General information on the seidozerite supergroup

There are currently fifty-two TS-block minerals (TS = Titanium Silicate) in the seidozerite supergroup (Memorandum 56-SM/16, Sokolova and Cámara, Reference Sokolova and Cámara2017), forty-five of which are listed in Sokolova and Cámara (Reference Sokolova and Cámara2017); since then, surkhobite has been discredited (IMA 20-A, Sokolova et al., Reference Sokolova, Day, Hawthorne, Agakhanov, Cámara, Uvarova and Della Ventura2021), and eight new mineral species have been added: shkatulkalite, Na₂Nb₂Na₃Ti(Si₂O₇)₂O₂(FO)(H₂O)₄(H₂O)₃ (Men’shikov et al., Reference Men’shikov, Khomyakov, Polezhaeva and Rastsvetaeva1996, Zolotarev et al., Reference Zolotarev, Selivanova, Krivovichev, Savchenko, Panikorovskii, Lyalina, Pautov and Yakovenchuk2018, Sokolova et al., Reference Sokolova, Day, Hawthorne and Cámara2022a), selivanovaite, Fe³⁺□Ti₂Na□Ti₂(Si₂O₇)₂O₄(H₂O)₄ (Pakhomovsky et al., Reference Pakhomovsky, Panikorovskii, Yakovenchuk, Ivanyuk, Mikhailova, Krivovichev, Bocharov and Kalashnikov2018; Sokolova et al., Reference Sokolova, Day, Hawthorne and Cámara2022b), rinkite-(Y), Ca₂(CaY)Na(NaCa)Ti(Si₂O₇)₂(OF)F₂ (Pautov et al., Reference Pautov, Agakhanov, Karpenko, Uvarova, Sokolova and Hawthorne2019), fluorbarytolamprophyllite, (BaK)Ti₂Na₃Ti(Si₂O₇)₂O₂F₂ (Filina et al., Reference Filina, Aksenov, Sorokhtina, Chukanov, Kononkova, Belakovskiy, Britvin, Kogarko, Chervonnyi and Rastsvetaeva2019), bortolanite, Ca₂(Ca_1.5Zr_0.5)Na(NaCa)Ti(Si₂O₇)₂(OF)F₂ (Day et al., Reference Day, Sokolova, Hawthorne, Horváth and Pfenninger-Horváth2022), nacareniobsite-(Y), Ca₂(CaY)Na₃Nb(Si₂O₇)₂(OF)F₂ (Agakhanov et al., Reference Agakhanov, Day, Sokolova, Karpenko, Hawthorne, Pautov, Pekov, Kasatkin and Agakhanova2023), nacareniobsite-(Nd), Ca₂(CaNd)Na₃Nb(Si₂O₇)₂(OF)F₂ (Agakhanov et al., Reference Agakhanov, Day, Sokolova, Karpenko, Hawthorne, Pautov, Kasatkin, Pekov, Belakovskiy and Britvin2025) and ferroinnelite (IMA2024-029).

The TS block is the main structural unit in all seidozerite-supergroup structures; it consists of a central O (Octahedral) sheet and two adjacent H (Heteropolyhedral) sheets; Si₂O₇ groups occur in the H sheets. The TS block is characterised by a planar minimal cell based on translation vectors, t₁ and t₂, the lengths of which are t ₁ ≈ 5.5 and t ₂ ≈ 7 Å, and t₁ ^ t₂ is close to 90°. Minerals of the seidozerite supergroup are divided into four groups based on the content of Ti and the topology and stereochemistry of the TS block: in the rinkite, bafertisite, lamprophyllite and murmanite groups, Ti (+ Nb + Zr + Fe³⁺ + Mg + Mn) = 1, 2, 3 and 4 apfu (atoms per formula unit) per (Si₂O₇)₂, respectively (Sokolova, Reference Sokolova2006; Sokolova and Cámara, Reference Sokolova and Cámara2013, Reference Sokolova and Cámara2018). The seidozerite supergroup is based on a quantitative classification as minerals of each group have a fixed content of Ti (+ Nb + Zr + Fe³⁺ + Mg + Mn). Each group of minerals has a TS block with a different (1) content of Ti (+ Nb + Zr + Fe³⁺ + Mg + Mn), (2) topology defined by the type of linkage of H and O sheets, and (3) stereochemistry of Ti (+ Nb + Zr + Fe³⁺ + Mg + Mn) (Sokolova, Reference Sokolova2006). There are three topologically distinct TS blocks based on three types of linkage of two H sheets and the central O sheet (Sokolova, Reference Sokolova2006). Linkage 1 occurs where two H sheets connect to the O sheet such that two Si₂O₇ groups on opposite sides of the O sheet link to trans edges of the same polyhedron of the O sheet. Linkage 2 occurs where two Si₂O₇ groups link to two octahedra of the O sheet adjacent along t₂. Linkage 3 occurs where two Si₂O₇ groups link to two octahedra adjacent approximately along t₁. All TS-block structures consist either solely of TS blocks or of two types of block: the TS block and an I (Intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, H₂O groups and oxyanions (PO₄)^3–, (SO₄)^2– and (CO₃)^2–. The general formula of the TS block is as follows: A^P ₂B^P ₂M^H₂M^O₄(Si₂O₇)₂X_4+n, where M^H₂ and M^O₄ are cations of the H and O sheets; M^H = Ti, Nb, Zr, Y, Mn, Ca + REE and Ca; M^O = Ti, Zr, Nb, Fe³⁺, Fe²⁺, Mg, Mn, Zn, Ca and Na; A^P and B^P are cations at the peripheral (P) sites = Na, Ca + REE, Ca, Zn, Ba, Sr and K; X are anions = O, OH, F and H₂O; X_4+n = X^O₄ + X^P _n, n = 0, 1, 1.5, 2 and 4; X^P = X^P _M and X^P _A = apical anions of the M^H and A^P cations at the periphery of the TS block. There are four types of self-linkage between adjacent TS blocks (Sokolova, Reference Sokolova2006): [1] TS blocks link directly through common edges of M^H and A^P polyhedra, and common vertices of M^H, A^P and Si polyhedra of the H sheets belonging to two TS blocks. [2] TS blocks link through common vertices of Ti(+ Nb) octahedra, and the I block has one layer of cations (m = 1). [3] TS blocks do not link directly, additional cations do not occur in the I space (m = 0), and TS blocks are connected through hydrogen bonds of H₂O groups at the X^P sites. [4] TS blocks do not link directly, and there are additional layers of cations in the I block (m = 1–6).

Sokolova and Cámara (Reference Sokolova and Cámara2013) introduced the concept of basic and derivative structures for TS-block minerals. A basic structure has the following four characteristics: (1) There is only one type of TS block. (2) The two H sheets of the TS block are identical. (3) There is only one type of I block or it is absent. (4) There is only one type of self-linkage of TS blocks. Basic structures obey the general structural principles of Sokolova (Reference Sokolova2006). A derivative structure has one or more of the three following characteristics: (1) There is more than one type of TS block. (2) There is more than one type of I block. (3) There is more than one type of self-linkage of TS blocks. A derivative structure is related to two or more basic structures of the same group: it can be derived by adding these structures via sharing of the central O sheet of the TS blocks of adjacent structural fragments which represent basic structures. There are five seidozerite-supergroup minerals with derivative structures, four of them occur in the lamprophyllite group.

In the lamprophyllite group, the TS block is characterised by Ti (+ Nb + Fe³⁺ + Mg) = 3 apfu; Ti occurs in the H sheets (2 apfu) and the O sheet (1 apfu). Linkage 1 occurs: Si₂O₇ groups of the two H sheets link to trans edges of the Ti octahedron of the O sheet. In the crystal structures of the lamprophyllite-group minerals, TS blocks either alternate with I blocks, self-linkage [4] occurs or connect via hydrogen bonding between H₂O groups at the X ^P sites, self-linkage [3] occurs.

Previous work on innelite

Kravchenko et al. (Reference Kravchenko, Vlasova, Kazakova, Ilyukhin and Abrashev1961) described innelite, (Na,Ca,Mg,Fe³⁺,Fe²⁺)₃(Ba,K,Mn)₄(Ti,Al)₃Si₄O₂₄ (OH,F)_1.5·1.15S, from the Inagli massif, Yakutia, Russia. They reported triclinic symmetry, a chemical analysis (they did not analyse innelite for phosphorous) and deposited the holotype sample at the Fersman Mineralogical Museum, Moscow, Russia (sample #81801, collections of S.M. Kravchenko and A.F. Efimov).

Chernov et al. (Reference Chernov, Ilyukhin, Maksimov and Belov1971) solved the crystal structure of innelite on a single crystal from the holotype sample #81801, described the main features of the structure topology and wrote its ideal formula as Na₂Ba₃(Ba,K,Mn)(Ca,Na)Ti(TiO₂)₂ [Si₂O₇]₂(SO₄)₂. Based on the structure work of Chernov et al. (Reference Chernov, Ilyukhin, Maksimov and Belov1971), Sokolova (Reference Sokolova2006) considered innelite a TS-block mineral and wrote its ideal formula as Na₂CaBa₄Ti₃[Si₂O₇]₂(SO₄)₂O₄.

Pekov et al. (Reference Pekov, Chukanov, Kulikova and Belakovsky2006) reported a chemical composition for innelite from the Inagli and Kovdor massifs, emphasising that innelite contains P (with S > P) and wrote an ‘ideal’ formula of innelite as Ba₄(Na,Ca)₃Ti₃Si₄O₁₄(SO₄)₂(O,OH,F)₄; however, they did not list H₂O in the chemical analyses and did not include (PO₄) in the formula.

In 2008, we obtained a fragment of the holotype sample of innelite (#81801, Inagli, Yakutia, Russia, Fersman Mineralogical Museum) for a comprehensive study using single-crystal X-ray diffraction, electron microprobe analysis (EMPA) and high-resolution transmission electron microscopy (HRTEM). Transmission electron microscopy showed that the sample is an intergrowth of two innelite polytypes, and the crystal structures of innelite-1T (space group P $\bar 1$) and innelite-2M (space group P2/c; c2_M = 2c1_T) were solved and refined (Sokolova et al., Reference Sokolova, Cámara and Hawthorne2011). The crystal structure of innelite is a combination of a TS block which consists of HOH sheets and an I block. The I block contains T sites, statistically occupied by S and P, and Ba atoms. Innelite-1T is innelite as described by Kravchenko et al. (Reference Kravchenko, Vlasova, Kazakova, Ilyukhin and Abrashev1961) and Chernov et al. (Reference Chernov, Ilyukhin, Maksimov and Belov1971). Sokolova et al. (Reference Sokolova, Cámara and Hawthorne2011) wrote the ideal structural formula of innelite as Ba₄Ti₂ Na(NaM²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)], M²⁺ = Mn > Fe > Mg > Ca.

Sokolova and Cámara (Reference Sokolova and Cámara2017) considered innelite a lamprophyllite-group (seidozerite-supergroup) TS-block mineral, redefined its formula based on the work of Sokolova et al. (Reference Sokolova, Cámara and Hawthorne2011) and renamed innelite-1T as innelite-1A (Memorandum 56-SM/16).

Later on, in the IMA list of minerals, the redefined formula of innelite was changed to Ba₄Ti₂ Na(NaCa)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)]. Recently, we pointed out that Mn²⁺ is the dominant divalent cation at the M ^O3 site, not Ca. Thus, the formula of innelite was corrected to Ba₄Ti₂Na(NaMn²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)] (Bosi et al., Reference Bosi, Hatert, Pasero and Mills2024).

Occurrence and associated minerals

As we found ferroinnelite in a sample of ‘phosphoinnelite’ (see above), information on occurrence, associated minerals and origin of ferroinnelite is taken from Pekov et al. (Reference Pekov, Chukanov, Kulikova and Belakovsky2006). Ferroinnelite occurs in an agpaitic pegmatite vein cross-cutting calcite carbonatite at the Phlogopite deposit, Kovdor alkaline massif, Kola Peninsula, Russia (67°35′21″N and 30°28′4″E). Associated minerals are cancrinite (partly altered to thomsonite-Ca), orthoclase, aegirine–augite, magnesio-arfvedsonite, golyshevite and fluorapatite. The mineral formed in a pegmatite as a result of hydrothermal activity.

Physical properties

Ferroinnelite occurs as transparent elongated platy to tabular crystals up to 0.15 mm long. Figure 1 shows a single crystal of ferroinnelite (0.140 × 0.070 × 0.035 mm) on a glass fibre. Ferroinnelite is yellow to yellow brown; very small grains are pale yellow. The mineral has a white streak and a vitreous lustre. Ferroinnelite does not fluoresce under cathode rays or ultraviolet light. The mineral is brittle, cleavage is {001} medium, the fracture is uneven; no parting was observed. Mohs hardness is ∼5 by analogy with innelite [4.75] (Kravchenko et al., Reference Kravchenko, Vlasova, Kazakova, Ilyukhin and Abrashev1961). The calculated density, D _calc., is 4.088 g/cm³.

Figure 1. Pale brownish-yellow crystal of ferroinnelite (0.140 × 0.070 × 0.035 mm) on a glass fibre. This crystal was used for IR-spectroscopy at INFN-Laboratori Nazionali di Frascati (Roma), Italy.

Ferroinnelite is biaxial (+). The calculated mean refractive index of ferroinnelite is <n> = 1.823. Canadian safety regulations do not allow use of refractive-index liquids of 1.800 and above and so we cannot measure them. The optic angle, 2V = 87(2)°, was measured on a spindle stage using the method of extinction angles. Ferroinnelite is slightly pleochroic, very pale yellow to yellow brown.

FTIR and Raman spectroscopies

Fourier-transform infrared (FTIR) and Raman single-crystal spectra for ferroinnelite were collected on crystal 2. In particular, the IR spectrum was collected at INFN, Laboratori Nazionali di Frascati (Rome) using a Bruker Hyperion 3000 microscope attached to a Vertex V66 optical bench. Operating conditions were: 4 cm^–1 nominal resolution on both peak and background, KBr beamsplitter, MCT N₂-cooled detector and an average of 128 scans. The beam size was ∼50 × 50 μm. Raman measurements were done at the Raman Spectra Lab, Department of Science, Roma Tre University, at room temperature using an inVia Renishaw spectrometer equipped with a diode laser (532 nm and output power 100 mW), an edge filter, a 1800 lines per mm diffraction grating and a Peltier-cooled 1024 × 256 pixel CCD detector; the sample was mounted on the manual stage of a Leica DM2700 M confocal microscope. Spectral acquisitions (20 accumulations at 30 s each) were done with a 50× objective and a laser power of 3.5 mW.

A selected FTIR spectrum in the principal (OH)-stretching region of ferroinnelite is given in Fig. 2a, where it is compared with the spectrum collected for holotype innelite 81801 from Yakutia (Fig. 2b). Due to the very small size of the analysed samples, both patterns are very noisy and poorly resolved, however both show undoubtedly the presence of OH groups in the structure. Both spectra consist of a peak at ∼3630 cm^–1 and a broad band due to several overlapping components, centred around 3550 cm^–1 in ferroinnelite and 3590 cm^–1 in innelite; these are superimposed on a very broad absorption centred at ∼3310 cm^–1 in ferroinnelite and ∼3469 cm^–1 in innelite that can be assigned to H₂O groups and eventually moisture adsorbed on the analysed grains.

Figure 2. Infrared spectra of (a) ferroinnelite (crystal 9530_2) and (b) holotype innelite (crystal 81801_1) in the principal OH-stretching region; Raman spectra of (c) ferroinnelite and (d) holotype innelite from ∼100 to 4000 cm^–1; the OH stretching range is shown in the inset where the pattern decomposition is given as a guide to the eye.

The Raman spectra (Fig. 2c and 2d) show a weak but well-defined (OH)-stretching pattern (see insets in Fig. 2c and 2d) where relatively sharp peaks are resolved at 3567–3591 cm^–1 (for ferroinnelite and innelite, respectively) and 3643–3650 cm^–1; the higher wavenumber peak of ferroinnelite is split with a second evident component at 3671 cm^–1 (Fig. 2c). Two additional broad components at 3416–3439 cm^–1 (for ferroinnelite and innelite, respectively), and 3153–3167 cm^–1 can be related to H₂O. In the low-frequency range (<1200 cm^–1), the spectra are very similar and extremely complex and show five intense to relatively intense peaks at frequency > 800 cm^–1 plus many lower-intensity peaks at frequencies < 800 cm^–1. Based on the literature data for similar TS-block minerals (e.g. Sokolova et al., Reference Sokolova, Abdu, Hawthorne, Genovese, Camara and Khomyakov2015; Cámara et al., Reference Cámara, Sokolova, Abdu and Pautov2016), peaks at frequencies > 800 cm^–1 can be assigned to Si–O stretching, whereas peaks at frequencies < 800 cm^–1 can be assigned to Si–O bending and to M–O modes. Moreover, the spectra are complicated by the presence of SO₄ and PO₄ tetrahedra in the I block whose modes typically occur as intense scattering at frequencies > 900 cm^–1.

Chemical composition

The ferroinnelite crystal 1 previously used for single-crystal X-ray data collection was analysed with a Cameca SX-100 electron-microprobe operating in wavelength-dispersion mode with an accelerating voltage of 15 kV, a specimen current of 20 nA, a beam size of 10 μm and count times on peak and background of 20 and 10 s, respectively. The following standards were used for Kα or Lα X-ray lines (analysing crystals are given in brackets): F(LTAP): F-bearing riebeckite; Na(TAP): albite; Si(TAP), Ca(LPET): diopside; Nb(PET): Ba₂NaNb₅O₁₅; P(LPET): apatite; Ti(LLiF): titanite; Sr(LPET): SrTiO₃; Ta(LLiF): MnNb₂Ta₂O₉; Mn(LLiF): spessartine; Mg(TAP): forsterite; Fe(LLiF): fayalite; Zr(LPET): zircon; S(LPET), Ba(LiF, L _β): baryte; Al(LTAP): andalusite; K(LPET): orthoclase. Data were reduced using the ϕ(ρZ) procedure of Pouchou and Pichoir (Reference Pouchou, Pichoir and Armstrong1985). The chemical composition of ferroinnelite is given in Table 1 and is the mean of 11 determinations.

Table 1. Chemical composition (wt.%) and unit formula (apfu) for ferroinnelite¹ and innelite²

¹ this work: EMPA; ²Sokolova et al. (Reference Sokolova, Cámara and Hawthorne2011): EMPA;

* calculated from structure-refinement results: ¹[(OH)_1.36 and (H₂O)_0.17] and ²[(OH)_1.50 and (H₂O)_0.17] pfu;

** monovalent anions of the O sheet;

*** content of H⁺ in the I block from: ¹[(OH)_0.51(H₂O)_0.17] and ²[(OH)_0.51(H₂O)_0.17] pfu.

The empirical formula calculated on 26 (O + F) atoms per formula unit (apfu) is (Na_1.95Fe²⁺_0.64Mg_0.21Mn²⁺_0.19Ca_0.02)_Σ3.01(Ba_3.84Sr_0.13Na_0.03)_Σ4.00(Ti_2.91Nb_0.05Al_0.02Zr_0.01Mg_0.01)_Σ3.00 Si_4.02S_0.94P_0.89H_1.70O_25.89F_0.11, Z = 1. The structural formula based on assigned site-populations (see below) is (Ba_3.84Sr_0.13Na_0.03)_Σ4(Ti_1.98Al_0.02)_Σ2Na(Na_0.95Fe²⁺_0.64Mg_0.21Mn²⁺_0.19Ca_0.01)_Σ2(Ti_0.93Nb_0.05Zr_0.01Mg_0.01)_Σ1(Si₂O₇)₂[(SO₄)_0.94(PO₄)_0.89(OH)_0.51(H₂O)_0.17]O₂[O_1.04(OH_0.85F_0.11)_Σ0.96]_Σ2. The simplified formula is (Ba,Sr)₄Ti₂Na(Na,Fe²⁺,Mg,Mn²⁺)₂(Ti,Nb)(Si₂O₇)₂[(SO₄)(PO₄)]O₂(O,OH)₂. The ideal formula of ferroinnelite, Ba₄Ti₂Na(NaFe²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)], requires (wt.%): SO₃ 5.77; P₂O₅ 5.12; TiO₂ 17.28; SiO₂ 17.32; BaO 44.21; FeO 5.18; Na₂O 4.47; H₂O 0.65; Total 100.00.

Powder X-ray diffraction

Powder X-ray diffraction data were obtained by collapsing the entire dataset collected from the single crystal into one dimension. Data (in Å for MoKα) are listed in Table 2. Unit-cell parameters are therefore the same as for the single-crystal data (Table 3).

Table 2. Simulated powder X-ray diffraction data* for ferroinnelite

* the entire dataset of images was collapsed in 1D.

Table 3. Miscellaneous refinement data for ferroinnelite

* twin matrix is ( $\overline 1 $ 0 0, 0 1 0, 0 0 $\overline 1 $)

Crystal structure: experimental

Data collection and structure refinement

X-ray single-crystal data for ferroinnelite were collected from a twinned crystal (0.127 × 0.097 × 0.046 mm) with a Rigaku XtaLAB Synergy diffractometer, equipped with a PhotonJet-S (Mo) X-ray source operating at 50 kV and 1 mA, with monochromatised MoKα (0.71073 Å) radiation and equipped with a HyPix-6000HE Hybrid Photon Counting (HPC) detector working at 62 mm from the sample. The intensities of reflections with –9 ≤ h ≤ 9, –12 ≤ k ≤ 12 and –26 ≤ l ≤ 27 were collected with a frame width of 0.5° and a frame time of 40 s up to 2θ ≤ 81.44°, and an empirical absorption correction (SCALE3 ABSPACK, Rigaku Oxford Diffraction, 2018) was applied. There were few observed reflections at high 2θ, and refinement of the structure was done for 2θ ≤ 54°, –7 ≤ h ≤ 7, –9 ≤ k ≤ 9 and –19 ≤ l ≤ 19. The crystal structure was refined to R ₁ = 0.0745 on the basis of 4485 unique reflections (F _o > 4σ|F|) using atom coordinates of innelite-1A (Sokolova et al., Reference Sokolova, Cámara and Hawthorne2011) with Bruker SHELXTL version 2014/3 software (Sheldrick, Reference Sheldrick2015). The ratio of twin components is 0.466(2): 0.534(2), and the twin matrix is ( $\bar 1$ 0 0, 0 1 0, 0 0 $\bar 1$). The occupancies of the Ti-dominant M ^H and M ^O1 sites were refined with the scattering factor of Ti; alkali-cation sites M ^O2 with the scattering factor of Na and M ^O3 with the scattering factor of Fe; Ba-dominant A^P and B^P sites with the scattering factor of Ba; and the T site with the scattering factor of S. The refined occupancy of the Ti-dominant M ^O1 site is greater than 1 due to the presence of heavier cations such as Nb and Zr. For the M ^O2 site, site-occupancy refinement using the scattering factor of Na converged to 1.0 and was fixed at the last stages of the refinement. Neutral scattering factors were taken from the International Tables for X-ray Crystallography (Wilson, Reference Wilson1992). In the last stages of refinement, two subsidiary peaks were included in the refinement with the scattering factor of Ba. The presence of subsidiary peaks is a common feature in Ti silicates with the TS block; they are due to intimate intergrowths of these minerals and the presence of additional structure domains (see HRTEM results for innelite in Sokolova et al., Reference Sokolova, Cámara and Hawthorne2011). Refined site-occupancies of subsidiary peaks were 2 and 3%. The details of X-ray data collection and structure refinement are given in Table 3, final atom parameters are given in Table 4, selected interatomic distances and angles in Table 5, refined site-scattering values and assigned site-populations in Table 6, and bond-valence values in Table 7. A crystallographic information file and a list of observed and calculated structure factors for ferroinnelite have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary Material (see below).

Table 4. Atom coordinates and anisotropic displacement parameters (Ų) for ferroinnelite

* Site occupancies for 1 and 2 are 0.02 and 0.03, respectively (scattering factor of Ba).

** U _iso.

Table 5. Selected interatomic distances (Å) and angles (°) for ferroinnelite

Notes: φ = O, OH and F; Symmetry codes – a: x+1, y, z; b: –x+1, –y, –z+1; c: –x+1, –y+1, –z+1; d: x, y, z–1; e: x–1, y, z; f: –x+1, –y+1, –z+2; g: x, y+1, z+1; h: x, y+1, z; i: –x+2, –y+1, –z+1; j: x+1, y+1, z.

Table 6. Refined site-scattering values (epfu) and assigned site-populations (apfu) for ferroinnelite

* Coordination numbers are given for non-[6]-coordinated sites;

** φ = O, OH, F, H₂O;

*** Compositions and ideal compositions at all sites in ferroinnelite are respectively similar and equivalent to the ones in innelite-1A (shown in blue), except for the M ^O3 site: (NaFe²⁺) (ferroinnelite) and (NaMn²⁺) in innelite-1A (Sokolova et al., Reference Sokolova, Cámara and Hawthorne2011; Sokolova and Cámara, Reference Sokolova and Cámara2017).

Table 7. Bond-valence* (vu) values for ferroinnelite

* Bond-valence parameters are from Brown (Reference Brown, O’Keeffe and Navrotsky1981);

** Coordination numbers are given for non-[4]-coordinated anions.

Structure description

Cation sites

The cation sites are divided into 3 groups: the M ^O sites of the O sheet, the M ^H and Si sites of the H sheet, and the peripheral A^P and B^P sites and the T sites in the I block. In accord with Sokolova (Reference Sokolova2006), we label X^O_M and X^O_A two anions of the O sheet which do not coordinate Si atoms; X^O_M = anion at the common vertices of 3 M^O and ^[5]M^H polyhedra; X^O_A = anion at the common vertices of 3 M^O octahedra.

There are three cation sites in the O sheet: the M ^O1, M ^O2 and M ^O3 sites are occupied by Ti_0.93Nb_0.05Zr_0.01Mg_0.01, Na and [Na_0.95(Fe²⁺_0.64Mg_0.21Mn²⁺_0.1964Ca_0.01)] apfu, respectively, and the ideal compositions of the M ^O1, M ^O2 and M ^O3 sites are Ti, Na and (NaFe²⁺) apfu (Table 6). The M^O1 and M^O3 cations are coordinated by four O atoms and two X^O_A anions (see Anion considerations below) with <M^O1–φ> = 2.00 Å and <M^O3–φ> = 2.27 Å (φ = O,OH,F) (Tables 4–6; Fig. 3a). The structure diagrams were drawn using Atoms 6.4 software (Dowty, Reference Dowty2016). The M^O2 cation is coordinated by six O atoms, with <M^O2–O> = 2.43 Å (Table 5). The cations of the O sheet sum ideally to Na(NaFe²⁺)Ti apfu.

Figure 3. The TS block in the crystal structure of ferroinnelite viewed down [001]: (a) O sheet and (b) linkage of H and O sheets; SiO₄ tetrahedra are orange, Ti⁴⁺-dominant polyhedra are yellow, Na octahedra and (Na,Fe²⁺) octahedra are navy blue and bluish-green, respectively; X^O_M [O] and X^O_A [O,(OH)] anions are shown as small white circles with green rims and pink spheres. Labels 1 and 2 (on orange) correspond to Si(1,2) tetrahedra, respectively. Labels 1–3 correspond to M^O(1–3) octahedra in the O sheet, respectively. Inset on the right shows a possible hydrogen bond X^O_A–O3 where the M ^O3 site is occupied by Na and the X ^O_A site is occupied by an OH group shown as a small red sphere. The structure diagrams were drawn using Atoms 6.4 software (Dowty, Reference Dowty2016).

In the H sheet, there are two tetrahedrally coordinated Si sites, occupied by Si, with <Si–O> = 1.63 Å (Table 6, Fig. 3b). The [5]-coordinated M ^H site is occupied mainly by Ti (Table 6) and is coordinated by O atoms, with <M^H–O> = 1.92 Å; the very short M^H–X^O_M distance of 1.70 Å (Table 5) is in accord with the structure topology of lamprophyllite-group minerals (Sokolova, Reference Sokolova2006). For the two H sheets, the sum of the cations is ideally Ti₂Si₄ apfu.

In the I block, there are two distinct peripheral A^P and B^P sites (Fig. 4). The [9]-coordinated A^P and [11]-coordinated B^P sites are occupied mainly by Ba (Table 6). The A^P and B^P sites are coordinated by O atoms of the H sheet and O atoms (or OH and H₂O groups) of the I block (see below), with <A^P–φ> = 2.87 and <B^P–φ> = 2.88 Å, respectively (Fig. 5a,b). The ideal composition of the two A^P and B^P sites is Ba₄ apfu. There is one T site occupied by S_0.94P_0.89□_0.17 pfu, ideally (SP) apfu, and coordinated by four O atoms, with <T–O> = 1.50 Å (Tables 5, 6; Figs 4, 5a). The three sites in the I block ideally give Ba₄(SP) apfu.

Figure 4. The crystal structure of ferroinnelite projected onto (100). Legend as in Fig. 3; Ba atoms at the A^P and B^P sites are shown as raspberry-coloured spheres, TO₄ (T = S,P) tetrahedra are yellow. The three vertical lines show the positions of the cation layers (m = 3) in the I block.

Figure 5. Details of short-range order of anions in the I block for ferroinnelite: (a) linkage of Ba atoms (at the ^[9]A^P and ^[11]B^P sites) and TO₄ tetrahedra where T tetrahedra are fully occupied by S and P; (b) linkage of Ba atoms at the A^P and B^P sites via OH [O(8,9,10)] and H₂O groups [O11] where T sites are vacant. Legend as in Fig. 4; Ba atoms at the A^P and B^P sites are shown as raspberry-coloured spheres and labelled A and B, respectively. Anions in the I block are (a) not shown where they are O atoms which coordinate T sites occupied by S and P and (b) shown as small red OH and large red H₂O spheres where T sites are vacant. Thin black lines in (a) and (b) show Ba–O bonds; dashed lines in (b) show possible hydrogen bonds.

We write the cation part of the ideal formula as the sum of the cations of the I block + O sheet + two H sheets: Ba₄(SP) + Na(NaFe²⁺)Ti + Ti₂Si₄ = Ba₄Na(NaFe²⁺)Ti₃Si₄(SP) with a total charge of 51⁺.

Chemical formula

For ferroinnelite, we write the ideal structural formula of the form A^P ₂B^P ₂M^H₂M^O₄(Si₂O₇)₂(TO₄)₂(X^O_M)₂(X^O_A)₂ as the sum of the ideal compositions of the TS block and I blocks: {Ti₂Na(NaFe²⁺)Ti(Si₂O₇)₂O₂[O(OH)]}^3– + {Ba₄[(SO₄)(PO₄)]}³⁺ = Ba₄Ti₂Na(NaFe²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)], where M^H₂ = cations of the H sheet = Ti₂; M^O₄ = cations of the O sheet = Na(NaFe²⁺)Ti; X^O₄ = anions in the O sheet = O₂[O(OH)]; A^P ₂B^P ₂ = cations at the P (peripheral sites) which belong to the I block = Ba₄; central part of the I block = (TO₄)₂ = [(SO₄)(PO₄)].

Above, we reported the empirical formula for ferroinnelite (Table 1) calculated on the basis of 26 (O + F) anions: (Na_1.95Fe²⁺_0.64Mg_0.21Mn²⁺_0.19Ca_0.02)_Σ3.01(Ba_3.84Sr_0.13Na_0.03)_Σ4.00(Ti_2.91Nb_0.05Al_0.02Zr_0.01Mg_0.01)_Σ3.00Si_4.02S_0.94P_0.89H_1.70O_25.89F_0.11, Z = 1 and the structural formula based on assigned site-populations (Ba_3.84Sr_0.13Na_0.03)_Σ4(Ti_1.98Al_0.02)_Σ2Na(Na_0.95Fe²⁺_0.64Mg_0.21Mn²⁺_0.19Ca_0.01)_Σ2(Ti_0.93Nb_0.05Zr_0.01Mg_0.01)_Σ1(Si₂O₇)₂[(SO₄)_0.94(PO₄)_0.89(OH)_0.51(H₂O)_0.17]O₂[O_1.04(OH_0.85F_0.11)_Σ0.96]_Σ2. The validity of the ideal formula is supported by the close agreement between the charges for specific groups of cations in the ideal and structural formulae: 8⁺ for Ba₄ versus 7.97⁺ for (Ba_3.84Sr_0.13Na_0.03)_Σ4; 8⁺ for Ti₂ versus 7.98⁺ for (Ti_1.98Al_0.02); 4⁺ for Na(NaFe²⁺) versus 4.05⁺ for Na(Na_0.95Fe²⁺_0.64Mg_0.21Mn²⁺_0.1919Ca_0.01); 5^– for [(SO₄)(PO₄)] versus 5.06^– for [(SO₄)_0.94(PO₄)_0.89(OH)_0.51H₂O_0.17].

Summary

(1) The chemical composition of ferroinnelite has been determined. The ideal structural formula of ferroinnelite of the form A^P ₂B^P ₂M^H₂M^O₄(Si₂O₇)₂(TO₄)₂(X^O_M)₂(X^O_A)₂ is Ba₄Ti₂Na(NaFe²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)], where S:P = 1:1.

(2) Ferroinnelite, ideally Ba₄Ti₂Na(NaFe²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)], is a new representative of titanium disilicate minerals with the TS block and a new member of the lamprophyllite group (Ti + Nb + Fe³⁺ + Mg = 3 apfu) of the seidozerite supergroup. Ferroinnelite belongs to the structure type B2(LG): B = basic, LG = lamprophyllite group.

(3) There are two substitutions in the O sheet of the TS block: Na⁺ ↔ Fe²⁺ at the M ^O3 site adjacent to the X ^O_A site and OH^– ↔ O^2– at the X ^O_A site, and these couple as follows: Na⁺ + OH^– ↔ Fe²⁺ + O^2–.

(4) There are two substitutions in the I block: ^T(S,P) ↔ ^T□ at the T site and O^2– ↔ OH^– + minor H₂O at the anion sites which coordinate the T sites; they couple as follows: ^T(S,P) + O^2– ↔ ^T□ + OH^– + minor H₂O.

(5) Ferroinnelite is isostructural with innelite-1A, ideally Ba₄Ti₂Na(NaMn²⁺)Ti(Si₂O₇)₂[(SO₄)(PO₄)]O₂[O(OH)] (Sokolova et al., Reference Sokolova, Cámara and Hawthorne2011; Sokolova and Cámara, Reference Sokolova and Cámara2017) (Table 8). Ferroinnelite and innelite are related by the following cation substitution at the M ^O3 site in the O sheet in the TS block: Fe²⁺_fer ↔ Mn²⁺_inn.

Table 8. Comparison of ferroinnelite and innelite-1A

¹ this work;

² ideal structural formula, D _calc., unit-cell parameters and space group are from Sokolova et al. (Reference Sokolova, Cámara and Hawthorne2011); D _meas. and optical data are from Kravchenko et al. (Reference Kravchenko, Vlasova, Kazakova, Ilyukhin and Abrashev1961), power-diffraction data are from Raade and Berg (Reference Raade and Berg2000) (powder diffractometer Siemens D5005, left) and Pekov et al. (Reference Pekov, Chukanov, Kulikova and Belakovsky2006) (RKU-86 mm camera, right).

(6) IR and Raman spectroscopies confirmed the presence of OH and H₂O groups in ferroinnelite and the holotype sample of innelite (sample 81801, Fersman Mineralogical Museum, Moscow, Russia).