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Eudialyte-group minerals (EGM) are trigonal zircono- and titanosilicates with a large unit cell (a∼14A, c∼30 or 60A) and with the general formula A(1)3 A(2)3 A(3)3 A(4)3 A(5)3 B3-6 C D {M6Z3[T24O72]} (O,OH)4-6 X2, where the A(1)-(5) sites are typically dominated by Na, K, Sr, REE; B = Fe2+, Fe3+, Mn, Zr, Na, K, C = Nb, W, Ti, Mn, Si; D = Si, Nb, M = Ca, Mn, Sr, REE, Y, Na; Z = Zr, Ti, T = Si; X = Cl, F, H2O, CO3. The basis of their zeolite-like structure is a framework of M, Z and T layers alternating along [001] in the sequence MTZT (members with a 12-layer structural motif) or MTZTM*T*Z*T* (members with a 24-layer structural motif). The M layers are made of six-membered Ca6O24 rings, T layers are made of Si9O27 and Si3O9 rings, and Z layers are made of discrete Zr(Ti) octahedra. EGMs have been found to contain about half of the elements of the periodic table.
Most of them are concentrated in the cavities of eudialyte structures, which, in addition to high-occupancy sites, host a whole series of low-occupancy sites located at close distances to each other. Different members of the eudialyte group show significant differences both in the total number of atoms occupying these sites and in the variants of their spatial arrangement. Thus, eudialytes as a whole can be regarded as multicomponent isodimorphous series or nonstoichiometric compounds with a varying number and composition of atoms in the structural sites. According to our research results, the entire diversity of such minerals observed in nature can be satisfactorily described as combinations of a limited set of stoichiometrically complete structural and chemical end members, for which the author proposed the terms "structals" and "chemals," respectively (see the Abstract "Structals" and "chemals": two fundamental end-member types of solid solutions...). On this basis, the 24 currently known EGMs have been assigned to the following 15 structure types (additional minerals of each type are listed in parentheses): (1) eudialyte R-3m, (2) kentbrooksite (andrianovite (?), feklichevite (?), ferrokentbrooksite, georgbarsanovite, khomyakovite, manganokhomyakovite, taseqite), (3) carbokentbrooksite (zirsilite-Ce, johnsenite-Ce (?)), (4) golyshevite, (5) ikranite, (6) mogovidite, (7) eudialyte R3m, (8 ) oneillite, (9) raslakite, (10) aqualite, (11) voronkovite, (12) alluaivite, (13) rastsvetaevite, (14) dualite, and (15) labyrinthite. In accordance with author's previously proposed crystal chemical systematics for EGMs, the above structure types can be grouped into the following six families of structure types based on combinations of 12- and 24-layer structural motifs with each of the three space groups R-3m, R3m, and R3 identified in eudialytes (structure type numbers are given in parentheses): eudialyte family (1), kentbrooksite family (2-7), oneillite family (8-11), alluaivite family (12), rastsvetaevite family (13, 14), and labyrinthite family (15).
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