Saranovskite, SrCaFe²⁺₂(Cr₄Ti₂)Ti₁₂O₃₈, a new crichtonite-group mineral

Abstract

The new crichtonite-group mineral saranovskite, ideally SrCaFe²⁺₂(Cr₄Ti₂)Ti₁₂O₃₈, was discovered in the Glavnoe Saranovskoe deposit, Middle Urals, Russia, and named after the type locality. The associated minerals are chromite, Cr-bearing clinochlore, and calcite. Saranovskite forms black crude equant crystals about 2 mm across. The lustre is submetallic, and the streak is brownish-gray. Cleavage is not observed. The Mohs hardness is 6. Density calculated using the empirical formula is equal to 4.501 g cm^–3. The reflectance spectra in visible range are given. The IR spectrum shows the absence of H-, B- and C-bearing groups. The Raman spectrum of saranovskite confirms the absence of H-bearing groups and indicates a rather high degree of ordering of Ti⁴⁺ and lower-valence cations. The chemical composition of saranovskite is (wt.%; electron microprobe, total iron apportioned between FeO and Fe₂O₃ taking into account charge balance): MgO 2.01, CaO 1.43, MnO 0.21, FeO 8.14, SrO 3.27, BaO 2.18, Al₂O₃ 0.53, Sc₂O₃ 0.69, Cr₂O₃ 10.27, Fe₂O₃ 2.19, Y₂O₃ 1.56, La₂O₃ 0.94, Ce₂O₃ 0.91, Pr₂O₃ 0.14, Nd₂O₃ 0.35, TiO₂ 64.25, ZrO₂ 0.58, total 99.65. The crystal chemical formula of saranovskite is (Sr_0.55Ba_0.25Ln_0.10Ca_0.10)(Ca_0.36Y_0.25Ln_0.16Fe²⁺_0.08Zr_0.10Mn_0.05) (Fe²⁺_1.12Mg_0.88)(Cr³⁺_2.34Ti_2.28Fe²⁺_0.91Fe³⁺_0.11Al_0.18Sc³⁺_0.18)(Ti_5.82Fe³⁺_0.18)Ti_6.0O₃₈. The idealized formula is SrCaFe²⁺₂(Cr₄Ti₂)Ti₁₂O₃₈. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0243. The new mineral is isostructural to other crichtonite-group members. Saranovskite is trigonal, space group R$\overline{3}$, with a = 10.3553(2) Å, c = 20.7301(4) Å, V = 1925.12(8) Å³ and Z = 3. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.398 (75) (024), 2.881 (100) (– 126), 2.842 (65) (– 234), 2.247 (67) (– 144). 2.137 (76) (– 345), 1.799 (63) (– 348), 1.597 (72) (– 1.4.10, 152), 1.439 (76) (520).

Introduction

Minerals belonging to the crichtonite group crystallize in the space group R$\overline{3}$ and have the general crystal chemical formula ^XIIM0^VIM1^IVM2₂^VIM3₆^VIM4₆^VIM5₆O₃₈ in which Roman numerals indicate coordination numbers. The M4 and M5 sites have octahedral coordination and are predominantly occupied by Ti in all crichtonite-group minerals except paseroite in which these sites may be V⁵⁺-dominant (Mills et al. 2012). The cations which were identified at other sites are: M0 = Ba, K, Pb, Sr, La, Ce, Na, Ca; M1 = Mn²⁺, Y, U, Fe²⁺, Zr, Ca, Sc; M2 = Fe²⁺, Mn²⁺, Mg, Zn; M3 = Fe³⁺, Cr³⁺, Mn³⁺, V⁵⁺, Al (Mills et al. 2012). The new mineral saranovskite described in this paper is the first chromium member of the crichtonite group which is Sr-dominant at the M0 site.

Saranovskite is named after its discovery locality, the famous, historical Saranovskoe chromite deposit (now Glavnoe Saranovskoe). The new mineral and its name were approved by the IMA CNMNC (IMA no. 2020-015). The holotype specimen is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia with the registration number 5558/1.

Experimental methods

Five electron microprobe analyses were carried out using a digital scanning electron microscope Tescan VEGA-II XMU equipped by an Oxford INCA Wave 700 spectrometer (WDS mode, accelerating voltage of 20 kV, electron beam current of 20 nA, beam diameter of 0.3 μm). External standards used for calibration are reported in Table 3. All experimental data including vibrational spectra and X-ray diffraction patterns have been obtained on fragments of the crystal used for the WDS analyses.

To obtain an IR absorption spectrum, saranovskite powder was mixed with anhydrous KBr, pelletized, and analyzed using an ALPHA FTIR spectrometer (Bruker Optics) at a resolution of 4 cm^–1. 16 scans were collected. The IR spectrum of an analogous pellet of pure KBr was used as a reference.

Raman spectra were collected in the range of 100–3900 cm^–1 using a Horiba XploRa confocal Raman spectrometer with the exciting wavelength of 532 nm and 1800 T diffraction grating providing a spectral resolution ~ 1 cm^–1. Spectra were collected for 30 s for each spectral window.

Reflectance values for saranovskite have been measured in air using WTiC as a standard by means of a Universal Microspectrophotometer UMSP 50 (Opton-Zeiss, Germany).

Powder X-ray diffraction data were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer equipped with cylindrical image plate detector using Debye–Scherrer geometry (d = 127.4 mm). CoKα radiation with λ = 1.79021 Å was employed. The data were integrated using the software package osc2tab (Britvin et al. 2017).

Single-crystal X-ray studies were carried out at 293 K with an Xcalibur Eos CCD Oxford Diffraction diffractometer using MoKα radiation. A total of 32 004 reflections in the θ range of 3.936–51.887° were measured. The crystal structure of saranovskite was solved by direct methods using the AREN program package (Andrianov 1987) and refined by the full-matrix least squares method in an anisotropic approximation for all atoms using SHELXL package (Sheldrick 2015) to R = 0.0243. For most cationic sites, mixed atomic scattering curves were used. Crystal data, data collection information and structure refinement details are given in Table 1.

Table 1 Crystal data, data collection information and structure refinement details for saranovskite

Results

Occurrence, general appearance and physical properties

Saranovskite was found in the single specimen collected in the Rudnaya underground chromite mine (depth 400 m) operating at the Glavnoe Saranovskoe deposit which belongs to the Saranovskaya group of chromite deposits. The mine is located in the town of Sarany, 5 km to the north of the Laki railway station, Gornozavodskiy district, Perm Krai, Middle Urals, Russia (58° 30′ North, 58° 52′ East). For the description of the deposit, see papers by Ivanov (1990, 1997, 2016).

The new mineral occurs in a hydrothermal calcite vein 3 cm thick which crosscut a rock (chromium ore) mainly composed by Al-rich chromite (80–90 vol.%) cemented by fine-grained Cr-bearing clinochlore. Saranovskite crystals overgrow a crust of purple platy crystals of Cr-bearing clinochlore (i.e. clinochlore of second generation which crystallized at the hydrothermal stage on the walls of a crack in the ore). Saranovskite forms crude equant crystals about 2 mm across (Fig. 1). Only three crystals were found.

Fig. 1

Crystals of saranovskite. The crystal sizes are 1.5 ×  2 × 2 mm

Saranovskite is black, the lustre is submetallic, and the streak is brownish-gray. Cleavage is not observed. The fracture is conchoidal. Saranovskite is brittle. The VHN hardness determined by micro-indentation at load of 200 g is equal to 850 kg/mm² (range 799–890 kg/mm², n = 6) The Mohs hardness is 6. Density calculated using the empirical formula and unit-cell parameters obtained from single-crystal X-ray diffraction data is equal to 4.501 g cm^–3.

Reflectance spectroscopy in visible range

Under the microscope, saranovskite is gray, with brown internal reflections. Bireflectance is very weak, ∆R = 0.17% (589 nm). Anisotropism is weak. Pleochroism is not observed.

Reflectance values are given in Table 2 (the reference wavelengths required by the Commission on Ore Mineralogy are given in bold type).

Table 2 Reflectance values of saranovskite, %

Infrared absorption spectroscopy

The IR spectrum of saranovskite (Fig. 2) is rather close to the IR spectra of other crichtonite-group minerals (Chukanov 2014; Chukanov and Chervonnyi 2016). Wavenumbers of absorption bands in the IR spectrum of saranovskite and their assignments are (cm^–1, s—strong band, w—weak band, sh—shoulder): 1175w, 1087w (overtones), 726, 625sh, 604s (Ti–O stretching vibrations), 528s, 505sh, 412, 401 (mainly, M–O stretching vibrations where M = Cr, Fe, Mg, Al, Sc).

Fig. 2

Powder infrared absorption spectrum of saranovskite

The absence of absorption bands above 1200 cm^–1 indicates the absence of H-, B- and C-bearing groups.

Raman spectroscopy

The Raman spectrum of saranovskite (Fig. 3) confirms the absence of bands of O–H stretching vibrations (in the range of 2500–3900 cm^–1). The assignment of the Raman bands is as follows:

• 654 to 800 cm^–1–Ti–O stretching vibrations;

• 440 cm^–1–R1³⁺–O stretching vibrations (R1 = Cr, Fe, Sc, Al);

• 417 cm^–1–R2²⁺–O stretching vibrations (R2 = Mg, Fe);

• Below 400 cm^–1–lattice modes involving stretching vibrations of lower force-strength cations (Ca, Sr, REE) and different kinds of bending vibrations.

• Weak bands in the range of 1200–1600 cm^–1—overtones and combination modes.

Fig. 3

Raman spectrum of saranovskite

The assignment of the bands at 654, 735 and 800 cm^–1 to Ti–O stretching vibrations is based on the fact that Ti⁴⁺ is the most high force–strength cation occurring in saranovskite. The strongest and the most high-frequency band in this region observed at 800 cm^–1 may correspond to symmetric stretching vibrations of the Ti–O–Ti bridges with the shortest Ti–O bonds. Analogous strong bands are observed in the Raman spectra of other crichtonite-group minerals: crichtonite (at 809 cm^–1: RRUFF database, R060314), dessauite-(Y) and mathiasite (at 812 cm^–1 for both: Bittarello et al. 2014). However, in the Raman spectra of metamict samples (Frost and Reddy 2011) and synthetic crychtonite-related compounds with disordered structures, synthesized at high temperatures (Konzett et al. 2005), bands in the range of 770–850 cm^–1 are very weak or are observed as broad shoulders, respectively. Consequently, the presence of the strong band at 800 cm^–1 in the Raman spectrum of saranovskite indicates a high degree of ordering of Ti⁴⁺ and other cations having octahedral coordination.

Chemical composition

Analytical data for saranovskite based on five spot analyses of a polished section are given in Table 3. Contents of other elements are below detection limits. In BSE, the mineral is completely uniform. The rim is slightly enriched in Sr and Cr and depleted in Ba as compared to the core, but in all analyses, Sr prevails over Ba in atomic units. H₂O and CO₂ were not measured because no bands corresponding to CO₃^2– anions and H-bearing groups are observed in the IR spectrum.

Table 3 Chemical composition of saranovskite

The empirical formula could not be calculated based on 38 O atoms per formula unit (apfu) because the Fe²⁺:Fe³⁺ ratio was not determined experimentally. For this reason, formula coefficients given in Table 3 were calculated based on the sum of 22 metal cations pfu, in accordance with structural data (see below). The Fe²⁺:Fe³⁺ ratio was calculated from the charge-balance requirement, taking into account the fact that the IR spectrum shows the absence of OH groups.

The simplified formula derived based on structural data is (Sr,Ba)(Ca,Y,Ln)(Fe²⁺,Mg)₂(Cr,Ti)₆Ti₁₂O₃₈. The idealized end-member formula is SrCaFe²⁺₂(Cr₄Ti₂)Ti₁₂O₃₈.

X-ray diffraction data and crystal structure

Powder X-ray diffraction data of saranovskite are given in Table 4. The hexagonal unit-cell parameters refined from the powder data are: a = 10.372(1), c = 20.808(4) Å; V = 1938.8(8) Å³. The systematic absences of reflections are in agreement with the space group R$\overline{3}$ determined from the single-crystal X-ray diffraction data.

Table 4 Powder X-ray diffraction data (d in Å) of saranovskite

The crystal structure of saranovskite (Fig. 4, Tables 5, 6 and 7) is consistent with that reported for all other members of the crichtonite group. It is based on a close-packed framework with a nine-layer stacking sequence [chhchhchh]_∞. Large cations, namely Sr, Ba and subordinate Ca and LREE, occur at the 12-coordinated M0 site. The M1 octahedron is occupied by the largest octahedral cations, Ca, REE³⁺, and minor Zr, Mn²⁺ and Fe²⁺; Fe²⁺ and subordinate Mg occupy the M2 tetrahedron. The smallest, distorted M4- and M5-centered octahedra are mainly occupied by Ti atoms. The bond valence sums (BVS) at the M4 and M5 sites are equal to 4.04 and 3.99, respectively (Table 7). The M3 site with a more regular octahedral coordination and BVS of 3.29 concentrates trivalent cations with ionic radii in the range of 0.62–0.73 Å (Shannon 1976), as well as remaining Ti⁴⁺ and Fe²⁺ cations.

Fig. 4

Mutual arrangement of the polyhedral c and h layers in the crystal structure of saranovskite (a) and separate layers viewed down the c axis: the levels c ≈ 0.06 (b), c ≈ 0.16 (c), c ≈ 0.26 (d), c ≈ 0.39 (e), and c ≈ 0.49 (f). The unit cell is outlined

Table 5 Site coordinates and multiplicities (Q), equivalent displacement parameters of atoms (U_eq, Ų) and site compositions for saranovskite

Table 6 Selected cation-oxygen distances (Å) in saranovskite

Table 7 Bond-valence calculations for saranovskite

The crystal chemical formula of saranovskite can be written as follows (Z = 3): ^M0(Sr_0.55Ba_0.25Ln_0.10Ca_0.10)^{XII M1}(Ca_0.36Y_0.25Ln_0.16Fe²⁺_0.08Zr_0.10Mn_0.05)^{VI M2}(Fe²⁺_1.12Mg_0.88)^{IV M3}(Cr³⁺_2.34Ti_2.28Fe²⁺_0.91Fe³⁺_0.11Al_0.18Sc³⁺_0.18)^{VI M4}(Ti_5.82Fe³⁺_0.18)^{VI M5}Ti_6.0^VI O₃₈ where Roman numerals indicate coordination numbers of cations and the Fe²⁺:Fe³⁺ ratio was calculated based on the charge-balance requirement.

The mean charges of cationic sites are in a good agreement with the BVS values (Table 7). In particular, the M3 site is dominated by trivalent cations among which Cr is the most abundant: Fe²⁺_0.15(Cr³⁺_0.39Al³⁺_0.03Sc³⁺_0.03Fe³⁺_0.02)_Σ0.47Ti⁴⁺_0.38. Thus, according to the dominant valency rule, Ca and Cr are the species-defining components, and the idealized formula of saranovskite is SrCaFe²⁺₂(Cr₄Ti₂)Ti₁₂O₃₈.

Discussion

In all crichtonite-group minerals, the M4 and M5 sites are Ti-dominated. Thus, the crystal-chemical diversity of these minerals is mainly determined by the components occurring at the M0–M3 sites. The dominant components at the species-defining key sites of different members of the crichtonite group are given in Table 8.

Table 8 Dominant (species-defining) components at the key sites in crichtonite-group minerals

Saranovskite is the Ca-dominant (at the M1 site) and Cr³⁺-dominant (at the M4 site) analogue of crichtonite, ideally SrMn²⁺Fe²⁺₂(Fe³⁺₄Ti₂)Ti₁₂O₃₈ (Grey et al. 1976). Another crichtonite-group mineral chemically related to saranovskite is dessauite-(Y), (Sr,Pb)(Y,U)(Ti,Fe³⁺)₂₀O₃₈ (Orlandi et al. 1997). Comparative data for saranovskite and some related (Sr-dominant) crichtonite-group minerals are given in Table 9. It is to be noted that saranovskite is not the first member of the crichtonite group with species-defining Cr. Other two Cr-dominant (at the M3 site) crichtonite-group minerals are lindsleyite (Zhang et al. 1988) and mathiasite (Gatehouse et al. 1983) (see Table 8).

Table 9 Comparative data for Sr-dominant crichtonite-group minerals

Saranovskite is a hydrothermal mineral probably formed as a result of the interaction of chromite ore (chromitite) with fluid derived by a diabase intrusion (Ivanov 1990, 2016). During the formation of veins which crosscut chromitite bodies at the Glavnoe Saranovskoe deposit and the neighboring Yuzhno-Saranovskoe (Biserskoe) deposit, chromium activity in hydrothermal solutions was very high which resulted in the crystallization of a unique, in both diversity and gross amount, assemblage of silicates and oxides/hydroxides containing Cr³⁺ as a species-defining component or important admixture (several wt.%). We did not find in literature data on another locality with a similar species diversity of hydrothermal Cr-rich minerals.

The Cr-bearing silicates found in hydrothermal veins of Glavnoe Saranovskoe and Yuzhno-Saranovskoe chromite deposits are grossular–uvarovite series garnets, pumpellyite-group members forming the pumpellyite-(Mg)–shuiskite-(Mg)–shuiskite-(Cr) series, celadonite–chromceladonite series micas and Cr-rich varieties of titanite, zoisite, muscovite, clinochlore, and amesite. Hydrothermal chromium-rich oxides and hydroxides are represented here by two polymorphs of CrOOH, namely grimaldiite and guyanaite, redledgeite, saranovskite, stichtite and Cr-rich varieties of rutile, diaspore, kassite, and pyroaurite (minerals with species-defining Cr³⁺ are marked with bold italic) (Ivanov 2016; Lykova et al. 2018, 2020; Sustavov et al. 2019; this work). Saranovskite is the first Sr- and REE-enriched mineral in these hydrothermal veins. For this mineral, chromite was the source of Cr, diabase was the most probable source of Ti, while other components including Ca, Sr, Ba and REE could be extracted by the hydrothermal fluid from diabase or/and carbonate-bearing host rocks.

Cr-rich crichtonite-group minerals are quite common in ultramafic rocks (chromitites, gabbronorites, pyroxenites, kimberlites etc.) (Gatehouse et al. 1983; Zhang et al. 1988; Chukanov et al. 2019). In particular, Cr-rich loveringite was described in the Western Laouni layered complex, Southern Hoggar, Algeria (with 7.0% Cr₂O₃, i.e. 1.54 apfu Cr: Lorand et al. 1987), in Last-Yavr mafic–ultramafic intrusion, Kola Peninsula (with 7.11% Cr₂O₃, i.e. 1.67 apfu Cr: Barkov et al. 1996) and in the Koitelainen layered intrusion, Northern Finland (with 7.6% Cr₂O₃, i.e. 1.94 apfu Cr: Tarkian and Mutanen 1987). Mathiasite chromium analogue from the Obnazhennaya kimberlite pipe contains 12.38 wt.% Cr₂O₃ which corresponds to 2.60 apfu Cr (Chukanov et al. 2019). However, there are no data on the finds of crichtonite-group minerals in primary chromite ores of the Saranovskoe deposit.