Crystal Chemistry of Eudialyte Group Minerals from Rouma Island, Los Archipelago, Guinea

Abstract

We herein present a comprehensive investigation of the eudialyte group minerals from the nepheline syenites of Rouma Island in the Los Archipelago, Conakry region, Guinea. Two distinct mineral phases were identified: an oneillite-like phase, associated with the agpaitic rock suite, and, for the first time in this locality, kentbrooksite, occurring in pegmatites. The oneillite-like phase crystallizes in the trigonal system (space group R3), with unit cell parameters a = 14.1489(2) Å, c = 30.1283(5) Å and an idealized crystal chemical formula of Na₁₅(Mn,REE)₃(Ca,Mn)₃(Fe,Mn)₃Zr₃(Zr,Si,Al,Nb,Ti)₁ (Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl,F)₂. Kentbrooksite also exhibits trigonal symmetry (space group R3m), with unit cell parameters a = 14.2037(3) Å c = 30.1507(9) Å and an idealized formula of (Na,REE)₁₅(Ca,Mn)₆(Mn,Fe)₃Zr₃(Nb,Si)₁(Si₂₅O₇₃)(O,OH,H₂O)₃(F,Cl,OH)₂. Compared to the oneillite-like phase, kentbrooksite is markedly enriched in Mn and rare earth elements (REE). This geochemical distinction aligns with the progressive mineralogical evolution of the system, transitioning from the miaskitic to agpaitic suite (oneillite-like phase) and subsequently to pegmatites (kentbrooksite). These findings are consistent with the broader-scale observations regarding the syenite ring structure of the Los Archipelago.

1. Introduction

1.1. Mineralogy of the Los Archipelago

The Los Archipelago, located in Guinea, predominantly consists of nepheline syenites emplaced along the continental margin of West Africa during the Albian period [1]. The mineralogical characteristics of this magmatic complex exhibit marked similarities with other alkaline intrusions, such as Mont Saint-Hilaire in Quebec, Canada [2], and the Khibiny and Lovozero massifs in the Kola Peninsula, Russia [3].

The mineralogical study of the Los Archipelago was first initiated by Lacroix [4], who described the mineral species villiaumite [5] and serandite [6]. Subsequently, Parodi and Chevrier [7] documented the presence of unidentified Zr-Ti silicates, one of which was later recognized as the new mineral roumaite by Biagioni et al. [8]. Currently, the Los Archipelago includes two type localities: Rouma Island, which is the type locality for roumaite, serandite, and villiaumite, and a fourth locality for odinite [9], found within marine sediments of the archipelago’s waters. In total, 56 distinct mineral species have been identified within the region.

Eudialyte was first reported in the Los Archipelago by Lacroix [10], who identified it within the nepheline syenites of Rouma Island. The only available chemical analyses of eudialyte from this region were provided by Moreau et al. [1], albeit these analyses were incomplete, as rare earth elements (REE) were not measured. In addition to eudialyte, EPMA data were reported by Moreau et al. [1] for the zirconosilicates låvenite, astrophyllite, and kupletskite.

In their study of Los geology, Moreau et al. [1] identified two principal petrographic suites: a miaskitic suite, characterized by hastingsite-augite nepheline syenite, and an agpaitic suite, distinguished by arfvedsonite-aegirine nepheline syenite. They also reported a gradational transition from agpaitic to pegmatitic facies on Rouma and Kassaa Islands. Within this geological framework, eudialyte is associated with the agpaitic suite, alongside other Zr- and REE-rich phases [1].

The lack of a comprehensive study on Los eudialyte prompted this investigation, undertaken as part of the SYNTHESYS + FR-TAF-Call3-034 project, “The Rare Zirconium-Niobium Silicates of the Alfred Lacroix Collections”. Material collected during the 19th century from former French colonies and preserved within the mineralogical collections of the Muséum national d’Histoire naturelle (MNHN) represents a scientifically and historically significant heritage. This collection, comprising over 600 samples studied by Lacroix and catalogued as nepheline syenites and associated acid differentiates, potentially hosts Zr-, Ti-, and Nb-bearing silicates. In many of these samples, minerals such as eudialyte, låvenite, and catapleiite are reported. However, following Lacroix’s analytical methodology, most of these identifications were based on optical studies and limited chemical analyses conducted via wet analytical techniques. Given the prevalence of isomorphic substitutions within these mineral groups, optical and limited chemical data alone cannot definitively determine species identities. Our objective is to investigate a series of rare Zr-Nb-Ti disilicates and accurately characterize their compositions and structures using a multidisciplinary approach. During this study, a phase belonging to the eudialyte group was identified in a pegmatitic sample from Rouma Island. Here, we provide a detailed description of two distinct eudialyte-group mineral phases using Raman spectroscopy, electron microprobe analyses, and single-crystal X-ray diffraction techniques.

1.2. Crystal Chemistry of Eudialyte

Beginning with the foundational studies conducted by Giuseppetti et al. [11] and Golyshev et al. [12], minerals belonging to the eudialyte group (hereafter referred to as EGMs) have been extensively investigated in the scientific literature [13,14,15,16]. The crystal-chemical formula of EGMs, as derived from the International Mineralogical Association (IMA)-approved formula, can be expressed as follows (Z = 3):

{N1₃N2₃N3₃N4₃N5₃}{[M1a₃M1b₃]₃M(2)₃M(3)M(4)Z₃[Si₂₄O₇₂]O’₀₋₆}X(1)X(2)

where

• N(1–5) = Na, Ca, REE, (H₃O), K, Sr, □;
• M(1) = Na, Ca, Mn²⁺, Fe²⁺, REE;
• M(2) = Na, Mn²⁺, Fe²⁺, Fe³⁺, Zr⁴⁺;
• M(3) and M(4) = [4] (Si,Al) [6] (Nb, Zr,Ti,W);
• Z = Zr, Ti, Nb, Hf;
• O’ = O (OH);
• X = Cl, F, (OH), H₂O, CO₃, SO₄.

The group of EGMs currently comprises 33 recognized species [17]. Their crystal structures are defined by a heteropolyhedral framework (Figure 1) organized along the c-axis through the repetition of three distinct structural “slabs” or “layers”:

-

T layer: Composed of three-membered (Si₃O₉) and nine-membered (Si₉O₂₇) silicate rings. Polyhedra M3 and M4 are located within this layer at the centers of the silicate rings;

-

M layer: Characterized by six-membered rings of edge-sharing M(1)O₆ octahedra, which are further cross-linked by ^[4,5]M2 polyhedra;

-

Z layer: Composed of isolated ZO₆ octahedra.

Repeating the sequence TMTZ along the c axis according to a R lattice builds up the “12 layers” EGMs structures, with cell parameters a ≈ 14.2 Å, c ≈ 30 Å. The N(1-5) polyhedra are positioned in the cavities of the zeolite-like heteropolyhedral framework, present in the Z layer and between the M and Z layers. The X sites, mainly hosting Cl, F, and (OH), are located at the center of the hexagonal rings of the T layer.

Various authors [14,15] have noticed that, despite the complexity of the structure, part of it, {[M(1)₆Z₃(Si₃O₉)₂(Si₉O₂₇)₂]^–24}, remains relatively stable in all phases, while the part [N₁₅ M(2)₃] instead shows a wide variability, with variable populations and occupancy degrees in these sites.

Most EGMs are characterized by R-3m or R3m space group symmetry, but in the oneillite subgroup, the ordering of cations in the M1 octahedral sites leads to a symmetry reduction to R3 [13,14].

2. Materials and Methods

2.1. Materials

The material investigated in this study was obtained from mineralogical specimens in the collections of the Muséum National d’Histoire Naturelle (MNHN) of Paris (France), identified by inventory numbers MNHN_MIN_129.15 and Los-174, hereafter referred to as Los-129.15 and Los-174, respectively.

Sample Los-129.15 is labeled as “eudialyte within nepheline-syenite”. This sample (Figure 2) was donated to Alfred Lacroix in 1929 by Jules Numa Mugnier Serand, the lighthouse keeper from Rouma Island, who provided Lacroix with a suite of samples from the Los peralkaline complex.

In 1931, Lacroix honored Serand by naming the mineral species serandite after him. The sample originates from an agpaitic suite and primarily comprises anhedral centimeter-sized eudialyte crystals with an intense pink coloration, associated with aegirine, arfvedsonite, feldspar, fluorite, and nepheline.

Sample Los-174 is a pegmatite characterized by the presence of aegirine, catapleiite, plagioclase, nepheline, and serandite, along with sub-millimeter yellow tabular crystals. This sample is part of a collection of approximately 200 specimens sent to Alfred Lacroix by Charles Maxime Villiaume, to whom the mineral villiaumite is dedicated, along with Henri Hubert, then chief of the geological survey of “L’Afrique Occidentale Française” in 1922; Henri Pobéguin, a French explorer; and Jules Numa Mugnier Serand, as mentioned above.

2.2. Analytical Methods

The selected crystals were initially analyzed using a combination of scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to detect chemical deviations from the expected compositions of known Zr-Nb-Ti disilicate species from Rouma (MNHN analytical platform—PtME). Subsequently, the retained samples were further investigated using Raman spectroscopy, electron probe microanalysis (EPMA), and single-crystal structural studies.

Micro-Raman spectra of the studied minerals (Figure 3) were collected using polished samples in nearly backscattered geometry using a Horiba Jobin-Yvon XploRA Plus apparatus, with a motorized x–y stage and an Olympus BX41 microscope with a 50× objective (Dipartimento di Scienze della Terra, Università di Pisa).

The Raman spectra were excited with a 532 nm line from a solid-state laser attenuated to 25% to prevent any potential sample damage. The minimum lateral and depth resolution was set to a few micrometers. The system was calibrated using the 520.6 cm^–1 Raman band of silicon before each experimental session. Spectra were collected through three acquisitions with variable counting times ranging from 60 to 120 s. Back-scattered radiation was analyzed with a 1200 gr/mm grating monochromator. The experimental precision is estimated at ±2 cm^–1.

Quantitative chemical analyses were conducted on polished, carbon-coated sections of eudialyte fragments from samples Los-129.15 and Los-174. For each sample, five analysis points were examined using a JEOL JXA8200 electron microprobe in wavelength dispersion mode at the laboratory of the Department of Earth Sciences “A. Desio”, University of Milan (ESD-MI). The instrument was operated with an accelerating voltage of 15 kV, a beam current of 5 nA, and a spot size of 20 microns. To minimize errors in elemental quantification, the analytical approach outlined in Atanasova et al. [18] was followed. The following standards were employed: hornblende (Hbl123) for F, K-feldspar for K, omphacite for Na, grossular for Al, Ca, and Si, zircon for Zr and Hf, pure element for Nb, celestine for S, apatite for P, cancrinite for Cl, rhodonite for Mn, ilmenite for Ti, fayalite for Fe, and orthophosphates [19] for REE.

Matrix effects in the raw data were corrected using the ϕρZ method from the JEOL series of programs. Experimental conditions are detailed in Supplementary Material Table S1.

Intensity data for the two eudialyte samples, Los-129.15 and Los-174, were collected using a Bruker D8 Venture diffractometer, equipped with an air-cooled Photon III CCD detector and microfocus MoKα radiation (C.I.S.U.P., University of Pisa, Pisa, Italy). Experimental conditions are detailed in Supplementary Material Table S2.

3. Results

3.1. Raman Spectroscopy

The Raman spectra of the two samples under study are presented in Figure 4. Even considering the influence of the orientation effect affecting the intensity of Raman bands of the two spectra, significant differences are however observed between the spectra, particularly with the presence of strong absorption bands in the 600–800 cm⁻¹ region of the Los-129.15 sample, which underscores the distinct nature of the two minerals. The band assignments in the Raman spectra, following Chukanov et al. [20], are as follows:

• 900–1300 cm^–1: Si–O stretching modes, observed in both samples;
• 807, 737 and 696 cm^–1: Mixed vibrations of SiO₄ tetrahedral rings, observed in Sample Los-174;
• 500–600 cm^–1: Mixed vibrations of SiO₄ tetrahedral rings combined with Zr–O stretching modes, observed at 568 cm⁻¹ in Los-174 and 542 cm⁻¹ in Los-129.15;
• Below 400 cm^–1: Lattice modes.

3.2. EPMA Analyses

EPMA chemical analyses of Los-129.15 and Los-174 are reported in

Table 1. EPMA chemical analysis of Los-129.15 and Los-174 EGMs. a.p.f.u. calculated on the basis of (Si+Al+Ti+Zr+Hf+Nb+Ta+W) = 29.

		Los-129.15			Los-174
	wt%	s.d. (n = 5)	a.p.f.u	wt%	s.d. (n = 5)	a.p.f.u
SiO2	47.62	0.17	25.22	45.19	0.31	25.29
P2O5	b.d.l.	-	-	0.01	0.02	<0.01
SO3	0.10	0.02	0.00	0.01	0.01	<0.01
TiO2	0.3	0.03	0.12	0.09	0.05	0.04
Al2O3	0.32	0.02	0.20	0.01	0.02	<0.01
CaO	4.08	0.07	2.32	6.99	0.06	4.19
MnO	5.9	0.11	2.65	7.22	0.15	3.42
FeO	3.88	0.08	1.72	1.50	0.19	0.70
Na2O	14.4	0.15	14.78	12.55	0.21	13.62
K2O	0.39	0.04	0.26	0.39	0.01	0.28
Y2O3	0.06	0.02	<0.01	0.49	0.05	0.14
La2O3	2.00	0.06	0.39	2.99	0.12	0.62
Ce2O3	2.77	0.06	0.54	3.47	0.18	0.71
Pr2O3	0.50	0.05	0.10	0.68	0.06	0.14
Nd2O3	0.30	0.07	0.06	0.50	0.07	0.10
Sm2O3	0.03	0.03	<0.01	b.d.l.	-	-
Gd2O3	0.10	0.05	0.02	0.13	0.03	0.03
Dy2O3	b.d.l.	-	-	0.23	0.22	0.04
Yb2O3	0.13	0.05	0.02	b.d.l.	-	-
ZrO2	12.65	0.36	3.27	10.72	0.21	2.93
HfO2	0.06	0.03	<0.01	0.08	0.03	<0.01
Nb2O5	0.73	0.06	0.17	2.87	0.32	0.73
F	0.19	0.02	0.32	0.07	0.06	0.12
Cl	0.73	0.03	0.66	0.04	0.03	0.04
O = F	−0.25			−0.04
Total	96.89			96.24

.

Based on previous studies [13,14,16], the analyses were recalculated on the basis of the sum of cations: Si+Al+Ti+Zr+Hf+Nb+Ta+W = 29. This recalculation method differs slightly from other recalculation approaches, such as O = 78.

The empirical formula for sample Los-129.15 eudialyte is (Na_14.78K_0.26)₁₅(Ca_2.32Mn_0.68)₃(Mn_1.87REE_1.13)₃(Fe_1.72Mn_0.10)_1.82Zr₃(Zr_0.27Si_0.22Al_0.20Nb_0.17Ti_0.12)Si₁(Si₂₄O₇₂)O_1.48(F_0.32Cl_0.66)_0.98, whereas the empirical formula for the sample Los-174 eudialyte, assuming a 2.1% water content for valency balance criteria is (Na_13.62REE_1.1K_0.28)₁₅(Ca_4.19Mn_1.16REE_0.54Y_0.11)₆(Mn_2.26Fe_0.7)_2.96Nb_0.73(Si_1.28P_0.01)(Zr_2.93Ti_0.04Y_0.03)(Si₂₄O₇₂)[(OH)_3.2(H₂O)_2.6]_5.8(OH_2.34F_0.12Cl_0.04)_2.5.

The chemical data confirm the quite distinct nature of the two eudialytes suggested by the Raman spectroscopy data.

3.3. X-Ray Crystallography

The crystal structures of Los-129.15 and Los-174 eudialytes were solved using direct methods with SHELXTL [21] and refined with SHELXL-2018 [21]. Neutral scattering curves from the International Tables for Crystallography [22] were utilized.

3.3.1. Los-174 Sample

The crystal structure of Los-174 was refined in the R3m space group, with data collection and refinement details also provided in

Table 2. Crystal data, data collection, and structure refinement information for Los-174 sample.

Crystal Data	Los-174
Crystal size (mm)	0.08 × 0.08 × 0.13
Cell setting, space group	Trigonal, R3m
a (Å), c (Å)	14.2037(3), 30.1507(9)
V (Å3)	5267.8(2)
Z	3
Data collection and refinement
Radiation, wavelength (Å) MoKα	0.71073
Temperature (K)	293
2θmax (°)	66.28
Measured reflections	39336
Unique reflections	4880
Reflections with Fo > 4σ(Fo)	7707
Rint	0.0361
Rσ	0.0207
Range of h, k, l	−21 ≤ h ≤ 21, −21 ≤ k ≤ 21, −46 ≤ l ≤ 46
R [Fo > 4σ(Fo)]	0.0261
R (all data)	0.0262
wR (on Fo2)	0.0652
Goof	1.122
Number of least-squares parameters	267
Maximum and minimum residual peak (e Å−3)	1.57 (at 1.25 Å from Cl1) −1.01 (at 0.98 Å from N4)

.

Table 3. Atomic coordinates, occupancies, equivalent isotropic displacement parameters (Ų), and bond valence sums for Los-174 crystal structure refinement.

	x/a	y/b	z/c	Occupancy	Ueq	BVS
Z	0.50233(2)	0.49767(2)	0.48718(2)	Zr1.0	0.0095(1)	4.02
M1	0.59190(5)	0.66440(7)	0.65317(2)	Ca0.91Ce0.09	0.0176(2)	2.09
M2a	0.84944(8)	0.69888(15)	0.65076(3)	Mn0.9	0.0133(3)	1.82
M2b	0.8275(11)	0.6551(22)	0.6564(5)	Mn0.07	0.0133(3)	0.13
M3	0	0	0.61621(3)	Nb0.85	0.0126(3)	4.68
M4a	0	0	0.41015(10)	Si0.81	0.0123(5)	3.22
M4b	0	0	0.3687(5)	Si0.19	0.0123(5)	1.09
N1	0.22203(15)	0.44407(30)	0.49993(11)	Na1.0	0.0349(7)	1.02
N2a	0.7773(2)	0.5546(4)	0.4721(2)	Na0.84	0.03834(9)	0.84
N2b	0.7462(6)	0.4924(12)	0.4891(4)	Ca0.16	0.03834(9)	0.80
N3	0.43226(3)	0.56774(3)	0.36712(2)	Na0.59Ce0.41	0.0161(2)	2.23
N4	0.5629(4)	0.4371(4)	0.6097(2)	Na0.95Ca0.05	0.104(40)	1.25
N5	−0.07126(18)	0.07126(18)	0.50790(15)	Na1.0	0.060(12)	0.99
Si1	0.27689(8)	0.33947(8)	0.41753(3)	Si1.0	0.0100(2)	4.05
Si2	0.59635(6)	0.40365(6)	0.40008(5)	Si1.0	0.0154(3)	4.08
Si3	0.24893(12)	0.12446(6)	0.39503(5)	Si1.0	0.0109(2)	4.02
Si4	0.40379(6)	0.59621(6)	0.57131(5)	Si1.0	0.0128(3)	4.01
Si5	0.74555(12)	0.87278(6)	0.57614(5)	Si1.0	0.0110(2)	4.21
Si6	0.72135(8)	0.65754(8)	0.55733(3)	Si1.0	0.0098(2)	3.90
O1	0.3656(3)	0.4363(3)	0.4471(1)	O1.0	0.0244(67)	1.98
O2	0.2956(3)	0.3710(3)	0.3658(1)	O1.0	0.0147(49)	2.00
O3	0.5510(2)	0.4490(2)	0.3626(2)	O1.0	0.0320(12)	1.96
O4	0.7286(2)	0.4573(4)	0.3946(2)	O1.0	0.0244(89)	1.92
O5	0.5736(2)	0.4264(2)	0.4498(2)	O1.0	0.0274(10)	2.05
O6	0.3094(4)	0.1547(2)	0.3488(1)	O1.0	0.0163(71)	2.05
O7	0.2833(3)	0.2294(3)	0.4272(1)	O1.0	0.0216(62)	1.94
O8	0.1197(4)	0.0598(2)	0.3882(2)	O1.0	0.0354(13)	2.02
O9	0	0	0.4626(3)	O1.0	0.0266(16)	1.98
O10	0.15741(19)	0.31482(39)	0.43428(15)	O1.0	0.0171(8)	2.04
O11	0.4318(2)	0.5682(2)	0.5234(2)	O1.0	0.0264(10)	1.97
O12	0.4564(3)	0.7282(2)	0.5741(2)	O1.0	0.0203(8)	1.98
O13	0.6985(4)	0.8492(2)	0.6253(1)	O1.0	0.0183(8)	1.83
O14	0.8748(4)	0.9374(2)	0.5711(2)	O1.0	0.0232(9)	1.90
O15	0.7023(3)	0.7605(2)	0.5468(1)	O1.0	0.0166(5)	1.95
O16	0.84710(18)	0.69420(36)	0.54180(14)	O1.0	0.0154(7)	2.18
O17	0.7079(3)	0.6307(3)	0.6093(1)	O1.0	0.0167(5)	2.08
O18	0.6412(3)	0.5586(3)	0.5262(1)	O1.0	0.0191(6)	2.02
O19	0.9358(2)	0.8716(4)	0.6503(2)	O0.92	0.0235(9)	1.64
O20	0.4451(2)	0.5549(2)	0.6115(2)	O1.0	0.0271(10)	2.04
OH1	0	0	0.3143(10)	O0.5Cl0.5	0.62(10)	0.50
OH2	0	0	0.2019(13)	O0.65Cl0.35	0.221(23)	0.20
Cl3	0.3333	0.6667	0.7418(11)	Cl0.26	0.0822(71)	0.39
OH3	0.427(2)	0.714(1)	0.413(1)	O0.42	0.0822(71)	0.45

presents the atomic coordinates, equivalent isotropic displacement parameters (Ų), occupancy values, and bond valence sums (BVS), calculated based on the refined occupancy for the Los-174 refinement.

Table 4. Refined occupancies of mixed cationic sites, site scattering values, and site scattering calculated from the assigned atomic populations for the Los-174 crystal structure refinement.

Mixed Site	Occupancy	s.s.	s.s.calc	Population
Z	Zr1.0	40	39.75	Zr0.98 Ti0.01 Y0.01
M1	Ca0.91, Ce0.09	23.42	23.29	Ca0.60 Mn0.21 REE0.1 Na0.09
M2a	Mn0.9	22.5	22.75	Mn0.91
M2b	Mn0.07	1.75	1.64	Fe0.04 Ca0.03
M3	Nb0.85	34.85	33.71	Nb0.73 Si0.27
M4a	Si0.81	11.34	11.34	Si0.81
M4b	Si0.19	2.66	2.66	Si0.19
N1	Na1.0	11	11	Na1.0
N2	Na0.84, Ca0.16	12.44	12.42	Na0.83 K0.09 Ca0.08
N3	Na0.59, Ce0.41	30.30	30.18	Na0.59 REE0.41
N4	Na0.95, Ca0.05	11.45	11.72	Na0.91 Ca0.09
N5	Na1.0	11	11	Na1.0

includes the refined occupancies of mixed cationic sites, site scattering values, and site scattering calculated from the assigned atomic populations for the Los-174 crystal structure refinement, which show satisfactory agreement with each other.

Table 5. Selected bond lengths for independent silicate groups in Los-174 kentbrooksite.

Si1		Si2		Si3
O1	1.595(4)	O3	1.588(5)	O6	1.581(5)
O2	1.605(3)	O5	1.600(5)	O8	1.603(6)
O10	1.632(4)	O4(x2)	1.646(3)	O7(x2)	1.635(5)
O7	1.637(5)
	1.617		1.620		1.613
Si4		Si5		Si6
O20	1.583(5)	O13	1.592(5)	O18	1.597(4)
O11	1.600(5)	O14	1.597(6)	O17	1.602(3)
O12(x2)	1.637(5)	O15	1.650(4)	O15	1.646(5)
		O15	1.650(3)	O16	1.659(5)
	1.614		1.622		1.626

,

Table 6. Selected bond lengths for independent coordination polyhedra in Z and M layers of Los-174 kentbrooksite.

Z		M1		M2a		M2b
O11)	2.050(4)	O20	2.258(4)	O17	2.110(30)	O19)	2.123(6)
O1(x2)	2.073(4)	O3	2.295(4)	O17	2.108(18)	O2(x2)	2.140(4)
O18(x2)	2.078(4)	O6	2.341(3)	O2	2.113(16)	O17(x2)	2.142(4)
O5	2.084(4)	O1	2.341(5)	O2	2.117(23)
		O2	2.364(4)
		O13	2.432(6)
	2.073		2.339		2.111		2.138
M3		M4a		M4b
O19(x3)	1.886(6)	O9	1.581(10)	O8(x3)	1.585(8)
O14(x3)	2.055(5)	O8(x3)	1.614(6)	OH1	1.641(40)
	1.970		1.606		1.599

and

Table 7. Selected bond lengths for N large coordination polyhedra in Los-174 kentbrooksite.

N1		N2a		N2b
O10	2.539(6)	O18(x2)	2.552(7)	O18(x2)	2.401(11)
O12	2.550(6)	O5(x2)	2.621(7)	OH2	2.400(30)
O15(x2)	2.627(6)	O4	2.626(8)	O5(x2)	2.448(16)
O1(x2)	2.632(6)	O16	2.714(7)	O4	2.882(14)
O11(x2)	2.689(5)	O7(x2)	2.714(6)	O16	2.948(15)
OH3(x2)	3.070(30)
	2.712		2.639		2.561
N3		N4		N5
O2(x2)	2.481(3)	O17(x2)	2.478(6)	O9	2.222(7)
OH3(x2)	2.52(3)	O6	2.512(8)	O16	2.254(6)
O13	2.599(5)	OH1	2.797(15)	O14(x2)	2.524(6)
O19(x2)	2.619(5)	O20	2.899(8)	O15(x2)	3.031(5)
Cl3	2.735(16)	O18(x2)	2.938(8)	O7(x2)	3.236(6)
O1(x2)	2.904(4)
O3	2.924(4)
	2.664		2.721		2.757

report selected bond lengths for Los-174 crystal structure refinement.

3.3.2. 129.15 Sample

Attempts to solve and refine the structure of Los-129.15 in the R-3m and R3m space groups were unsuccessful, and refinement was carried out in the R3 space group. A significant improvement in the refinement was achieved by introducing merohedral twinning, with the twin operator corresponding to the mirror plane in the R3m space group. Details of data collection and crystal structure refinement are provided in

Table 8. Crystal data, data collection, and structure refinement informations for Los-129.15 sample.

Crystal Data	Los-129.15
Crystal size (mm)	0.095 × 0.04 × 0.02
Cell setting, space group	Trigonal, R3
a (Å), c (Å)	14.1489(2), 30.1283(5)
V (Å3)	5223.4(1)
Z	3
Data collection and refinement
Radiation, wavelength (Å) MoKα	0.71073
Temperature (K)	293
2θmax (°)	65.08
Measured reflections	38102
Unique reflections	8443
Reflections with Fo > 4σ(Fo)	7707
Rint	0.0374
Rσ	0.0339
Range of h, k, l	−18 ≤ h ≤ 21, −21 ≤ k ≤ 19, −45 ≤ l ≤ 45
R [Fo > 4σ(Fo)]	0.0322
R (all data)	0.0376
wR (on Fo2)	0.0747
Goof	1.052
Number of least-squares parameters	443
Maximum and minimum residual peak (e Å−3)	0.87 (at 0.59 Å from M1b) −0.63 (at 0.62 Å from Si3)

.

Table 9. Atomic coordinates, occupancies, equivalent isotropic displacement parameters (Ų), and bond valence sums for Los-129.15 crystal structure refinement.

Atom	x/a	y/b	z/c	Uequiv.	Occupancy	BVS
Z	0.50808(8)	0.50842(8)	0.01537(7)	0.0112(5)	Zr1.0	3.98
M1a	0.39467(16)	0.06115(15)	0.84859(11)	0.0143(6)	Mn0.97Ce0.03	2.04
M1b	0.42296(15)	0.33333(29)	0.84882(11)	0.0184(7)	Ca0.83Ce0.17	2.19
M2a	0.14837(107)	0.83412(74)	0.85082(22)	0.0307(94)	Fe0.44	0.94
M2b	0.17286(98)	0.81588(132)	0.84690(27)	0.0307(94)	Fe0.36	0.77
M3a	0.0	0.0	0.89948(26)	0.0261(17)	Nb0.22	1.30
M3b	0.0	0.0	0.88278(25)	0.0261(18)	Nb0.23	1.60
M3c	0.0	0.0	0.93486(73)	0.0261(19)	Si0.22	1.00
M4a	0.0	0.0	0.14204(43)	0.0349(28)	Si0.85	2.89
M4b	0.0	0.0	0.09815(180)	0.0349(29)	Si0.15	0.81
N1a	0.77067(100)	0.21771(92)	1.00387(29)	0.0344(33)	Na0.78	0.76
N1b	0.73689(178)	0.25523(200)	1.02384(57)	0.0344(33)	Na0.22	0.26
N2a	0.54799(86)	0.44453(103)	0.69659(30)	0.0295(30)	Na0.73	0.76
N2b	0.57038(158)	0.4173(20)	0.68330(68)	0.0295(31)	Na0.27	0.27
N3	0.90196(46)	0.10566(50)	0.80037(15)	0.0407(23)	Na0.59Ca0.41	1.84
N4	0.43937(40)	0.56780(41)	0.89619(14)	0.0156(17)	Na0.83Ca0.17	1.43
N5	0.2765(21)	0.7279(20)	0.69067(51)	0.0169(14)	Na0.64	0.16
Si1	0.75019(36)	0.27689(33)	0.76812(11)	0.0126(14)	Si1.0	4.09
Si2	0.60804(39)	0.41544(40)	0.92967(12)	0.0163(18)	Si1.0	4.06
Si3	0.45851(38)	0.54220(38)	0.77450(11)	0.0205(14)	Si1.0	4.07
Si4	0.87320(37)	0.12654(36)	0.92438(10)	0.0130(11)	Si1.0	4.26
Si5	0.06331(36)	0.32900(37)	0.75269(14)	0.0121(15)	Si1.0	3.96
Si6	0.26985(37)	−0.06810(37)	0.94516(14)	0.0115(14)	Si1.0	4.08
Si7	0.04847(39)	0.72485(37)	0.75208(15)	0.0130(14)	Si1.0	4.12
Si8	0.05794(35)	0.71439(39)	0.94508(15)	0.0127(15)	Si1.0	3.94
O1	0.59485(96)	0.3846(10)	0.77296(38)	0.0238(44)	O1.0	1.95
O2	0.80124(90)	0.24234(84)	0.80731(38)	0.0247(36)	O1.0	2.00
O3	0.77922(86)	0.2578(10)	0.71849(33)	0.0225(37)	O1.0	1.92
O4	0.71646(92)	0.26035(97)	0.92563(41)	0.0256(49)	O1.0	1.89
O5	0.5688(12)	0.4613(12)	0.89023(41)	0.0378(59)	O1.0	2.01
O6	0.9123(14)	0.1071(15)	0.64409(33)	0.0422(56)	O1.0	2.06
O7	0.37812(93)	−0.0634(11)	0.74260(36)	0.0430(57)	O1.0	2.07
O8	0.39460(85)	0.42781(80)	0.74261(36)	0.0184(33)	O1.0	1.80
O9	0.4954(11)	0.5199(12)	0.82143(34)	0.0262(46)	O1.0	1.95
O10	0.3845(13)	0.5939(13)	0.78125(51)	0.0644(64)	O1.0	1.42
O11	0.28494(80)	0.05189(78)	0.95525(35)	0.0225(29)	O1.0	1.82
O12	−0.06172(80)	0.7047(10)	0.95300(38)	0.0277(45)	O1.0	2.08
O13	0.85218(96)	0.1614(11)	0.87759(32)	0.0254(42)	O1.0	1.89
O14	0.9288(11)	0.0497(11)	0.92342(38)	0.0353(42)	O1.0	1.71
O15	0.7015(10)	0.1080(90)	0.64287(44)	0.0229(42)	O1.0	1.98
O16	0.35585(116)	−0.0757(13)	0.97417(51)	0.0332(54)	O1.0	2.10
O17	0.28871(105)	0.92240(91)	0.89352(36)	0.0211(43)	O1.0	1.96
O18	0.07054(82)	0.70166(84)	0.89394(36)	0.0214(30)	O1.0	2.13
O19	0.15167(99)	0.83615(94)	0.96231(27)	0.0234(36)	O1.0	1.94
O20	0.2653(12)	0.2278(12)	0.72111(53)	0.0381(57)	O1.0	2.02
O21	−0.0228(10)	0.2300(12)	0.72219(41)	0.0230(50)	O1.0	2.00
O22	0.0496(11)	0.3028(11)	0.80552(39)	0.0261(44)	O1.0	1.96
O23	0.0279(12)	0.73560(10)	0.80437(33)	0.0345(59)	O1.0	1.92
O24	0.1716(11)	0.81445(91)	0.73792(34)	0.0264(35)	O1.0	2.06
O25	0.0	0.0	0.18493(51)	0.022(18)	O0.69Cl0.31	1.73
O26	0.0637(12)	0.12659(85)	0.84928(32)	0.0302(45)	O0.51	0.67
O27	0.0	0.0	1.0426(11)	0.0302(45)	O1.0	2.06
Cl1	0.0	0.0	0.2788(28)	0.0614(69)	Cl0.75O0.25	1.23
Cl2	0.0	0.0	0.74738(51)	0.0554(33)	Cl0.5	0.87
OH2	0.0	0.0	0.7688(15)	0.0554(33)	O0.5	0.48

presents the atomic coordinates, equivalent isotropic displacement parameters (Ų), occupancy values, and bond valence sums (BVS) based on the refined occupancy for the Los-129.15 refinement.

Table 10. Refined occupancies of mixed cationic sites, site scattering values, and site scattering calculated from the assigned atomic populations for the Los-129.15 refinement.

	Occupancy	s.s.	s.s.calc	Population
Z	Zr1.0	40	40	Zr1.0
M1a	Mn0.97 Ce0.03	25.99	27.56	Mn0.8 REE0.13
M1b	Ca0.83 Ce0.17	26.46	26.68	Ca0.67 REE0.17 Mn0.08 Na0.06 Y0.02
M2a	Fe0.44	11.50	11.43	Fe0.27 Na0.30 Mn0.08 Zr0.03
M2b	Fe0.36	9.36	9.44	Fe0.31 Na0.13
M3a	Nb0.22	9.02	9.50	Al0.1 Zr0.09 Nb0.08 Ti0.06
M3b	Nb0.23	9.43	9.51	Al0.1 Nb0.09 Zr0.08 Ti0.06
M3c	Si0.22	3.08	3.08	Si0.22
M4a	Si0.85	11.90	11.90	Si0.85
M4b	Si0.15	2.10	2.10	Si0.15
N1a	Na0.78	8.58	8.58	Na0.78
N1b	Na0.22	2.42	2.42	Na0.22
N2a	Na0.73	8.03	8.03	Na0.73
N2b	Na0.27	2.97	2.97	Na0.27
N3	Na0.59 Ca0.41	14.69	14.69	Na0.82 K0.08 REE0.06 Ca0.04
N4	Na0.83 Ca0.17	12.53	12.39	Na0.92 Ca0.07 REE0.01
N5	Na0.64	7.04	7.7	Na0.7

presents the refined occupancies, site scattering, and site scattering calculated from EPMA-derived atomic populations for the mixed sites in Los-129.15 crystal structure. These values exhibit fairly good agreement with one another.

Selected bond lengths of coordination polyhedra in Los-129.15 crystal structure are presented in

Table 11. Selected bond lengths for independent silicate groups in Los-129.15 crystal structure refinement.

Si1		Si2		Si3		Si4
O2	1.583(15)	O5	1.582(18)	O10	1.563(24)	O13	1.570(13)
O3	1.608(12)	O6	1.584(15)	O9	1.592(13)	O12	1.575(16)
O1	1.617(12)	O4	1.621(12)	O7	1.613(16)	O14	1.630(21)
O1	1.658(15)	O4	1.656(13)	O8	1.702(12)	O11	1.632(14)
	1.617		1.611		1.618		1.602
Si5		Si6		Si7		Si8
O21	1.607(13)	O16	1.545(20)	O20	1.594(19)	O18	1.573(13)
O7	1.620(17)	O17	1.596(14)	O24	1.618(13)	O15	1.595(13)
O22	1.623(13)	O19	1.623(11)	O8	1.618(11)	O12	1.645(15)
O24	1.631(12)	O11	1.631(14)	O23	1.625(12)	O19	1.646(11)
	1.620		1.599		1.614		1.615

,

Table 12. Selected bond lengths for Z and M coordination polyhedra in Los-129.15 crystal structure refinement.

Z		M1a		M1b
O6	2.050(21)	O2	2.120(11)	O5	2.314(13)
O20	2.056(15)	O5	2.195(14)	O2	2.396(11)
O21	2.066(14)	O17	2.234(11)	O23	2.424(12)
O16	2.077(15)	O22	2.239(17)	O9	2.449(16)
O15	2.081(13)	O9	2.251(14)	O13	2.451(14)
O3	2.093(15)	O13	2.283(14)	O18	2.468(11)
	2.070		2.220		2.417
M2a		M2b
O22	2.079(17)	O22	2.078(19)
O18	2.086(13)	O18	2.090(16)
O23	2.104(15)	O17	2.113(16)
O17	2.161(16)	O23	2.192(17)
O26	2.30(2)
	2.147		2.118
M3a		M3b		M3c
O14(x3)	1.655(19)	O26(x3)	1.851(11)	O14(x3)	1.529(20)
O26(x3)	2.166(12)	O14(x3)	1.927(18)
	1.910		1.889
M4a		M4b
O25	1.30(19)	O10(x3)	1.60(3)
O10(x3)	1.73(2)	O27	1.68(7)
	1.622		1.621

and

Table 13. Selected bond lengths for extra-framework N polyhedra in Los-129.15 crystal structure.

N1a		N1b		N2a		N2b
O21	2.562(15)	O21	2.44(3)	O8	2.485(19)	O3	2.372(21)
O20	2.564(16)	O20	2.48(3)	O3	2.486(17)	O15	2.406(21)
O6	2.621(21)	Cl2(x3)	2.50(3)	O19	2.522(18	O16	2.526(26)
O24	2.637(18)	O6	2.54(3)	O15	2.600(18	O3	2.629(27)
O4	2.642(17)	O6	2.61(3)	O16	2.639(17)	O19	2.660(31)
O12	2.663(19)	O24	2.93(4)	O1	2.649(18)	O1	2.791(25)
O6	2.733(22)	O4	2.98(3)	O7	2.805(18)	Cl1	3.072(68)
O11	2.819(18)	OH2(x3)	2.98(5)	O3	2.883(17)	O8	3.13(3)
mean	2.655		2.701		2.634		2.698
N3		N4		N5
O23	2.435(14)	O17	2.471(17)	O27	1.547(35)
O22	2.518(13)	O18	2.509(15)	O24	2.751(34)
O26	2.611(17)	O9	2.589(14)	O7	2.997(26)
O26	2.657(18)	O16	2.837(18)	O14	3.038(19)
O13	2.662(14)	O25	2.848(10)	O10	3.062(23)
OH2	2.672(19)	O15	2.869(13)	O10	3.155(24)
O21	2.811(14)	O5	2.900(22)	O11	3.169(29)
O20	2.870(16)	Cl1	2.920(45)
O2	2.927(17)
Cl2	2.963(12)
mean	2.713		2.743		2.817

.

4. Discussion

4.1. Literature Chemical Data on Los Zirconosilicates

Moreau et al. [1] reported EPMA analyses for zirconosilicates eudialyte, låvenite, a phase of the wöhlerite group [23], and for astrophyllite and kupletskite, two phases of the astrophyllite group [24,25].

For eudialyte, the authors stated that only “partial analyses of eudialyte (without REE determinations)” are reported. The data presented in Table 9 by Moreau et al. [1] point to the occurrence of a Ca-poor zirconium eudialyte [16] with an anomalously low Na content (7.5 and 11.6 a.p.f.u.) compared to the values normally measured in EGMs. The measured values for the cations that can substitute Na in the N [1,2,3,4,5] sites, such as K and Ca, are completely insufficient to provide an acceptable occupancy for these sites. A possible presence of H₃O⁺ groups as a substitute for Na, as occurs in the minerals aqualite [26] and ilyukhinite [27], would result in an amount of H₂O incompatible with the measured data and with the amount of H₂O proposed by Moreau et al. [1].

A low Na content is obtained also in the recalculation of the EPMA data given by Moreau et al. [1] in their Table 5 for astrophyllite, ideally K₂NaFe₇Ti₂Si₈O₂₆(OH)₄F, and kupletskite with the ideal formula (K,Na)₃(Mn,Fe)₇Ti₂Si₈O₂₆(OH)₄F. As it can be seen from Table 6 from Moreau et al. [1], this problem does not affect instead the chemical data reported for låvenite, ideally Na₂Ca₂Mn₂Zr₂(Si₂O₇)₂O₂F₂.

A possible explanation derives from the effects of migration of light elements such as Na and F associated with beam-induced heating and charging effects, commonly detected in alkali-bearing silicates [28,29]

These effects have been observed and carefully considered both in EGMs [13,18] and in minerals of the astrophyllite group [24,25]. It can be hypothesized that these effects are enhanced from the way Na bonds within the crystal structure of these minerals. In fact, Na is much more stably bound in låvenite compared to EGMs and phases of the astrophyllite group. Låvenite belongs to the wöhlerite group [23], which is characterized by a pattern of four-columns-wide octahedral “ribbons”, which interconnect through corner sharing and via the disilicate groups to create a three-dimensional framework. In låvenite, Na is therefore firmly bound in octahedral coordination, unlike in EGMs, where it is located in extra-framework cages with a large coordination number, or in astrophyllite group minerals, where Na is hosted in sites with [10,11,12,13] coordination weakly bonded in interlayer space [23,24].

4.2. Crystal Structure Description of Los-174

The crystal-chemical formula derived from the structural refinement expressed as (Z = 3) is as follows:

^N1-5(Na_13.14Ce_1.23Ca_0.63)₁₅ ^M1(Ca_5.46Ce_0.54)₆ ^M2Mn_2.91 M3Nb0_.85 ^M4(Si₁) ^ZZr₃ ^T(S_i24O₇₂) ^O’[OH_2.68(H₂O)_1.58Cl_0.5]_4.76 ^X(OH_1.91Cl_0.61)_2.51.

A water content of 2.1% can be estimated on valency balance criteria leading to the EPMA-based empirical formula:

(Na_13.62REE_1.1K_0.28)₁₅(Ca_4.19Mn_1.16REE_0.54Y_0.11)₆(Mn_2.26Fe_0.7)_2.96Nb_0.73(Si_1.28P_0.01)(Zr_2.93Ti_0.04Y_0.03)(Si₂₄O₇₂)[(OH)_3.2(H₂O)_2.6]_5.8(OH_2.34F_0.12Cl_0.04)_2.5.

On the basis of chemical and structural data, according to the indications of Johnsen et al. [14], the Los-174 sample can be classified as kentbrooksite, with the ideal formula (Na,REE)₁₅(Ca,Mn)₆(Mn,Fe)₃Zr₃(Nb,Si)₁(Si₂₅O₇₃)(O,OH,H2O)₃(O,F,Cl)₂.

The phase can be effectively compared with the kentbrooksite from the Kangerdlugssuaq intrusion [30], with notable differences primarily characterized by higher Ca and REE contents and lower Mn and Na concentrations in comparison to the material studied by Johnsen et al. [30].

4.2.1. T Silicate Layer

As stated in the introduction, the mineral structure of the eudialyte group consists of 3- and 6-membered tetrahedral rings that combine to form T layers. These T layers intercalate along the c-axis, positioned above and below the layers containing the M polyhedral (see Figure 1). The average bond distances for Si tetrahedra, ranging from 1.613 to 1.626 Å (Table 5), and the BVS values, ranging from 3.9 to 4.21 (Table 3), align with those typically found in Si tetrahedra lacking a substantial Al component.

4.2.2. Z and M Layers

The Z polyhedron, (Table 6) as observed in Kangerdlugssuaq kentbrooksite, adopts a regular octahedral geometry with an average bond length of 2.073 Å (2.068 Å according to Johnsen et al. [30]).

These polyhedra connect consecutive T layers along the c-axis. The M1 polyhedra are predominantly occupied by Ca, with a minor substitution by REE, exhibiting an average bond distance of 2.339 Å (Table 6), which is in close agreement with the 2.324 Å value reported by Johnsen et al. [30] for kentbrooksite from Kangerdlugssuaq. This observation is consistent with both the EPMA data and the bond valence sum (BVS) of 2.09 for this site (Table 3). The M1 octahedra are arranged into six-membered rings through edge-sharing, which are interconnected by M2 polyhedra, forming the M layers (Figure 1). The M2 site, as described by Johnsen et al. [30], is a split site: M2a exhibits an occupancy of Mn_0.9 and has a coordination number of [4], while M2b is fivefold coordinated in a distorted square pyramidal geometry, with an occupancy of Mn_0.07. The average bond lengths (Table 6) are 2.111 Å for M2a and 2.138 Å for M2b. The M3 and M4 sites are situated along the threefold axis at the center of the Si₉O₂₇ rings (Wyckoff position 3a). The M3 site has an occupancy of Nb_0.85 and an octahedral coordination, with an off-center displacement leading (Table 6) to a triple set of shorter bonds [1.886(6) Å] and a triple set of longer bonds [2.055(5) Å]. The mean bond length is 1.970 Å, which is comparable to the 1.937 Å reported by Johnsen et al. [30]. The M4 site, similar to that in Kangerdlugssuaq kentbrooksite, is a split site with tetrahedral coordination (Table 6), exhibiting full occupancy and a mean bond length of 1.600 Å for the two sites. The two M4a and M4b tetrahedra (Figure 1) are oriented in opposite directions along the z-axis.

4.2.3. Extra-Framework N Polyhedra

The N1–N5 sites are located within large cavities in the heteropolyhedral framework, accommodating extra-framework cations such as Na, K, REE, water molecules, and additional anions. The N1 site in Los-174 exhibits coordination [10] with a mean bond length of 2.712 Å, and its full Na occupancy (Table 3, Table 4 and Table 7) is consistent with the description of this site. The N2 site is a split site, with Na_0.84 occupying N2a and Ca_0.16 occupying N2b.

In agreement with the refined occupancies, the N2a site has coordination [8] and a mean bond length of 2.639 Å, whereas N2b exhibits coordination [7] and a bond length of 2.563 Å. The N3 site has coordination [11] and a mean bond length of 2.664 Å, within a maximum bond length of 3.3 Å. The occupancy of Na_0.59Ce_0.41 fits well with site scattering data from EPMA (Table 4) and corresponds closely to the bond valence value (v.u.) of 2.23 calculated for this site. The N4 and N5 sites exhibit similar environments, with N4 having Na_0.95Ca_0.05 occupancy and N5 having Na_1.0 occupancy. The mean bond lengths are 2.721 Å for N4 and 2.757 Å for N5, with calculated bond valence sums of 1.25 and 0.99 v.u., respectively (Table 3 and Table 7).

4.2.4. X Anionic Sites

The low BVS values and elevated displacement factors (Table 3) suggest the presence of H₂O at the OH1, OH2, and OH3-Cl3 sites, which was not directly measured due to the limited material available.

The two distinct anionic sites (OH, Cl) in kentbrooksite from Los are located along the threefold axis in the T layers, as expected. A mixed O-Cl occupancy was modeled with a split Cl3-OH3 site, refined to occupancies of Cl_0.26 and O_0.42. Analysis of bond lengths (Figure 5) supports this model, as both Cl3 and OH3 are bonded to the N3 site, with Cl3 forming a notably longer bond (2.735 Å) compared to the N4-OH3 bond (2.52 Å).

A hydrogen bond between chlorine in Cl3 and oxygen in O19 is suggested (Figure 5), with an O19—Cl3 distance of 3.17(3) Å, which is consistent with the low bond valence sum of 1.64 v.u. calculated for O19.

4.3. Crystal Structure Description of Los-129.15

The crystal-chemical formula derived from the structural refinement can be written as follows (Z = 3):

^N1−5(Na_12.15Ca_2.85)₁₅[^M1a(Mn_2.91Ce_0.09)^M1b(Ca_2.49Ce_0.51)]₆[^M2a(Fe_1.32)^M2b(Fe_1.08)]_2.46^ZZr₃^M3(Nb_0.45Si_0.22)_0.67^M4(Si₁)^T(Si₂₄O₇₂)^O’(O₃)^X(O_0.23Cl_1.56)_1.79.

Comparing with the empirical formula from EPMA data:

(Na_14.78K_0.26)₁₅(Mn_1.87REE_1.13)₃(Ca_2.32Mn_0.68)₃(Fe_1.72Mn_0.10)_1.82Zr₃(Zr_0.27Si_0.22Al_0.20Nb_0.17Ti_0.12)₁Si₁(Si₂₄O₇₂)O_1.48(F_0.32Cl_0.66)_0.98.

On the basis of chemical and structural data, according to the indications of Johnsen et al. [14], it can be seen that the Los-129.15 sample is very close in chemical composition and crystal structure to oneillite.

By comparing the CNMNC-IMA accepted formula for oneillite, Na₁₅Ca₃Mn₃Fe²⁺₃Zr₃Nb(Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl)₂, with the EPMA chemical data reported for Los-129.15, one can notice that the dominant cation in oneillite M3 site is Nb, whereas a mixed-occupancy s (Zr_0.27Si_0.22Al_0.20Nb_0.17Ti_0.12)S₁, with Zr as the dominant cation, is suggested for the M3 site of the Los-129.15 phase. Given the structural complexity of the M3–M4 microregion in EGMs, the proposed complex mixed occupancy of the M3 site, and the faint differences in the amounts of Zr and Nb, it is not tenable to soundly hypothesize the occurrence of a new species with Zr as the dominant cation at the M3 site. New studies are presently in progress to spot crystals with distinctly higher Zr contents, suitable to soundly verify the possible existence of a new species of the eudialyte group.

The Los-129.15 phase will be therefore henceforth denoted as “oneillite-like phase”, with idealized chemical formula Na₁₅Mn₃Ca₃(Fe,Zr)₃Zr₃(Si,Al,Nb,Ti)₁ (Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl,F)₂.

4.3.1. T Silicate Layers

Selected bond lengths for the independent Si tetrahedra in the Los-129.15 oneillite-like phase are reported in Table 11. The mean bond lengths and the bond-valence sums (Table 9 and Table 11) are in agreement with the presence of Al-free tetrahedra.

4.3.2. Z and M Layers

The Z polyhedron is quite a regular one, with an average bond length of 2.070 Å (Table 12) and a bond valence sum of 3.98 v.u (Table 9), pointing to a full occupancy by Zr.

A key structural feature of the oneillite subgroup is the cation ordering within the M1 octahedral rings (Figure 6).

These rings consist of two distinct sites, M1a and M1b, which alternate within the six-fold M1 ring, thereby reducing its symmetry to the R3 space group. The M1a (Table 9, Table 10, Table 11 and Table 12) site is predominantly occupied by Mn²⁺, with an average bond length of <M1a-O> = 2.220 Å, while the M1b site is primarily occupied by Ca²⁺, with <M1b-O> = 2.417 Å. Minor REE quantities are also present at both sites, refined as 0.03 a.p.f.u. for M1a and 0.17 a.p.f.u. for M(1b) [31]. The M2 site (Table 12), which links adjacent octahedral rings (Figure 7), is split into two partially occupied positions: M2a, with [5] square pyramidal coordination, and M2b, with [4]-fold coordination.

Their respective refined occupancies are Fe_0.44 and Fe_0.36 (Table 12), with mean bond lengths (Table 8) of <M2a-O> = 2.147 Å and <M2bO> = 2.118 Å.

The M3 and M4 sites are located at the centers of nine-membered tetrahedral rings along the threefold axes (Wyckoff position 3a). In the Los-129.15 oneillite-like phase structure (Figure 8 and Figure 9), both sites are split.

The M3 site (Figure 8) includes two octahedral positions [M3a and M3b] and one tetrahedral position [M3c], while the M4 site (Figure 9) consists of two tetrahedral positions [M4a and M4b]. The M3a site, with a refined occupancy of Nb0.22, exhibits distorted octahedral coordination (Table 9), with bond distances of M3a-O14 = 1.655 Å (triplet) and M3a-O26 = 2.166 Å (triplet), resulting in <M3a-O> = 1.910 Å. In contrast, the M3b octahedral site, with refined occupancy Nb0.23, is more regular, with <M3b-O> = 1.889 Å. The M3c site, refined with Si0.22 occupancy, forms triplet M3c-O14 bonds at 1.529 Å. The M4a and M4b tetrahedral sites (Figure 9) can be described as coupled tetrahedra oriented in opposite directions along the z-axis [13], with mean bond distances of 1.622 Å and 1.621 Å, respectively (Table 12). Similar values have been reported for oneillite from Mont Saint-Hilaire [13], with <Si₇-O> = 1.68 Å and <Si₇-O> = 1.616 Å. The substitution of some Al³⁺ for Si⁴⁺ at these sites, coupled with the incorporation of (OH)⁻, could account for the low bond valence sum (BVS) of 1.42 v.u. observed (Table 9) at the O10 anion site.

4.3.3. Extra-Framework N Polyhedra

In the structure of the Los-129.15 oneillite-like phase, the N1 and N2 sites (Table 9, Table 10, Table 11, Table 12 and Table 13) are split and fully occupied by Na, with bond lengths ranging from <N2a-O> = 2.634 Å to <N1b-O> = 2.701 Å. The N3 and N4 sites exhibit mixed Na/Ca occupancy, with N3 containing a higher proportion of Ca (Na_0.59Ca_0.41) and shorter bond lengths [<N3-O> = 2.713 Å] compared to N4 (Na_0.83Ca_0.17) with <N4-O> = 2.743 Å. Slightly elevated BVS values for N3 and N4 (Table 9) suggest the potential presence of REE at these sites, a common feature in EGM phases [13,14]. The N5 site, with a refined occupancy of Na_0.64, is notably vacant (Table 9), consistent with the low BVS of 0.16 v.u. calculated for this site. The bond distances at N5 (Table 13) are asymmetric, with one short bond [N5-O27 = 1.547 Å] and six longer bonds ranging from 2.751 to 3.169 Å. This suggests that the N5 site is predominantly occupied by water molecules and Na⁺. A similar environment is reported for the N5 site in sergevanite, a phase of the oneillite subgroup [32], with an occupancy of (H₃O, H₂O)_0.75Na_0.25.

4.3.4. X Anionic Sites

The remarkably low bond valence sum (BVS) value for O10 (1.42 v.u.) and (Table 9) its relatively high isotropic displacement parameter (0.0644 Ų) are likely indicative of partial occupancy by (OH) at this anionic site, which is associated with the M4a–M4b sites. A similar, albeit less pronounced, trend is observed for the O14 anionic site (Table 9), which is linked to the M3a, M3b, and M3c sites.

Two distinct anion sites (OH, Cl) are located along the threefold axis within the T layers, exhibiting mixed O/Cl occupancy. The Cl1 site, with a Cl_0.75O_0.25 occupancy, is [6]-coordinated (Figure 10) by Na cations, forming (Table 13) two triplets of bonds with N4 [2.921(47) Å] and N2b [3.073(66) Å]. Cl1 is further connected to the O25 anion site via a hydrogen bond. O25 partially accommodates (OH)⁻ groups, as indicated by its low BVS of 1.73 v.u (Table 9).

The second X site (Figure 11) is split into two positions with refined occupancies (Table 9) of Cl_0.50 (Cl2) and O_0.50 (OH2). The OH2 position is shifted toward the N3 cation, while the Cl2 position is closer to the N1b site.

5. Crystal-Chemical Comparison of Oneillite-like Phase and Kentbrooksite and Their Geological Significance

5.1. Comparison Between Key Crystal Chemical Features of Los Kentbrooksite and Oneillite-like Phase

Two distinct EGM phases were identified in the Los nepheline syenites through combined Raman, EPMA, and single-crystal structural studies. The two phases exhibit significant differences in their crystal structure, which are closely related to their distinct chemical compositions.

Recalling the EPMA chemical formulae of Los-129.15 oneillite-like phase, (Na_14.78K_0.26)₁₅(Mn_1.87REE_1.13)₃(Ca_2.32Mn_0.68)₃(Fe_1.72Mn_0.10)_1.82Zr₃(Zr_0.27Si_0.22Al_0.20Nb_0.17Ti_0.12)₁Si₁(Si₂₄O₇₂)O_1.48(F_0.32Cl_0.66)_0.98, and of Los-174 kentbrooksite, (Na_13.62REE_1.1K_0.28)₁₅(Ca_4.19Mn_1.16REE_0.54Y_0.11)₆(Mn_2.26Fe_0.7)_2.96Nb_0.73(Si_1.28P_0.01)(Zr_2.93Ti_0.04Y_0.03)(Si₂₄O₇₂)O₃(O_1.24F_0.12Cl_0.04)_1.4, it can be seen that both the two phases contain significant amounts of Mn²⁺ and REE³⁺, with kentbrooksite being further enriched in these two components (3.42 Mn²⁺ and 1.75 REE³⁺ a.p.f.u.) with respect to the oneillite-like phase (2.65 Mn²⁺ and 1.13 REE³⁺ a.p.f.u.). A slight increase can also be noticed in the HFSE (Zr+Nb+Ti) amount, from 3.56 a.p.f.u. in the oneillite-like phase to 3.70 a.p.f.u. in kentbrooksite.

On the other hand, the oneillite-like phase is strongly Ca deficient and enriched in Fe²⁺. These variations in chemical composition are reflected in the primary crystal structural differences, mainly observed in the M layers (

Table 14. Comparative crystal-structural data for Los kentbrooksite and oneillite-like phase.

	Los-174 Kentbrooksite	Los-129.15 Oneillite-like Phase
Idealized formula	(Na,REE)15(Ca,Mn)6(Mn,Fe)3Zr3(Nb,Si)1(Si25O73)(O,OH,H2O)3(F,Cl,OH)2.	Na15Mn3Ca3(Fe,Zr)3Zr3(Si,Al,Nb,Ti)1 (Si25O73)(O,OH,H2O)3(OH,Cl,F)2
EGM group	Ca-rich zirconium eudialytes	Ca-poor zirconium eudialytes
Space group a,c, Å V, Å3	R3m 14.2037(3), 30.1507(9) 5267.8(2)	R3 14.1489(2), 30.1283(5) 5223.4(1)
Dominant cations at the key M sites
M1a	Ca+REE	Ca+Mn
M1b		Mn+REE
M2	Mn	Fe
M3	Nb	Nb+Si
M4	Si	Si

).

Referring to crystal structural data, in kentbrooksite, the six-membered rings of M1 polyhedra are predominantly occupied by Ca. In contrast, in the oneillite-like phase, a member of the Ca-poor EGM group, two distinct subsets of M1 polyhedra are identified: one primarily occupied by Ca and the other by Mn. Additionally, minor REE are present at the M1 sites in the oneillite-like phase [31]. The M2 polyhedra in kentbrooksite are almost completely occupied by Mn, with a refined occupancy of 0.97 a.p.f.u., while in the oneillite-like phase, these sites are more vacant (Fe 0.8 a.p.f.u.) and feature Fe as the dominant cation. The ^[6]M3 sites in kentbrooksite were refined as partially occupied by Nb, with a refined occupancy of Nb 0.85, whereas in the oneillite-like phase, the ^[6]M3 sites were refined as significantly Nb-deficient and more vacant, showing a refined occupancy of Nb_0.45 and Si_0.22.

5.2. Geological Significance of the Occurrence of Kentbrooksite and of an Oneillite-like Phase in Los Nepheline-Syenites

According to Moreau et al. [1], two main petrographic suites can be identified in Los nepheline-syenites, namely an agpaitic and a miaskitic suite, differentiated by their mineralogy. A gradational transition from agpaitic to pegmatitic facies on Rouma and Kassaa islands was reported by the same authors.

According to Moreau et al. [1], agpaitic syenites contain aegirine and arfvedsonite, often highly enriched in manganese (up to 9.2 wt% MnO), together with other Mn-rich minerals such as mica (up to 6.8 wt% MnO), pyrophanite, kupletskite, and låvenite. Agpaitic rocks are moreover characterized by the presence of villiaumite, serandite, steacyite, and REE-Zr-rich minerals such astrophyllite, catapleiite, eudialyte, kupletskite, pyrochlore, and rosenbuschite.

In contrast, miaskitic syenites are characterized by definitely lower Mn contents in hastingsitic amphibole and mica and by a significant amount of titanite. As already reported, the oneillite-like phase was identified within agpaitic rocks, whereas kentbrooksite occurs in pegmatites.

Within this geological framework, our data are in agreement with the conclusions of the studies on compositional variations of eudialyte group minerals by Schilling et al. [33] and Marks et al. [34].

In fact, these authors reported [33,34] that the Mn/Fe index is an optimal indicator for fractionation, with pegmatitic and aplitic rocks showing high X_Mn. Moreover, Marks et al. [34], in their study of eudialyte group minerals from the Ilímaussaq complex, observed that the highest REE contents are found in EGM hosted within the most evolved rock units of the Ilímaussaq complex.