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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

1

2

3

4

5

1

2

3

4

(Si

O

)Ø

. 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 (

) is a 3D quasi-framework consisting of the 

Si

O

, 

Si

O

, and 

1

O

 rings (

1 = Ca, Mn

, Fe

, Na, 

, Y, Sr; coordination numbers are denoted by Roman numerals) connected via 

2O

 polyhedra and 

O

 octahedra (

2 = Fe

, Fe

, Mn

, Mn

, Mg, Zr, Ta, Na; 

 = Zr, Ti, Nb) and containing additional 

3 and 

4 sites which are situated at the centers of two nonequivalent Si

O

 rings and can be occupied by 

Si, 

Nb, 

Ti, and 

W, as well as subordinate Al, Na, and other components whose charges vary from +1 to +6 ^[2][3] (

). In the structures of most EGMs, including eudialyte 

., 

1 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 (

1.1

1.2

O

) and symmetry lowering from the space group 

3

 or 

-3

 to 

3. In some samples, a splitting of the 

1 site ^[4] or one of the

1.1/

1.2 sites ^[5] into two sub-sites located at short distance of ~0.2 Å from each other takes place. The 

1–

4 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

, Ca

, Mn

, Sr

, Ba

, Pb

, Y

, 

, and H

O

) and (in some samples) water molecules occupy five sites, 

1–

5, which are typically split and can be partly vacant. Cations other than Na

 show a tendency to concentrate at the 

3 and 

4 sites. Some of these cations (K

, Ca

, Mn

, Sr

, Ce

, and H

O

) are species-defining ones in several representatives of the eudialyte group. The Ø anions (Ø = O, OH) coordinate the 

2, 

3, and 

4 sites. Additional anions (Cl

, F

, OH

, S

, SO

, and CO

) and water molecules occur at the 

1 and 

2 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

2, 

3, and 

4, as well as at the 

 and 

 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 

1–

5, 

2, 

3, 

4, and 

 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

 ~ 14.2 Å, 

 ~ 30 Å, members of the eudialyte group with modular structures and doubled 

parameter are known. Their unit cells contain two eudialyte-type modules which differ from each other by local situations around key sites ^[12].