Introduction
This article continues a series of papers on the mineralogy and crystal chemistry of new mineral species containing rare-earth elements (REE; lanthanoids + Y) as species-defining cations (below – ‘REE minerals’) discovered at the Mochalin Log deposit, Chelyabinsk Oblast, South Urals, Russia (55°48′42″N, 60°33′46″E). A brief historical outline, the general description, geological and mineralogical data for this deposit were provided in the first paper which also contained the characterisation of two new isostructural gatelite-group minerals, ferriperbøeite-(La) and perbøeite-(La) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Škoda, Polekhovsky, Agakhanov, Belakovskiy, Kuznetsov, Britvin and Pushcharovsky2020a). In the second article we described radekškodaite-(La) and radekškodaite-(Ce), two members of the epidote–törnebohmite polysomatic series with a novel-type structure including one epidote and two törnebohmite modules (ET2) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Ksenofontov, Agakhanov, Belakovskiy, Polekhovsky, Kuznetsov, Britvin, Pushcharovsky and Nestola2020b). The third paper contained data on the new mineral species percleveite-(La) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Škoda, Agakhanov, Belakovskiy, Plášil, Kuznetsov, Britvin and Pushcharovsky2020c). The fourth paper reported on three new isotypic minerals alexkuznetsovite-(La), alexkuznetsovite-(Ce) and biraite-(La) and the establishment of the biraite group (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Škoda, Agakhanov, Belakovskiy, Britvin and Pushcharovsky2021). Herein we describe another new REE mineral from the Mochalin Log deposit that is named zilbermintsite-(La) [Russian cyrilic зильберминцит-(La)] in honour of the outstanding Russian mineralogist and geochemist Professor Veniamin Arkadievich Zilbermints (other spelling: Silberminz) (1887–1939) who worked at the Moscow Mining Academy and in the All-Union Scientific Research Institute of Mineral Resources (VIMS), Moscow. In 1927 Prof. Zilbermints discovered an alluvial deposit of rare earth ores in the dumps of an outworked placer gold deposit from the 19th century in the Kyshtym region in the Urals that he called the “cerite deposit” and that is known now as the Mochalin Log REE deposit. Based on the results of fieldwork between 1927–1929, Prof. Zilbermints provided detailed geological and petrological description of the deposit, collected and studied numerous REE-rich nodules and reported on the main REE minerals composing them (Zilbermints, Reference Zilbermints1928, Reference Zilbermints1930; Silberminz, Reference Silberminz1929). His destiny was tragic: he was falsely accused during Stalin's times, executed and not exonerated for another 20 years. The Levinson modifier -(La) in the mineral name reflects the predominance of La among REE. The new mineral, its name and symbol (Zlb-La) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2023-063, Kasatkin et al., Reference Kasatkin, Zubkova, Škoda, Pekov, Agakhanov, Gurzhiy, Ksenofontov, Belakovskiy and Kuznetsov2023). The holotype specimen is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow with the catalogue number 98320.
Occurrence and general appearance
The polymineralic nodule containing zilbermintsite-(La) was collected in August 2018 during fieldwork made by a group of the authors (AVK, RŠ and AMK) at the placer dump no. 2bis (Fig. 1). The new mineral occurs as a very few anhedral grains, typically at the contact of ferriperbøeite-(La), törnebohmite-(La) and ferriallanite-(Ce) (Fig. 2). The largest grain found measures 0.65 × 0.20 mm. Apart from the above mentioned species, the very rich REE mineral assemblage of this nodule includes bastnäsite-(La), biraite-(La), ferriallanite-(La), ferriperbøeite-(Ce), fluorbritholite-(Ce), monazite-(La), perbøeite-(La), percleveite-(Ce), percleveite-(La), perrierite-(Ce) and perrierite-(La). Non-REE bearing minerals present are quartz, thorianite and thorite.
Figure 1. (a) Placer dump no. 2bis of the Mochalin Log REE deposit where the nodule with zilbermintsite-(La) was collected; (b) freshly collected nodules with numerous REE-bearing minerals.
Figure 2. Zilbermintsite-(La) (Zlb-La) closely associated with ferriperbøeite-(La) (Fpbo-La), ferriallanite-(Ce) (Faln-Ce) and törnebohmite-(La) (Tbh-La). Polished section. SEM (BSE) image, specimen no. ML 196-2bis.
According to the distribution scale of RΕΕ minerals found at the Mochalin Log deposit (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Škoda, Polekhovsky, Agakhanov, Belakovskiy, Kuznetsov, Britvin and Pushcharovsky2020a), zilbermintsite-(La) should be considered as very rare: among 300 nodules with REE-bearing minerals investigated by us, it was found in only one of them. In term of its internal structure, this nodule belongs to type 2 (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Škoda, Polekhovsky, Agakhanov, Belakovskiy, Kuznetsov, Britvin and Pushcharovsky2020a) where REE minerals form thin and chaotic intergrowths with each other. However, the occurrence of zilbermintsite-(La) is restricted to along the contact between törnebohmite-(La) and ferriperbøeite-(La) grains.
Physical properties and optical data
The new mineral is light brown and translucent in thin fragments, with a brown streak and vitreous lustre. It does not fluoresce under ultraviolet light. One direction of good cleavage and one direction of imperfect cleavage were observed under the scanning electron microscope (SEM); from the structure data (see below) we assume that good cleavage could be on {100}. Parting is not observed. Zilbermintsite-(La) is brittle with a fracture stepped in the cleavage direction and uneven across it (observed under the SEM). The Vickers micro-indentation hardness (load 150 g) is equal to 838 kg mm–2 (range 767–912, n = 4) corresponding to ca. 6 on the Mohs scale. Density could not be measured due to lack of sufficiently large monomineralic fragments and the absence of heavy liquids with suitable density. A density value calculated using the empirical formula and the unit-cell parameters from single-crystal X-ray diffraction (XRD) data is 4.684 g cm–3.
In transmitted plane-polarised light zilbermintsite-(La) is weakly pleochroic in brown tones. The absorption scheme is Z > Y > X. Optically it is biaxial (+), with α = 1.805(7), β = 1.812(7), γ = 1.867(8) (589 nm), 2Vmeas. = 40(15)° and 2Vcalc. = 40°. Dispersion of optical axes is very weak, r < v. Optical orientation was not determined due to the anhedral shape of the grains.
Raman spectroscopy
The Raman spectra of zilbermintsite-(La) (Fig. 3) were obtained from polished section by means of a Horiba Labram HR Evolution spectrometer. This dispersive, edge-filter-based system is equipped with an Olympus BX 41 optical microscope, a diffraction grating with 600 grooves per millimetre, and a Peltier-cooled, Si-based charge-coupled device (CCD) detector. After careful tests with different lasers (473, 532 and 633 nm), the 532 nm Nd:YAG diode laser with the beam power of ~5 mW at the sample surface was selected for spectra acquisition to minimise analytical artefacts. A Raman signal was collected in the range of 80–4000 cm–1 with a 100× objective (NA 0.9), the system was operated in confocal mode, beam diameter was ~1 μm and the axial resolution ~2 μm. No visual damage of the analysed surface was observed at these conditions after the excitation. Wavenumber calibration was done using the Rayleigh line and low-pressure Ne-discharge lamp emissions. The wavenumber accuracy was ~0.5 cm–1 and the spectral resolution was ~2 cm–1. Band fitting was done after appropriate background correction, assuming combined Lorentzian–Gaussian band shapes using the Voigt function.
Figure 3. The Raman spectrum of zilbermintsite-(La) excited by 532 nm laser: in the 100–1500 cm–1 region and in the 2500–4000 cm–1 region (inset in the upper right corner). The measured spectrum is shown by dots. The curve matching to dots is a result of spectral fit as a sum of individual Voigt peaks shown below the curve.
The assignment of the Raman bands is as follows: bands in the range of 3595, 3313 and 3199 cm–1 correspond to O–H-stretching vibrations; 1013 cm–1 corresponds to stretching vibrations of Si–O–Si fragments in Si2O7 groups; 845 to 968 cm–1 to stretching vibrations of apical Si–O bonds; 550 to 700 cm–1 are Al–O⋅⋅⋅H bending vibrations and stretching vibrations of Si–Ob–Si bonds; and 300 to 630 cm–1 are due to mixed modes and overlapping bands of (Al,Fe3+,Mg)–O-stretching vibrations, as well as bending vibrations of silicate groups. Below 300 cm–1 bands are due to lattice modes involving REE–O-, Ca–O- and Fe2+–O-stretching vibrations and librational vibrations of silicate groups.
The Raman spectrum of zilbermintsite-(La) is similar to those of radekškodaite-(La) and radekškodaite-(Ce) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Ksenofontov, Agakhanov, Belakovskiy, Polekhovsky, Kuznetsov, Britvin, Pushcharovsky and Nestola2020b). It is also similar to the spectra of västmanlandite-(Ce) (Holtstam et al., Reference Holtstam, Kolitsch and Andersson2005) and ferriperbøeite-(La) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Škoda, Polekhovsky, Agakhanov, Belakovskiy, Kuznetsov, Britvin and Pushcharovsky2020a) in the region of 300–1000 cm–1 but differs significantly from them in the region of stretching vibrations of Si–O–Si fragments (1000 to 1100 cm–1) and stretching vibrations involving REE and bivalent cations forming low-force-strength bonds (below 300 cm–1). In particular, in the zilbermintsite-(La) spectrum, the bands of stretching vibrations of Si–O–Si fragments are significantly weaker than in the ferriperbøeite-(La) spectrum that reflects a lower Si2O7:SiO4 ratio in the new mineral.
In the Raman spectrum of zilbermintsite-(La), three bands of O–H-stretching vibrations are observed, which corresponds to the number of independent sites occupied by OH groups. The band at 687 cm–1 in the Raman spectrum of zilbermintsite-(La) is close to the strong band at 689 cm–1 in the Raman spectrum of allanite-(Ce) (Andò and Garzanti, Reference Andò and Garzanti2014; Čopjaková et al., Reference Čopjaková, Škoda, Vašinová Galiová, Novák and Cempírek2015) and ferriallanite-(Ce) (Sobek et al., Reference Sobek, Losos, Škoda, Holá and Nasdala2023) and can be assigned to the symmetric stretching vibrations of Si–Ob–Si bonds of the epidote-type module (Wang et al., Reference Wang, Han, Guo, Yu and Zeng1994).
Chemical data
Chemical data (five spot analyses) were obtained using a Cameca SX-100 electron microprobe (WDS mode, acceleration voltage of 15 kV, a beam current of 20 nA and a 3 μm beam diameter). The spectral interference of FKα and CeMζ were manually corrected using empirically determined correction factors. The content of H2O was not determined directly due to the scarcity of pure material and was calculated by stoichiometry on the basis of O28(OH,F)3 taking into account that bond-valence sums for the O(1), O(2) and O(3) sites are close to 1 (see below). Both the crystal structure and Raman spectroscopy data confirm the presence of OH groups and the absence of B–O, C–O and N–O bonds in the mineral. Analytical data are given in Table 1. Contents of other elements with atomic numbers higher than carbon are below detection limits.
Table 1. Chemical composition of zilbermintsite-(La).
*Total Fe content corresponding to FeO content of 6.78 wt.% was divided into Fe2+ and Fe3+ based on the occupancies of the M1–M4 sites found from crystal structure refinement data: see Table 8. Thus, 2.82 wt.% FeO and 4.40 wt.% Fe2O3 were calculated based on the found Fe2+: Fe3+ atomic ratio: 0.64 Fe2+ and 0.90 Fe3+ pfu, respectively (Fe3+: Fe2+ = 1.40). **For total iron calculated as FeO. ***Calculated by stoichiometry: (OH + F) = 3 apfu. S.D. – standard deviation.
Zilbermintsite-(La) grains are chemically homogeneous and the WDS analyses reveal the uniform chemical composition with only minimal inter-REE chemical variation. Lanthanum prevails significantly among REE in all analyses. The empirical formula calculated on the basis of O28(OH,F)3 atoms per formula unit (apfu) is (Ca0.94La2.56Ce2.18Nd0.20Pr0.10Th0.02)Σ6.00(Al2.96Fe3+0.90Fe2+0.64Mg0.34Mn0.13Ti0.03)Σ5.00Si7.00 O28[(OH)2.42F0.58].
The ideal formula is (CaLa5)(Fe3+Al3Fe2+)[Si2O7][SiO4]5O(OH)3 which requires CaO 3.46, La2O3 50.18, FeO 4.43, Al2O3 9.43, Fe2O3 4.92, SiO2 25.92, H2O 1.66, total 100 wt.%.
The Gladstone–Dale compatibility index (1 – K p/K c) calculated for zilbermintsite-(La) using its empirical formula and the unit-cell parameters determined from single-crystal XRD data is 0.013 rated as superior (Mandarino, Reference Mandarino1981).
Zilbermintsite-(La) does not react with either cold hydrochloric or nitric acid.
X-ray crystallography and crystal structure
Powder XRD data were collected using a Rigaku R-AXIS Rapid II single-crystal diffractometer equipped with a cylindrical image plate detector (radius 127.4 mm) using Debye-Scherrer geometry, CoKα radiation (rotating anode with VariMAX microfocus optics), 40 kV and 15 mA. Angular resolution of the detector is 0.045°2θ (pixel size 0.1 mm). The data were integrated using the software package Osc2Tab (Britvin et al., Reference Britvin, Dolivo-Dobrovolsky and Krzhizhanovskaya2017). Powder XRD data for zilbermintsite-(La) are given in Table 2 in comparison to that calculated from single-crystal XRD data using the Atoms 5.1 program (Dowty, Reference Dowty2000). Parameters of monoclinic unit cell were calculated from the observed d spacing data using UnitCell software (Holland and Redfern, Reference Holland and Redfern1997) and are as follows: a = 8.965(4), b = 5.735(3), c = 25.096(9) Å, β = 116.68(4)° and V = 1152(8) Å3.
Table 2. Powder X-ray diffraction data (d in Å) of zilbermintsite-(La).
*For the calculated pattern, only reflections with intensities ≥2 are given.
**For the unit-cell parameters calculated from single crystal data.
The strongest reflections are given in boldtype.
For the single-crystal XRD study, a grain of zilbermintsite-(La), 0.04 × 0.13 × 0.16 mm in size, extracted from the polished section and analysed previously using electron microprobe and Raman spectroscopy, was mounted on a glass fibre and examined with an Xcalibur S single-crystal diffractometer equipped with a CCD detector. A full sphere of three-dimensional data was collected. Data reduction was performed using CrysAlisPro software Version 1.171.39.46 (Rigaku Oxford Diffraction, 2018). The data were corrected for Lorentz factor, absorption and polarisation effects.
The crystal structure of zilbermintsite-(La) was solved by direct methods and refined with the use of the SHELX software package (Sheldrick, Reference Sheldrick2015) to R 1 = 0.0757 for 2677 unique reflections with I > 2σ(I). Two O atoms (O1 and O19) were refined in isotropic approximation of displacement parameters. The crystal data, data collection information and structure refinement details are given in Table 3, atomic coordinates and thermal displacement parameters of atoms in Tables 4 and 5, and selected interatomic distances in Table 6. Bond valence calculations are given in Table 7. Assigned site occupancies (s.o.f.) based on the results of the crystal structure refinement are given in Table 8. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Table 3. Crystal data, data collection information and structure refinement details for zilbermintsite-(La).
* w = 1/[σ2(F o2) + (0.1030P)2 + 21.2454P]; P = {[max of (0 or F o2)] + 2F c2}/3.
Table 4. Atom coordinates and equivalent thermal displacement parameters (U eq, in Å2) and site occupancy factors (s.o.f.) for zilbermintsite-(La).
* Fixed during the refinement. For M(1) Al vs Fe was refined (e ref 20.15), for M(3) Fe vs Mg was refined (e ref 24.46) (see Table 8). Thus, on the basis of chemical data and e ref M(1) site was assumed to be occupied by (Fe0.57Mg0.26Al0.17) possibly with minor Ti4+ admixture and M(3) site by (Fe0.75Mn0.15Mg0.10).
** U iso.
Table 5. Anisotropic displacement parameters (in Å2) in zilbermintsite-(La).
Table 6. Selected interatomic distances (Å) and angle (°) in the structure of zilbermintsite-(La).
Table 7. Bond valences for zilbermintsite-(La).
Note. Parameters from Gagné and Hawthorne (Reference Gagné and Hawthorne2015) and, for possible H-bonding, from Ferraris and Ivaldi (Reference Ferraris and Ivaldi1988). The values were calculated taking into account the refined occupancies for the M(1–4) and A(1) sites. For better clarity the split character of the O(23) and O(24) sites was not taken into account and these sites were considered as averaged with no splitting [the cations–O(23)/O(24) distances for bond-valence calculations including H-bonding were taken for non-split sites]. Slightly low values of BVS for these sites are typical for the members of the radekškodaite group. For Fe cations in the M(3) site the bond-valence parameters of Fe2+ were used. The bond-valence parameters of Ce3+ were used for A(1–6) sites. The values of BVS for A(2–6) sites could be increased taking into account the presence of La3+ cations. Bond valences are calculated disregarding the minor occurrence of F.
Table 8. M1–M4 sites occupancies (s.o.) suggested for zilbermintsite-(La) based on the results of the crystal structure refinement.
Note. SSFexp and SSFcalc are the experimental and calculated site-scattering factors, respectively, normalised to unity.
The crystal structure of zilbermintsite-(La) (Fig. 4) consists of the chains of edge-sharing octahedra running along the b axis: single chains of the M(2)- and M(4)-centred octahedra and branched chains with the M(1)-centred octahedra in the central part, and the M(3)-centred octahedra attached to them from both sides. The chains are linked via isolated [SiO4] tetrahedra and disilicate groups [Si2O7] [the angle Si–O–Si in the disilicate group is equal to 149.6(9)°]. The A(1–6) sites occur in large cavities. Even though La3+ is a dominant REE in zilbermintsite-(La), the Ce3+ scattering curve was used during the structure refinement for the A(1–6) sites because of the significant content of Ce3+ cations, minor amounts of heavier Nd3+ and Pr3+, and very minor amount of Th4+. According to the s.o.f. refinement, the A(2–6) sites are fully occupied by REE cations. The A(1) site is Ca-dominant with minor REE admixture; Ca vs Ce was refined that gave Ca0.955(7)Ce0.045(7). There are four octahedrally coordinated M sites, M(1–4). For the M(1), M(2) and M(4) sites, Al vs Fe was refined, and for the M(3) site, Fe vs Mg was refined. The M(2) and M(4) sites are Al-dominant with the refined occupancy Al0.936(16)Fe0.064(16) and Al0.91(2)Fe0.09(2), respectively. These sites centre the smallest octahedra with the mean distances 1.912 Å [M(2)–O] and 1.918 Å [M(4)–O]. The M(1)-centred octahedron is slightly larger, with the mean M(1)–O distance of 1.995 Å. This site is Fe3+-dominant, with refined occupancy Fe0.55(2)Al0.45(2), corresponding to number of electrons (e ref) = 20.2(4). On the basis of the chemical information, the M(1) occupancy was therefore fixed to Fe0.57Al0.17Mg0.26 in the final stage of refinement. The largest M(3) octahedron with the mean M(3)–O distance of 2.136 Å is predominantly occupied by Fe2+ with subordinate Mn, Mg and Fe3+ in the atomic ratio Fe2+:Mn:Fe3+:Mg = 0.64:0.15:0.11:0.10, according to both electron microprobe data and refined number of electrons (e ref = 24.46). During the refinement, splitting was found for the O(23) and O(24) sites which deviate from the m plane as also occurs in the structures of radekškodaite-(La) and radekškodaite-(Ce).
Figure 4. The crystal structure of zilbermintsite-(La). SiO4 tetrahedra are red. Alternation of epidote-type slabs (E) and törnebohmite-type slabs (T) is shown. The unit cell is outlined. Drawn using Diamond version 3.2. (Crystal Impact, 2014).
Bond-valence calculations (Table 7) confirm the above conclusions about distribution of cations between different sites: specifically, Fe3+ at M(1) and Fe2+ at M(3).
Discussion
Zilbermintsite-(La) is isotypic to radekškodaite-(La) and radekškodaite-(Ce) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Ksenofontov, Agakhanov, Belakovskiy, Polekhovsky, Kuznetsov, Britvin, Pushcharovsky and Nestola2020b). They are considered as ET2-type polysomes in which structures can be described as a regular alternation of one module of the epidote-type structure (E) with two modules of the törnebohmite-type structure (T2) (Kasatkin et al., Reference Kasatkin, Zubkova, Pekov, Chukanov, Ksenofontov, Agakhanov, Belakovskiy, Polekhovsky, Kuznetsov, Britvin, Pushcharovsky and Nestola2020b). Zilbermintsite-(La) differs from radekškodaites by the Fe3+ predominance at the M(1) site which is Al-dominant in the latter. The E module in zilbermintsite-(La) has ferriallanite-(La) composition whereas in radekškodaite-(La) and radekškodaite-(Ce) it has allanite-(La) and allanite-(Ce) compositions, respectively. According to the mineral group nomenclature (Mills et al., Reference Mills, Hatert, Nickel and Ferraris2009), the radekškodaite group could be established that includes radekškodaite-(La), radekškodaite-(Ce) and zilbermintsite-(La). For their comparison see Table 9.
Table 9. Comparative data for minerals of the radekškodaite group.
One of the crucial points for the definition of zilbermintsite-(La) is the distribution of M cations including di- and trivalent iron. The arrangement of Fe2+, Fe3+, Al, Mg and Mn between M sites is suggested taking into account electron microprobe data (Table 1), refined numbers of electrons (Tables 4 and 8), M–O distances (Table 6) and BVS values (Table 7). Unfortunately, we could not determine the valence of iron directly, by means of wet chemical analysis or Mössbauer spectroscopy due to the scarcity of pure material. The methods using electron microprobe data (measurement of chemical shift of Fe analytical lines depending on iron valence state) are not effective in this case because of a relatively low content of total Fe in the mineral (ca. 5 wt.% Fe). However, the distribution of species-defining Fe2+ and Fe3+ between the two major iron M sites, M1 and M3, is reliably found from the structure data. The refined numbers of electrons demonstrate that both these sites are Fe-dominant (Tables 4 and 8) and interatomic distances show that the M1 site contains Fe3+ (M1–O = 1.995 Å) whereas Fe2+ is concentrated in M3 (M3–O = 2.136 Å): Table 6. This conclusion is clearly confirmed by bond valence calculations: the BVS values for M1 and M3 are 2.90 and 2.20 valence units, respectively (Table 7). Finally, the correctness of the site occupancies given in Table 8, as well as the Fe2+:Fe3+ ratio found, are confirmed by the lowest value of the Gladstone–Dale compatibility index.
The genetic position of zilbermintsite-(La) respects the general geochemical and crystal chemical trend in the mineral assemblage of some zonal REE nodules from Mochalin Log deposit. The REE content decreases and the silicate (Si, Al, Fe, Mg and Ca) content increases from central parts of the nodules outwards. The occurrence of zilbermintsite-(La) is situated at the contact between törnebohmite-(La) and ferriperbøeite-(La) and it is in accordance with the generalised genetic sequence (centre to rim) of the epidote–törnebohmite polysomatic series: törnebohmite → radekškodaite/zilbermintsite → perbøeite/ferriperbøeite → allanite/ferriallanite → REE-rich epidote. This mineral assemblage also reflects the changes in the trend of alternation between epidote (E) and törnebohmite (T) modules. The abovementioned mineral sequence can be schematised as T→ET2→ET→E. According to the textural relations observed from dozens of REE nodules investigated, the minerals of the epidote–törnebohmite polysomatic series are rather cogenetic and probably their distribution mirrors the different activity of REE and the other necessary elements (Si, Al, Fe, Ca, Mg, etc) in different parts of the nodules during crystallisation.
Acknowledgements
We thank Associate Editor Mihoko Hoshino, Referee Pietro Vignola and two anonymous reviewers and Principal Editor Stuart Mills for constructive comments that improved the manuscript. The PXRD studies have been performed at the X-ray Diffraction Centre of St. Petersburg State University.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2024.17.
Competing interests
The authors declare none.
