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New Data on the Isomorphism in Eudialyte-Group Minerals. 1. Crystal Chemistry of Eudialyte-Group Members with Na Incorporated into the Framework as a Marker of Hyperagpaitic Conditions
 
 
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Review

New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites

by
Ramiza K. Rastsvetaeva
1 and
Nikita V. Chukanov
2,3,*
1
Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics”, Russian Academy of Sciences, Leninskiy Prospekt 59, 119333 Moscow, Russia
2
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, 142432 Moscow, Russia
3
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(8), 720; https://doi.org/10.3390/min10080720
Submission received: 24 July 2020 / Revised: 14 August 2020 / Accepted: 14 August 2020 / Published: 17 August 2020
(This article belongs to the Special Issue Study of the Eudialyte Group Minerals)
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Abstract

:
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.

1. Introduction

Eudialyte-group minerals (EGMs) are typical components of some kinds of agpaitic igneous rocks and related pegmatites and metasomatic assemblages. Crystal-chemical features of these minerals are important indicators reflecting conditions of their formation (pressure, temperature, fugacity of oxygen and volatile species, and activity of non-coherent elements [1,2,3,4,5,6,7,8,9]).
A unique crystal-chemical diversity of EGMs is determined by a wide variability of their chemical composition involving more than 30 main elements and complex mechanisms of homovalent, heterovalent, and, especially, blocky isomorphism involving groups of atoms having different valency and coordination. 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.
According to the recommendation of the Commission on New Minerals, Nomenclature, and Classification of the International Mineralogical Association [3], the general formula of EGMs is N13N23N33N43N53M16M23–6M3M4Z3(Si24O72)Ø4–6X2. 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 M2O4–7 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 [1,2] (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 [10] or one of the M1.1/M1.2 sites [11] 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 [3,6,7,12,13].
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 [14]. Such substitutions are known for a large number of alkaline zircono- and titanosilicates [9]. 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 [4].

2. Blocky Isomorphism at the M2 Site

The most complex blocky isomorphism is realized in the M2 micro-region situated between rings of octahedra, M16O24 [15]. This micro-region can be populated by cations having different radii, charges, and coordination (from flat square formed by edges of octahedra belonging to two neighboring M16O24 rings to a 7- or 8-fold polyhedron): IVFe2+, VFe2+, VFe3+, VIFe3+, VMn2+, VIMn2+, IVZr, IVTa, IVNa, VNa, VINa, VIINa, VIK, VIIK, and VIIIK. The M2-cations occur in the plane of the six-membered rings which is perpendicular to the threefold axis (Figure 2). The coordination polyhedra M2O5 and M2O7 can have different orientations with respect to the plane of the square. Some examples of M2-centered polyhedra are shown in Figure 3.
The fourfold (nearly, flat-square) coordination is most typical for Fe2+ and, to a less extent, for Na. In particular, eudialyte s.s. is M2(IVFe2+)-dominant [16,17]. The crystal structures of M2(IVNa)-dominant minerals are described in [11,18,19,20,21,22,23,24]. Some other components (Zr, Hf, and Ta) can occur in the M2O4 polyhedron in subordinate amounts [1]. The mean M2–O distances vary from 2.04 Å for M2 = Fe2+ to 2.29 Å for M2 = Na (Table 1).
The fivefold M2-centered polyhedron is a square pyramid M2O4Ø (Figure 3) where O atoms belong to the neighboring M16O24 rings and Ø is OH group of the M3O3(OH)3 or M4O3(OH)3 octahedron or H2O molecule. Kentbrooksite (Na,REE)15(Ca,REE)6VMn2+3Zr3[SiNb](Si24O74)F2·2H2O (Figure 4a) was the first EGM in which such polyhedron was identified [29]. Later, VM2 site was found in different EGMs [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. Mn-dominant VM2 site occurs in the structure of andrianovite [30,31]. In the ilyukhinite structure [47,48] the M2 site has a fourfold flat-square coordination with the M2–O distances of 2.11–2.51 Å and is predominantly occupied by Mn with a minor Fe admixture. The replacement of manganese with iron at the fivefold M2 site has been established in a number of minerals close in structure to kentbrooksite including ferrokentbrooksite [32], georgbarsanovite [33,34], taseqite [35,36], feklichevite [37], golyshevite [38,39], as well as some varieties of other EGMs [1]. In the M2 micro-region of so-called “eucolite” (a variety of ferrokentbrooksite [40]), Fe atoms occupy two sites located at the distance of 0.49 Å from each other and having a joint occupancy about 100%. In most cases, differently oriented square pyramids occur in the M2 micro-region [1,41,42,43]. In siudaite [49], the M2 site is split into the M2a and M2b sub-sites which are located on both sides from the M2 square at the distance of 0.99 Å from each other and have fivefold square pyramid coordination. The population of the M2aO4Ø and M2aO4Ø polyhedra (Ø = O, OH) is Fe3+0.4Mn2+0.4 and Fe3+0.2, respectively. This conclusion was confirmed by means of Mössbauer spectroscopy. In some EGMs, M2O4Ø2 octahedra (Figure 3) are formed involving OH groups (or H2O) situated on both sides from the M2O4 square where M2 is Mn2+ (in manganoeudialyte) or Fe3+ (in ikranite: Figure 4b).
The VIIM2-centered sevenfold polyhedra are based on the flat square and include O atoms of the framework and large X anions. Positions of large cations (K+, Sr2+, or Ce3+) occurring in the M2 micro-region, are located at large distances from the plane of the O4 square. In these cases, the coordination polyhedron is supplemented by two inner O atoms of the Si9O27 ring and one or two (statistically) species anions (Cl, F, or H2O) situated at the X site on the threefold axis. As a result, a 7- or 8-fold polyhedron is formed. The coordination number of Na+ occurring in the M2 micro-region can vary from 4 to 7. The VIINa-centered polyhedron dominates in the M2 micro-region of some samples [28,44]. As a subordinate component, M2(VIINa) occurs in the structures of intermediate members of the manganoeudialyte–ilyukhinite [45] and eudialyte–sergevanite [46] solid-solution series, with the Na–O distances of 2.23(4)–2.96(3) and 2.33(1)–3.01(1) Å, respectively. Rarely, the VIIM2 polyhedron is centered by Mn or Zr.

3. Blocky Isomorphism at the M3 and M4 Sites

The micro-regions M3 and M4 at the centers of two nonequivalent Si9O27 rings can be vacant, but usually they contain tetrahedra (SiO4, rarely AlO4) or octahedra (typically, NbO6; rarely, TiO6, WO6, MnO6, and NaO6) [1] (Figure 5). The positions of M3 and M4 cations are located on the threefold axis, and their charge can vary from +1 (Na) to +6 (W). When SiO4 tetrahedron occurs in the M3 or M4 micro-region, corresponding Si9O27 ring transforms into a 10-membered disc consisting of three 5-membered rings (Figure 5b).
Three O atoms which form the base of each Si-centered tetrahedron occurring in the M3 or M4 micro-region are common with O atoms of the Si9O27 ring. The fourth O atom of this tetrahedron belongs to OH group. The M3,M4Si atoms can occur either above or below the triangular base of the tetrahedron, and OH groups belonging to the tetrahedra can be located on both sides of the Si9O27 ring in such a way that apical vertices of the neighboring tetrahedra are oriented in opposite directions. Different options involving M3 and M4 tetrahedra are shown in Figure 6. It is to be noted that the M3a and M3b tetrahedra, as well as M4a and M4b tetrahedra cannot exist in corresponding micro-regions simultaneously because of short M3aSi–M3bSi and M4aSi–M4bSi distances. The option (b) corresponds to a centrosymmetric structure, and the options (d) and (e) correspond to non-centrosymmetric ones.
The option (c) is most common, but corresponding local situation cannot be realized due to unacceptably short OH–OH distance of 1.4–1.5 Å between the vertices of the tetrahedra. However, this option can be realized statistically.
An example of the option (a) is the crystal structure of a high-silicon EMG from the Kovdor massif, Kola Peninsula, described as “low-iron eudialyte 2” [50]. In this sample, the M3 and M4 sites occupied by Si are split into pairs of partially occupied sub-sites located at short distances from each other: M3a–M3b = 0.92(6) Å and M4b–M4c = 1.04(1) Å. As noted above, a local situation when both M3b and M4a sites are occupied is impossible due to a short distance (of 1.41 Å) between OH groups belonging to corresponding tetrahedra.
Our investigations carried out on a large selection of EGMs [5,51] (Table 2) have shown that the total amount of Si atoms at the M3 and M4 sites (∑T) varies in wide ranges, from 0.88 to almost 2 atoms per formula unit (apfu) (Z = 3). The ratio of the amounts of differently oriented tetrahedra (∑Ta/∑Tb) varies from 0 to 2.31; only in two structures this ratio is equal to 1 which corresponds to the existence of center of symmetry. Both centrosymmetric structures are characterized by rather low ∑T values of 1.12 and 1.39 apfu, and only one of them corresponds to the option (a).
In Si-deficient EGMs, the M3 and M4 micro-regions contain vacancies (as e.g., in ikranite [57,62,63] and mogovidite [38,64]) and/or octahedra formed by three O atoms common with the Si9O27 rings and three OH groups. The M3 and M4 cations with octahedral coordination can occur in one of three sites located at short distances from the IVM3 and IVM4 sites which are usually occupied by Si. Consequently, the IVM3 and VIM3 sites, as well as the IVM4 and VIM4, cannot be occupied simultaneously in the same micro-region. In most cases, blocky isomorphism involving atoms with tetrahedral and octahedral coordination is realized in the M3 and M4 micro-regions. However, there are cases when these microregions contain only one type of polyhedra, namely, only tetrahedra (for example, in davinciite [52,53]), or only octahedra. The latter case is realized in the centrosymmetric high zirconium eudialyte described by Giuseppetti [65], in which the M3 and M4 sites have been refined with the same occupancy, □0.65VIZr0.35. However, the possibility of the occurrence of Zr in the M3 and M4 micro-regions was not confirmed by subsequent investigations. In most cases, octahedral sites in these micro-regions are occupied by Nb. In three EGMs (khomyakovite and manganokhomyakovite [66], and johnsenite-(Ce) [67]) the VIM3 site is occupied by W. Some examples of blocky isomorphism in the M3 and M4 micro-regions are presented in Figure 7, Figure 8 and Figure 9.
Unlike Si-centered tetrahedra, the NbO3(OH)3 octahedra located in the M3 and M4 micro-regions can be directed towards the N5 cavity between the neighboring Si9O27 rings only in the case when the N5 site is vacant. Otherwise this would result in unrealistic short distances between OH groups and N5 cations.
The option when the octahedra are oriented both outward and inward of the N5 cavity is extremely rare. This situation takes place in a low-silicon sample with high contents of niobium and titanium and a low content of zirconium, found in the Kovdor phlogopite deposit, Kola Peninsula (Figure 10). This centrosymmetric member of the eudialyte group was formed as the result of partial leaching of sodium from an earlier EGM [68]. In the micro-regions M3 and M4 of this mineral, there are two octahedral sites located at a distance of 2.19 Å from each other and populated by titanium with the occupancy factors of 50 and 30%, respectively. A TiO6 octahedron with a population of 50% is directed inside the N5 cavity, while the less populated one faces in the opposite direction from the 9-membered ring. At a distance of 0.29 Å from the Ti site, there is a site occupied by Si with the population of 20%. Thus, all positions in the M3 and M4 micro-regions are populated statistically.

4. Blocky Isomorphism at N Sites

At the N1–N5 extra-framework sites occurring in large cavities of the structures of EGMs, positional isomorphism of a statistical nature is most often observed. Below we will use the symbols N1–N5 to denote corresponding cavities (i.e., micro-regions containing several closely spaced extra-framework sites). In most cases, these positions are split and occupied by sodium; the set of closely spaced Na sites is typically coordinated by anions belonging to the same polyhedron. However, if significant amounts of other large cations enter these sites, ordering of different elements among closely spaced sites may result in changes in their anionic environment.
Most frequently, extra-framework cations other than Na occur at the N3 and N4 sites (Table 3). For example, in the structure of davinciite [52], the N3 and N4 cavities contain pairs of partially populated sites with the distances N3a–N3b = 0.65(1) Å and N4a–N4b = 0.42(1) Å. The N3a and N3b sites are partially occupied by K and (K + Na) and have 7- and 6-fold coordination, respectively. The 6-fold polyhedron is formed solely by O atoms of the framework, whereas the 7-fold polyhedron contains one H2O molecule. The N4a and N4b sites are partially occupied by (Na + Sr) and Sr and have 8- and 7-fold coordination, respectively. The larger Sr-centered 8-fold polyhedron involves two Cl atoms, and smaller one contains only one Cl atom.
A remarkable example of blocky isomorphism at the N sites is presented in a Fe- and Na-deficient EGM [41,50,69]. In this sample, sodium deficiency of almost 50% compared to eudialyte s.s. is mainly compensated by hydronium groups H3O+, as well as subordinate K+, Sr2+, and Ce3+. These cations occur in the N3 cavity and are statistically distributed over four sites, located at distances of 0.32(1) to 0.95(1) Å from each other. The coordination numbers and cation–anion distances (Å) for these sites are 10 and 2.91 for H3O, 10 and 2.87 for K, 9 and 2.73 for Ce, and 7 and 2.61 for Sr.
In most samples of EGMs, including eudialyte, manganoeudialyte, kentbrooksite, ferrokentbrooksite, carbokentbrooksite, voronkovite, raslakite, sergevanite, and oneillite, the N3 and N4 sites are Na-dominant. However, in some members of the eudialyte group the N4 site is predominantly occupied by other cations: Sr (in taseqite), REE (in zirsilite-(Ce) and johnsenite-(Ce)), Mn (in georgbarsanovite and siudaite), and K (in andrianovite: Figure 8), etc.
Large N4-cations form triads of edge-sharing polyhedra. The coordination number of the N4 site in kentbrooksite, taseqite, and georgbarsanovite is equal to 10 (Figure 11) and is characterized by the cation–anion distances from 2.471 to 2.943 Å. The N4 site in andrianovite has 8-fold coordination and the cation–anion distances from 2.557 to 2.980 Å. Mixed population is typical for the N4 site. For example, the composition of the N4 micro-region in georgbarsanovite and andrianovite is (Mn1.07Sr0.72Ce0.47Ca0.33K0.25Y0.16)∑3 and (K1.45Sr1.05Ce0.5)∑3, respectively.
In the majority of eudialyte-related minerals, the crystal chemical formula (Z = 3) contains 6 Ca atoms, which enter into the composition of the six-membered ring of octahedra, and the N3 and N4 sites are predominantly occupied by Na (Figure 12a). However, in specific environments, EGMs of a different stoichiometry are formed. In particular, there are three representatives of this group with a high content of calcium of 9 to 10 Ca apfu: feklichevite, golyshevite, and mogovidite (Table 1).
An example of EGM with an unusually high Ca content is feklichevite, Na11Ca9Fe2Zr3NbSi25O73(OH,H2O,O,Cl)5 [37]. In this mineral, Ca completely occupies the M1 site in the ring of octahedra and dominates at the neighboring N3 site. A cluster formed by three Ca-centered polyhedra sharing common vertices occurs at the threefold axis completed by six M1 octahedra (Figure 12b).
Golyshevite and mogovidite are the most Ca-rich EGMs (Table 3). In these minerals, Ca occupies the M1 site in the ring of octahedra and dominates over Na at N(4) (in golyshevite [39], Figure 12c) or at both N(3) and N(4) (in mogovidite [38]). In the latter case, a double layer of Ca-centered polyhedra is formed (Figure 12d).
The N3- and N4-centered polyhedra share common edges with the M1O6 octahedra. As a result, the substitution of Na by bi- or trivalent cations at the N3 and N4 sites results in changes of local situations in the M2 micro-region and formation of rigid layers formed by the M1-, M2-, N3-, and N4-centered polyhedra. Acentric character of the georgbarsanovite crystal structure is especially obvious because of the presence of bi- and trivalent cations at the N4 site. As a result, this mineral demonstrates a pronounced piezo-effect.
The following local situations around the M2 micro-region are possible in Na-deficient EGMs:
  • IVM2 + VIM1 + VIM1 + VI–VIIIN3 + VI–VIIIN3
  • VM2 + VIM1 + VIM1 + VIM3 + VI–VIIIN3 + VI–VIIIN3
  • VM2 + VIM1 + VIM1 + VIM4 + VI–VIIIN3 + VI–VIIIN3
  • VI-VIIM2 + VIM1 + VIM1 + VIM3 + VIM4 + VI–VIIIN3 + VI–VIIIN3
  • IVM2 + VIM1 + VIM1 + VI–VIIIN4 + VI–VIIIN4
  • VM2 + VIM1 + VIM1 + VIM3 + VI–VIIIN4 + VI–VIIIN4
  • VM2 + VIM1 + VIM1 + VIM4 + VI–VIIIN4 + VI–VIIIN4
  • VI-VIIM2 + VIM1 + VIM1 + VIM3 + VIM4 + VI–VIIIN4 + VI–VIIIN4
  • IVM2 + VIM1 + VIM1 + VI–VIIIN3 + VI–VIIIN3 + VI–VIIIN4 + VI–VIIIN4
  • VM2 + VIM1 + VIM1 + VIM3 + VI–VIIIN3 + VI–VIIIN3 + VI–VIIIN4 + VI–VIIIN4
  • VM2 + VIM1 + VIM1 + VIM4 + VI–VIIIN3 + VI–VIIIN3 + VI–VIIIN4 + VI–VIIIN4
  • VI–VIIM2 + VIM1 + VIM1 + VIM3 + VIM4 + VI–VIIIN3 + VI–VIIIN3 + VI–VIIIN4 + VI–VIIIN4

5. Blocky Isomorphism at the X Sites

In the structures of EGMs, the extra-framework X1 and X2 sites located on the threefold axis can be occupied by different anions (Cl−, F−, S2−, SO42−, and CO32−) and H2O molecules which coordinate large N and (in some samples) M2 cations. The most common X-components are Cl− (e.g., in eudialyte s.s.) and H2O (e.g., in feklichevite). Carbonate anion is a significant component in five EGMs: golyshevite, mogovidite, carbokentbrooksite, johnsenite-(Ce), and zirsilite. The only EGM with species-defining F− is kentbrooksite. S-bearing anions can occur at the X sites in subordinate amounts.
In zirsilite-(Ce) and carbokentbrooksite, the X1 site is split into two sub-sites, one of which is partly occupied by CO32− and another one contains Cl−. The total population of the X1 site is C0.43Cl0.3 in zirsilite-(Ce) and C0.58Cl0.27 in carbokentbrooksite. The X2 site in these minerals is occupied by H2O.
In golyshevite, the X1 site is split into two sub-sites located at the distance of 0.55 Å from each other and occupied by CO32− (with the total occupancy close to 100%), and the X2 site is occupied by H2O and subordinate Cl−. Both carbonate groups are flat triangles whose O atoms coordinate cations of the N4 micro-region (Figure 9).
In most S-bearing EGMs, sulfur occurs in the sulfate form. In these minerals, isolated SO4 tetrahedra occur at the sites located on the threefold axis (see Figure 10 and Figure 13). A potentially new EGM containing sulfur in the sulfide form has been discovered recently in the Lovozero alkaline massif, Kola Peninsula [72]. Hypothetically, this mineral crystallized at a low activity of oxygen. Its end-member formula is (Na14Sr)Ca6(Mn2Na)Zr3[Si26O72](OH)3S2−·2H2O. In the sulfide EGM, the X1 and X2 sites are split into pairs of closely spaced sub-sites with the following occupancies: X1a = 0.41(S2− + Cl), X1b = 0.39H2O, X2a = 0.33(S2− + Cl), X2b = 0.30H2O. The S2− anions belonging to the X1 and X2 micro-regions coordinate triads of Sr2+ and Na+ cations with the Sr–S and Na–S distances of 2.322 and 2.541 Å, respectively.

6. Ordered Blocky Isomorphism in Modular EGMs

Heteropolyhedral frameworks of EGMs consist of layers containing Si3O9 and Si9O27 rings of tetrahedra (T-layer), VIM16O24 rings of octahedra (M-layer), and isolated ZO6 octahedra (Z-layer) alternating in the order (…TMTZ…). Most EGMs have 12-layer structures, with the 2D repeat unit (TMTZ)3, unit-cell parameters a ~ 14 Å, c ~ 30 Å, and space groups R 3 ¯ m, R3m, or R3. However, in several members of the eudialyte group (so-called “megaeudialytes”), additional cation ordering takes place which results in the formation of the doubled 2D repeat units [(TMTZ)(T*M*T*Z*)]3 with the ordered 1:1 alternation of two nonequivalent TMTZ modules and doubling of the c parameter (c ~60–61 Å). Megaeudialytes are characterized by the same space groups as common 12-layer EGMs. Unlike crystalline compounds with superstructures, in megaeudialytes R-translation remains, and the unit-cell doubling is due to blocky isomorphism in the 30 Å module.
Megaeudialytes are known only in the Lovozero and Khibiny alkaline massifs, Kola Peninsula [1,4,73,74,75,76,77,78,79]. Four of them (alluaivite, dualite, labyrinthite, and rastsvetaevite) have been approved by the Commission on New Minerals, Nomenclature, and Classification of the IMA as valid mineral species (see Figure 14 and Figure 15).
Unlike zirconium, titanium rarely forms isolated octahedra in structures of silicate minerals [80]. The main cause of this distinction between the behavior of these elements is a different degree of distortion of Zr- and Ti-centered octahedra. As a result, the isomorphism between Ti and Zr is usually significantly restricted. Minerals of the eudialyte group are one of exceptions to this rule. In EGMs, titanium can occur at the M2, M3, and M4 sites (Table 4).
Alluaivite (Sample 1 in Table 4; Figure 15) [81] is the first megaeudialyte whose crystal structure was solved. In this mineral zirconium is absent, and the Z and Z* sites are occupied by Ti and subordinate Nb. The substitution of Zr4+ by the smaller cation Ti4+ results in the lowering of unit-cell parameters (by 0.1–0.2 Å for a and 0.1–0.3 Å for c). Both the M2 and M2* micro-regions (in the modules I and II, respectively) are occupied by Na. All Na in the M1 micro-region has fourfold flat-square coordination. In the M2 micro-region, Na is distributed among three sites: one site having flat-square coordination and two sites with 7-fold coordination located on both sides from the square. Another cause of the unit-cell doubling is blocky isomorphism in the M3 and M4 micro-regions; in each module, both additional SiO4 tetrahedra have the same orientation, but their orientation is different in different modules.
In the structure of dualite (Sample 3 in Table 4; see Figure 14a) [43], alternation of Zr and Ti in the modules I and II takes place. The M3 + M4 and M3* + M4* micro-regions of dualite are dominated by IVMn and Si, respectively. There are also significant differences in the populations of the VNa-dominant M2 and M2* sites of dualite.
The unit-cell doubling of rastsvetaevite (Sample 4 in Table 4; see Figure 14b) [78] is mainly due to the blocky isomorphism in the M2 and M2* micro-regions: in M2, Na has fourfold flat-square coordination and is surrounded by K atoms having sevenfold coordination; the micro-region M2* is predominantly occupied by Fe. There are also differences between the modules I and II of rastsvetaevite in the population of extra-framework sites. In particular, in the module I the N3 site is occupied by K, whereas in the module II all N sites are Na-dominant.
In most cases, the formation of megaeudialytes is regulated by the ordering of the M2-cations. For example, in hydrated rastsvetaevite (Sample 10 in Table 4; see Figure 14c) [75], different population of the M2 and M2* micro-regions (by IVNa2.7VFe0.3 and VFe2.0VMn0.8, respectively) is the main cause of the unit-cell doubling.

7. Combinations of Blocky Isomorphism at the M1–M4 Sites

The population of each of the M1–M4 micro-regions does not occur independently on the local situations at neighboring sites. Very often, M2-centered polyhedra share common edges with the M1O6 octahedra and common vertices with the M3O6 and M4O6 octahedra. As a result, only specific kinds of clusters (i.e., combinations of local situations around neighboring M1–M4 cations) can be realized with their statistical alteration in different unit cells. The examples of different clusters which can be involved in the combined blocky isomorphism in EGMs with the symmetry R3m (1–4) and R3 (5–8) are
  • IVM2 + VIM1 + VIM1
  • VM2 + VIM1 + VIM1 + VIM3
  • VM2 + VIM1 + VIM1 + VIM4
  • VI-VIIM2 + VIM1 + VIM1 + VIM3 + VIM4
  • IVM2 + VIM1.1 + VIM1.2
  • VM2 + VIM1.1 + VIM1.2 + VIM3
  • VM2 + VIM1.1 + VIM1.2 + VIM4
  • VI-VIIM2 + VIM1.1 + VIM1.2 + VIM3 + VIM4
The M1O6 octahedra can be occupied by different elements including Ca, Mn, Fe, Na, REE, Sr, and Y. For example, in one module of dualite, all the listed elements, except Y statistically substitute each other [43]. For each cluster involving the M1 and M2 polyhedra, only specific combinations of cations which answer the condition of local valence balance on the common O atom are possible. Some examples are presented in Figure 16 and Figure 17.
In kentbrooksite, barsanovite and some related EGMs, the M2Mn2+O5 and M2Fe2+O5 polyhedra are combined with the M3,M4Nb5+O6 octahedra. In W-dominant EGMs related to kentbrooksite (i.e., khomyakovite, manganokhomyakovite, and johnsenite-(Ce)), the M2Mn2+O5 and M2Fe2+O5 pyramids are combined with the M3,M4W6+O6 octahedra. In these cases, local charge balance is regulated by the isomorphism between O and OH at the common vertex of the square pyramid and octahedron.
The charge of M2-cations, which occur in different polyhedra having common edges with the M1O6 octahedra, can vary from 1 to 5, but cations with high charges (Ti4+, Zr4+, and Ta6+) cannot occupy the M2 micro-region in combination with trivalent M1 cations (Ln3+, Y3+). In high-calcium EGMs like mogovidite, feklichevite, golyshevite, and ferrofeklichevite, the M2 micro-region can contain bi-, tri-, tetra-, and pentavalent cations. In other cases, different combinations of uni-, bi-, and trivalent M1-cations with M2-cations having charges from +1 to +5 are realized.
A high degree of separation of different elements in the M2 micro-region is realized in a Fe-deficient sample from the Oleniy Ruchey deposit, Khibiny massif, Kola Peninsula [10]. Its crystal-chemical formula is (Z = 3):
N1–5[(Na11.1(H3O)2.4Ln0.3] N6[K0.5] M1[Ca4.9(Mn1.1)] M2[IVFe0.9VINa0.7VNa0.6VIIMn0.5V(Mn,Fe)0.3]M3,M4[Si1.8Nb0.2] Z[Zr2.6Ti0.4] [Si24O72] [SO4]0.25(OH)2.4Cl0.5(H2O,OH)·1.5H2O. In this mineral, four kinds of clusters (1 to 4 types) involving five kinds of M2-polyhedra are realized statistically.
A more complex situation takes place in Ca-depleted samples, in which ordering in the M2 micro-region is accompanied by the ordering in the M1 sites of the 6-membered ring of octahedra. In this case, the M1 site is transformed into two alternating sites, M1.1 and M1.2 (Figure 18), which are occupied differently (respectively, by Ca and Mn in oneillite and sergevanite, by Ca and Fe in raslakite, and by Mn and Na in voronkovite). Differently occupied octahedra have different sizes (Table 5) which results in variations of configurations of M2-polyhedra.

8. Conclusions

The uniqueness of the eudialyte-group minerals lies in the combination of complexity and variability of their crystalline structures and various schemes of homovalent, heterovalent, and blocky isomorphism, in which at least 26 different elements are involved. From the crystal chemical point of view, the most interesting are the complex mechanisms of blocky isomorphism, when, in a given micro-region of a unit cell, the substitution of one group of atoms and ions by another one is accompanied by significant changes of the local structure, including splitting of cationic and anionic sites, changes in the valence state and coordination numbers of cations. Such substitution schemes are realized both around the key sites of rigid high-force-strength cations of the framework (M2, M3, and M4), and in the N1–5 and X1–2 micro-regions, populated by extra-framework cations and anions, respectively.

Author Contributions

R.K.R. and N.V.C. analyzed literature data on the crystal chemistry of the eudialyte-group minerals and wrote the review. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Science and Higher Education within the State assignment FSRC «Crystallography and Photonics» RAS and state task, state registration number AAAA-A19-119092390076-7 (search and analysis of published data on EGMs), and the Russian Science Foundation, projects nos. 18-29-12005 (X-ray crystal structure analysis) and 18-29-12007-mk (crystal-chemical analysis).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Arrangement of key sites in the eudialyte-type structures viewed along (210).
Figure 1. Arrangement of key sites in the eudialyte-type structures viewed along (210).
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Minerals 10 00720 g002 550
Figure 2. Six-membered ring composed of M1 octahedra (a) and positions of some components in the M2 micro-region between neighboring rings of M1 octahedra (b).
Figure 2. Six-membered ring composed of M1 octahedra (a) and positions of some components in the M2 micro-region between neighboring rings of M1 octahedra (b).
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Minerals 10 00720 g003 550
Figure 3. Different polyhedra in the structures of eudialyte-group minerals: square (a), tetragonal pyramid (b), and octahedron (c), all viewed along (210), and the arrangement of 7-fold M2Na-centered polyhedra in a high-sodium EGM [28] (d).
Figure 3. Different polyhedra in the structures of eudialyte-group minerals: square (a), tetragonal pyramid (b), and octahedron (c), all viewed along (210), and the arrangement of 7-fold M2Na-centered polyhedra in a high-sodium EGM [28] (d).
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Minerals 10 00720 g004 550
Figure 4. Fragments of the crystal structures of kentbrooksite (a) and ikranite (b) viewed along (001).
Figure 4. Fragments of the crystal structures of kentbrooksite (a) and ikranite (b) viewed along (001).
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Figure 5. Local situations at the centers of the Si9O27 rings viewed along (001). (a) A ring with vacancy at the M3 site, (b) a ring centered with the SiO4 tetrahedron, (c) a ring centered with the NbO6 octahedron.
Figure 5. Local situations at the centers of the Si9O27 rings viewed along (001). (a) A ring with vacancy at the M3 site, (b) a ring centered with the SiO4 tetrahedron, (c) a ring centered with the NbO6 octahedron.
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Figure 6. (a–e) A scheme showing different options of mutual orientation of the M3 and M4 tetrahedra.
Figure 6. (a–e) A scheme showing different options of mutual orientation of the M3 and M4 tetrahedra.
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Figure 7. A fragment of the crystal structure of davinciite.
Figure 7. A fragment of the crystal structure of davinciite.
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Figure 8. A fragment of the crystal structure of andrianovite.
Figure 8. A fragment of the crystal structure of andrianovite.
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Figure 9. Blocky isomorphism in the M3 and M4 micro-regions of golyshevite viewed along (210).
Figure 9. Blocky isomorphism in the M3 and M4 micro-regions of golyshevite viewed along (210).
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Figure 10. The crystal structute of a Nb,Ti-rich EGM from the Kovdor phlogopite deposit viewed along (210) [68].
Figure 10. The crystal structute of a Nb,Ti-rich EGM from the Kovdor phlogopite deposit viewed along (210) [68].
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Minerals 10 00720 g011 550
Figure 11. The layer formed by the M1Ca6, N4Sr10, and N3Mn2+5 polyhedra in “eucolite” (Sr- and Mn-rich EGM viewed along (001) [27,34]).
Figure 11. The layer formed by the M1Ca6, N4Sr10, and N3Mn2+5 polyhedra in “eucolite” (Sr- and Mn-rich EGM viewed along (001) [27,34]).
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Minerals 10 00720 g012 550
Figure 12. Layers consisting of M1Ca-, N3Na-, and N4Na-polyhedra in Na-rich EGMs (a); M1Ca-, N3Ca-, and N4Na-polyhedra in feklichevite (b); M1Ca-, N3Na-, and N4Ca-polyhedra in golyshevite (c); and M1Ca-, N3Ca-, and N4Ca-polyhedra in mogovidite (d).
Figure 12. Layers consisting of M1Ca-, N3Na-, and N4Na-polyhedra in Na-rich EGMs (a); M1Ca-, N3Ca-, and N4Na-polyhedra in feklichevite (b); M1Ca-, N3Na-, and N4Ca-polyhedra in golyshevite (c); and M1Ca-, N3Ca-, and N4Ca-polyhedra in mogovidite (d).
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Figure 13. The crystal structure of aqualite viewed along (210).
Figure 13. The crystal structure of aqualite viewed along (210).
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Figure 14. Ordered isomorphism at the Z site in dualite [73] (a), at the M2 and N3 sites in rastsvetaevite holotype [23,74] (b), and at the M2 and N4 sites in hydrated rastsvetaevite [8,75] (c), viewed along (210).
Figure 14. Ordered isomorphism at the Z site in dualite [73] (a), at the M2 and N3 sites in rastsvetaevite holotype [23,74] (b), and at the M2 and N4 sites in hydrated rastsvetaevite [8,75] (c), viewed along (210).
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Figure 15. The crystal structures of alluaivite (differently oriented SiO4 tetrahedra at the M3 and M4 sites are shown with red) (a) and Mn-rich “megaeudialyte” with the alteration of M2(IVNa) and M2(VMn2+) in neighboring modules [24] (b).
Figure 15. The crystal structures of alluaivite (differently oriented SiO4 tetrahedra at the M3 and M4 sites are shown with red) (a) and Mn-rich “megaeudialyte” with the alteration of M2(IVNa) and M2(VMn2+) in neighboring modules [24] (b).
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Figure 16. Alternative variants of blocky isomorphism in the structure of manganoeudialyte [58], with the M2 micro-region occupied by differently oriented Mg- and Fe-centered square pyramids (a) and by Mn-centered octahedra (b) viewed along (001).
Figure 16. Alternative variants of blocky isomorphism in the structure of manganoeudialyte [58], with the M2 micro-region occupied by differently oriented Mg- and Fe-centered square pyramids (a) and by Mn-centered octahedra (b) viewed along (001).
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Figure 17. Pairs of differently oriented M2O5 pyramids in the structure of a VNa-dominant sample viewed along (001) [15].
Figure 17. Pairs of differently oriented M2O5 pyramids in the structure of a VNa-dominant sample viewed along (001) [15].
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Figure 18. A fragment of the crystal structure of voronkovite [83].
Figure 18. A fragment of the crystal structure of voronkovite [83].
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Table
Table 1. Geometric parameters of the M2O4 polyhedra for different cations.
Table 1. Geometric parameters of the M2O4 polyhedra for different cations.
CationM2–O Distances, Å (Ranges)O–O Distances, ÅReference
Na+2.29–2.293.28, 3.28, 3.13 × 2[22]
Zr4+2.040–2.1172.78, 2.81, 3.03, 3.08[25]
Ta5+2.035–2.1162.95, 3.11, 2.85 × 2[26]
Fe2+2.03–2.062.95, 2.98, 2.80 × 2[27]
Table
Table 2. Population of tetrahedral sites in the M3 and M4 micro-regions, total amounts of the IVM3 and IVM4 components (∑T, Z = 3) and atomic ratios of the contents of (IVM3a,IVM4a) and (IVM3b,IVM4b) cations (∑Ta/∑Tb). The samples are listed in the order of lowering of the ∑T value.
Table 2. Population of tetrahedral sites in the M3 and M4 micro-regions, total amounts of the IVM3 and IVM4 components (∑T, Z = 3) and atomic ratios of the contents of (IVM3a,IVM4a) and (IVM3b,IVM4b) cations (∑Ta/∑Tb). The samples are listed in the order of lowering of the ∑T value.
No.MineralM3aM3bM4aM4b∑T∑Ta/∑TbReferences
1.Davinciite0.5Si0.49Si0.41Si0.53Si + 0.04Al1.970.86[52,53]
2.Fe-poor EGM0.1Al0.4Si + 0.5Mn 0.9Si1.90.05[41]
3.Fe-poor EGM0.24Si0.57Si0.48Si0.47Si1.760.69[50]
4. H3O-analogue of eudialyte0.45Si0.34Si0.57Si0.21Si + 0.19Al1.761.38[19]
5.Aqualite0.45Si0.55Si0.5Si 1.722.13[21,54,55]
6. Ta-bearing eudialyte 0.5Si + 0.3Al0.43Si 0.35Si1.580.37[26]
7. Sergevanite0.55Si 0.51Si0.49Si1.552.16[44]
8. Sergevanite0.5Si0.5Al 0.5Si1.50.5[25,56]
9. Raslakite0.4Si + 0.1Al 0.4Si + 0.1Al0.5Si1.52.0[42,57]
10.H3O-analogue of eudialyte 0.44Si + 0.06Al0.5Si0.5Si1.50.5[19]
11.Mangano-eudialyte0.38Si0.25Al0.37Si0.44Si1.441.09[58]
12.H3O-analogue of eudialyte0.29Si0.2Si0.41Si0.49Si1.391.01[19]
13.Taseqite 0.3Si 1.0Si1.30[35,36,59,60,61]
14.Na-rich eudialyte0.56Si 0.56Si1.121.0[28]
15.H3O-rich and Si-deficient EGM0.5Si0.31Si0.07Al0.17Si1.051.19[19]
16.H3O-rich and Si-deficient EGM0.44Si0.1Al0.23Si0.19Si0.962.31[19]
17.Andria-
novite		 0.2Si0.75Si0.950.27[30,31]
18.H3O-rich and Si-deficient EGM0.18Si + 0.2Al 0.5Si0.880.76[19]
Table
Table 3. Dominant components at the N3 and N4 sites of Na-depleted EGMs.
Table 3. Dominant components at the N3 and N4 sites of Na-depleted EGMs.
MineralN3N4Reference
GolysheviteNaCa[38,39]
MogoviditeCaCa[38,64]
FeklicheviteCaNa[37]
“Ferrofeklichevite” CaNa[70]
TaseqiteNaSr[35,36]
GeorgbarsanoviteNaMn[33,34]
Zirsilite-(Ce)NaCe[71]
Johnsenite-(Ce)NaREE[67]
AbdrianoviteNaK, Sr[30]
DavinciiteKNa[52]
Table
Table 4. Ordered blocky isomorphism at the M2 and M3/M4 sites of EGMs with modular structures and doubled c parameter (Z = 3).
Table 4. Ordered blocky isomorphism at the M2 and M3/M4 sites of EGMs with modular structures and doubled c parameter (Z = 3).
No.MineralModule IModule IIReferences
M3+M4M2M3*+M4*M2*
1Alluaivite holotype
(Z = Ti)		Si2 ↑VIINa2.34
IVNa0.66		Si2 ↓IVNa3[81]
2Alluaivite
(Z = Ti)		Si2 ↑IVNa1.6VMn1.4Si2 ↓IVNa3[18]
3Dualite holotype
(Z = ZZr + ZTi)		IVMn1.64
Si0.36		VNa1.3VITi1.0
IVFe0.7		Si2VNa1.7VMn1
IVFe0.3		[43,73]
4Rastsvetaevite holotypeSi1.5VIAl0.2
VINb0.1		VIIK2VIIK2
IVNa1.0		Si2IVFe1.9VNa0.5
VMn0.3VFe0.3		[74,78]
5Labyrinthite
holotype		Si1.2VITi0.5IVFe2.2MnV0.8Si2IVNa1.2VIINa1.7
VIINa1.6		[23,77]
6K-deficient rastsvetaeviteSi2VIIK4.6IVNa0.5Si2IVFe2.2
VMn0.5VFe0.3		[8]
7“Hyper-Mn EGM”Si2VMn2.46VFe0.18
VINa1.86		Si2IVNa2.4VINa1.2[24]
8Labyrinthite R3m analogueSi1.3VITi0.5
VINb0.1		IVFe1.0
VK1.5VSr0.5		Si2VMn1.6IVFe0.9[79]
9Hydrated rastsvetaeviteSi1.4VINb0.2IVNa1.0IVSr0.6Si1.4VITi0.2IVFe2.2VMn0.6[8]
10Hydrated rastsvetaeviteSi2IVNa2.7VFe0.3Si1.7VITi0.3VFe2.0VMn0.8[75]
11Centrosymmetric EGMSi2IVFe1.23Si2IVNa2.01VMn0.63
VISr0.6		[76]
Note: Orientation of Si-centered tetrahedra at the M3 and M4 sites of alluaivite is shown with arrows.
Table
Table 5. Cation–oxygen distances (Å) in the M1a- and M1b-octahedra of ordered EGMs with the R3 symmetry.
Table 5. Cation–oxygen distances (Å) in the M1a- and M1b-octahedra of ordered EGMs with the R3 symmetry.
No.MineralDominant CationsReferences
CaMnFeNa
1Oneillite(2.431)(2.227) [82]
2Raslakite2.311–2.400
(2.362)		 2.225–2.351
(2.306)		 [57]
3Voronkovite 2.132–2.278
(2.22)		 2.393–2.497
(2.45)		[83,84]
4Sergevanite2.331–2.438
(2.381)		2.257–2.384
(2.309)		 [85]
5Sergevanite2.320–2.420
(2.38)		2.230–2.386
(2.303)		 [20]
6Ca-deficient EGM2.300–2.441
(2.37)		2.23–2.33
(2.29)		 [86]
7Ca-deficient EGM2.27–2.411
(2.36)		2.211–2.38
(2.29)		 [86]
8Zr-rich EGM2.30–2.41
(2.35)		 2.228–2.347
(2.285)		 [56,87]
9Zr-rich EGM2.297–2.38
(2.35)		2.211–2.38
(2.29)		 [88]
10 *Hydrated EGM2.28–2.50
(2.37)		 2.23–2.41
(2.34)		[89]
Note: Mean M1,2–O distances are given in brackets. The table includes holotypes of oneillite (1), raslakite (2), voronkovite (3), and sergevanite (4). * Megaeudialyte with c = 60.33 Å.

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Rastsvetaeva, R.K.; Chukanov, N.V. New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites. Minerals 2020, 10, 720. https://doi.org/10.3390/min10080720

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Rastsvetaeva RK, Chukanov NV. New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites. Minerals. 2020; 10(8):720. https://doi.org/10.3390/min10080720

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Rastsvetaeva, Ramiza K., and Nikita V. Chukanov. 2020. "New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites" Minerals 10, no. 8: 720. https://doi.org/10.3390/min10080720

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