Who Is Who in the Eudialyte Group: A New Algorithm for the Express Allocation of a Mineral Name Based on the Chemical Composition

Mikhailova, Julia A.; Stepenshchikov, Dmitry G.; Kalashnikov, Andrey O.; Aksenov, Sergey M.

doi:10.3390/min12020224

Open AccessEditor’s ChoiceArticle

Who Is Who in the Eudialyte Group: A New Algorithm for the Express Allocation of a Mineral Name Based on the Chemical Composition

¹

Geological Institute, Kola Science Centre, Russian Academy of Sciences, 14 Fersman Str., 184209 Apatity, Russia

²

Laboratory of Arctic Mineralogy and Material Sciences, Kola Science Centre, Russian Academy of Sciences, 14 Fersman Str., 184209 Apatity, Russia

^*

Author to whom correspondence should be addressed.

Minerals 2022, 12(2), 224; https://doi.org/10.3390/min12020224

Submission received: 25 January 2022 / Revised: 7 February 2022 / Accepted: 7 February 2022 / Published: 9 February 2022

(This article belongs to the Special Issue Study of the Eudialyte Group Minerals)

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Abstract

:

Eudialyte-group minerals (EGMs) are Na-Ca zirconosilicates typical for peralkaline plutonic rocks. In the zeolite-like crystal structure of these minerals, there are many sites of different volumes and configurations, and therefore EGMs can include up to one-third of the periodic table. Although there are preferred sites for many elements in the crystal structure of eudialyte-group minerals, the same element can appear in several sites. In addition, many sites may be partially or fully vacant. Currently, 30 mineral species are established in the eudialyte group. However, this diversity is, in fact, limited to holotype specimens. To name any mineral from the eudialyte group, you need to solve its crystal structure and compare it with holotypes. Meanwhile, the composition (and, therefore, the name) of any mineral of the eudialyte group is an excellent indicator of the composition of the mineral-forming media, which is very important to petrological and mineralogical studies. In this article, we propose a diagnostic scheme for minerals of the eudialyte group, based only on the chemical composition. The scheme includes five consecutive steps, each of which evaluates the content of a species-forming element (or the sum of such elements). This scheme can be supplemented by new members without changing its hierarchical structure.

Keywords:

eudialyte group; systematic; classification; diagnostics; mineral; peralkaline rocks

1. Introduction

Eudialyte-group minerals (EGMs) are alkaline zirconium-calcium silicates that accommodate many different elements in their complex zeolite-like structure. In addition to Si, Na, Ca, and Zr EGMs comprise significant amounts of Fe, Mn, REE, Y, Nb, Hf, Ti, K, Sr, and Cl, F, H₂O and OH groups [1,2,3,4]. The crystal structures of EGMs are based on a heteropolyhedral framework (Figure 1) formed by three types of layers: the T-layer containing three- and nine-membered rings of SiO₄-tetrahedra [(Si₃O₉) and (Si₉O₂₇), respectively]; the M-layer represented by six-membered [M(1)₆O₂₄] rings of edge-shared M(1)O₆-octahedra; and the Z-layer of isolated ZO₆ octahedra. These layers alternate along the c parameter in the …|TMTZ|… sequence (the thickness of the TZTM fragment is ~10 Å), and according to an R-lattice they form “12-layer” eudialytes (a~14.2 Å, c~30 Å) [5]. In “24-layer” “megaeudialytes”, a doubling of the c-period of the unit cell (a~14.2 Å, c~60 Å) is observed.

The eudialyte group combines 30 mineral species and a wide range of their varieties have been described. The crystal chemical formula of EGMs (derived from the IMA approved formula [2]) can be written as (Z = 3):

{N(1)₃N(2)₃N(3)₃N(4)₃N(5)₃}{[M(1a)₃M(1b)₃]₃M(2)₃M(3)M(4)Z₃[Si₂₄O₇₂]Ø_0-6}X(1)X(2),

where

N(1–5) = Na, H₃O⁺, K, Sr, REE, Y, Ba, Mn, Ca, □ (vacancy);

M(1) = Ca, Mn, REE, Na, Fe;

M(2) = ^IV,VFe²⁺, ^V,VIFe³⁺, ^V,VIMn²⁺, ^V,VINa⁺, ^IV,VZr⁴⁺;

M(3) and M(4) = ^IVSi, ^VINb, ^VITi, ^VIW⁶⁺, □;

Z = Zr, Ti, Nb;

Ø = O, (OH);

X = Cl, F, S²⁻, H₂O, CO₃, and SO₄.

The M(3) and M(4) sites are located at the centers of the (Si₉O₂₇) rings and are predominantly occupied by Si, Nb, W, and some other components or can be partly vacant. In addition, the M(2) site is located at M(2)O₄Ø_n-polyhedra (n = 0–2) between the octahedral six-membered rings. The M(2–4) sites form microregions of split “subsites” with close distance to each other. These “subsites” can differ by their chemical composition and/or coordinational environment [6]. The large cavities of the zeolite-like framework are filled by extraframework cations and water molecules located at N(1)–N(5) sites.

The principles of nomenclature of the eudialyte-group minerals have been developed by the Eudialyte Nomenclature Subcommittee established by the Commission on New Minerals and Mineral Names of the International Mineralogical Association [2]. The following nomenclature schemes have been tested:

(1): Linnean biological principle;
(2): Hierarchical system with root names modified by using the modifiers and Levinson suffixes;
(3): A unique-name system using the modifiers.

Conventional unique names with a maximum of one cation prefix are recommended for the eudialyte-group minerals, and this prefix should refer to the M(2) site. As in other groups of minerals, a new mineral in the eudialyte group is considered to be one in which at least one structural position is occupied by a different chemical element than in other minerals with a similar structure. Empirical and CNMNC formulas of the IMA-approved members of the eudialyte group are presented in Table 1, and site occupancies are shown in Table 2.

R \bar{3}

R \bar{3}

EGMs are widely used in petrological studies. Together with other Na-Ca-HFSE minerals (i.e., rinkite, wöhlerite, aenigmatite, astrophyllite, catapleiite, etc.) they are considered as mineralogical markers of agpaitic rocks [33] which attract economic interest as the most promising sources for future high-field-strength elements (HFSE) and rare earth elements (REE) supply [34,35]. Moreover, the highly variable composition of EGMs may be used to study the compositional and/or physical–chemical changes during the evolution of magmatic and hydrothermal systems [36,37,38,39].

In this case, because of the great economic, petrological, and mineralogical significance of the EGMs, an express diagnostic based on microprobe data is especially important. However, the large number of framework and extraframework sites in the crystal structure (which can be partial or completely vacant), the different types of cation distributions, and the presence and predominance of the same element at one or more sites make it difficult to give a name to a sample of EGMs based only on the chemical composition (without additional single-crystal X-ray diffraction analysis, IR- and Mössbauer spectroscopy). Originally. a possible way to determine the mineral species based on the empirical formula was proposed by Johnsen and Grice [1]. Pfaff with coauthors [40] presented an extended and improved scheme for site assignment using IMA-approved end-members also based on microprobe analyses (however, the different valence states of Fe and Mn and undetermined H₂O-contents were ignored in these studies).

Later, Rastsvetaeva and Chukanov [3] proposed a hierarchical crystal chemical scheme for the classification of EGMs (without formal approval by the CNMNC IMA) which contains the following features steps:

The value of c parameter (~30 Å or ~60 Å for “12-layer” and “24-layer” members, respectively);
Cation ordering in octahedral six-membered [M(1)₆O₂₄] rings, i.e., ring of six calcium atoms vs. ring with alternating atoms of different elements (Ca + Mn, Ca + Fe, Mn + Na);
What elements (or vacancies) prevail in centers of [Si₉O₂₇] rings: Si, Nb, Ti, W, Mn;
What elements prevail M(2)O_n polyhedra;
What elements prevail N(3) and N(4) polyhedra;
The water content [Na > (H₂O + H₃O⁺) or Na < (H₂O + H₃O⁺)].

Based on this classification scheme and taking into account the data on the site occupancy in the crystal structures of different EGMs (Table 2), we propose a new algorithm that makes it possible to determine minerals based on chemical data only without a direct site assignment procedure. Our diagnostic scheme consists of the following steps:

(1): Zr or Ti in Z site;
(2): Division according to the assignment of the M(1) site;
(3): Division according to the occupation of M(3) and M(4) sites;
(4): Division according to population of the M(2), N(3), N(4) sites.

2. Methods

The diagnostic scheme has been developed in accordance with the principles and rules of the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The list of eudialyte-group minerals was taken from IMA list of minerals 2021 (http://cnmnc.main.jp/imalist.htm accessed on 20 November 2021).

3. Diagnostic Scheme of EGMs

The proposed diagnostic scheme is based on the values of the formula coefficients of the elements, which must be normalized on Si+Al+Ti+Zr+Hf+Nb+Ta+W = 29 [1]. After that, the apfu values of species-forming elements are considered step-by-step.

3.1. Framework Cations: Zirconium or Titanium in Z Site (Step 1)

In all “12-layer” minerals, as well as “24-layer” rastsvetaevite and labyrintite, ZO₆ octahedra are occupied by zirconium and there are no other structural sites in which zirconium dominates. In the crystal structure of “24-layer” dualite, a half of the zirconium is replaced by titanium, while in the “24-layer” alluaivite, titanium fully occupies the ZO₆ octahedra. To establish the corresponding values of apfu for Zr and Ti at Z site, the following case should be considered:

(1): Zirconium fully occupies Z site;
(2): Titanium fully occupies Z site;
(3): Half of zirconium is replaced by titanium.

Let us assume that Z = Zr₃Ti₃ in dualite (=Zr_1.5 Ti_1.5 in comparison with “12-layer”), Z = Ti₆ in alluaivite (=Ti₃ in comparison with “12-layer”), and Z = Zr₃ in other minerals (as well as in “24-layer” rastsvetaevite and labyrintite). In accordance with IMA–CNMNC dominant–constituent rule [41], the content of Zr in Z site should be: is 0.75 < Zr < 2.25 apfu in dualite; Zr < 0.75 apfu in alluaivite; Zr > 2.25 apfu in other minerals. Thus, at the first step, all members of the eudialyte group are divided into zirconium-titanium eudialytes (represented only by “24-layer” dualite and alluaivite), and zirconium eudialytes (all “12-layer” minerals and “24-layer” rastsvetaevite and labyrintite).

3.2. Framework Cations: Calcium at the M(1) Site (Step 2)

Calcium is normally the predominant element at M(1) site with Mn and REE as the common substituents and in most “12-layer” members of the eudialyte group, the M(1) site is occupied by Ca. In low-calcium members of the EGM (oneillite, raslakite, sergevanite, voronkovite, and their varieties) the Ca/(Ca + Mn + REE) ratio in M(1) site decreases and the M(1)O₆ octahedron (<M(1)–O> ~2.35(2) Å) becomes more distorted. In this case, the M(1) site splits into two symmetrically nonequivalent M(1a) and M(1b) sites at the centers of small M(1a)O₆ octahedra (<M(1a)–O> ~2.227 Å) occupied predominantly by Mn²⁺ (oneillite and sergevanite) or Fe²⁺ (raslakite) and a relatively large M(1b)O₆ (<M(1b)–O> ~2.431Å) octahedra occupied by Ca ¹. If Ca atoms predominate over other cations at M(1) site, the condition Ca > 4.5 apfu must be satisfied. This value is the boundary for dividing zirconium eudialytes (Figure 2) into Ca-rich zirconium eudialytes (Ca > 4.5 apfu) and Ca-poor zirconium eudialytes (Ca ≤ 4.5 apfu).

It should also be noted that in the holotype samples of kentbrooksite and ikranite, the M(1) site is not fully occupied by calcium, but the corresponding split of this site (due to the symmetry reducing like in oneillite, raslakite, and voronkovite) has not been observed. In holotype kentbrooksite, the composition of the M(1) site is (Ca_0.545Mn_0.297REE_0.103Na_0.05) or (Ca_3.27Mn_1.78REE_0.62Na_0.33)_Σ6 for the whole [M(1)₆O₂₄] ring). The IMA-formula of kentbrooksite is approved without specification of the amount of calcium, therefore, kentbrooksite should be attributed to the Ca-rich zirconium eudialytes. The ikranite holotype sample also contains a low amount of calcium without split of the M(1) site due to symmetry reducing. Ikranite will be attributed to Ca-poor zirconium eudialytes.

3.3. Framework Cations: Occupation of the M(3) and M(4) Sites (Step 3)

The following variants of filling the M(3) and M(4) sites are possible: two additional SiO₃Ø-tetrahedra (T + T), a SiO₃Ø-tetrahedron and MO₃Ø₃-octahedron (T + M), a SiO₃Ø-tetrahedron and a vacancy (T + □), and two vacancies (□ + □). In the case of M(3) + M(4) = T + M, M is either niobium or tungsten (in Ca-rich zirconium eudialytes) or only niobium (in Ca-poor zirconium eudialytes).

For T + T members, the following conditions are required:

Si > 25.5 apfu {[Si₂₄O₇₂] + ^M⁽³⁾Siφ₄ + ½^M⁽⁴⁾Siφ₄};
Nb ≤ 0.5 apfu
W ≤ 0.5 apfu

For T + M members, the following condition is required: 24.5 < Si ≤ 25.5 apfu.

Minerals T + M with a predominance of niobium must additionally meet the condition Nb > 0.5 ≥ W. Tungsten is the species-forming element if the following conditions are met: 24.5 < Si ≤ 25.5; W > 0.5 ≥ Nb.

It should be noted that the normalization Si + Al + Ti + Zr + Hf + Nb + Ta + W = 29 implies complete filling of the positions Z₃M(3)M(4)Si₂₄ [1]. However, in the case of the presence of vacancies in either the M(3) or the M(4) sites, such a calculation may lead to unreliable results.

The presence of vacancies can only be accurately established using additional methods, for example, spectroscopic ones. In the IR spectra of EGMs with additional SiO₄ tetrahedra located at the centers of nine-membered rings, there are bands in the range 905–940 cm⁻¹. Bands in the range 676–689 cm⁻¹ indicate additional NbO₆ and WO₆ octahedra at the centers of the nine-membered rings. Figure 3 shows for comparison the IR spectra of holotype samples of manganoeudialyte, oneillite, mogovidite, and ikranite. Obviously, IR spectroscopy data increase the accuracy of diagnostics of EGMs.

Let us consider different options for calculating the formula for holotype samples of mogovidite (vacancy in M(3) site) and ikranite (vacancies in M(3) and M(4) sites) (Table 3).

As can be seen from Table 3, the choice of the basis for calculating the formula significantly affects the values of the formula coefficients for minerals with a predominance of vacancies in the M(3) and/or M(4) sites. The value of Si pfu increases most significantly, since its share in the basis for calculating of the EGM formula is the highest (at least 24 apfu). However, neither in the case of mogovidite, nor in the case of iranite, does the Si pfu exceed 25.5. Normalization on Si + Al + Ti + Zr + Hf + Nb + Ta + W = 29 for mogovidite and ikranite shows the Si content as in T + M eudialytes, but at the same time a low niobium content. This is because the M(3) site in the mogovidite and the M(3) and M(4) sites in the ikranite are partially occupied. So, in the crystal–chemical formula of mogovidite, the populations of the M(3) and M(4) sites are as follows: ^M³[□_0.58Nb_0.21 Si_0.21] ^M⁴[Si_0.5 Nb_0.2 Ti_0.1□_0.2] [18], and in ikranite ^M³[□_0.56 Si_0.21 Zr_0.08 Nb_0.03 Ti_0.03] ^M⁴[□_0.54 Zr_0.32 Nb_0.07 Ti_0.07] [19]. Therefore, the presence of vacancies in the M(3) and M(4) sites is indirectly indicated by the values 24.5 < Si ≤ 25.5 and Nb ≤ 0.5; W ≤ 0.5. Since mogovidite and ikranite differ significantly in calcium content, their separation in the proposed diagnostic scheme does not cause difficulties.

So, based on the occupancies of the M(3) and M(4) sites, the Ca-rich zirconium eudialytes are divided into T+T, T+Nb, T+W, T+□ eudialytes, while the Ca-poor zirconium eudialytes are divided into T+T, T+Nb, □+□ eudialytes (Figure 2).

3.4. Ca-Rich Zirconium Eudialytes: Оccupation of the M(2) Site (Step 4)

Before the consideration of the assignment of the M(2) site by manganese and/or iron, one should take into account that part of the manganese can be included in the M(1)₆O₂₄ ring. For minerals which contain 4.5 apfu ≤ ^TotalCa < 6 apfu, the lack of cations in M(1) = 6 apfu − ^TotalCa is first calculated. The lack of cations is compensated by manganese, and if it is not sufficient, then REE, then Na, and Fe [1]. The remainder of manganese ^M⁽²⁾Mn = ^TotalMn − ^M⁽¹⁾Mn belongs to M(2) site. Accordingly, the remainder of the REE is ^NREE = ^TotalREE − ^M⁽¹⁾REE. For other elements: ^M⁽²⁾Fe = ^TotalFe − ^M⁽¹⁾Fe [where ^M⁽²⁾Fe = ^M⁽²⁾(Fe²⁺ + Fe³⁺)], ^NNa = ^TotalNa − ^M⁽¹⁾Na.

If manganese or iron dominate in the crystal structure of eudialyte only in the M2 position, then the condition 4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5 must be satisfied; if the M2 position is vacant, then the condition ^M⁽²⁾Mn + ^M⁽²⁾Fe ≤ 1.5 must be satisfied. Additional conditions, such as ^M⁽²⁾Fe > ^M⁽²⁾Mn or ^M⁽²⁾Mn > ^M⁽²⁾Fe, as well as Fe^{3 +} > Fe²⁺ or Fe²⁺ > Fe³⁺, allow the separation of Mn-dominant and Fe-dominant end terms, as well as eudialytes with a predominance of ferrous or ferric iron.

Thus, T + T eudialytes include the eudialyte subgroup (4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5; ^M⁽²⁾Fe > ^M⁽²⁾Mn; Fe²⁺ > Fe³⁺), fengchengite subgroup (4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5; ^M⁽²⁾Fe > ^M⁽²⁾Mn; Fe³⁺ > Fe²⁺), manganoeudialyte subgroup (4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5; ^M⁽²⁾Mn > ^M⁽²⁾Fe), and aqualite subgroup (^M⁽²⁾Mn + ^M⁽²⁾Fe ≤ 1.5). Among the T + M eudialytes in the case of 4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5; ^M⁽²⁾Fe > ^M⁽²⁾Mn, ferrokentbrooksite and khomyakovite subgroups are distinguished, and in the case of 4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5; ^M⁽²⁾Mn > ^M⁽²⁾Fe are kentbrooksite and mangankhomyakovite subgroups. Mogovidite is representative of T + □ eudialytes with 4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5; ^M⁽²⁾Fe > ^M⁽²⁾Mn, and its potential manganese analog will have the ratio ^M⁽²⁾Mn > ^M⁽²⁾Fe.

In siudaite and georgbarsanovite, ^M⁽²⁾Mn + ^M⁽²⁾Fe > 4.5, which corresponds to the complete filling of the M(2) site and the predominance of one of these elements in the N(4) site (siudaite and georgbarsanovite subgroups). In minerals of the siudaite subgroup, iron is trivalent, and in minerals of the georgbarsanovite subgroup, it is bivalent. It is important to note that in our scheme, the values of ^M⁽²⁾Fe or ^M⁽²⁾Mn are used only to assess the deficiency or excess of cations in the considered sites. Therefore, the value of ^M⁽²⁾Mn + ^M⁽²⁾Fe > 4.5 with a maximum content of cations in M(2) equal to three, indicates only that M(2) is completely populated and there is an excess (1.5 apfu and more) content of the sum Mn and Fe.

EGMs can be enriched by calcium (Ca > 6 apfu) and in addition to M(1) site the excess of Ca can occupy the extraframework N(3) and N(4) sites. The excess calcium (^NCa = ^TotalCa − 6 apfu) will then be considered together with the elements assigned the N sites: Sr, REE, and K. In feklichevite, golyshevite, and mogovidite, ^NCa > 1.5 apfu, since calcium in these minerals not only fills the six-membered [M(1)₆O₂₄] rings, but also predominates in N(3) site (feklichevite, golyshevite) or N(3) and N(4) sites (mogovidite).

Schematically, variants for filling positions in the structure of EGMs with iron and/or manganese are shown in Figure 4.

3.5. Ca-Poor Eudialytes: Occupation of the M(1) and M(2) Sites (Step 4)

Eudialytes □ + □ are not divided into subgroups, since they contain only ikranite, for which 4.5 ≥ ^M⁽²⁾Mn + ^M⁽²⁾Fe > 1.5 and ^M⁽²⁾Fe > ^M⁽²⁾Mn.

For T + M and T + T Ca-poor eudialytes, the sequence of calculations is as follows:

An estimate of the sum Ca + Mn + Fe, i.e., the total content of cations in the M(1a), M(1b), and M(2) sites. If only Ca, Mn, or Fe prevail in the listed sites, then the sum will be from 7.5 to 10.5 apfu. If the listed positions contain other species-forming elements (for example, sodium in the M(2) site in raslakite and sergevanite), then the Ca + Mn + Fe sum will be in the range from 7.5 to 4.5 apfu.
Assessment of the calcium content. If calcium dominates in one of the positions, for example, M(1a), then the condition Ca > 1.5 apfu must be satisfied. In voronkovite, calcium is not a species-forming element and in this mineral Ca < 1.5 apfu.
If Ca > 1.5 apfu, then the following is an estimate of the content of manganese and iron in position M(1). First, the difference between calcium content and 6 is compensated by manganese ^M(1)Mn = 6 − ^TotalCa, and then, if manganese is insufficient, by iron ^M⁽¹⁾Fe = 6 − (^M⁽¹⁾Mn + ^TotalCa). The remainder of manganese and iron is assigned to position M(2): ^M⁽²⁾Mn = ^TotalMn − ^M⁽¹⁾Mn and ^M⁽²⁾Fe = ^TotalFe − ^M⁽¹⁾Fe.

T + M eudialytes include the oneillite subgroup, the only representative of which, oneillite, contains 10.5 ≥ Ca + Mn + Fe > 7.5; Ca > 1.5; ^M⁽¹⁾Mn > ^M⁽¹⁾Fe; ^M⁽²⁾Fe > ^M⁽²⁾Mn (manganese and iron fill the M1a and M2 positions). The T + T eudialytes include the sergevanite subgroup (7.5 ≥ Ca + Mn + Fe > 4.5; Ca > 1.5; ^M⁽¹⁾Mn > ^M⁽¹⁾Fe; ^M⁽²⁾Fe > ^M⁽²⁾Mn), the raslakite subgroup (7.5 ≥ Ca + Mn + Fe > 4.5; Ca > 1.5; ^M⁽¹⁾Fe > ^M⁽¹⁾Mn; ^M⁽²⁾Fe > ^M⁽²⁾Mn) and the voronkovite subgroup (7.5 ≥ Ca + Mn + Fe > 4.5; Ca < 1.5; ^M⁽¹⁾Mn > ^M⁽¹⁾Fe).

3.6. Extraframework Cations (N Sites)

Further separation of EGMs should be performed in accordance with the filling of N sites, namely, according to values of ^NNa, ^NCa, K, Sr, ^NREE apfu. If potassium predominates in one of the N sites (for example, andrianovite), the conditions ^NNa > 10.5; K > 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5 must be satisfied simultaneously. If strontium predominates, as in odikhinchaite, then the conditions ^NNa > 7.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr > 1.5 must be met. At this step, in some subgroups, it is possible to select series.

3.7. Synthesizing the Diagnostic Scheme

The general diagnostic scheme of minerals of the eudialyte group minerals is shown in Table 4. It can be supplemented by new members without changing its hierarchical structure. This scheme does not require writing the complete empirical formula of the mineral, but uses some rules for writing it, for example, the sequence of filling the M(1) position with different elements when the calcium content is below 6 pfu. It is important to note that many positions in the crystal structure of EGMs (for example, M3 and M4) are filled statistically and therefore an empirical formula can only be obtained by solving the structure of the mineral. The approach we propose makes it possible, in conditions of limited data (only on chemical composition), to navigate in the virtually infinite variety of minerals of the eudialyte group.

4. Conclusions

We propose a diagnostic scheme for the eudialyte-group minerals based on their chemical composition (and, in four cases, by using IR spectroscopy, for detection manganoeudialyte, oneillite, mogovidite and ikranite). The scheme includes five consecutive steps, each of which evaluates the content of a species-forming element (or the sum of such elements). So, it is possible name any eudialyte-group mineral sample without applying a time-consuming crystallographic investigation.

Author Contributions

Conceptualization, J.A.M. and S.M.A.; methodology, D.G.S. and A.O.K.; software, D.G.S.; writing—original draft preparation, J.A.M.; writing—review and editing, S.M.A. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful to Sergey V. Krivovichev for the fruitful discussion and valuable comments. The research was supported by the Russian Science Foundation, grant No. 20-77-10065 (contribution to the crystal-chemical analysis of EGMs, S.M.A.) and Ministry of Science and Higher Education of the Russian Federation, projects No. 0226-2019-0051 (design and development of the classification scheme of EGMs, J.A.M., D.G.S., A.O.K.) and 122011300125-2 (search and analysis of literature sources related to EGMs, S.M.A.).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The general view of crystal structure of “12-layer” EGMs.

Figure 2. Schematic sketch of the second and third steps of EGMs diagnostic. Based on Rastsvetaeva and Chukanov (2012) [3].

Figure 3. IR spectra of holotype samples of manganoeudialyte [25] (a), oneillite [27] (b), mogovidite [18] (c), and ikranite [19] (d). The spectra of manganоeudialite, oneillite, and mogovidite contain bands corresponding to additional SiO₄ tetrahedra located at the centers of nine-membered rings (933, 925, 928 cm⁻¹, respectively).

Figure 4. Options for filling M(1), M(2) and N(3) sites with iron and manganese.

Table 1. Empirical and corresponding CNMNC formulas of the of the IMA-approved members of the eudialyte group.

Mineral	Empirical Formula */IMA Formula	Ref.
Alluaivite	(Na_17.47K_0.12La_0.03Ce_0.14Sr_0.28Ba_0.11)(Ca_4.46Mn_1.47)Si_25.85O_73.26 (Ti_2.18Zr_0.05Nb_0.85)Cl_0.66∙2.75H₂O Na₁₉(Ca,Mn²⁺)₆(Ti,Nb)₃Si₂₆O₇₄Cl·2H₂O	[7]
Andrianovite	Na_12.09(K_1.40Sr_0.97REE_0.60Ba_0.04)_Σ3.01(Ca_5.90Y_0.08)_Σ5.98(Mn_1.81Fe²⁺_1.19)_Σ2.90(Zr_2.96Hf_0.04)_Σ3.0 (Nb_0.69Si_0.27Ti_0.05Al_0.01)_Σ1.02(Si₂₅O₇₃)[O_2.14(OH)_0.52]_Σ2.66[(H₂O)_1.30(CO₃)_0.42Cl_0.28]_Σ2.0 Na₁₂(K,Sr,Ce)₃Ca₆Mn₃Zr₃Nb(Si₂₅O₇₃)(O,H₂O,OH)₅	[8]
Aqualite	[(H₃O)_7.94Na_2.74K_1.20Sr_0.49Ba_0.46Fe_0.23Mn_0.12]_Σ13.18(Ca_5.79REE_0.19)_Σ5.98(Zr_2.92Ti_0.08)_Σ3.0 (Si_25.57Ti_0.21Al_0.19Nb_0.03)_Σ26.0[O_66.46(OH)_5.54]_Σ72.0[(OH)_2.77Cl_1.23]_Σ4.0 (H₃O)₈(Na,K,Sr)₅Ca₆Zr₃Si₂₆O₆₆(OH)₉Cl	[9]
Carbokentbrooksite	(Na_9.43Ca_0.90K_0.36)_Σ10.69[Na_1.39(Ce_0.60La_0.36Nd_0.15Pr_0.05)_Σ1.16Sr_0.45]_Σ3(Ca_5.34Mn_0.54Y_0.12)_Σ6 (Mn_1.98Fe_1.02)_Σ3(Zr_2.96Ti_0.04)_Σ3(Nb_0.88Ti_0.13)_Σ1.01Si₂₅H_4.32O_77.64(CO₃)_0.58Cl_0.27 (Na,☐)₁₂(Na,Ce)₃Ca₆Mn₃Zr₃NbSi₂₅O₇₃(OH)₃(CO₃)·H₂O	[10]
Davinciite	(Na_1l.75Sr_0.29Ba_0.03)_Σ12.07(K_2.28Na_0.72)_Σ3Ca_5.99(Fe_2.26Mn_0.16)_Σ2.42(Zr_2.80Ti_0.15Hf_0.03Nb_0.02)_Σ3 (Si_1.96Al_0.04)_Σ2[Si₃O₉]₂[Si₉O₂₇]₂[(OH)_1.42O_0.58]_Σ2[Cl_1.62(H₂O)_0.38]_Σ2⋅0.48H₂O Na₁₂K₃Ca₆Fe²⁺₃Zr₃(Si₂₆O₇₃OH)Cl₂	[11]
Dualite	(Na_29.79Ba_0.1K_0.10)_Σ30(Ca_8.55Na_1.39REE_1.27Sr_0.79)_Σ12(Na_3.01Mn_1.35Ti_0.77)_Σ6(Zr_2.61Nb_0.39)_Σ3 (Ti_2.52Nb_0.48)_Σ3(Mn_0.82Si_0.18)_Σ1(Si_50.77Al_0.23)_Σ51O₁₄₄[(OH)_6.54(H₂O)_1.34Cl_0.98]_Σ8.86 Na₃₀(Ca,Na,Ce,Sr)₁₂(Na,Mn,Fe,Ti)₆Zr₃Ti₃MnSi₅₁O₁₄₄(OH,H₂O,Cl)₉	[12]
Eudialyte	– Na₁₅Ca₆Fe₃Zr₃Si(Si₂₅O₇₃)(O,OH,H₂O)₃(Cl,OH)₂	[13]
Feklichevite	Na_10.85Ca_9.15REE_0.08Fe_2.00Mn_0.25Ti_0.05Zr_3.05Hf_0.04Nb_0.62Si_25.61O_75.89Cl_0.21 Na₁₁Ca₉(Fe³⁺,Fe²⁺)₂Zr₃Nb(Si₂₅O₇₃)(OH,H₂O,Cl,O)₅	[14]
Fengchengite	[(Na_3.00Na_3.00)_Σ6.00(Na_5.28K_0.33□_0.39)]Σ_12.00(□_2.71Sr_0.20REE_0.09)_Σ3.00(Ca_4.80Sr_0.82Fe²⁺_0.29 Mg_0.05Mn_0.04)_Σ6.00(Fe³⁺_2.25Mn_0.35Cr_0.08□_0.32)_Σ3.00(Zr_2.86Ti_0.09Nb_0.05)_Σ3.00(Si_0.87Ti_0.05□_0.08)_Σ1.00 Si(Si_24.00O_73.00)[(H₂O)_2.93(OH)_0.07]_Σ3.00[(OH)_1.04Cl_0.96]_Σ2.00 Na₁₂☐₃Ca₆Fe³⁺₃Zr₃Si(Si₂₅O₇₃)(H₂O)₃(OH)₂	[15]
Ferrokentbrooksite	(Na_13.05REE_0.99K0.32Ca_0.23Sr_0.15)_14.74(Ca_4.59Mn_1.24Y_0.17)₆(Fe_2.39Mn_0.61)₃ (Zr_3.00Ti_0.04Hf_0.03)_3.07(Nb_0.64Si_0.23Zr_0.07Ta_0.02)_0.96(Si_24.93Al_0.07)₂₅O₇₃ (O,OH,H₂O)_2.47(Cl_0.89F_0.71OH_0.40)₂ Na₁₅Ca₆Fe²⁺₃Zr₃Nb(Si₂₅O₇₃)(O,OH,H₂O)₃(F,Cl)₂	[16]
Georgbarsanovite	(Na_11.74K_0.24)_Σ11.97(Mn_1.19Sr_0.71REE_0.63Ca_0.30Y_0.13Ba_0.03)_Σ2.99Ca₆(Fe²⁺_2.55Zr_0.23Ti_0.05Hf_0.04)_Σ2.87 Zr₃Nb_0.92Si_25.25O_76.48Cl_1.11F_0.61∙0.88H₂O Na₁₂(Mn,Sr,REE)₃Ca₆Fe²⁺₃Zr₃NbSi₂₅O₇₆Cl₂·H₂O	[17]
Golyshevite	(Na_9.02Ca_0.43K_0.30)_Σ9.75Ca₃(Ca_5.92Ce_0.05La_0.03)_Σ6.00Fe³⁺_1.69Fe²⁺_0.50Mn_0.29Zr_2.97 (Nb_0.60Si_0.66Al_0.08)Si₂₄O₇₂(OH)_2.37(CO₃)_1.05Cl_0.21∙1.01H₂O Na₁₀Ca₉Zr₃Fe₂SiNb(Si₃O₉)₂(Si₉O₂₇)₂(OH)₃(CO₃)·H₂O	[18]
Ikranite	Na_7.56(H₃O)_6.64K_0.27Ca_3.31Sr_0.46Ce_0.27La_0.11Nd_0.03Mn_1.41Fe²⁺_0.16Fe³⁺_1.77Zr_3.33Ti_0.14Hf_0.04 Nb_0.06Si₂₄O₇₂Cl_0.74∙2.64H₂O (Na,H₃O)₁₅(Ca,Mn,REE)₆Fe³⁺₂Zr₃Si₂₄O₆₆(O,OH)₆Cl·nH₂O	[19]
Ilyukhinite	H_36.04(Na_3.82K_0.20)(Ca_5.65Ce_0.22La_0.14Nd_0.07)(Mn_1.285Fe_0.48)(Zr_2.645Ti_0.34) Nb_0.31Si_25.41S_0.42Cl_0.23O_86.82 (H₃O,Na)₁₄Ca₆Mn₂Zr₃Si₂₆O₇₂(OH)₂·3H₂O	[20]
Johnsenite-(Ce)	Na_11.74([Ce_0.64La_0.33Dy_0.03]_Σ1.00Sr_0.54Ca_0.51Y_0.22K_0.19Hf_0.01)_Σ2.47(Ca_5.06 [Pr_0.24Nd_0.18Gd_0.06Sm_0.02]_Σ0.50Mn_0.44)_Σ6(Mn_2.22Fe_0.78)_Σ3(Zr_2.71Ti_0.32)_Σ3.03 (W_0.78Nb_0.21)_Σ0.99Si_24.97O₇₃(CO₃)(OH,Cl_0.75)_Σ2 Na₁₂Ce₃Ca₆Mn₃Zr₃WSi₂₅O₇₃(CO₃)(OH)₂	[21]
Kentbrooksite	(Na_14.93REE_0.44Y_0.42K_0.30Sr_0.15)_Σ16.24(Ca_3.27Mn_1.78REE_0.62Na_0.33)_Σ6.00 (Mn_1.90Fe_0.72Al_0.13Mg_0.05)_Σ2.80(Nb_0.55Zr_0.12Ti_0.10)_Σ0.77Si_0.60(Zr_2.81Hf_0.06Ti_0.13)_Σ3 [(Si₃O₉)₂(Si₉O₂₇)₂O₂](F_1.51Cl_0.27OH_0.22)_Σ2∙2.3H₂O (Na,REE)₁₅(Ca,REE)₆Mn₃Zr₃Nb(Si₂₅O₇₃)(O,OH,H₂O)₃(F,Cl)₂	[22]
Khomyakovite	(Na_12.26Ca_0.33K_0.38Sr_0.13REE_0.08)_Σ13.05(Sr_2.78Na_0.22)_Σ3Ca₆(Fe_2.05Mn_0.78Mg_0.03)_Σ2.86 (Zr_2.94Ti_0.05Hf_0.03)_Σ3.02(W_0.56Nb_0.34)_Σ0.90(Si_24.78Al_0.06)_Σ24.84 O₇₃(O,OH,H₂O)_Σ3.70(OH_1.36Cl_0.64)_Σ2 Na₁₂Sr₃Ca₆Fe₃Zr₃W(Si₂₅O₇₃)(O,OH,H₂O)₃(Cl,OH)₂	[23]
Labyrinthite	Na_33.19K_0.98Ca_11.68Sr_1.57Fe_2.19Mn_0.77Zr_5.95Ti_0.49Si_51.26Cl_3.04H_10.8 (Na,K,Sr)₃₅Ca₁₂Fe₃Zr₆TiSi₅₁O₁₄₄(O,OH,H₂O)₉Cl₃	[24]
Mangankhomyakovite	(Na_11.51K_0.30Ca_0.25Sr_0.04REE_0.07)_Σ12.17Sr₃Ca₆(Mn_2.04Fe_1.23)_Σ3.27(Zr_2.91Hf_0.03Ti_0.01)_Σ2.95 (W_0.66Nb_0.41Ta_0.01)_Σ1.08(Si_24.60Al_0.01)_Σ24.61O₇₃(O,OH,H₂O)_3.70(OH_1.19Cl_0.81)_Σ2 Na₁₂Sr₃Ca₆Mn₃Zr₃W(Si₂₅O₇₃)(O,OH,H₂O)₃(Cl,OH)₂	[23]
Manganoeudialyte	H_12.08Na_12.05Sr_0.90K_0.39La_0.03Ce_0.02Ca_5.93(Mn_1.54Fe_1.18)Zr_3.03Nb_0.28Al_0.25 Hf_0.04Ti_0.18Si_25.20O_79.40Cl_0.87F_0.13 Na₁₄Ca₆Mn₃Zr₃[Si₂₆O₇₂(OH)₂](H₂O,Cl,O,OH)₆	[25]
Mogovidite	(Na_9.87Ca_4.05K_0.24Ce_0.06La_0.03)_Σ14.25Ca_6.00Fe³⁺_1.48Fe²⁺_0.58Mn_0.30Zr_3.02Ti_0.09(Nb_0.40Si_0.71) Si₂₄O₇₂(OH)_2.86(CO₃)_1.03Cl_0.46⋅0.74H₂O Na₉(Ca,Na)₁₂Fe₂Zr₃Si₂₅O₇₂(CO₃)(OH)₄	[18]
Odikhinchaite	H_8.22Na_9.97K_0.42Ca_7.59Sr_1.87Ce_0.08La_0.08Nd_0.03Mn_2.58Fe_0.35Mg_0.20Ti_0.03Zr_3.01Nb_1.05 Si_24.87Al_0.05Cl_0.65C_0.68O_81.71 Na₉Sr₃[(H₂O)₂Na]Ca₆Mn₃Zr₃NbSi(Si₂₄O₇₃)(OH)₃(CO₃)·H₂O	[26]
Oneillite	(Na_14.37REE_1.53K_0.20Sr_0.03)_Σ16.13(Ca_1.77REE_0.59Na_0.66)_Σ3.02(Mn_2.76Y_0.24)_Σ3.00 (Fe_1.43Mn_0.96Zr_0.25)_Σ2.64(Zr_2.93Nb_0.05Hf_0.03)_Σ3.01(Nb_0.85Ta_0.02)_Σ0.87(Si_24.77Al_0.12)_Σ24.89 O₇₃(O,OH,H₂O)_3.09(OH_1.27Cl_0.73)_Σ2.00 Na₁₅Ca₃Mn₃Fe₃Zr₃Nb(Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl)₂	[27]
Raslakite	Na_16.02K_0.32Ca_3.l3Sr_0.21Mg_0.22Fe_2.17Mn_0.88Ce_0.16La_0.08Ti_0.14Zr_3.80Hf_0.06Nb_0.17Al_0.l6 Si_25.40Cl_1.18H_4.66O_76.465 Na₁₅Ca₃Fe₃(Na,Zr)₃Zr₃(Si,Nb)Si₂₅O₇₃(OH,H₂O)₃(Cl,OH)	[19]
Rastsvetaevite	Na_27.10(K_7.93Ba_0.03)_Σ7.96(Ca_11.29Sr_0.74Ce_0.04)_Σ12.07(Fe_2.32Mn_0.42)_Σ2.74(Zr_5.69Ti_0.30Hf_0.04)_Σ6.03 (Si_51.53Al_0.20Nb_0.16Ta_0.01)_Σ51.90O₁₄₄O_2.14(OH)_1.86Cl_2.29⋅1.71H₂O Na₂₇K₈Ca₁₂Fe₃Zr₆Si₅₂O₁₄₄(OH,O)₆Cl₂	[28]
Sergevanite	H_14.46Na_13.64K_0.92Ca_4.22Ce_0.27La_0.17Nd_0.12Pr_0.06Sm_0.02Mn_1.81Fe²⁺_0.58Ti_0.46Zr_2.67 Hf_0.04Nb_0.38Si_25.5S_0.30Cl_0.20O₈₁ Na₁₅(Ca₃Mn₃)(Na₂Fe)Zr₃Si₂₆O₇₂(OH)₃∙H₂O	[29]
Siudaite	[Na_7.57(H₂O)_1.43]_Σ9(Mn_1.11Na_0.88Ce_0.31La_0.20Nd_0.05Pr_0.04K_0.41)_Σ3(H₂O)_1.8(Ca_5.46Mn_0.54)_Σ6 (Fe³⁺_1.76Mn²⁺_1.19)_Σ2.95Nb_0.65(Ti_0.20Si_0.50)_Σ0.71(Zr_2.95Hf_0.04Ti_0.01)_Σ3 Si_24.00Cl_0.47O₇₀(OH)₂Cl_0.47·1.82H₂O Na₈(Mn₂Na)Ca₆Fe³⁺₃Zr₃NbSi₂₅O₇₄(OH)₂Cl·5H₂O	[30]
Taseqite	(Na_8.81Sr_4.78K_0.17Ce_0.02)_13.78(Ca_5.17Mn_0.59Y_0.09)_5.85(Fe_1.93Mn_0.92)_2.85(Zr_2.84Nb_0.11Hf_0.05)_3.00 (Nb_1.06Ta_0.04Sn_0.03)_1.13Si_24.55O₇₃(O_1.65OH_0.75(H₂O)_0.74)_3.14(Cl_1.91OH_0.09)_2.00 Na₁₂Sr₃Ca₆Fe₃Zr₃NbSi₂₅O₇₃(O,OH,H₂O)₃Cl₂	[31]
Voronkovite	(Na_13.96Sr_0.54K_0.19)_Σ14.69(Na_1.64Ca_0.92Ce_0.26La_0.18)_Σ3.00(Mn_2.06Ca_0.81Nd_0.13)_Σ3.00 (Fe_1.54Zr_0.60Na_0.48Nb_0.21Ti_0.13Hf_0.04)_Σ3.00Zr_3.00(Si_1.91Al_0.09)_Σ2.00 (Si₂₄O₇₂)[(OH)_2.98O_1.02]_Σ4(Cl_0.39F_0.35)_Σ0.74·1.23H₂O Na₁₅(Na,Ca,Ce)₃(Mn,Ca)₃Fe₃Zr₃Si₂₆O₇₂(OH,O)₄Cl·H₂O	[32]
Zirsilite-(Ce)	(Na_9.01Ca_0.82K_0.32)_Σ10.15[(Ce_0.76La_0.47Nd_0.16Pr_0.06)_Σ1.45Na_1.12Sr_0.43]_Σ3(Ca_5.36Mn_0.50Y_0.14)_Σ6 (Mn_2.13Fe_0.87)_Σ3(Zr_2.80Ti_0.18)_Σ2.98Nb_0.93Si₂₅H_5.54(CO₃)_0.43Cl_0.30 (Na,☐)₁₂(Ce,Na)₃Ca₆Mn₃Zr₃NbSi₂₅O₇₃(OH)₃(CO₃)·H₂O	[10]

* as it was published in the paper with the description of the new mineral species.

Table 2. Site occupancies of the IMA-approved members of the eudialyte group.

Mineral	Space Group	N(1)	N(2)	N(3)	N(4)	N(5)	Z	M(1a)	M(1b)	M(2)	M(3)	M(4)	X(1)	X(2)
Alluaivite	$R \bar{3}$ m	Na	Na	Na	Na	Na	Ti	Ca		□, Na	Si	Si	H₂O	Cl
Alluaivite	$R \bar{3}$ m	Na	Na	Na	Na	Na	Ti	Ca		Na	Si	Si	H₂O	Cl
Andrianovite	R3m	Na	Na	Na	K	Na	Zr	Ca		Mn	Nb	Si	H₂O	CO₃
Aqualite	R3	H₃O	Na	H₃O	H₃O	□, OH	Zr	Ca	Ca	□, Na	Si	Si	Cl, SO₄	Cl
Carbokentbrooksite	R3m	Na	Na	Na	Na	Na	Zr	Ca		Mn	Nb	Si	CO₃	H₂O
Davinciite	R3m	Na	Na	K	Na	Na	Zr	Ca		Fe²⁺	Si	Si	H₂O	Cl
Dualite	R3m	Na	Na	Na	Na	Na	Zr	Ca		Na	Si	Si	H₂O	Cl
Dualite	R3m	Na	Na	Na	Na	Na	Ti	Ca		Na	Mn	Si	H₂O	Cl
Eudialyte	R3m	Na	Na	Na	Na	Na	Zr	Ca		Fe²⁺	Si	Si	Cl	H₂O
Feklichevite	R3m	Na	Na	Ca	Na	Na	Zr	Ca		Fe³⁺	Nb	Si	H₂O	Cl
Fengchengite	$R \bar{3}$ m	Na	Na	□	Na	Na	Zr	Ca		Fe³⁺	Si	Si	Cl	OH
Ferrokentbrooksite	R3m	Na	Na	Na	Na	Na	Zr	Ca		Fe²⁺	Nb	Si	Cl	F
Georgbarsanovite	R3m	Na	Na	Na	Mn	Na	Zr	Ca		Fe²⁺	Nb	Si	Cl	H₂O
Golyshevite	R3m	Na	Na	Na	Ca	□, Na	Zr	Ca		Fe³⁺	Si	Nb	CO₃	H₂O
Ikranite	R3m	H₃O	Na	Na	Na	□, OH	Zr	Ca		Fe³⁺	□	□	H₂O	H₂O
Ilyukhinite	R3m	H₃O	H₃O	H₃O	Na	H₃O	Zr	Ca		Mn	Si	Si	H₂O	H₂O
Johnsenite-(Ce)	R3m	Na	Na	Na	REE	Na	Zr	Ca		Mn	W	Si	CO₃	H₂O
Kentbrooksite	R3m	Na	Na	Na	Na	Na	Zr	Ca		Mn	Nb	Si	F	F
Khomyakovite	R3m	Na	Na	Na	Sr	Na	Zr	Ca		Fe	W	Si	OH	Cl
Labyrinthite	R3	Na	Na	Na	Na	Na	Zr	Ca	Ca	Na	Ti	Si	Cl, F	Cl
Labyrinthite	R3	Na	Na	Na	Na	Na	Zr	Ca	Ca	Fe	Si	Si	H₂O	Cl
Mangankhomyakovite	R3m	Na	Na	Na	Sr	Na	Zr	Ca		Mn	W	Si	OH	Cl
Manganoeudialyte	R3m	Na	Na	Na	Na	Na	Zr	Ca		Mn	Si	Si	H₂O	H₂O
Mogovidite	R3m	Na	Na	Ca	Ca	Na	Zr	Ca		Fe³⁺	□, Nb	Si	CO₃	H₂O
Odikhinchaite	R3m	Na	Na	Sr	Na	H₂O	Zr	Ca		Mn	Nb	Si	CO₃	H₂O
Oneillite	R3	Na	Na	Na	Na	Na	Zr	Mn	Ca	Fe	Nb	Si	H₂O	H₂O
Raslakite	R3	Na	Na	Na	Na	Na	Zr	Ca	Fe	Na	Si	Si	H₂O	Cl
Rastsvetaevite	R3m	Na	Na	Na	Na	Na	Zr	Ca		K	Si	Si	H₂O	Cl
Rastsvetaevite	R3m	Na	Na	K	Na	Na	Zr	Ca		Fe	Si	Si	Cl	Cl
Sergevanite	R3	Na	Na	Na	Na	H₂O	Zr	Ca	Mn	Na	Si	Si	H₂O	Cl
Siudaite	R3m	Na	Na	Na	Mn	H₂O	Zr	Ca		Fe³⁺	Nb	Si	Cl	H₂O
Taseqite	R3m	Na	Na	Na	Sr	Na	Zr	Ca		Fe²⁺	Nb	Si	Cl	Cl
Voronkovite	R3	Na	Na	Na	Na	Na	Zr	Na	Mn	Fe	Si	Si	H₂O	H₂O
Zirsilite-(Ce)	R3m	Na	Na	Na	Ce	Na	Zr	Ca		Mn	Nb	Si	CO₃	H₂O

Table 3. Calculation of formula coefficients for holotype samples of ikranite and mogovidite.

Cation/Anion	Ikranite *		Mogovidite **
Cation/Anion	Si = 24¹⁹	Si + Al + Ti + Zr + Hf + Nb + Ta + W = 29	Si = 24.71¹⁸	Si + Al + Ti + Zr + Hf + Nb + Ta + W = 29
Nb	0.06	0.06	0.40	0.42
Si	24	25.28	24.71	25.39
Ti	0.14	0.14	0.09	0.09
Zr	3.33	3.51	3.02	3.10
Hf	0.04	0.04	-	-
Fe³⁺	1.77	1.87	1.48	1.52
La	0.11	0.12	0.03	0.03
Ce	0.27	0.29	0.05	0.05
Nd	0.03	0.04	-	-
Fe²⁺	0.16	0.16	0.57	0.59
Mn	1.41	1.49	0.30	0.31
Ca	3.31	3.48	10.05	10.33
Sr	0.46	0.48	-	-
Na	7.56	7.97	9.87	10.14
K	0.27	0.29	0.24	0.25
Cl	0.74	0.78	0.46	0.47
F	0.15	0.16	-	-

* Ikranite chemical composition [19]: SiO₂ 48.91, TiO₂ 0.37, Fe₂O₃ 4.80, FeO 0.38, MnO 3.40, CaO 6.29, Na₂O 7.95, K₂O 0.44, Cl 0.89, F 0.10, H₂O 7.70, SrO 1.61, ZrO₂ 13.94, Nb₂O₅ 0.28, La₂O₃ 0.62, Ce₂O₃ 1.53, Nd₂O₃ 0.19, HfO₂ 0.28, sum 99.68. ** Mogovidite chemical composition [18]: SiO₂ 47.49, TiO₂ 0.23, Fe₂O₃ 3.78, FeO 1.32, MnO 0.68, CaO 18.03, Na₂O 9.78, K₂O 0.36, Cl 0.52, ZrO₂ 11.90, Nb₂O₅ 1.72, La₂O₃ 0.15, Ce₂O₃ 0.28, H₂O 1.25, CO₂ 1.42 sum 98.91.

Table 4. Calculation of formula coefficients for holotype samples of ikranite and mogovidite.

Zirconium eudialytes ΣZr > 2.25	Ca-rich zirconium eudialytes ΣCa > 4.5	T + T eudialytes Si > 25.5 Nb ≤ 0.5 W ≤ 0.5	Eudialyte subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Fe > ^M(2)Mn; Fe²⁺ > Fe³⁺	Davinciite series* ^NNa > 10.5; K > 1.5; ^NCa < 1.5; ^NREE < 1.5; ^NSr < 1.5	DAVINCIITE
					RASTSVETAEVITE
				Eudialyte series ^NNa > 13.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5	EUDIALYTE Ti < 0.25
					LABYRINTHITE Ti > 0.25
			Fengchengite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Fe > ^M(2)Mn; Fe³⁺ > Fe²⁺	FENGCHENGITE ^NNa < 13.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5
			Manganoeudialyte subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Mn > ^M(2)Fe	ILYUKHINITE ^NNa ≤ 13.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5
				MANGANOEUDIALYTE ^NNa > 13.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5
			Aqualite subgroup ^M(2)Mn + ^M(2)Fe ≤ 1.5	AQUALITE ^NNa < 4.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5;
		T + Nb eudialytes 24.5 < Si ≤ 25.5; Nb > 0.5 ≥ W	Kentbrooksite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Mn > ^M(2)Fe	KENTBROOKSITE ^NNa > 13.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5; CO₃ ≤ 0.5
				CARBOKENTBROOKSITE ^NNa > 13.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5; CO₃ > 0.5
				ANDRIANOVITE ^NNa > 10.5; K > 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5; CO₃ ≤ 0.5
				ODIKHINCHAITE ^NNa > 7.5; K < 1.5; ^NCa < 1.5; Sr > 1.5; ^NREE < 1.5; CO₃ ≤ 0.5
				ZIRSILITE-(Ce) ^NNa > 10.5; K < 1.5; ^NCa < 1.5; ^NREE > 1.5; Sr < 1.5; CO₃ ≤ 0.5
			Ferrokentbrooksite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Fe > ^M(2)Mn;	Feklichevite series ^NCa > 1.5	FEKLICHEVITE ^NNa > 10.5; K < 1.5 ^NREE < 1.5; Sr < 1.5
					GOLYSHEVITE ^NNa > 7.5; K < 1.5; ^NREE < 1.5; Sr < 1.5
				Ferrokentbrooksite series ^NCa ≤ 1.5	TASEQITE ^NNa > 10.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr > 1.5;
					FERROKENTBROOKSITE ^NNa > 13.5; K < 1.5; ^NREE < 1.5; Sr ≤ 1.5
			Siudaite subgroup ^M(2)Mn + ^M(2)Fe > 4.5; Fe³⁺ > Fe²⁺	SIUDAITE ^NNa > 7.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5
			Georgbarsanovite subgroup ^M(2)Mn + ^M(2)Fe > 4.5; Fe²⁺ > Fe³⁺	GEORGBARSANOVITE ^NNa > 10.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5
		T + W eudialytes 24.5 < Si ≤ 25.5; W > 0.5 ≥ Nb	Khomyakovite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Fe²⁺ > ^M(2)Mn	KHOMYAKOVITE ^NNa > 10.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr > 1.5
			Manganokhomyakovite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Mn > ^M(2)Fe	JOHNSENITE-(Ce) ^NNa > 10.5; K < 1.5; ^NCa < 1.5; ^NREE > 1.5; Sr < 1.5
				MANGANKHOMYAKOVITE ^NNa > 10.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr > 1.5
		T + □ eudialytes 24.5 < Si ≤ 25.5; Nb ≤ 0.5; W ≤ 0.5	Mogovidite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Fe²⁺ > ^M(2)Mn	MOGOVIDITE ^NNa > 7.5; K < 1.5; ^NCa ≥ 4.5; ^NREE < 1.5; Sr < 1.5
	Ca-poor zirconium eudialytes ΣCa ≤ 4.5	□ + □ eudialytes 24.5 < Si ≤ 25.5; Nb ≤ 0.5	Ikranite subgroup 4.5 ≥ ^M(2)Mn + ^M(2)Fe > 1.5; ^M(2)Fe > ^M(2)Mn	IKRANITE ^NNa < 10.5; K < 1.5; ^NCa < 1.5; ^NREE < 1.5; Sr < 1.5
		T + M eudialytes 24.5 < Si ≤ 25.5; Nb > 0.5	Oneillite subgroup 10.5 ≥ Ca + Mn + Fe > 7.5; Ca > 1.5 ^M(1)Mn > ^M(1)Fe ^M(2)Fe > ^M(2)Mn	ONEILLITE Na > 13.54; K < 1.5; REE < 1.5; Sr < 1.5
		T + T eudialytes Si > 25.5; Nb ≤ 0.5	Raslakite subgroup 7.5 ≥ Ca + Mn + Fe > 4.5; Ca > 1.5 ^M(1)Fe > ^M(1)Mn ^M(2)Fe > ^M(2)Mn	RASLAKITE Na > 16.5; K < 1.5; REE < 1.5; Sr < 1.5
			Sergevanite subgroup 7.5 ≥ Ca + Mn + Fe > 4.5; Ca > 1.5 ^M(1)Mn > ^M(1)Fe ^M(2)Fe > ^M(2)Mn	SERGEVANITE Na < 13.5; K < 1.5; REE < 1.5; Sr < 1.5
			Voronkovite subgroup 7.5 ≥ Ca + Mn + Fe > 4.5; Ca < 1.5	VORONKOVITE Na > 16.5; K < 1.5; REE < 1.5; Sr < 1.5
DUALITE 0.75 < ΣZr ≤ 2.25
ALLUAIVITE ΣZr ≤ 0.75

* IR research is necessary. Rastsvetaevite differs from davinciite in the presence of additional splitting of bands in the IR spectrum.

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Mikhailova, J.A.; Stepenshchikov, D.G.; Kalashnikov, A.O.; Aksenov, S.M. Who Is Who in the Eudialyte Group: A New Algorithm for the Express Allocation of a Mineral Name Based on the Chemical Composition. Minerals 2022, 12, 224. https://doi.org/10.3390/min12020224

AMA Style

Mikhailova JA, Stepenshchikov DG, Kalashnikov AO, Aksenov SM. Who Is Who in the Eudialyte Group: A New Algorithm for the Express Allocation of a Mineral Name Based on the Chemical Composition. Minerals. 2022; 12(2):224. https://doi.org/10.3390/min12020224

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Mikhailova, Julia A., Dmitry G. Stepenshchikov, Andrey O. Kalashnikov, and Sergey M. Aksenov. 2022. "Who Is Who in the Eudialyte Group: A New Algorithm for the Express Allocation of a Mineral Name Based on the Chemical Composition" Minerals 12, no. 2: 224. https://doi.org/10.3390/min12020224

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Who Is Who in the Eudialyte Group: A New Algorithm for the Express Allocation of a Mineral Name Based on the Chemical Composition

Abstract

1. Introduction

2. Methods

3. Diagnostic Scheme of EGMs

3.1. Framework Cations: Zirconium or Titanium in Z Site (Step 1)

3.2. Framework Cations: Calcium at the M(1) Site (Step 2)

3.3. Framework Cations: Occupation of the M(3) and M(4) Sites (Step 3)

3.4. Ca-Rich Zirconium Eudialytes: Оccupation of the M(2) Site (Step 4)

3.5. Ca-Poor Eudialytes: Occupation of the M(1) and M(2) Sites (Step 4)

3.6. Extraframework Cations (N Sites)

3.7. Synthesizing the Diagnostic Scheme

4. Conclusions

Author Contributions

Funding

Conflicts of Interest

References

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