Chirvinskyite, (Na,Ca)₁₃(Fe,Mn,□)₂(Ti,Nb)₂(Zr,Ti)₃-(Si₂O₇)₄(OH,O,F)₁₂, a New Mineral with a Modular Wallpaper Structure, from the Khibiny Alkaline Massif (Kola Peninsula, Russia)

Abstract

Chirvinskyite, (Na,Ca)₁₃(Fe,Mn,□)₂(Ti,Nb)₂(Zr,Ti)₃(Si₂O₇)₄(OH,O,F)₁₂, is a new wöhlerite–related zirconotitano–sorosilicate. It is triclinic, P $\overline{1}$, a = 7.0477(5), b = 9.8725(5), c = 12.2204(9) Å, α = 77.995(5), β = 82.057(6), γ = 89.988(5)°, V = 823.35(9) ų, Z = 1. The mineral was found in albitized alkaline pegmatites in a foyaite of the Mt. Takhtarvumchorr (Khibiny alkaline massif, Kola Peninsula, Russia, N 67°40′, E 33°33′). Chirvinskyite forms sheaf–like and radiated aggregates (up to 6 mm in diameter) of split fibrous crystals hosted by saccharoidal fluorapatite and albite. The mineral is pale cream in color, with a silky luster and a white streak. The cleavage is not recognized. Mohs hardness is 5. Chirvinskyite is biaxial (–), α 1.670(2), β 1.690(2), γ 1.705(2) (589 nm), 2V_calc = 80.9°. The calculated and measured densities are 3.41 and 3.07(2) g·cm⁻³, respectively. The empirical formula based on Si = 8 apfu is (Na_9.81Ca_3.28K_0.01)_∑13.10(Fe_0.72Mn_0.69□_0.54Mg_0.05)_∑2.00 (Ti_1.81Nb_0.19)_∑2.00(Zr_2.27Ti_0.63)_∑2.90(Si₂O₇)₄{(OH)_5.94O_3.09F_2.97}_∑12.00. Chirvinskyite belongs to a new structure type of minerals and inorganic compounds and is related to the wöhlerite-group minerals. Its modular “wallpaper” structure consists of disilicate groups Si₂O₇ and three types of “octahedral walls”. The mineral is named in honor of Petr Nikolaevich Chirvinsky (1880–1955), Russian geologist and petrographer, head of the Petrography Department of the Perm’ State University (1943–1953), for his contributions to mineralogy and petrology, including studies of the Khibiny alkaline massif.

1. Introduction

Zircono- and titanosilicates of the wöhlerite family are characteristic minerals of the alkaline complexes, including the Khibiny massif in Kola Peninsula, Russia. These minerals have complex modular crystal structures formed by Si₂O₇ groups and complex zigzag octahedral walls similar to those observed in borates with wallpaper structures [1,2]. The chemical composition and configuration of these walls vary widely [3,4], which results in about 10 approved wöhlerite-related minerals, with five of them (wöhlerite, låvenite, normandite, cuspidine, and chirvinskyite) occuring in the Khibiny massif [5].

The thinnest split fibers of an unknown låvenite-like mineral were found by late Yuri Men’shikov (1934–2013) during an investigation of the apatite/molybdenite/parakeldyshite deposit at Mt. Takhtarvumchorr [5]. Men’shikov obtained chemical and powder XRD data for this mineral and proposed to name it in honor of Prof. Petr Nikolaevich Chirvinsky (1880–1955), Russian geologist and petrographer, head of the Petrography Department of the Perm State University (1943–1953), for his contributions to mineralogy and petrology, including studies of the Khibiny alkaline massif in 1931–1941 [6,7]. However, the quality of crystals available at the time did not allow solution of the crystal structure of the mineral, which was necessary for its approval as a new mineral species. In 2015, we collected much larger radiated aggregates of this mineral in albitized alkaline pegmatites of the same locality, which allowed us to solve its crystal structure and finish the chirvinskyite investigation.

Both the mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature, and Classification of the International Mineralogical Association (IMA 2016–051). Type material is deposited in the collections of the Mineralogical Museum of St. Petersburg State University, Russia, under catalogue number 19657 and in the Geological and Mineralogical Museum of the Geological Institute of the Kola Science Centre, Apatity, Russia, under catalogue number GIM 7609.

2. Materials and Methods

The mineral was found and investigated in hand-sized specimens collected in 2000–2015 for most investigations. Fragments of fibrous chirvinskyite aggregates (up to 4 × 2 × 2 mm³) were used.

Optical properties of chirvinskyite were determined by using an Amplival optical microscope (Carl Zeiss Microscopy, Jena, Germany) and standard immersion liquids. The mineral density was determined by the float-sink method in Clerici solution (equal parts of TlHCO₂ and TlC₃H₃O₄ plus water), with the liquid density estimated by its refractive index.

The identification of associated minerals as well as the study of chirvinskyite morphology were performed using a LEO-1450 scanning electron microscope (Carl Zeiss Microscopy, Oberkochen, Germany) with a Quantax 200 energy-dispersive X-ray spectrometer (Bruker, Ettlingen, Germany).

The chemical composition of chirvinskyite was determined by wavelength-dispersive spectrometry on a Cameca MS-46 electron microprobe (Geological Institute, Kola Science Center, Russian Academy of Sciences, Apatity, Russia) operating at 20 kV, 20–30 nA. The H₂O content was calculated from the structural data and confirmed by the Penfield method [8] for the hand-cleaned material. Other elements with atomic numbers exceeding 5 were not detected. The analyses were performed with the beam size of 5 µm and the counting time of 10–20/10 s on peaks/background for every chemical element. The standards used included fluorite (F), lorenzenite (Na), pyrope (Mg), diopside (Si, Ca), wadeite (K), rutile (Ti), synthetic MnCO₃ (Mn), hematite (Fe), synthetic ZrSiO₄ (Zr), and LiNbO₃ (Nb). The coefficients in crystal chemical formulas were calculated by using the MINAL program [9].

The Raman spectrum of chirvinskyite was measured using a Jobin–Yvon LabRam HR 800 spectrometer (Horiba, Kyoto, Japan) with a 514 nm laser.

The powder X-ray diffraction pattern of chirvinskyite was obtained using a Rigaku R-AXIS RAPID II diffractometer equipped with a cylindrical image plate detector, with the Debye-Scherrer geometry (d = 127.4 mm, CoKα radiation). The data were integrated using the software package OSC2XRD [10]. Unit-cell dimensions were determined from the corrected X-ray powder-diffraction by Rietveld refinement using the program Topas [11].

The single-crystal X-ray diffraction study was performed using the Bruker Kappa APEX DUO diffractometer operated at 45 kV and 0.6 mA and equipped with a CCD area detector by means of a monochromatic MoKα X-radiation, frame widths of 1° in ω, and 200 s counting time for each frame. The intensity data were reduced and corrected for Lorentz, polarization, and background effects using the Bruker software APEX2 [12]. A semi-empirical absorption-correction based upon the intensities of equivalent reflections was applied [13,14]. The structure was solved and refined with the ShelX program package [14,15] within the Olex2 shell [16] to R₁ = 0.0736 for 3826 independent reflections with F² > 4σ(F²). Occupancies of the cation sites were calculated from the experimental site-scattering factors considering empirical chemical composition. Images of the chirvinskyite crystal structure were drawn with the “Atoms 6.5” program [17]. Further details about the crystal structure of the mineral are provided in the Crystallographic Information File (CIF) available as a Supplementary Information.

3. Results

3.1. Occurrence

The Khibiny alkaline massif (Kola Peninsula, Russia) is the world largest alkaline complex that was protruded through the Archaean granite–gneisses, Proterozoic amphibole schists, and Devonian traps about 370 Ma [18]. The stock-like foyaite protrusion (about 30 km in diameter) was sharply expanded near the day surface (to about 50 km in diameter) with the formation of ring faults filled by residual foidolite melts (Figure 1a).

Numerous xenoliths of olivine basalt and tuff interlayered with quartzite and sandstone are widespread within the foyaite massif. They are comparatively fresh far from the foidolite rings and strongly fenitized near the rings [19]. Fenitization of the basalt and tuff produced Ca–Ti-rich hydrothermal solutions that made surrounding foyaites rich in Ca and Ti with the formation of corresponding mineral assemblages (fluorapatite, titanite, rinkite, rosenbushite, etc.) including minerals of the wöhlerite group: cuspidine, låvenite, normandite, wöhlerite [5], and chirvinskyite.

The type locality of chirvinskyite (Figure 1b) is situated at the Mt. Takhtarvumchorr near the large xenolith of deeply fenitized basaltic tuff consisting mainly of orthoclase, cordierite, phlogopite, and pyrrhotite. Surrounding host foyaite is intensively albitized and contains fluorapatite, molybdenite, graphite, and zircon–parakeldyshite mineralization of minor economic significance [5,6]. Numerous veins of albitized/natrolitized nepheline–microcline–aegirine-augite pegmatites (up to 6 m wide and 200 m long) are abundant in this region (localities nos. 53 and 54 in [5]), and chirvinskyite occurs in one of the veins (N 67°40′, E 33°33′).

Chirvinskyite was found in eluvial blocks of a thick (above 1 m) albitized and natrolitized pegmatite with indistinct contact with the host foyaite (due to intensive metasomatic alteration of both pegmatite and the host rock). The pegmatite blocks consist mainly of white fine–grained albite with corroded microcline laths (up to 3 cm long), fine-grained natrolite replacing nepheline, prismatic crystals of aegirine–augite and arfvedsonite (up to 4 cm long), flattened prismatic crystals of astrophyllite (up to 5 cm long), radiating and stream–like aggregates of black needle–like crystals of aegirine (up to 2 cm long), and relics of altered light brown eudialyte (up to 1 cm in diameter). Aegirine aggregates are often impregnated with thin graphite lamella. In the axial zones of pegmatite, there are lens–shaped clusters (up to 1 × 0.50 m) of light–green saccharoidal fluorapatite with inclusions of flattened prismatic crystals of aenigmatite (up to 3 cm long), lamellar crystals of molybdenite, ilmenite, and pyrrhotite (up to 7 mm in diameter), which is partially replaced by pyrite, marcasite, and goethite, rounded grains of galena (up to 5 mm) and sphalerite (up to 1 cm), and reddish–orange prismatic crystals of låvenite (up to 3 mm long). In places, there are lemon–yellow radiating aggregates (up to 4 cm in diameter) and prismatic crystals (up to 1.5 cm long) of titanite, partially replaced by lorenzenite, as well as rich impregnations of eudialyte and zircon grains (up to 6 mm) surrounded by 1–3 mm wide snow–white fringes of parakeldyshite, sodalite, and cancrinite [5].

Chirvinskyite forms sheaf-like and radiating aggregates up to 6 mm in diameter (Figure 2a). They consist of separated fibers of the mineral with numerous inclusions of natrolite, albite, aegirine, parakeldyshite, lorenzenite, and fluorcalciopyrochlore between them (Figure 2b).

3.2. Appearance and Physical Properties

Chirvinskyite forms sheaf-like and radiated aggregates (up to 6 mm in diameter) of split fibrous metacrysts (up to 0.02 mm) hosted in fine-grained natrolite, fluorapatite, and/or albite (Figure 2). No idiomorphic crystals have been found. The a:b:c ratio calculated from the unit-cell parameters (single-crystal data) is close to 0.7:1:1.2. No twinning was observed. Cleavage and parting were not observed due to the splitting of chirvinskyite crystals into nanometer-sized fibers. The mineral is brittle and has a fibrous fracture. Mohs hardness is about 5 (hardly scratched with a steel needle). The density measured is 3.07(2) g·cm⁻³, the structural density is 3.33 g·cm⁻³, and the density calculated using the empirical formula and single-crystal unit-cell parameters is 3.41 g·cm⁻³. The difference between calculated and observed values may be caused by numerous inclusions of surrounding minerals (mostly natrolite, albite, aegirine, parakeldyshite, and fluorapatite).

Macroscopically, chirvinskyite is pale creamy. Its luster is silky along the fibers, and dull in cross-sections. The mineral is transparent in separate fibers, translucent in aggregates, and has a white streak. It is biaxial, negative, α = 1.670(2), β = 1.690(2), γ = 1.705(2) (589 nm), 2V_calc = 80.9°. The optical orientation is Z = b, X^c = 14°, and further details remain unclear. In transmitted light, chirvinskyite is colourless, without pleochroism or dispersion. The mineral is non-fluorescent. The Gladstone–Dale calculation provides a compatibility index of 0.028, which is regarded as excellent.

3.3. Chemical Composition

Table 1. Chemical composition of chirvinskyite.

Sample	Mean	SD	Min	Max	n
H2O *, wt%	3.14				1
F	3.32	0.20	3.12	3.52	3
Na2O	17.85	1.04	16.90	20.59	17
MgO	0.13	0.09	0.00	0.29	17
SiO2	28.22	0.49	27.76	29.44	17
K2O	0.03	0.02	0.00	0.05	17
CaO	10.80	0.34	10.21	11.36	17
TiO2	11.46	0.98	9.79	12.80	17
MnO	2.87	0.50	2.17	3.89	17
FeO	3.03	0.39	2.32	3.99	17
ZrO2	16.43	1.71	13.63	19.43	17
Nb2O5	1.46	0.35	0.88	2.06	17
F=O	−1.40
Total	97.34

* Water content calculated from structural data (H₂O content determined by the Penfield method is about 7 wt%).

provides the mean analytical results for 17 chirvinskyite aggregates. Taking into account structural data, the average empirical formula of chirvinskyite (based on Si = 8 apfu) can be written as (Na_9.81Ca_3.28K_0.01)_∑13.10 (Fe_0.72Mn_0.69□_0.54Mg_0.05)_∑2.00 (Ti_1.81Nb_0.19)_∑2.00 (Zr_2.27Ti_0.63)_∑2.90 (Si₂O₇)₄ {(OH)_5.94O_3.09 F_2.97}_∑12.00. Taking into account the structural data, the simplified formula of the mineral is (Na,Ca)₁₃(Fe,Mn,□)₂(Ti,Nb)₂(Zr,Ti)₃(Si₂O₇)₄(OH,O,F)₁₂, or A₁₃B₂C₂D₃(T₂O₇)₄X₁₂, where A = Na, Ca, K, B = Fe, Mn, Mg, vacancy, C = Ti, Nb, D = Zr, Ti, T = Si, X = O, OH, F. The formula of a theoretical end member is (Na₈Ca₅)Fe²⁺₂Ti₂Zr₃(Si₂O₇)₄O₆(OH)₆.

Despite the fact that iron predominates in the B-position of chirvinskyite, manganese is its significant constituent as well. The total content of Fe, Mn, and Mg in chirvinskyite linearly decreases with the increase of the Ca content (Figure 3a). The Zr content decreases with the increase of the Ti content from Zr₃Ti₂ to Zr₂Ti₃ (Figure 3b). Therefore, the main isomorphic substitutions in chirvinskyite can be written as the following formula shown below.

Fe²⁺_B → Mn²⁺_B;

2Na⁺_A + (Fe, Mn, Mg)²⁺_B → 2Ca²⁺_A + □_B;

Zr⁴⁺_D → Ti⁴⁺_D.

In limiting cases, these substitutions can lead to the formation of the following compounds, correspondingly.

(Na₈Ca₅)Mn₂Ti₂Zr₃(Si₂O₇)₄O₆(OH)₆;

(Na₈Ca₅)□₂Ti₂Zr₃(Si₂O₇)₄O₆(OH)₆;

(Na₈Ca₅)Fe₂Ti₅(Si₂O₇)₄O₆(OH)₆.

Chirvinskyite does not dissolve in cold hydrochloric acid.

3.4. Raman Spectroscopy

The Raman spectrum of chirvinskyite (Figure 4) contains characteristic bands of the O–H stretching vibrations (

Table 2. Raman bands in the chirvinskyite spectrum and their interpretation.

Raman Shift, cm−1	Assignment	Type
2918, 3073, 3054sh, 3554	OH	v1
704s, 729sh, 802, 848	Si2O7	v1
898, 924sh, 960, 997	Si2O7	v3
330, 347w, 413w, 445	Si2O7	v2
607, 636s, 652	Si2O7	v4
481s, 497, 538, 593	MeO6	v1, v2
177, 196	MeO6	v3, v4
233, 256s, 272, 289, 303	MeO7, MeO8	v1, v2
91s, 123, 145		lattice vibrations

sh = shoulder. s = strong intensity. w = weak.

) in the range from 2900 to 3600 cm⁻¹. The intensive bands at 607, 636s, and 652 as well as 330, 347w, 413w, and 445 cm⁻¹ can be assigned to the Si–O–Si and O–Si–O asymmetric and symmetric modes of bending vibrations in the [Si₂O₇]–groups. The bands observed in the ranges of 900 to 1000 and 700 to 850 cm⁻¹ can be related to the asymmetric and symmetric vibrations of the same bonds [20,21]. The bands at 481s, 497, 538, 593, 177, and 196 cm⁻¹ are assigned to the bending and stretching vibrations of the Ti‒O, Zr‒O, Mn‒O, and Fe‒O bonds in respective cation-centered octahedra [22,23]. The bands at the 233, 256s, 272, 289, and 303 cm⁻¹ correspond to the bending/stretching vibrations of the Ca‒O and Na‒O bonds of the Ca(Na)O₈ and Ca(Na)O₇ coordination polyhedra. The bands below 150 cm⁻¹ can be assigned to the lattice vibrations. The absence of absorption bands in the range of 1100 to 1650 cm⁻¹ indicates the absence of boron-bearing groups and water molecules. The d_calc(O^…O) distances calculated according to the Libowitzky [24] formula for the peaks at 3554, 3504, 3073, and 2918 cm⁻¹ are equal to 3.32, 3.21, 2.97, and 2.94 (Å). The band at 3554 cm^‒1 (d_calc = 3.32 Å) corresponds to the X(18)^…O(15) bond. The respective O^…O distance in the structure is 3.282 Å. The band at 3504 cm⁻¹ (d_calc = 3.21 Å) may be associated with the following O^…O distances corresponding to weak hydrogen bonds: 3.205 Å for (X(18)^…O(8) and 3.206 Å for X(19)^…O(12), respectively. The bands at 3073 and 2918 cm^-1 with d_calc = 2.97 and 2.94 Å may be related to short contacts X(16)^…O(12) and X(16)^…O(8), with corresponding distances in the structure of 2.710 and 2.736 Å. These two bands may also be associated with an organic impurity.

3.5. Powder X-ray Diffraction

The unit-cell parameters of chirvinskyite determined from the powder pattern (

Table 3. X-ray powder-diffraction data for chirvinskyite.

Imeas	dmeas, Å	dcalc, Å	hkl	Imeas	dmeas, Å	dcalc, Å	hkl
5	11.86	11.83	001	2	2.382	2.381	134
5	9.67	9.63	010	12	2.326	2.327	025
2	8.40	8.39	011	2	2.276	2.277	302
34	7.00	6.98	100	9	2.193	2.195	−115
2	6.41	6.42	101	4	2.131	2.132	035
17	5.907	5.916	002	5	2.109	2.107	−233
1	5.197	5.179	−111	9	2.018	2.018	016
9	4.844	4.832	021	2	2.007	2.004	−2–32
6	4.632	4.638	−1–11	3	1.9878	1.9846	2–15
3	4.222	4.228	−102	8	1.9677	1.9664	0–25
5	4.139	4.137	−112	3	1.9275	1.9268	050
15	4.039	4.024	121	3	1.9029	1.8985	−151
23	3.956	3.955	013	11	1.8610	1.8617	216
1	3.724	3.718	122	7	1.7670	1.7651	154
5	3.656	3.651	−1–12	6	1.7528	1.7514	−2–15
2	3.489	3.489	200	25	1.7407	1.7413	402
33	3.416	3.427	023	2	1.6927	1.6927	−341
7	3.245	3.243	−103	1	1.6814	1.6833	−411
3	3.212	3.210	202	1	1.6722	1.6712	3–41
2	3.143	3.140	−211	16	1.6461	1.6458	037
11	3.012	3.012	014	2	1.6260	1.6262	−146
6	2.985	2.982	−2–11	4	1.6117	1.6144	14–4
4	2.932	2.941	−1–22	7	1.6041	1.6057	060
57	2.886	2.890	132	6	1.5724	1.5727	−315
100	2.796	2.798	024	6	1.5536	1.5528	30–5
3	2.706	2.708	124	4	1.5475	1.5461	33–3
2	2.637	2.632	−2–12	6	1.4943	1.4961	−3–42
7	2.614	2.618	2–13	13	1.4854	1.4849	4–24
4	2.588	2.589	−222	7	1.4677	1.4676	4–15
2	2.522	2.531	−133	1	1.4382	1.4368	−20–8
12	2.451	2.448	034	1	1.4182	1.4187	−31–7
9	2.413	2.415	042	6	1.3974	1.3982	4–25

The strongest X-ray powder-diffraction lines are given in bold.

) are: 7.0485(6), b = 9.8557(8), c = 12.2233(8) Å, α = 77.95(1), β = 82.05(1), γ = 90.03(1)°, V = 822.1(1) ų, Z = 1, which are in good agreement with the single-crystal data. The strongest X-ray powder-diffraction lines (listed as d in Å (I) (hkl)) are as follows: 7.00(34)(100), 5.907(17)(002), 3.956(23)(013), 3.416(33)(023), 2.886(57)(132), 2.796(100)(024), 1.7407(25)(402), and 1.6461(16)(037).

3.6. Crystal Structure

Crystal data, data-collection information, and structure-refinement details are given in

Table 4. Data and structure refinement parameters for chirvinskyite.

Crystal system	Triclinic
Space group	P–1
a, Å	7.0477(5)
b, Å	9.8725(5)
c, Å	12.2204(9)
α, °	77.995(5)
β, °	82.057(6)
γ, °	89.988(5)
Volume, Å3	823.35(9)
Z	1
Dcalc, g/cm3	3.332
μ, mm1	3.260
F(000)	802.0
Radiation	MoKα (λ = 0.71073)
2Θ range for data collection, °	3.506 to 75.382
Index ranges	−9 ≤ h ≤ 9, −13 ≤ k ≤ 13, −11 ≤ l ≤ 16
Reflections collected	11456
Independent reflections	4394 [Rint = 0.0384, Rsigma = 0.0345]
Data/restraints/parameters	4394/0/319
GooF	1.111
Final R indexes [I>=2σ (I)]	R1 = 0.0736, wR2 = 0.2016
Final R indexes [all data]	R1 = 0.0813, wR2 = 0.2067
Largest difference peak/hole/e Å−3	2.23/−1.47

, atom coordinates, occupancies, bond valence sums, and displacement parameters are given in

Table 5. Atomic coordinates, occupancies, bond valence sums, experimental and calculated site-scattering factors (SSF_exp and SSF_calc, respectively), and equivalent isotropic displacement parameters (Ų ×10⁻³) of chirvinskyite.

Site	x	y	z	Occupancy	SSFexp (e−)	SSFcalc (e−)	BVS *	Uiso
A1	−0.0800(2)	0.9365(1)	0.3232(1)	Ca	20.00	20.00	2.37	13.7(3)
A2	−0.0613(3)	0.4585(2)	0.2439(2)	Ca0.59Na0.41	16.31	16.31	1.96 (2.35)	16.0(7)
A3	−0.1393(4)	0.3604(2)	0.5589(2)	Na0.78Ca0.22	12.98	12.98	1.46 (1.32)	17.3(10)
A4	−0.2550(4)	0.5007(3)	−0.0016(2)	Na	11.00	11.00	1.32	19.9(7)
A5	−0.2851(3)	0.2105(2)	0.1348(2)	Na0.77Ca0.23	14.27	14.27	1.51 (1.36)	14.6(10)
A6	0.2193(4)	0.2111(3)	0.1328(2)	Na0.90Ca0.10	11.90	11.90	1.38 (1.32)	15.2(11)
A7	0	0	0	Ca0.54Na0.46	15.86	15.86	1.82 (2.24)	18.3(11)
B **	−0.3655(2)	0.6437(1)	0.4443(1)	Fe0.36Mn0.345 □0.27Mg0.025	20.80	18.29	2.17	11.1(4)
C	0.5607(2)	0.5380(1)	0.74501(9)	Ti0.94Nb0.06	23.14	23.14	3.76 (3.71)	8.9(4)
D1	0.5	0	0	Zr0.53Ti0.47	31.54	31.54	3.58 (4.17)	8.1(4)
D2	0.5777(1)	0.06435(8)	0.68359(6)	Zr0.58Ti0.42	32.44	32.44	3.59 (4.12)	8.2(3)
T1	0.2745(3)	0.2522(2)	0.8359(2)	Si	14.00	14.00	4.09	8.9(4)
T2	0.8063(3)	0.2503(2)	0.8348(2)	Si	14.00	14.00	4.10	8.4(4)
T3	0.3281(3)	0.8055(2)	0.6049(2)	Si	14.00	14.00	4.15	12.5(4)
T4	−0.1382(3)	0.8016(2)	0.6059(2)	Si	14.00	14.00	4.15	9.2(4)
O1	0.3024(9)	0.1492(6)	0.9513(4)	O	8.00	8.00	2.04 (2.17)	19.3(11)
O2	0.3360(9)	0.4102(6)	0.8257(5)	O	8.00	8.00	2.02 (2.05)	23.1(12)
O3	0.3512(9)	0.1947(5)	0.7235(4)	O	8.00	8.00	2.00 (2.06)	19.0(11)
O4	0.039(1)	0.2577(7)	0.8365(6)	O	8.00	8.00	2.08 (2.11)	32.5(14)
O5	0.7206(8)	0.1492(6)	0.9520(4)	O	8.00	8.00	2.01 (2.13)	18.5(11)
O6	0.7874(8)	0.1889(5)	0.7244(4)	O	8.00	8.00	2.10 (2.17)	17.0(11)
O7	0.7468(9)	0.4085(6)	0.8230(5)	O	8.00	8.00	1.98 (2.01)	23.6(12)
O8	0.3643(9)	0.6787(5)	0.7045(5)	O	8.00	8.00	2.10 (2.11)	22.0(12)
O9	0.3743(9)	0.9497(6)	0.6342(5)	O	8.00	8.00	2.11 (2.21)	22.9(12)
O10	0.3976(8)	0.7876(5)	0.4788(4)	O	8.00	8.00	1.96 (1.99)	15.4(10)
O11	0.0896(9)	0.8051(8)	0.6215(6)	O	8.00	8.00	2.10 (2.11)	35.1(15)
O12	−0.2231(9)	0.6766(6)	0.7078(5)	O	8.00	8.00	2.08 (2.10)	23.2(12)
O13	−0.200(1)	0.9498(6)	0.6309(5)	O	8.00	8.00	2.01 (2.10)	26.0(13)
O14	−0.149(1)	0.7765(6)	0.4828(5)	O	8.00	8.00	2.10 (2.05)	25.0(13)
X15	0.5817(8)	0.4853(6)	0.6104(4)	O0.70F0.30	8.00	8.30	1.76 (1.68)	19.2(11)
X16	0.5228(6)	0.6215(4)	0.9006(4)	(OH)	8.00	8.00	1.32 (1.28)	6.2(8)
X17	0.5369(7)	0.9556(5)	0.8415(4)	O0.89F0.11	8.00	8.11	1.83 (2.03)	15.8(10)
X18	−0.1189(8)	0.5239(5)	0.3995(4)	(OH)0.90O0.10	8.00	8.00	1.48 (1.51)	14.3(10)
X19	−0.0248(7)	0.3710(4)	0.0870(4)	(OH)0.95O0.05	8.00	8.00	1.34 (1.39)	9.4(9)
X20	−0.0394(6)	0.0669(5)	0.1592(4)	O0.70F0.30	8.00	8.30	1.76 (1.83)	8.1(8)

* BVS—bond valence sums calculated using bond-valence parameters taken from Reference [25]. The values in square brackets are for the site compositions for the end-member with the formula Na₈Ca₅Fe₂Ti₂Zr₃(Si₂O₇)₄O₆(OH)₆. ** B occupancy based on the chemical data.

and

Table 6. Anisotropic displacement parameters (Ų) for chirvinskyite.

Atom	U11	U22	U33	U23	U13	U12
A1	0.0167(7)	0.0087(6)	0.0139(7)	0.0010(5)	−0.0015(5)	0.0011(5)
A2	0.0204(11)	0.0103(10)	0.0175(11)	−0.0016(7)	−0.0052(7)	0.0007(6)
A3	0.0229(16)	0.0116(14)	0.0149(14)	0.0018(9)	−0.0006(9)	0.0022(9)
A4	0.0230(16)	0.0207(16)	0.0148(15)	−0.0015(12)	−0.0021(12)	0.0005(13)
A5	0.0153(14)	0.0126(13)	0.0169(14)	−0.0056(9)	−0.0022(9)	0.0005(8)
A6	0.0178(17)	0.0125(15)	0.0173(16)	−0.0072(10)	−0.0031(10)	0.0018(10)
A7	0.0240(17)	0.0151(15)	0.0150(15)	−0.002(1)	−0.0025(10)	0.0024(10)
B	0.0137(7)	0.0088(6)	0.0107(6)	−0.0009(4)	−0.0028(4)	−0.0004(4)
C	0.0112(6)	0.0048(5)	0.0097(6)	0.0007(4)	−0.0017(4)	0.0006(4)
D1	0.0091(6)	0.0051(6)	0.0093(6)	0.0007(4)	−0.0019(4)	0.0003(4)
D2	0.0100(4)	0.0050(4)	0.0099(4)	−0.0020(3)	−0.0021(3)	0.0004(3)
T1	0.0158(9)	0.035(7)	0.0069(8)	0.0000(6)	−0.0008(6)	−0.0003(6)
T2	0.0137(8)	0.046(7)	0.0065(8)	−0.0004(6)	−0.0013(6)	0.0009(6)
T3	0.023(1)	0.056(8)	0.0091(8)	−0.0017(6)	−0.0022(7)	0.0011(7)
T4	0.0143(9)	0.048(7)	0.0090(8)	−0.0018(6)	−0.0030(6)	0.0020(6)
O1	0.033(3)	0.017(2)	0.008(2)	−0.0018(18)	−0.001(2)	0.004(2)
O2	0.038(3)	0.014(2)	0.017(3)	−0.003(2)	−0.005(2)	−0.002(2)
O3	0.034(3)	0.014(2)	0.009(2)	−0.0016(18)	−0.001(2)	0.002(2)
O4	0.027(3)	0.037(4)	0.035(3)	−0.008(3)	−0.008(3)	0.001(3)
O5	0.029(3)	0.019(3)	0.007(2)	−0.0016(18)	0.0000(2)	−0.003(2)
O6	0.029(3)	0.011(2)	0.011(2)	−0.0034(18)	−0.003(2)	−0.002(2)
O7	0.036(3)	0.014(2)	0.020(3)	−0.004(2)	−0.003(2)	0.002(2)
O8	0.042(4)	0.009(2)	0.015(2)	−0.0014(19)	−0.007(2)	0.008(2)
O9	0.037(3)	0.011(2)	0.022(3)	−0.006(2)	−0.004(2)	−0.004(2)
O10	0.025(3)	0.013(2)	0.007(2)	−0.0023(17)	0.0031(19)	−0.0040(19)
O11	0.020(3)	0.045(4)	0.039(4)	−0.008(3)	−0.004(3)	0.003(3)
O12	0.037(3)	0.012(2)	0.018(3)	−0.001(2)	0.0000(2)	−0.005(2)
O13	0.042(4)	0.014(3)	0.024(3)	−0.006(2)	−0.008(3)	0.005(2)
O14	0.045(4)	0.019(3)	0.012(2)	−0.005(2)	−0.006(2)	0.001(2)
X15	0.026(3)	0.017(3)	0.013(2)	−0.0026(19)	0.001(2)	0.005(2)
X16	0.009(2)	0.0025(18)	0.0059(19)	0.0023(15)	−0.0006(15)	−0.0009(15)
X17	0.012(2)	0.015(2)	0.019(2)	0.0006(19)	−0.0041(19)	−0.0011(18)
X18	0.025(3)	0.010(2)	0.008(2)	−0.0024(17)	−0.0007(18)	−0.0008(19)
X19	0.014(2)	0.0021(19)	0.011(2)	0.0018(16)	−0.0028(17)	−0.0007(16)
X20	0.008(2)	0.008(2)	0.007(2)	0.0023(16)	−0.0015(16)	−0.0007(16)

, and selected interatomic distances are given in

Table 7. Selected interatomic distances (Å) for the crystal structure of chirvinskyite.

B	O34	2.310(5)	T2	O4	1.643(7)	A3	O65	2.348(6)
B	O105	2.245(6)	T2	O5	1.606(5)	A3	O102	2.484(6)
B	O14	2.174(6)	T2	O6	1.612(5)	A3	O112	2.990(8)
B	X152	2.242(6)	T2	O7	1.599(6)	A3	O142	2.493(7)
B	X155	2.275(5)	<T2–O>	1.615	A3	X155	2.400(6)
B	X18	2.173(5)				A3	X18	2.243(5)
	<B1–φ*>	2.237	T3	O8	1.602(6)	A3	X182	2.318(6)
			T3	O9	1.583(5)		<A3–φ>	2.468
C	O25	2.031(6)	T3	O10	1.597(5)
C	O7	2.025(6)	T3	O11	1.665(7)	A4	O24	2.484(6)
C	O8	1.998(6)		<T3–O>	1.612	A4	O721	2.496(6)
C	O1211	1.988(6)				A4	X2	2.298(5)
C	X15	1.813(5)	T4	O11	1.644(7)	A4	X21	2.286(5)
C	X16	2.209(4)	T4	O12	1.607(6)	A4	X19	2.313(6)
	<C1–φ>	2.011	T4	O13	1.604(6)	A4	X1920	2.338(5)
			T4	O14	1.587(6)		<A4–φ>	2.369
D1	O15	2.080(6)		<T4–O>	1.611
D1	O16	2.080(6)				A5	O518	2.428(6)
D1	O57	2.078(6)	A1	O34	2.503(6)	A5	O813	2.440(6)
D1	O5	2.078(6)	A1	O44	2.994(7)	A5	O913	2.919(6)
D1	X178	2.055(5)	A1	O64	2.468(6)	A5	X1613	2.376(5)
D1	X179	2.055(5)	A1	O913	2.404(6)	A5	X1713	2.363(6)
	<D1–φ>	2.071	A1	O1113	2.770(8)	A5	X1916	2.353(5)
			A1	O1313	2.458(7)	A5	X20	2.250(5)
D2	O315	2.118(6)	A1	O14	2.234(6)		<A5–φ>	2.447
D2	O616	2.098(5)	A1	X20	2.127(5)
D2	O9	2.056(6)		<A1–φ>	2.495	A6	O118	2.417(6)
D2	O1012	2.192(5)				A6	O1213	2.437(6)
D2	O1311	2.041(6)	A2	O24	2.479(7)	A6	O1313	2.977(7)
D2	X17	1.986(5)	A2	O44	2.766(7)	A6	X1612	2.391(5)
	<D2–φ>	2.082	A2	O74	2.522(7)	A6	X1712	2.386(6)
			A2	O82	2.463(7)	A6	X1916	2.379(5)
T1	O1	1.593(5)	A2	O112	2.766(8)	A6	X20	2.259(5)
T1	O2	1.594(6)	A2	O122	2.479(7)		<A6–φ>	2.464
T1	O3	1.619(5)	A2	X18	2.120(5)
T1	O4	1.662(7)	A2	X19	2.246(5)	A7	O118	2.521(6)
	<T1–O>	1.617		<A2–φ>	2.480	A7	O14	2.521(6)
						A7	O418	2.876(7)
						A7	O44	2.876(7)
						A7	O518	2.520(6)
						A7	O54	2.520(6)
						A7	X2019	2.164(4)
						A7	X20	2.164(4)
							<A7–φ>	2.520

¹1+X,−1+Y,1+Z; ²−X,1−Y,1−Z; ³+X,−1+Y,1+Z; ⁴1−X,1−Y,1−Z; ⁵−1+X,+Y,+Z; ⁶2−X,−Y,2−Z; ⁷1−X,−Y,2−Z; ⁸+X,−1+Y,+Z; ⁹1−X,1−Y,2−Z; ¹⁰1+X,+Y,1+Z; ¹¹1+X,+Y,+Z; ¹²1−X,2−Y,1−Z; ¹³−X,2−Y,1−Z; ¹⁴1+X,1+Y,+Z; ¹⁵−1+X,1+Y,+Z; ¹⁶+X,1+Y,+Z; ¹⁷−1−X,1−Y,1−Z; ¹⁸−1+X,1+Y,−1+Z; ¹⁹−X,2−Y,−Z; ²⁰−X,1−Y,−Z; ²¹−1+X,+Y,−1+Z; ²²+X,1+Y,−1+Z; ²³1+X, −1+Y,+Z; *φ = O, OH.

.

The simplified formula of chirvinskyite is A₁₃B₂C₂D₃(T₂O₇)₄X₁₂, where A = Na, Ca, K, B = Fe, Mn, Mg, vacancy, C = Ti, Nb; D = Zr, Ti; T = Si, and X = O, OH, F. There are seven different A sites (Table 5, Figure 5) with coordination numbers varying from 6 (A4) and 7 (A3, A5, A6) to 8 (A1, A2, A7) (Table 7) with the average <A-φ> bond lengths (φ = O, OH) from 2.369 to 2.520 Å. There are one pure Ca (A1) and one pure Na (A4) sites, whereas the other five A sites have mixed Na-Ca occupancies with different Na/Ca ratios (Table 5). The A2 and A7 sites are dominantly occupied by Ca, whereas Na is the dominant amount in the A3, A5, and A6 sites. Fe is prevalent in the B site even though it also contains a significant amount of Mn. The B site has an octahedral coordination with the average <B-φ> bond length equal to 2.237 Å. The octahedral C site is occupied by Ti with the minor admixture of Nb with the average <C-φ> bond length of 2.011 Å. The CO₆ octahedra are strongly distorted due to the electronic second-order Jahn-Teller effect [26]. There are one short (1.813 Å) and one long (2.209 Å) C-φ apical bonds and four intermediate (1.988–2.031 Å) equatorial C-O bonds (Table 7). There are two octahedrally coordinated D sites, D1 and D2, which are predominantly occupied by Zr. The D sites have nearly regular octahedral coordinations with the average <D1-φ> and <D2-φ> distances of 2.071 and 2.082 Å, respectively. The four T-sites are occupied by Si and have tetrahedral coordination with the average <T-O> bond lengths ranging from 1.611 to 1.617 Å. The T-O-T bridges in the Si₂O₇ groups are almost linear with T1-O4-T2 = 166° and T3-O11-T4 = 175°.

There are six anion X sites, X15–X20, not bonded to Si and occupied by different proportions of O, OH, and F. Taking into account the bond-valence sums and local coordination environments, it can be suggested that the fourfold coordinated X15, X17, and X20 sites are occupied by O²⁻ and F⁻ anions, whereas threefold coordinated hydroxyl groups are dominant in the X16, X18, and X19 sites (Table 5). For the ideal end-member formula, the amounts of O and OH/F should be equal, which results in the composition of Na₈Ca₅Fe₂Ti₂Zr₃(Si₂O₇)₄O₆(OH,F)₆.

Chirvinskyite belongs to the new structure type of Ti-Zr disilicates. Its crystal structure can be described in modular terms adopted for the minerals of the wöhlerite family [3,4]. From this point of view, the crystal structure of chirvinskyite consists of octahedral walls and Si₂O₇ groups (Figure 6). Note that this description is valid in the case that only the A–O bonds shorter than 2.6 Å are taken into account and, therefore, the coordination of the A sites may be treated as octahedral. There are three kinds of walls consisting of four chains, three chains, and five chains of edge-sharing octahedra (Figure 7). The three-membered and five-membered octahedra walls share a common chain of edge-sharing octahedra to form a continuous layer with a zigzag section parallel to (001). The four-membered walls are linked to the layers by sharing common X15 sites and through the disilicate groups. Each Si₂O₇ group is linked to three adjacent octahedral walls. The crystal structure of chirvinskyite considered in this aspect belongs to the new structure type of wallpaper structures that are very common in borates [2,27]. Belov [28] pointed out that wallpaper borates and silicates of the wöhlerite-cuspidine group have much in common in terms of the structure architecture. Therefore, chirvinskyite can be described as a new type of a titanozircono-sorosilicate with the new kind of wallpaper structure. Its structural formula, Na₈Ca₅(Fe,Mn)₂Ti₂(Zr,Ti)₃(Si₂O₇)₄O₆(OH)₆, is in good agreement with the simplified formula determined by the chemical analysis.

4. Discussion

As already noted, the chirvinskyite-bearing pegmatite initially had the nepheline–microcline–aegirine-augite composition (with minor arfvedsonite, aenigmatite, astrophyllite, and eudialyte) characteristic for other pegmatites at Mt. Takhtarvumchorr. Later, both pegmatite and host foyaite were metasomatically altered by low-temperature hydrothermal solutions, with replacement of nepheline, microcline, and aegirine-augite by natrolite, albite, and aegirine, respectively.

3Nph + Ab + 4H₂O = 2Ntr

Mc + Na+ = Ab + K⁺

Aeg-Au + Na+ = 2Aeg + Ca²⁺

where Ab—albite, NaAlSi₃O₈, Aeg—aegirine, NaFeSi₂O₆, Aeg-Au—aegirine-augite, NaCaFe₂Si₄O₁₂, Mc—microcline, KAlSi₃O₈, Nph—nepheline, NaAlSiO₄, and Ntr—natrolite, Na₂Al₂Si₃O₁₀·2H₂O.

The origin of chirvinskyite can be related to the metasomatic alteration of accessory låvenite by Na-rich hydrothermal solutions, which was accompanied by the replacement of zircon and eudialyte by parakeldyshite and titanite by lorenzenite [5].

Lvt + 4Ttn + 2H₂O + Zr⁴⁺ + 6Na⁺ = Chv + 2Ti⁴⁺ + Ca²⁺

40Zrn + 2Eud + 64Na⁺ + 7O₂ = 46Pkd + 12H₂O + 6Fe²⁺ + 12Ca²⁺

2Ttn + 2Na⁺ + H₂ = Lor + H₂O + 2Ca²⁺

where Chv—chirvinskyite, Na₈Ca₅Fe₂Ti₂Zr₃Si₈O₃₄(OH)₆, Eud—eudialyte, Na₁₄Ca₆Fe₃Zr₃Si₂₆O₇₂(OH)₄· 4H₂O, Lor—lorenzenite, Na₂Ti₂Si₉O₉, Lvt—låvenite, Na₂Ca₂FeMnZr₂Si₄O₁₆(OH)₂, Pkd—parakeldyshite, Na₂ZrSi₂O₇, Ttn—titanite, CaTiSiO₅, and Zr—zircon, ZrSiO₄. Calcium as a characteristic product of all these reactions is concentrated in fluorapatite.

Under such conditions, there is no wonder that such an H₂O-rich sorosilicate as chirvinskyite was crystallized. This mineral may contain some additional non-structural molecular H₂O, which is detected by the Penfield method and Raman spectroscopy. However, by analogy with similar minerals (such as murmanite, selivanovaite, etc.), the crystal-chemical formula of chirvinskyite does not take these H₂O molecules into account.

Chirvinskyite belongs to a new structure type of minerals and inorganic compounds. The related structures are those of låvenite and other “octahedral-wall” titanosilicates. According to the Nickel-Strunz classification, it belongs to the group 9.BE (Sorosilicates with Si₂O₇ groups, with additional anions as well as cations in octahedral and greater coordination). According to the Dana classification, it belongs to the group 56 (Sorosilicates with Si₂O₇ groups, with additional O, OH, F, and H₂O).

In this study, we define chirvinskyite as a mineral with Fe dominant in the B site. However, large contents of Mn and Ca in some aggregates allows us to predict Mn-dominant and vacancy-dominant analogues of chirvinskyite, which await further investigation.