1. Introduction
Frankamenite, K
3Na
3Ca
5[Si
12O
30]F
3(OH)(H
2O), is a complex alkaline silicate and unique mineral found today in only one deposit in the world (Murun Massif, Russia). According to the silicate minerals hierarchy of Day and Hawthorne [
1], frankamenite is a tube silicate with a one-dimensional tetrahedral polymerization. The [Si
12O
30]
12− -tubes in frankamenite extend along the
c-axis and consist of two linked ribbons of six-membered rings. This tube is topologically identical to the
3V12 in the charoite structure (see [
2] and therein). The silicon–oxygen radical has the designation
3T12, where T means “tetrahedron”, 3 is the connectivity of the tetrahedron, and 12 is the number of such tetrahedra in the geometrical repeat unit [
1]. The tubes are connected to corrugated sheets of Ca
2+ and Na
+- octahedra parallel to the
c-axis. The internal channels are occupied by K
+ ions and additional H
2O groups. The silicate tubes and sheets of (CaO
4(OH)F)
8−- and (NaO
4F
2)
9−-octahedra occur in layers, alternating along the
c-axis.
The same type of tube (
3T12) can be found in the crystal structures of canasite [
3] and fluorcanasite [
4], insofar as frankamenite is the triclinic polymorph of the above minerals. Canasite contains four (OH)
− sites, while in fluorcanasite there are two F sites, an (OH) site, and a mixed (F,OH) site, all of which are bound to the Na
+ and Ca
2+-octahedra.
The minerals of the canasite group are listed for comparison in
Table 1. The crystal structure of canasite, discovered in the Khibiny Massif in the 1950s, has been determined over several decades [
5,
6] and was successfully refined by Rozhdestvenskaya et al. in 1988 [
3]. They also noted differences in the chemical compositions of the canasite minerals from the Khibiny and Murun Massifs [
3], the latter of which was also characterized by Evdokimov and Regir in 1994 [
7]. In 1992, Nikishova et al. presented the results of crystal structure refinement of the mineral from Yakutian charoitites (Murun Massif), calling it triclinic canasite [
8]. The structure of this variety was refined within the space group
P1, which explained the difficulties [
3] encountered while working with these samples and the presence of additional reflections in the diffraction patterns. The crystal structure and interatomic distances slightly differ from those of monoclinic canasite from the Khibiny Massif. In 1994, triclinic canasite was approved by the CNMNC as a new mineral called frankamenite [
9]. It was named after the Russian mineralogist and crystallographer, professor of St. Petersburg State University, V.A. Frank-Kamenetsky. An interesting fact is that charoite was also initially mistaken for lilac canasite, but after a detailed study, it became recognized as another mineral species [
10]. The last paper on the refinement of its structural features was published in 1996 [
11]. In 2003, Rastsvetaeva et al. [
12] reported on canasite from the Khibiny Massif containing a high content of fluorine. This species differs from canasite in symmetry (space group
Cm vs.
C2/
m of canasite) and occupancies of the octahedral positions. These characteristics enabled its approval in 2007 by the CNMNC as a new mineral called fluorcanasite [
4]. According to [
12], fluorcanasite can be considered either as a fluorine analogue of canasite or as a monoclinic analogue of frankamenite. The intensity of studies of minerals of the canasite group has noticeably declined over the past 15 years, despite the fact that, for example, the optical and vibrational properties of frankamenite are yet to be studied.
Deciphering and interpreting the vibrational modes of frankamenite remain questionable. However, having structural data (
P1 (No. 1), point group
C1 (1)), it is possible to calculate a set of phonon modes in the center of the Brillouin zone based on factor–group analysis [
13]: Γ = 183A (using
https://www.cryst.ehu.es/rep/sam.html (accessed on 15 June 2023)). The latter indicates the polarity of the frankamenite crystal.
Within the last few decades, canasite-based materials have received unprecedented interest. These compounds are potential glass–ceramic materials. Glass–ceramics are polycrystalline solids obtained by the controlled crystallization of glass during a heat treatment process that contain one or several crystalline phases, and in most cases, a residual glassy phase. Bioactive silicate glass–ceramics can be used as long-term implants due to their relatively high mechanical strength and only negligible and slow solubility of silicates in human body fluids [
14]. Studies concerning the development of glass–ceramics, which are potentially suitable for biomedical applications, have reported that CaO, P
2O
5, and F, regardless of their origin, must be the essential compositional components of glass–ceramic systems ([
15] and therein). One of the potential glass–ceramic materials is canasite [
16]. The stoichiometric canasite forms a stable glass, requiring only a few percent excess fluoride to achieve efficient nucleation, and it is easy to produce essentially monophase glass–ceramics [
17].
According to [
18], an ideal osteoconductive bioglass–ceramic should be free of Al
2O
3. Fluorcanasite (with composition K
2Na
4Ca
5Si
12O
30F
4) glass–ceramics were obtained by controlling the glass crystallization in the SiO
2–K
2O–Na
2O–CaO–CaF
2 system [
19]. Bandyopadhyay-Ghosh et al. (2010) [
20] reported modified fluorcanasite glass–ceramic compositions based on changing the Na
2O and CaO molar ratios and adding P
2O
5.
Due to good castability combined with excellent cell response in vitro, modified fluorcanasites have great potential for use as load-bearing, osteoconductive biomaterials in orthopedics, implantology, and reconstructive facial surgery [
18,
20,
21], whereas stoichiometric fluorcanasite glass–ceramics showed poor mechanical properties and crumbled during mechanical processing [
22]. By modifying the compositions, the biological activity of the modified glass–ceramics can be significantly increased [
23,
24]. The addition of lithium disilicate increases the chemical resistance of fluorcanasite-based glass–ceramics [
25,
26]. Vyas et al. (2022) [
27] found out that with increased fluorcanasite content in the composition, hydrophilicity also increases over the entire surface of the sample, and, subsequently, cell adhesion and proliferation is raised.
Fluorcanasite glass–ceramics have excellent mechanical properties [
28]. The type, size, and volume ratio of the primary crystalline phases in the material directly affect the properties of canasite-based glass–ceramics [
29]. Phase evolution in canasite-based compositions is complex, and small compositional modifications significantly change the crystallizing product and the nucleation mechanism [
30]. The main crystalline phases are canasite and frankamenite, often having an interpenetrating lattice structure [
31]. At relatively low temperatures, frankamenite predominantly nucleates homogeneously throughout the glass [
30]. According to published data [
30,
31], the nucleation temperature for the phases of the canasite group is ~520 °C, and the temperature of crystal growth is ~780 °C.
Glass–ceramics find applications in cookware, hermetic sealing, and electronic substrates, among others. One of the applications of glass–ceramics is for sealing solid oxide fuel cell (SOFC) components, mainly using alkaline earth-metal-based aluminosilicate glass–ceramics [
32].
It is important to understand that in order to create such relevant modern materials, the widest possible knowledge of the compounds used is necessary. Recently, Kaneva et al. (2021) [
33] made the first attempt to study the vibrational properties of this mineral. However, the data are only descriptive. This work reveals new insights into the crystal–chemical and optical properties of frankamenite.
4. Discussion
In the crystal structure of minerals belonging to the canasite group, listed in
Table 1, the
3T12 tubes [
1] connect to ridged sheets of M-octahedra (occupied by Ca
2+ and Na
+) and alternate along the
c-axis. Inside each tube, there are K
+ ions and an additional (H
2O) group. In the crystal structure of frankamenite and canasite [
3], there are three positions of K atoms, while in fluorcanasite, one of the three positions is split into two around the center of symmetry [
12]. Canasite has four (OH) sites [
3], frankamenite has two F and two (OH) sites, while fluorcanasite has two F sites, an (OH) site, and a mixed site that usually contains more F than (OH) [
12]. All of these sites are bonded to
M-octahedra. Frankamenite (sp. gr.
P1) contains eight independent octahedral positions (
Table 5), while canasite (sp. gr.
C2/
m) and fluorcanasite (sp. gr.
Cm) crystal structure has six
M-sites. In frankamenite, one
M-site is occupied by Na, one position is filled by Ca, and the remaining octahedral positions are occupied simultaneously by Na and Ca. Fluorcanasite has two Na positions, three Ca sites, and a mixed Na+Ca position [
12]. According to the data in [
3], in the crystal structure of canasite, Ca is ordered over four positions, while Na occupies the remaining two M-positions. Thus, the minerals of the canasite group exhibit significant crystal chemical differences, expressed primarily in symmetry and chemical composition, and they occupy M-octahedral sites and positions of additional anions while having a similar [Si
12O
30]
12− tube tetrahedral framework.
It should be noted that the Si–O values for Si-tetrahedra in the model do not exhibit significant deviations (0.31–2.51%,
Table S6 of Supplementary Materials). Effective coordination numbers of Si-tetrahedra have minor deviations from the experimental ones: 0.03–3.63%. The average deviations of the tetrahedron volume and volume of the sphere fitted to the positions of the vertices of the tetrahedron are 3.4%. The above parameters of deviations for M-octahedra are slightly higher: <cation–anion> distance deviation = 0.08–3.78%, mean deviation for M1–M8 octahedra volumes and fitted sphere volumes are 4.38 and 4.61%, respectively, and ECoN deviation = 4.12%, which is the average of eight M-octahedra. The largest deviations in geometrical parameters are noted for K-polyhedra (ECoN and Vp values,
Table S6 of Supplementary Materials). However, for r and Vs values, the deviations lay in the ranges of 0.13–2.11 and 1.64–5.53%, respectively, except for a few single values of deviation that do not exceed 16%. In general, it can be concluded that the simulated structural model is very close to the experimentally obtained model of the natural frankamenite crystal structure. Moreover, in the optimized crystal structure model, the coordinates of the H positions for the hydroxyl groups and water molecules are established.
The two types of channels are distinguished inside the crystal structure of frankamenite. Channel I is extended along the
c-axis and delimited by the eight-membered rings of tetrahedra (
Figure 12a). The shortest distances between oppositely located oxygen atoms in the ring are 7.435(8) × 6.066(10) Å. Channel II is delimited by the eight-membered tetrahedral rings along the
a-axis (
Figure 12b). The ring cross section has free diameters of 4.793(9) × 4.134(7) Å. The
effective channel width (
ecw)—defined as the distance between the oxygen atoms in the smallest
n-ring or the smallest free aperture subtracted by 2.7 Å when the oxygen ionic radius is assumed to be 1.35 Å [
58]—for channel I is 4.74 × 3.37 Å, and for channel II, it is 2.09 × 1.43 Å. According to [
59], a minimum
ecw of 3.2 Å is required for a crystalline substance to be defined as microporous. In the frankamenite structure, only channel I is suitable for this parameter. The pores inside this channel of frankamenite have larger dimensions with respect to the channel aperture and, therefore, theoretically may contain guest atoms larger than K and water molecules, occupying these cavities in the natural mineral.
In the FTIR spectra of frankamenite, the three bands attributed to O–H stretching modes at 3500, 3555, and 3608 cm
−1 were found (
Figure 3). According to the Libowitzky equations [
60], the O···O distances corresponding to these bands are 2.90 Å and 3.00 Å. The O···H distances are approximately 2.05, 2.15, and 2.55 Å. Beckenkampet al. (1992) [
61] proposed the equation for the determination of the shortest distance from OH-anion to metal. Following this equation, the distances attributed to the absorption bands at 3608 cm
−1 are equal to 2.40 Å, which is close to the distance between metal and OH given in
Table S2 of Supplementary Materials. Therefore, the band at 3698 cm
−1 could be attributed to two types of OH-anions in frankamenite. The bands at 3500 and 3555 cm
−1 are decreased when heated. The aperture of channel I is larger than that of channel II. The water molecules could easily move within channel I (
Figure 12), and dehydration occurs there at a lower temperature than within channel II [
51,
62].
According to the general concepts, the polarized spectra maintain a constant relationship between the intensity of any mode and the orientation of the corresponding chemical bond in the crystal [
63]. H
2O molecules and OH-radicals at 30 °C are predominantly oriented along the crystallographic direction
b (schematic image in
Figure 8c). Apparently, H
2O molecules undergo rotation when the crystal is cooled (one of the supposed schemes of H
2O reorientation is shown in
Figure 8). At T = −70 °C, ν
3(H
2O) are characterized by the lowest relative intensity (
Figure 8b and
Figure 9), which presumably indicates the orientation of the H
2O molecule orthogonal to the laser polarization vector. The subsequent increase in Iν
3 (H
2O) indicates the continued rotation of the molecule. OH-radicals reorient in a similar way; however, their turn to the position orthogonal to the polarization vector occurs at lower temperatures (at ~−150 °C). The latter is likely due to the peculiarities and unequal environments of H
2O and OH (as mentioned above).
The Ce
3+ may substitute Ca
2+ ions in the M3–M8 positions. The average energies of the Ce
3+ f-d transitions in the silicates containing polyhedral layers of Ca cations correlate with the average cation–oxygen distance (
Figure 13). Taking into account the observed cation and anion distribution, the following isomorphous substitution scheme could be suggested for frankamenite: Ca
2+ + OH
−/F
– ↔ Ce
3+ + O
2− or 2Ca
2+ + O
2− ↔ Ce
3+ + □ + OH
−.
Frankamenite contains cationic positions (M sites) that can be easily doped with transition metal (e.g., Mn2+, Cr3+, Fe2+, Fe3+) and lanthanide ions with other average energy of f-d transitions that could be useful for the tuning of Ce3+ luminescence. Therefore, frankamenite could be a prospective material for ion exchanger, novel phosphors, and luminophores.
5. Conclusions
In this study, the crystal–chemical and optical properties of frankamenite, a member of the canasite group, were examined in detail. This rare and unique alkaline silicate mineral is currently only known to exist in the Murun Massif deposit (Russia). A comprehensive crystal–chemical analysis was conducted on mineral samples, and for the first time, the optical and vibrational properties of frankamenite were investigated. The crystal–chemical formula of frankamenite is K2.97Ba0.01Na2.74Ca5.03Mn0.08Sr0.03Fe0.01[Si11.99Al0.01O30](F3(OH))·0.64H2O.
In the crystal structure of compound, the [Si12O30]12− tetrahedral tubes are connected to M-octahedral sheets, alternating along the c-axis. There are eight independent octahedral positions in frankamenite, and the site population of each position has been proposed based on chemical and structural studies. The pores within the channels of frankamenite structure have larger dimensions compared with the channel apertures. As a result, the mineral has the potential to contain additional guest atoms and groups (such as K or water molecules), which can move within the channel during heating. In fact, dehydration of frankamenite occurs at temperatures above 250 °C. Thermo-Raman spectroscopy detected the thermally induced reorientation of H2O molecules and OH− groups in the structure of frankamenite.
Furthermore, optical absorption and luminescence investigations revealed that Mn2+ and Ce3+ ions may substitute Ca2+ ions in the M3–M8 positions of frankamenite. Consequently, the cationic positions (M sites) in frankamenite can easily be doped with transition metal and lanthanide ions. As a result, frankamenite holds potential as a material for ion exchange, novel phosphors, and luminophores.
Thus, the data obtained in this work show that, in addition to glass–ceramic production, the mineral frankamenite is a promising material for the use of compounds based on its crystal chemistry in the research field of photonics. The results also demonstrated the significant potential of utilizing ab initio calculations in the examination of natural compounds. Through the integration of SCXRD, EMPA, and Raman and IR spectroscopy techniques, alongside ab initio calculations, a comprehensive analysis of the material’s spectroscopic features can be carried out, accounting for its structural characteristics.