Shkatulkalite, a Rare Mineral from the Lovozero Massif, Kola Peninsula: A Re-Investigation

Abstract

The crystal structure of shkatulkalite has been solved from the crystal from the Lovozero alkaline massif, Kola Peninsula, Russia. The mineral is monoclinic, P2/m, a = 5.4638(19), b = 7.161(3), c = 15.573(6) Å, β = 95.750(9)°, V = 606.3(4) ų, R₁ = 0.080 for 1551 unique observed reflections. The crystal structure is based upon the HOH blocks consisting of one octahedral (O) sheet sandwiched between two heteropolyhedral (H) sheets. The blocks are parallel to the (001) plane and are separated from each other by the interlayer space occupied by Na1 atoms and H₂O groups. The Na2, Na3, and Ti sites are located within the O sheet. The general formula of shkatulkalite can be written as Na₅(Nb_1−xTi_x)₂(Ti_1−yMn²⁺_y)[Si₂O₇]₂O₂(OH)₂·nH₂O, where x + y = 0.5 and x ≈ y ≈ 0.25 for the sample studied. Shkatulkalite belongs to the seidozerite supergroup and is a member of the lamprophyllite group. The species most closely related to shkatulkalite are vuonnemite and epistolite. The close structural relations and the reported observations of pseudomorphs of shkatulkalite after vuonnemite suggest that, at least in some environments, shkatulkalite may form as a transformation mineral species.

1. Introduction

Titanosilicates constitute an important group of minerals that have found many applications as materials, including ion-exchange, sorption, catalysis, optics, biocide technologies, etc. [1,2,3,4,5]. Of particular interest are layered titanosilicates of the seidozerite supergroup that currently contains more than forty-five mineral species [6] with several new minerals described very recently [7,8,9,10,11,12,13]. These species occur mainly in alkaline massifs such as those in Kola Peninsula, Russia, above the Polar circle. Belov and Organova [14] were the first who considered the crystal chemistry of several minerals of the current murmanite group of seidozerite supergroup [6]. Belov [15] and Pyatenko et al. [16] made further generalizations and called minerals with a “seidozerite block” (=TS block, [17]) and astrophyllite-group minerals titanosilicate analogues of micas. The modular approach to these minerals has been developed by Egorov-Tismenko and Sokolova [18,19], who described a homologous series of Ti-analogues of micas, and Ferraris [20,21,22], who named those minerals heterophyllosilicates and described them as members of a single polysomatic series. Sokolova [17] quantitatively divided TS-block minerals into four groups based on the content of Ti, structural topology and stereochemistry of the TS block.

Shkatulkalite, Na₁₀MnTi₃Nb₃(Si₂O₇)₆(OH)₂F·12H₂O, was described by Menshikov et al. [23] from the pegmatite “Shkatulka” of the Lovozero alkaline massif, Kola Peninsula, Russia. The mineral was considered to be a Ti–Nb–sorosilicate of the “epistolite group” (now considered a part of the lamprophyllite group [24]). In their review on the seidozerite-supergroup minerals, Sokolova and Cámara [6] pointed out that shkatulkalite is a potential member of the supergroup, but the final assignment of the mineral to a particular group remained unclear, due to the fact that its crystal structure was unknown until now. Menshikov et al. [23] established that the mineral is monoclinic, a = 5.468(9), b = 7.18(1), c = 31.1(1) Å, β = 94.0(2)°, V = 1218(8) ų, Z = 1, and commented on the proximity of the a and b parameters of shkatulkalite to those typical for other known Ti and Nb sorosilicates. On the basis of systematic absences, the space groups Pm, P2, P2/m were proposed as possible for the mineral. However, due to the poor quality of single-crystal X-ray diffraction data, the structure of the mineral could not be solved at the time. Németh et al. [25] investigated syntactic intergrowths of epistolite, murmanite and shkatulkalite using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED), and reported on the absence of the l = 2n + 1 reflections for the latter mineral, pointing out that its c parameter is halved with respect to the value of 31.1 Å reported by Menshikov et al. [23]. Later, shkatulkalite was described in nepheline syenites of the alkaline sill of St. Amable, Quebec, Canada [26], as forming prismatic crystals and radial intergrowth of crystals in small miarolic voids as well as in a hydrothermal cavity in the southeastern part of the Demix quarry. The authors [26] noted that the shkatulkalite is visually indistinguishable from vuonnemite and epistolite found in the same voids. At the same time, Menshikov et al. [23] pointed out that shkatulkalite sometimes forms pseudomorphs after vuonnemite and thus can be considered as a transformation mineral species [27,28], that is, mineral species that forms as a result of a secondary transformation of a primary proto-phase. However, this hypothesis could not be confirmed until the crystal structure of the mineral is solved.

The aim of the present paper is to report the results of crystal-structure determination of shkatulkaite and to re-consider its status as both seidozerite-supergroup mineral and transformation mineral species.

2. Materials and Methods

2.1. Occurrence

The ultra-agpaitic pegmatite body “Shkatulka” located in the western part of the Alluaiv mountain of the Lovozero alkaline massif was discovered by underground excavations in 1990 [29]. Shkatulkalite was found in the marginal zone of the ussingite core of the pegmatite and in the adjacent aegirine zone [23]. The mineral was represented by three morphological varieties: (1) rectangular plates and tabular crystals; (2) aggregates of nacreous mica-like flakes; (3) partial pseudomorphs after vuonnemite. The first variety has the best quality of the material and was used as a holotype sample. In the present work we used material provided by the Museum of the Geological Institute of the Kola Science Center of the Russian Academy of Sciences, sample No. GIM 5968/2-1. It turned out that, in addition to the three types of shkatulkalite identified previously by Menshikov et al. [23], there is also fourth, which is not visually different from vuonnemite and only sometimes has a slightly lighter tone. Some areas of matte from the white or slightly yellowish to cream-colored large vuonnemite plates turned out to be shkatulkalite, and this material proved to be suitable for the single-crystal X-ray diffraction studies. The main difficulty of studying this material was in the preparation of samples for microprobe analysis, due to the poor polishability of its crystals.

2.2. Chemical Composition

The chemical composition of shkatulkalite was studied in three independent laboratories. Initially, the analyzes were performed at the Resource Center “Geomodel” of St. Petersburg State University using the AzTec Energy 350 energy dispersive attachment to the Hitachi S-3400N scanning electron microscope (Hitachi, Tokyo, Japan), operating at 20 kV, 20–30 nA, with a 5 μm beam diameter. The standards used were: lorenzenite (Na), periclase (Mg), diopside (Ca), quartz (Si), microcline (K), barite (Ba), rutile (Ti), rhodonite (Mn), corundum (Al), celestine (Sr), hematite (Fe), and Nb (Nb). Alternatively the chemical composition of shkatulkalite also was determined by the wavelength-dispersive spectrometry on a Cameca MS-46 electron microprobe (Geological Institute, Kola Science Centre, Russian Academy of Sciences, Apatity) operating at 22 kV (for Sr at 30 kV), 20–40 nA. Due to the easy dehydration of the mineral in a vaccum, and its deterioration under an electron beam, the measurements were carried out with a defocused beam up to 20 μm, while manually moving the sample. The standards used were: lorenzenite (NaKα, TiKα), wollastonite (CaKα, SiKα), orthoclase (KKα), synthetic MnCO₃ (MnKα), Y₃Al₅O₁₂ (AlKα), hematite (FeKα), apatite (PKα) and Nb (NbLα). The fluorine was determined using an Xflash-5010 Bruker Nano Gmbh energy dispersive X-ray spectrometer (Bruker, Bremen, Germany) mounted on a scanning electron microscope LEO-1450 at 20 kV and 0.5 nA. A non-standard procedure for the P/B-ZAF method of Quantax-200 was used.

Table 1. Chemical composition of shkatulkalite ^1.

Component	1		2		3		4
	Mean	Range	Mean	Range	Mean	Range	Mean	Range
Chemical Composition in wt %
Na2O	13.90	13.25–14.30	14.69	13.75–15.39	16.14	15.60–16.84	16.15	14.70–17.57
CaO	0.43	0.28–0.61	0.50	0.43–0.56	0.44	0.40–0.52	0.55	0.49–0.62
SrO	2.07	1.59–2.51	1.03	0.98–1.07	0.46	0.26–0.60	1.01	0.65–1.85
MnO	1.58	1.14–1.90	1.86	1.79–1.91	1.70	1.54–1.81	1.92	1.70–2.28
K2O	0.27	0.23–0.33	0.18	0.17–0.22	-	-	0.25	0.20–0.41
BaO	1.08	0.82–1.38	n.d.	n.d.	-	-	0.06	0.01–0.29
FeO	n.d.	n.d.	0.08	0.06–0.10	-	-	0.12	0.02–0.19
Fe2O3	-	-	-	-	0.07	0.05–0.08	-	-
Al2O3	n.d.	n.d.	0.13	0.11–0.16	0.24	0.21–0.28	0.16	0.08–0.62
SiO2	31.52	31.24–32.07	32.24	31.59–32.60	35.70	35.58–35.84	32.71	31.65–33.74
TiO2	10.68	10.04–11.41	11.59	11.51–11.64	11.12	11.03–11.23	10.87	10.60–11.19
Nb2O5	22.72	21.49–23.92	22.55	21.89–23.05	21.93	21.72–22.22	22.65	21.30–24.14
P2O5	n.d.	n.d.	0.32	0.26–0.38	-	-	0.43	0.21–0.98
F	not measured		0.96	0.77–1.17	0.94	-	1.51	0.77–1.83
H2O	15.75 2	-	14.67 2	-	11.66 2	-	12.25	-
–O=F2	-	-	0.40	-	0.40	-	0.64	-
Total	100.00	-	100.00	-	100.00	-	100.00	-
Chemical Composition in Atoms per Formula Unit (apfu)
Basis	Si = 4		Si + Al = 4		Si + Al + Fe = 4		Si + Al + Fe = 4
Na	3.42	-	3.52	-	3.47	-	3.80	-
Ca	0.06	-	0.07	-	0.05	-	0.07	-
Sr	0.15	-	0.07	-	0.03	-	0.07	-
Mn	0.17	-	0.19	-	0.16	-	0.20	-
K	0.06	-	0.03	-	-	-	0.04	-
Ba	0.05	-	-	-	-	-	-	-
Fe	-	-	0.01	-	0.01	-	0.01	-
Al	-	-	0.02	-	0.03	-	0.02	-
Si	4.00	-	3.98	-	3.96	-	3.97	-
Ti	1.02	-	1.08	-	0.93	-	0.99	-
Nb	1.30	-	1.26	-	1.10	-	1.24	-
P	-	-	0.03	-	-	-	0.04	-
F	-	-	0.37	-	0.33	-	0.58	-
H	13.34	-	12.09	-	8.64	-	9.91	-

¹1–3—this work (1—Hitachi S-3400N; 2—Cameca MS-46, 3—JEOL 733); 4—Menshikov et al. [23]; ² calculated by difference to 100%.

provides the summary of analytical results.

The chemical composition of shkatulkalite also was studied by the wavelength-dispersive spectrometry on a JEOL Superprobe 733 electron microprobe (Fersman Geological Museum, Russian Academy of Sciences, Moscow) operating at 15 kV and 15 nA. Due to the easy dehydration of the mineral in a vaccum and its deterioration under an electron beam, the measurements were carried out with a defocused beam up to 20 μm. The standards used were: chkalovite (NaKα), diopside (CaKα), microcline (KKα), tephroite (MnKα), almandine (FeKα, SiKα, AlKα), synthetic AlPO₄ (PKα), synthetic BaSO₄ (BaKα), synthetic SrTiO₃ (SrKα, TiKα), fluorphlogopite (FKα), and synthetic Cs₂Nb₄O₁₁ for Nb (NbLα). It was observed that shkatulkalite contains rutile inclusions oriented parallel to the (001) plane (Figure 1a). Shkatulkalite shows significant variations of different elements (Figure 1b–f). No elements other than those mentioned above were detected.

The resulting empirical formulae obtained in our study can be written as (Na_3.42Sr_0.15Ca_0.06K_0.06Ba_0.05)_3.84(Nb_1.30Ti_1.02Mn²⁺_0.17)_2.49(Si₂O₇)₂O_0.47(OH)₂·5.67H₂O (for sample 1), (Na_3.52Sr_0.07Ca_0.07K_0.03)_3.69(Nb_1.26Ti_1.08Mn²⁺_0.19)_2.53(Si_1.99Al_0.01O₇)₂[(OH)_1.63F_0.37]O_0.44·5.23H₂O (for sample 2) and (Na_3.47Sr_0.03Ca_0.05)_3.69(Nb_1.10Ti_0.93Mn²⁺_0.16)_2.53(Si_1.98Al_0.02O₇)₂[(OH)_0.80.F_0.33]_1.13·3.49H₂O (for sample 3). These results are in very general agreement with the results of the crystal-structure study discussed below, taken into account the high instability of the mineral under electron beam.

2.3. Single-Crystal X-ray Diffraction

Single-crystal X-ray diffraction study of shkatulkalite was performed at the Resource Center “X-ray Diffraction Methods” of St. Petersburg State University using Bruker Kappa APEX DUO diffractometer operated at 45 kV and 0.6 mA and equipped with a CCD (charge-coupled device) area detector. The study was done by means of a monochromatic MoKα X-radiation (λ = 0.71073 Å), frame widths of 0.5° in ω and 30 s counting time for each frame. The intensity data were reduced and corrected for Lorentz, polarization and background effects using the Bruker software APEX2 [30]. A semiempirical absorption-correction based upon the intensities of equivalent reflections was applied (SADABS [31]). The unit-cell parameters were refined by least square techniques using 4463 reflections. In general, the unit-cell parameters obtained in this study are in agreement with those reported by Menshikov et al. [23], but with the c parameter halved (15.573 instead of 31.1 Å), thus confirming the observations by Németh et al. [25] (see above). The structure was solved and refined in space group P2/m to R₁ = 0.080 (wR₂ = 0.195) for 1378 unique observed reflections using ShelX program package [32] within the Olex2 shell [33]. Crystal data, data collection and structure refinement details are given in

Table 2. Crystal data and structure refinement for shkatulkalite.

Crystal System	Monoclinic
Space group	P2/m
a, Å	5.4638(19)
b, Å	7.161(3)
c, Å	15.573(6)
β, °	95.750(9)
V, Å3	606.3(4)
Z	1
ρcalc, g/cm3	2.370
μ, mm−1	1.988
Crystal dimensions, mm	0.13 × 0.10 × 0.05
F(000)	418.0
Radiation	MoKα (λ = 0.71073)
2Θ range, deg.	2.63–56.00
Index ranges	−7 ≤ h ≤ 7, −8 ≤ k ≤ 9, −17 ≤ l ≤ 20
Reflections collected	4463
Independent reflections	1551 [Rint = 0.0550, Rsigma = 0.0652]
Data/restraints/parameters	1551/0/126
GOF (goodness-of-fit)	1.202
Final R indexes [I ≥ 2σ(I)]	R1 = 0.0801, wR2 = 0.1947
Final R indexes [all data]	R1 = 0.0892, wR2 = 0.1997
Largest diff. peak/hole/e−Å−3	1.88/−1.16

; atom coordinates, occupancies and displacement parameters in

Table 3. Atomic coordinates, experimental and calculated site-scattering factors (SSF_exp and SSF_calc, respectively) and isotropic displacement parameters (Ų) for shkatulkalite.

Atom	x	y	z	Occupancy	SSFexp [e−]	SSFcalc [e−]	Ueq
Nb	0.5901(2)	0	0.30252(7)	Nb0.75Ti0.25	36.3(7)	36.25	0.0102(4)
Ti	0	½	½	Ti0.77Mn0.20□0.03	22.0(4)	22.00	0.027(1)
Si	0.0947(3)	0.7115(3)	0.3227(1)	Si	14.0(2)	14.00	0.0117(6)
Na1	0.5803(8)	½	0.2572(7)	Na0.60□0.25Sr0.07 Ba0.06K0.02	13.0(3)	13.00	0.079(4)
Na2	0.5	0.7437(6)	½	Na0.87Mn0.07Ca0.06	12.5(2)	12.52	0.019(2)
Na3	0	0	½	Na	11.0(3)	11.00	0.018(1)
O1	0.331(1)	0.8064(9)	0.2873(4)	O	8.0(1)	8.00	0.024(1)
O2	0.085(2)	½	0.2832(6)	O	8.0(1)	8.00	0.021(2)
O3	0.841(1)	0.8087(9)	0.2841(4)	O	8.0(1)	8.00	0.023(1)
O4	0.1206(9)	0.7024(7)	0.4268(3)	O	8.0(1)	8.00	0.013(1)
O5	0.627(1)	0	0.4167(5)	O	8.0(1)	8.00	0.017(2)
X6	0.684(1)	½	0.4282(5)	(OH)0.82F0.18	8.0(1)	8.18	0.018(2)
Ow1	0.542(2)	0	0.1551(7)	(H2O)0.94	7.5(3)	7.52	0.040(3)
Ow2	0.536(7)	0.410(5)	0.134(2)	(H2O)0.37	2.7(2)	2.96	0.09(1)
Ow3	0.044(3)	0	0.158(1)	(H2O)0.92	7.4(4)	7.36	0.085(8)
Ow4	½	½	0	(H2O)0.19	1.5(5)	1.52	0.04(3)
Ow5	0.771(8)	½	0.001(3)	(H2O)0.24	1.9(3)	1.92	0.04(2)
Ow6	0.019(5)	0.380(4)	0.058(2)	(H2O)0.14	1.1(1)	1.12	0.003(9)
Ow7	0	0.259(7)	0	(H2O)0.31	2.5(4)	2.48	0.06(2)
Ow8	0.65(1)	0	0.001(5)	(H2O)0.25	2.0(4)	2.00	0.09(3)

and

Table 4. Anisotropic displacement atom parameters for shkatulkalite (Ų).

Atom	U11	U22	U33	U23	U13	U12
Nb	0.0033(5)	0.0060(6)	0.0213(7)	0	0.0004(3)	0
Ti	0.044(2)	0.005(2)	0.038(2)	0	0.029(2)	0
Si	0.0078(9)	0.006(1)	0.021(1)	−0.0003(7)	0.0007(7)	0.0008(7)
Na1	0.012(2)	0.006(3)	0.22(1)	0	0.027(3)	0
Na2	0.009(2)	0.016(2)	0.032(3)	0	−0.004(1)	0
Na3	0.010(3)	0.019(3)	0.024(3)	0	−0.006(2)	0
O1	0.018(3)	0.025(3)	0.032(3)	−0.004(3)	0.008(2)	−0.014(2)
O2	0.029(4)	0.005(4)	0.029(5)	0	0.001(3)	0
O3	0.020(3)	0.022(3)	0.028(3)	0	0.001(2)	0.012(2)
O4	0.012(2)	0.009(3)	0.020(3)	−0.001(2)	0.002(2)	−0.001(2)
O5	0.011(3)	0.016(4)	0.024(4)	0	0.001(3)	0
X6	0.016(4)	0.014(4)	0.025(4)	0	0.006(3)	0
Ow1	0.039(6)	0.048(7)	0.033(6)	0	0.007(4)	0
Ow2	0.11(3)	0.09(3)	0.05(2)	−0.03(2)	0.01(2)	0
Ow3	0.041(9)	0.07(1)	0.15(2)	0	0.02(1)	0

, selected interatomic distances in

Table 5. Selected bond lengths (Å) in the crystal structure of shkatulkalite.

Nb–O1 4,18	1.980(6)	Si1–O1	1.603(6)	Na1–Ow2 9,18	2.01(3)
Nb–Ow1	2.29(1)	Si1–O2	1.634(4)	Na1–O3 9,18	2.641(7)
Nb–O3 2,5	1.979(6)	Si1–O3	1.612(6)	Na1–O1 7,10	2.650(7)
Nb–O5	1.770(8)	Si1–O4	1.615(6)	Na1–X6	2.67(1)
<Nb–O>	1.995	<Si–O>	1.616	Na1–O2 7,18	2.774(9)
				<Na1–X,O>	2.534
Ti–O4 8,9,12,18	1.996(5)	Na2–O4 6,18	2.282(5)
Ti–X64,18	1.959(8)	Na2–X6 2,8	2.353(6)	Na3–O5 13,18	2.305(8)
<Ti–O,X>	1.984	Na2–O5 1,18	2.389(6)	Na3–O4 1,2	2.535(5)
		<Na2–O,X>	2.342	Na3–O45,6	2.535(5)
				<Na3–O>	2.458

¹ 1 − x, 2 − y, 1 − z; ² 1 + x, +y, +z; ³ 1 + x,1 + y, +z; ⁴ +x, 2 − y, +z; ⁵ 1 + x, 2 − y, +z; ⁶ 1 − x, +y, 1 − z; ⁷ −1 + x, +y, +z; ⁸ −x,1 − y, 1 − z; ⁹ +x, 1 − y, +z; ¹⁰ −1 + x, 1 − y, +z; ¹¹ −1 + x,−1 + y, +z; ¹² −x, +y, 1 − z; ¹³ 2 − x, 2 − y,1 − z; ¹⁴ − 1 − x, 1 − y, −z; ¹⁵ −x, 1 − y, −z; ¹⁶ −x, +y, −z; ¹⁷ −1 − x, −y, −z; ¹⁸ x, y, z.

. Occupancies of the cation sites were calculated from the experimental site-scattering factors taking into account empirical formulae.

Table 6. Bond-valence analysis (v.u. = valence units) for shkatulkalite.

Atom	Nb	Ti	Si	Na1	Na2	Na3	Total
O1	0.83↓ × 2		1.06	0.10			1.99
O2			0.97→ × 2	0.07, 0.08			2.09
O3	0.83↓ × 2		1.03	0.10↓ × 2			1.96
O4		0.61↓ × 4	1.03		0.27↓ × 2	0.14↓ × 4	2.05
O5	1.47				0.20↓→ × 2	0.26↓ × 2	2.13
X6		0.68↓ × 2		0.10	0.22↓→ × 2		1.22
O1w	0.36						0.36
O2w				0.56↓ × 2			0.56
Total	5.15	3.81	4.09	1.77	1.40	1.06

provides the results of bond-valence analysis with bond-valence parameters taken from [34].

3. Results

The crystal structure of shkatulkalite is based upon the HOH blocks consisting of one octahedral (O) sheet sandwiched between two heteropolyhedral (H) sheets (Figure 2a). According to Sokolova [17], the HOH blocks are of the type III with one Ti apfu present in the O sheet (Figure 2b). The blocks are parallel to (001) and are separated from each other (Figure 3a) with interlayer space occupied by Na1 atoms and H₂O groups. The Na2, Na3, and Ti sites are located within the O sheet. The H sheets are formed by NbO₆ octahedra and Si₂O₇ groups sharing common O atoms.

The Na1 site is coordinated by eight anions (taking into account the disorder observed for the O_w2 site with adjacent O_w2 sites located at 1.318 Å) and is located approximately in the center of a six-membered ring in the H layer (Figure 4). The relatively high coordination number (8) and the long average <Na1–O> bond length of 2.534 Å indicate the capability of this site to accumulate large cations such as K⁺, Sr²⁺ and Ba²⁺ present in shkatulkalite. The structure refinement indicated the presence of rather short Na1–O_w2 contact of 2.01 Å, which we explain by the static disorder observed for both Na1 and O_w2 sites (note also the high values of their atomic displacement parameters). The Na2 and Na3 sites are octahedrally coordinated by six anions each. The Na2O₆ octahedron is more compact, with the mean <Na2–O> bond length of 2.342 Å, the bond-valence sum (BVS) of 1.40 valence units (v.u.) and the site-scattering factor (SSF) of 12.54 e⁻ all pointing out that, in addition to Na⁺, this site also incorporates Ca²⁺ and Mn²⁺ cations (Table 3). The refinement of the SSF of the Na3 site is consistent with its full occupancy by Na⁺ cations, which is also confirmed by its BVS (1.06 v.u.) and the <Na3–O> bond length of 2.458 Å.

The Nb and Ti sites are both octahedrally coordinated by O atoms. The Nb site contains significant amount of Ti (its refined SSF corresponds to the composition Nb_0.75Ti_0.25). The NbO₆ octahedron is essentially distorted with one short (1.770 Å) Nb–O5 bond opposite to one long Nb–O_w1 (2.29 Å) bond, and four intermediate (~1.98 Å) Nb–O bonds. This kind of octahedral distortion is typical for NbO₆ octahedra in H sheets and was observed, for instance, in vuonnemite, Na₁₁TiNb₂(Si₂O₇)₂(PO₄)₂O₃F [35,36], and epistolite, Na₄TiNb₂(Si₂O₇)₂O₂(OH)₂(H₂O)₄ [37], two minerals most closely related to shkatulkalite (see below). The refined SSF of the Ti site is close to 22 e⁻, which would account for the full occupancy of this site by Ti. However, this would contradict the predominance of Nb over Ti observed in the chemical analyses (Table 1), which prompted us to suggest that the Ti site also accommodates Mn²⁺ cations present in shkatulkalite. It is rather common for divalent cations and, in particular, Mn²⁺ to substitute for Ti in the octahedral sites of the O sheet in heterophyllosilicates. Such a substitution was reported, for instance, for sobolevite, Na₁₃Ca₂Mn₂Ti₃(Si₂O₇)₂(PO₄)₄O₃F₃ [38,39].

The BVS for the X6 site in shkatulkalite is 1.22 v.u., which is compatible with its occupancy by (OH)⁻ or F⁻ anions. The (OH)_0.82F_0.18 assigned to this site in Table 3 was calculated to conform with the results of the chemical analyses that demonstrate the presence in shkatulkalite of 0.36 F apfu.

The interlayer between the adjacent HOH blocks in shkatulkalite is occupied by a number of partially occupied O_w sites that belong to H₂O molecules. The O_w1 atom is bonded to the Nb site, forming a long apical Nb-H₂O bond in the NbO₆ octahedra. The O_w2 site is linked to the Na1 site and is split into two sites, with the total occupancy of 68%. The O_w3–O_w8 sites are purely interlayer with the occupancies in the range from 19% to 92%. However, there are several H₂O positions that are mutually excluding (O_w4–O_w5, O_w6–O_w7, etc.). The maximum amount of H₂O in the formula considering all incompatible sites is 10 H₂O per formula unit. Therefore, the amount of H₂O molecules in the ideal formula can be written as n, where n ≤ 10. It seems that the cohesion among different TS-blocks is ensured through hydrogen bonding between the H₂O molecule of the O_w1 site (apical anion of NbO₆ octahedra) and the H₂O molecule of the O_w8 site. However, no precise picture of the hydrogen bonding in shkatulkalite can be derived, owing to the high degree of disorder observed for the interlayer sites.

According to the crystal-structure refinement, the crystal-chemical formula of shkatulkalite can be written as [Na_3.94☐_0.50Sr_0.14Mn_0.14Ba_0.12Ca_0.12K_0.04][Nb_1.50Ti_1.27Mn²⁺_0.20☐_0.03][Si₂O₇]₂O₂ [(OH)_0.82F_0.18]₂·7.43H₂O, which is in very general agreement with the empirical chemical formula given above, taking into account the difficulties associated with the study of the mineral by the electron-microprobe analysis. It is noteworthy that, according to the chemical analyses, the H₂O content in shkatulkalite ranges from 4.32 to 6.67 apfu. Thus, by analogy to selivanovaite and murmanite [11,25] the interlayer H₂O content in shkatulkalite is variable and may be defined as nH₂O. Taking into account the cations prevalent at different atomic sites in shkatulkalite, its ideal chemical formula should be written as Na₅Nb₂Ti[Si₂O₇]₂O₂(OH)₂]·nH₂O, where n ≤ 10, which is not electroneutral and contains one extra positive charge. Therefore, we assume that the incorporations of Ti into Nb and Mn into Ti sites are important for charge-balance considerations, and suggest the general formula should be written as Na₅(Nb_1−xTi_x)₂(Ti_1−yMn²⁺_y)[Si₂O₇]₂O₂(OH)₂·nH₂O, where x + y = 0.5, n ≤ 10. For the sample of shkatulkalite under investigation x ≈ y ≈ 0.25. For the two critical cases of x = 0.5 or y = 0.5, the end-member formulae would be Na₅(NbTi)Ti[Si₂O₇]₂O₂(OH)₂·nH₂O and Na₅Nb₂(Ti_0.5Mn²⁺_0.5)[Si₂O₇]₂O₂(OH)₂·nH₂O (or Na₁₀Nb₄TiMn²⁺[Si₂O₇]₄O₄(OH)₄·nH₂O), n ≤ 10, respectively.

4. Discussion

The structure of shkatulkalite represents the basic structure type B5(GIII), according to Sokolova and Cámara [40] that was inferred by inverse prediction. In fact, Sokolova and Cámara [40] introduced the concept of basic and derivative structures for TS-block minerals and stated that a derivative structure is related to two or more basic structures of the same group. Hence a derivative structure can be built by adding basic structures via sharing the central O sheet of the TS blocks of adjacent structural fragments. The inverse prediction of the structure of shkatulkalite to B5(GIII) is now completely confirmed with the present results. Incidentally, the ideal formula of B5(GII) predicted by Sokolova and Cámara [40] is □₂Nb₂Na₂M²⁺Ti (Si₂O₇)₂O₂(OH)₂(H₂O)₈ (with M²⁺ = Mn, Ca), while the proposed ideal formula by us for shkatulkalite is Na₅(Nb_1−xTi_x)₂(Ti_1−yMn²⁺_y)[Si₂O₇]₂O₂(OH)₂·nH₂O, where x + y = 0.5 and n ≤ 10, which is a remarkable agreement. It is worth noting that Cámara et al. [41] have already confirmed the right prediction of B7(GIV) structure type with the crystal structure of kolskyite.

In agreement with the suggestions by Németh et al. [25] and Sokolova and Cámara [6], shkatulkalite belongs to the seidozerite supergroup, since its structure is based upon the HOH [22] or TS [17] blocks. More precisely, shatulkalite is a member of the lamprophyllite group, having Ti + (Nb + Mn) = 3 apfu, from which the H and O sheets has 2 and 1 (Ti + Nb + Mn) apfu, respectively. The most closely related species to shkatulkalite are vuonnemite, Na₁₁TiNb₂(Si₂O₇)₂(PO₄)₂O₃F [35,36], and epistolite, Na₄TiNb₂(Si₂O₇)₂O₂(OH)₂(H₂O)₄ [37,38], crystal structures of which are shown in Figure 3b,c, respectively. The three minerals are based upon the same topological type of the HOH blocks (Figure 2b–d) with different layer symmetries (p2/m in shkatulkalite, and p − 1 in vuonnemite and epistolite). In all three minerals, the HOH blocks are parallel to (001), but the c parameters are different and equal to 15.573, 14.450 and 12.041 Å for shkatulkalite, vuonnemite and epistolite, respectively. In fact, both shkatulkalite and epistolite can be considered derivatives of vuonnemite and can be obtained from the latter at least through the gedanken experiment by removing the some Na⁺ and all (PO₄)³⁻ ions and subsequent hydration of the interlayer space. The hypothesis that shkatulkalite is a transformation mineral species that forms at the expense of vuonnemite (or some vuonnemite-related proto-mineral) seems quite reasonable, taking into account its high hydration state, which results in the very open packing of adjacent HOH blocks that is manifested in the large value of the c parameter and can be clearly seen in Figure 3a. In contrast, in the crystal structure of epistolite (Figure 3b), the packing of the HOH blocks is quite dense, resulting in the shrinkage of the c parameter. The syntactic intergrowths of shkatulkalite and epistolite reported by Németh et al. [25] may point out to the following growth scenarios: (1) both minerals are primary phases that crystallize simultaneously; (2) both minerals are transformation species that form according to the sequence “vuonnemite (or vuonnemite-related proto-mineral) → shkatulkalite → epistolite”; or (3) both minerals are transformation species that form along two different pathways, “vuonnemite → shkatulkalite” and “vuonnemite → epistolite”. The reported observations of pseudomorphs of shkatulkalite after vuonnemite suggest that, at least in some environments, shkatulkalite is indeed a transformation mineral species that inherits basic structural features from vuonnemite in accordance with Khomyakov’s structural inheritance principle [42]. The secondary nature and the status of shkatulkalite as a transformation mineral species may also account for the difficulties encountered when investigating the mineral by means of electron microprobe and crystal-structure analysis. The absence of precise agreement between the chemical and structural studies (the obvious cation deficiency observed in three series of independent chemical analyses and in the original report [23]) remains an issue that we cannot resolve at the present time.