are considered as the main species-defining “key sites” in the nomenclature of EGMs ^{[1][6][7][8][9]}.

Figure 1. Arrangement of key sites in the eudialyte-type structures viewed along (210).

       Extra-framework cations (Na⁺, K⁺, Ca²⁺, Mn²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Y³⁺, Ln³⁺, and H₃O⁺) and (in some samples) water molecules occupy five sites, N1–N5, which are typically split and can be partly vacant. Cations other than Na⁺ show a tendency to concentrate at the N3 and N4 sites. Some of these cations (K⁺, Ca²⁺, Mn²⁺, Sr²⁺, Ce³⁺, and H₃O⁺) are species-defining ones in several representatives of the eudialyte group. The Ø anions (Ø = O, OH) coordinate the M2, M3, and M4 sites. Additional anions (Cl⁻, F⁻, OH⁻, S²⁻, SO₄²⁻, and CO₃²⁻) and water molecules occur at the X1 and X2 sites located on the three-fold axis.

       Blocky isomorphism is defined as the ability of groups of atoms or ions having different configurations to replace each other in crystal structures ^[10]. Such substitutions are known for a large number of alkaline zircono- and titanosilicates ^[11]. In EGMs this kind of isomorphism is realized at the key sites M2, M3, and M4, as well as at the N and X sites. The eudialyte group is the only group of minerals in which blocky isomorphism is realized at several sites containing high-force-strength cations. Below we will use the symbols N1–N5, M2, M3, M4, and X to denote corresponding cavities (i.e., micro-regions which can contain several closely spaced sites).

       In addition to EGMs with the rhombohedral unit-cell parameters a ~ 14.2 Å, c ~ 30 Å, members of the eudialyte group with modular structures and doubled c parameter are known. Their unit cells contain two eudialyte-type modules which differ from each other by local situations around key sites ^[12].