minerals Review New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites 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; [email protected] 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 * Correspondence: [email protected] Received: 24 July 2020; Accepted: 14 August 2020; Published: 17 August 2020 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. Keywords: eudialyte group; crystal chemistry; blocky isomorphism; peralkaline rocks 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 [1–9]). 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 IV IV VI (Figure1) is a 3D quasi-framework consisting of the Si3O9, Si9O27, and M16O24 rings (M1 = Ca, Mn2+, Fe2+, Na, Ln, Y, Sr; coordination numbers are denoted by Roman numerals) connected via 2+ 3+ 2+ 3+ M2O4–7 polyhedra and ZO6 octahedra (M2 = Fe , Fe , Mn , Mn , Mg, Zr, Ta, Na; Z = Zr, Ti, Nb) and containing additional M3 and M4 sites which are situated at the centers of two nonequivalent IV VI VI VI Si9O27 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 [1,2] (Figure1). In the structures of most EGMs, including eudialyte s.s., M1 cations can be disordered, but in some representatives of this mineral group Minerals 2020, 10, 720; doi:10.3390/min10080720 www.mdpi.com/journal/minerals Minerals 2020, 10, x FOR PEER REVIEW 2 of 24 group they alternate in the ring of octahedra, which results in its transformation into the ring (М1.13М1.23O24) 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. MineralsBlocky2020, 10isomorphism, 720 is defined as the ability of groups of atoms or ions having different2 of 23 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 they alternate in the ring of octahedra, which results in its transformation into the ring (M1.13M1.23O24) realizedand symmetry at the key lowering sites M from2, M the3, and space M4, group as wellR3 masor atR the-3m Nto andR3. X In sites. some The samples, eudialyte a splitting group ofis the onlyM1 site group [10] orof oneminerals of the Min1.1 which/M1.2 blocky sites [11 isomorph] into twoism sub-sites is realized located at at several short distance sites containing of ~0.2 Å high-force-strengthfrom each other takes cations. place. Be ThelowM 1–weM will4 sites use are the considered symbols asN1– theN5, main M2,species-defining M3, M4, and X “keyto denote sites” correspondingin the nomenclature cavities of EGMs(i.e., micro-regions [3,6,7,12,13]. which can contain several closely spaced sites). FigureFigure 1. 1. ArrangementArrangement of of key key sites sites in in the the eudialyte-type eudialyte-type structures structures viewed along (210). + + 2+ 2+ 2+ 2+ 2+ 3+ 3+ + InExtra-framework addition to EGMs cations with (Nathe rhombohedral,K , Ca , Mn unit-cell, Sr parameters, Ba , Pb a ,Y∼ 14.2, LnÅ, c ,∼ and 30 Å, H 3membersO ) and of(in the some eudialyte samples) group water with molecules modular occupy structures five sites, and Ndoubled1–N5, which c parameter are typically are known. split and Their can unit be partly cells + containvacant. two Cations eudialyte-type other than modules Na show which a tendency differ from to concentrate each other by at thelocalN situatio3 and Nns4 around sites. Some key sites of these [4]. + 2+ 2+ 2+ 3+ + cations (K , Ca , Mn , Sr , Ce , and H3O ) are species-defining ones in several representatives 2.of Blocky the eudialyte Isomorphism group. Theat the Ø M anions2 Site (Ø = O, OH) coordinate the M2, M3, and M4 sites. Additional 2 2 2 anions (Cl−,F−, OH−,S −, SO4 −, and CO3 −) and water molecules occur at the X1 and X2 sites located on theThe three-fold most complex axis. blocky isomorphism is realized in the M2 micro-region situated between ringsBlocky of octahedra, isomorphism M16O24 [15]. is defined This micro-region as the ability can ofbe groupspopulated of by atoms cations or having ions having different diff radii,erent charges,configurations and coordination to replace each (from other flat in square crystal fo structuresrmed by [edges14]. Such of octahedra substitutions belonging are known to two 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]. Minerals 2020, 10, 720 3 of 23 2. Blocky Isomorphism at the M2 Site The most complex blocky isomorphism is realized in the M2 micro-region situated between rings Mineralsof octahedra, 2020, 10, Mx FOR16O PEER24 [15 REVIEW]. This micro-region can be populated by cations having different3 radii, of 24 charges, and coordination (from flat square formed by edges of octahedra belonging to two neighboring IV 2+ V 2+IV V 2+ 3V+ VI2+ V 3+3+VVI 23++ VVI 2+2+VI IV 2+ IVIV neighboringM16O24 rings M to16 aO 7-24 rings or 8-fold to a polyhedron):7- or 8-fold polyhedron):Fe , Fe ,FeFe, Fe, ,Fe Fe, , MnFe ,, MnMn, ,MnZr,, Zr,Ta, IVTa,Na, IVVNa,Na, VNa,VINa, VINa,VIINa, VIINa,VIK, VIK,VII VIIK,K, and andVIII VIIIK.K. TheThe MM2-cations2-cations occuroccur inin thethe planeplane of the six-membered rings which is perpendicular to the threefoldthreefold axisaxis (Figure(Figure2 2).). TheThe coordinationcoordination polyhedra polyhedra MM2O2O55 and M2O7 cancan have different different orientations with respect to thethe planeplane ofof thethe square.square. Some examples of M2-centered polyhedra are shown in Figure3 3.. (a) (b) Figure 2. Six-memberedSix-membered ring composed of M1 octahedra ( a) and positions of some components in the Minerals 2020, 10, x FOR PEER REVIEW 4 of 24 M2 micro-region between neighboring rings of M11 octahedraoctahedra ((bb).). 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].