The review considers various complex mechanisms of isomorphism in the eudialyte-group minerals, involving both key positions of the heteropolyhedral framework and extra-framework components. In most cases, so-called blocky isomorphism is realized when one group of atoms and ions is replaced by another one, which is accompanied by a change in the valence state and/or coordination numbers of cations. The uniqueness of these minerals lies in the fact that they exhibit ability to blocky isomorphism at several sites of high-force-strength cations belonging to the framework and at numerous sites of extra-framework cations and anions.

A review of the crystal chemistry of Fe-deficient eudialyte-group minerals is given. Specific features of cation distribution over key sites in the crystal structure, including partial substitution of Fe²⁺ with Na, Mn and Zr at the M2 site are discussed. It is concluded that Na-dominant (at the M2 site) eudialyte-group members (^M2Na-EGMs) are markers of specific kinds of specific peralkaline (hyperagpaitic) igneous rocks and pegmatites. New data are obtained on the chemical composition, IR spectra and crystal chemistry for two samples of ^M2Na-EGMs with disordered M1 cations, which are a potentially new mineral species with the simplified formula (Na,H₂O)₁₅Ca₆Zr₃[Na₂(Fe,Zr)][Si₂₆O₇₂](OH)₂Cl·nH₂O.

An intermediate member of the raslakite–sergevanite solid-solution series has been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR spectroscopy. The trigonal unit-cell parameters of the mineral are found to be a = 14.208(1) Å, c = 30.384(1) Å, and V = 5311.80(6) ų; sp. gr. R3. The crystal structure is refined to the final reliability factor R = 4.59% in the anisotropic approximation of atomic displacements using 3394 reflections with F > 3σ(F). The idealized formula of the mineral (Z = 3) is [Na₁₂(H₃O)₂][Ca₃(Fe_1.5Mn_1.5)]Zr₃[Na₂Zr][Si₂₆O₇₂]OH₄. Splitting of the site of six-membered ring of octahedra, occupied mainly by Fe and Mn, has been established for the first time. The problems of the genesis of calcium-poor eudialyte-group minerals are discussed.

The crystal structure of a high-symmetry representative of the eudialyte group from the Norra Kärr 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) Å, c = 30.286(1) Å, and V = 5271.3 (3) ų; 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)_13.5REE_1.5]Ca₆Mn₂Zr₃[Si_25.5O_73.5]Cl · H₂O. The problems of hydrolysis and central symmetry of minerals with eudialyte-type structure are discussed.

The crystal structure and crystallochemical features of Fe-deficient manganese-rich eudialyte from the Khibiny massif (Kola Peninsula) have been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR spectroscopy. The trigonal unit-cell parameters of the mineral are found to be a = 14.199(1) Å, c = 30.305(1) Å, and V = 5291.2(2) ų; sp. gr. R3m. The crystal structure is refined to the final reliability factor R = 4.46% in the anisotropic approximation of atomic displacements, using 1937 reflections with F > 3σ(F). The idealized crystallochemical formula of the mineral (Z = 3, Roman numerals indicate the coordination numbers (CNs) of cations in the key sites) is ^N1–5(Na,H₃O)\(_{{15}}^{{{\text{V}}-{\text{X}}}}\) ^M1(Ca,Mn)\(_{6}^{{{\text{VI}}}}\) ^M2(Fe^IV,Na^VI,Na^V,Mn^VII,Mn^V)₃ ^M3(Si^IV,Nb^VI) ^M4Si^{IV Z}(Zr,Ti)\(_{3}^{{{\text{VI}}}}\) [Si₂₄O₇₂] (OH,Cl,SO₄)₃ (H₂O,OH) ⋅ 1.5H₂O. The regularities of block isomorphism in eudialyte-group minerals are discussed.