Introduction
Fluorcarletonite, KNa4Ca4Si8O18(CO3)4(F,OH)⋅H2O, from the Malyy Murun syenite massif, Aldan Shield, Siberia, Russia, was approved recently as a new mineral species by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2019-038, Kaneva et al., Reference Kaneva, Radomskaya, Suvorova and Mitichkin2019). The first description of this mineral was provided by Kaneva et al. (Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a), who also explained the difference with respect to the structural analogue carletonite, KNa4Ca4Si8O18(CO3)4(OH,F)⋅H2O (Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023). The latter is a rare mineral named after Carleton University in Ottawa, and found in the Poudrette quarry, Mont Saint-Hilaire, Canada (Chao, Reference Chao1971, Reference Chao1972). Fluorcarletonite and carletonite undergo replacement of F ions with OH groups (or vice versa) which causes no significant structural differences as the fluorine and OH radii are very close (1.30 and 1.36 Å, Shannon, Reference Shannon1976) and the hydrogen does not significantly affect the local environment. The reported fluorine content in the crystal chemical formula of fluorcarletonite varies in the range of 0.53–0.98 atoms per formula unit (apfu) (Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a, Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) whereas for carletonite from Mont Saint-Hilaire massif F = 0.41 apfu (Chao, Reference Chao1971) or 0.36 ≤ F ≤ 0.41 apfu (Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) has been reported.
In the peripheral regions of zoned grains of Russian fluorcarletonite, the carletonite composition has been detected (Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a, Reference Kaneva, Radomskaya and Shendrik2022) which implies that, conversely, fluorcarletonite might occur in the rocks of Mount Saint-Hilaire Massif, Canada. However, pure hydroxyl or fluorine end-members have not yet been found. The occurrence of carletonite in lateritic soils covering carbonatites, ijolites, nepheline syenites and fenites of the Bingo carbonate complex, Democratic Republic of Congo (Kasay, Reference Kasay2018; Kasay et al., Reference Kasay, Bolarinwa, Aromolaran, Nzolang and Mambo2021) should also be confirmed by more accurate chemical and X-ray diffraction investigations.
Fluorcarletonite and carletonite are described as double-layer sheet silicates based on the [4.82]16 net with an arrangement of upward-pointing tetrahedra (u) and downward-pointing tetrahedra (d) showing (u2d2)4(udududud)4 configuration and 1:2.25 T:O ratio (Hawthorne et al., Reference Hawthorne, Uvarova and Sokolova2019). In detail, [Si8O18]4– sheets, extending in the a–b plane, consist of four-member rings with two upward- and two downward-pointing tetrahedra, and eight-member rings with alternately upward- and downward-pointing tetrahedra. Two adjacent single layers are interconnected by sharing a common oxygen of downward-pointing tetrahedra. The [Si8O18]4– sheets are connected with sheets consisting of NaO42–F–(H2O) and NaO52–(H2O) octahedra, NaO82–, CaO72–F– or CaO82– and KO102– polyhedra. The structural role of H2O molecules is different, as one molecule is bonded to an interstitial cation and acts as a bond-valence transformer, the other one is non-transformer (Hawthorne, Reference Hawthorne1992). Two independent CO3-groups are linked to Na- and Ca-polyhedra.
The crystal structure of fluorcarletonite was determined experimentally at low temperature (100 K; Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a) and room temperature (RT, Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) but was also simulated in order to define the position of the hydrogens and the most energetically favourable orientations of the H2O (Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023). The geometrical parameters of fluorcarletonite were found very similar to those of carletonite although they are distinct mineral species with individual peculiarities. In particular, the carletonite structure exhibits a splitting of the K atom over two sites, with 89% and 6% approximate occupancy, whereas the K position in fluorcarletonite is ordered (Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023).
In Kaneva et al. (Reference Kaneva, Radomskaya and Shendrik2022, Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) the luminescence and the gemmological properties of fluorcarletonite were also completely characterised. The multi-coloured appearance of the fluorcarletonite-containing rocks, which is due to the association of various minerals, makes these samples very attractive gem materials for incorporation into jewellery and ornamental items (Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a, Reference Kaneva, Radomskaya and Shendrik2022).
The thermal behaviour of fluorcarletonite has not yet been assessed thoroughly. Chao (Reference Chao1971) studied the decomposition of carletonite by means of ex situ high-temperature X-ray powder diffraction (HT XRPD) analyses and differential thermal analysis. The structure breakdown was associated to the release of CO2 (but also of H2O) evidenced by a strong endothermic peak in the DTA curve at 692°C. Kaneva et al. (Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a) used thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses on fluorcarletonite and found that the mineral underwent dehydration from 45 to 315°C, loss of CO2 from 630 to 1134°C and defluorination from 1136 to 1500°C.
In the present study, the thermal behaviour of fluorcarletonite was appraised via a combination of experimental (in situ high-temperature single-crystal X-ray diffraction, HT SCXRD; ex situ high-temperature Fourier transform infrared spectroscopy, HT FTIR) and theoretical (ab initio simulations) methods, thus filling the gaps from previous studies and providing new insights into the structural variations of minerals with complex bond topology in relations to temperature changes.
Geological context and sample description
The fluorcarletonite studied occurs in charoitites of the Severny district at the Malyy Murun massif, Murun complex, NW Aldan Shield, Siberia, Russia. The ultra-agpaitic alkaline Murun complex was dated 137–128 Ma and had a complex geological history characterised by four igneous stages (early intrusive, main intrusive, volcanic and late intrusive) and by hydrothermal activity represented by quartz veins with rutile–brookite–anatase mineralisation (see more details in Vladykin, Reference Vladykin2009; Borovikov et al., Reference Borovikov, Vladykin, Tretiakova and Dokuchits2018; Ivanov et al., Reference Ivanov, Vladykin, Demonterova, Gorovoy and Dokuchits2018; Vladykin et al., Reference Vladykin, Borokovikov, Dokuchits and Thomas2018).
Kaneva et al. (Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a, Reference Kaneva, Radomskaya and Shendrik2022) carried out the mineralogical and petrographic examination of a polished slab of a fluorcarletonite-containing rock sample from the Malyy Murun massif and found that the mineral paragenesis is defined by the crystallisation of alkali silicates (fluorcarletonite, charoite, apophyllite-(KF), aegirine and pectolite), quartz, apatite and microcline, by the formation of copper and lead sulfides as well as native copper, and by the alteration of primary rock-forming minerals with formation of secondary apophyllite-(KF) and wollastonite caused by supergene conditions. Fluorcarletonite appears as allotriomorphic grains, blue in colour, and forming close intergrowths with fluorapophyllite-(K) and pectolite.
In the present study, fluorcarletonite from the same rock sample as in Kaneva et al. (Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a, Reference Kaneva, Radomskaya and Shendrik2022, Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) was considered. The authors reported for fluorcarletonite the following average crystal chemical formula: K0.99Na3.86Ca3.87Sr0.02Si7.99Al0.01O18(CO3)3.81(F0.60OH0.40)⋅1.42H2O.
Methods
In situ HT SCXRD
Single crystals of fluorcarletonite suitable for the X-ray diffraction investigation were selected under an optical microscope, glued on the tip of a glass fibre and mounted on a goniometer head. The in situ high-temperature X-ray diffraction experiment was carried out with a Bruker AXS APEX II diffractometer equipped with MoKα radiation (λ = 0.71073 Å), a CCD area detector and a home-made heating device (Zema et al., Reference Zema, Ventruti, Tarantino and Micelli2022). A crystal (labelled Fcarl_1, 0.50 × 0.21 × 0.20 mm3) with good diffraction behaviour was heated in air from 25 to 450°C in steps of 25°C, with a heating rate of 1°C/min. After each heating step, the crystal was equilibrated for ca. 30 min before the acquisition of a new data collection. A total of 18 data collections was acquired: those at 25 and in the T range 125–450°C lasted ca. 6 hours (2θmax = 66° and d hkl = 0.65 Å) whereas those from 50 to 100°C lasted 1 hour (2θmax = 60° and d hkl = 0.7 Å). The experiment was repeated by heating a second crystal (labelled Fcarl_2, 0.63 × 0.35 × 0.30 mm3) from 25 to 550°C with the same operating conditions. However, in this case all the data collections lasted ca. 1 hour (2θmax = 72° and d hkl = 0.60 Å). Attempts were also made to heat the Fcarl_2 crystal up to 625°C but loss of crystallinity was observed.
The diffractometer operated at 50 kV and 30 mA. The collection strategies were optimised with the Apex program suite (Bruker Reference Bruker2010); data reductions were done using the software SAINT (Bruker, Reference Bruker2007); empirical absorption corrections were applied using SADABS (Bruker, Reference Bruker2009); and structure refinements were performed with the program CRYSTALS (Betteridge et al., Reference Betteridge, Carruthers, Cooper, Prout and Watkin2003) in the space group P4/mbm using reflections with I > 3σ(I) and starting from the atomic coordinates of fluorcarletonite reported in Kaneva et al. (Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023). The refined parameters were: scale factor, atomic positions and anisotropic displacement parameters. The occupancies of the cation sites were constrained to 1 whereas the H2O occupancies (O11w, O12w oxygen atoms) were refined for all the data collections. The analysis of the difference-Fourier maps showed the presence of residual electron density peaks <0.8 e −/Å3.
Details on data collection and structure refinements at 25, 100, 200, 300 and 450°C for the Fcarl_1 crystal, and at 25 and 550°C for the Fcarl_2 crystal are reported in Table 1. Selected bond distances at 25, 450 and 550°C are provided in Table 2. Selected anion distances and angles are listed in Table 3. Crystallography Information Files of all the crystal structures refined for Fcarl_1 and those at 25 and 550°C for Fcarl_2 are submitted as supplementary material together with atomic coordinates, site occupancy and displacement parameters at selected temperature (Table S1) and bond-valence data at 25 and 550°C (Table S2). Bond valence calculations were performed using the parameters from Brown and Altermatt (Reference Brown and Altermatt1985), and Breese and O'Keeffe (Reference Brese and O'Keeffe1991) for the cation–fluorine bonds.
Table 1. Crystallographic data and experimental conditions for fluorcarletonite at 25, 100, 200, 300 and 450°C for the Fcarl_1 crystal, and at 25 and 550°C for the Fcarl_2 crystal.
Notes: aGoodness of fit = [Σ[w(F o2–F c2)2]/(N–p)]1/2, where N and p are the number of reflections and parameters, respectively. bR 1 = Σ[|Fo| – |F c|]/Σ|F o|. cwR 2 = [Σ[w(F o2–F c2)2]/Σ[w(F o2)2]]1/2, w = 1.0/[A[0]*T[0]'(X)+A[1]*T[1]'(X) … +A[NP–1]*T[NP–1]'(X)] (Chebyshev optimised weights). The optimised parameters for each refinement are reported in the deposited Crystallography Information Files.
Table 2. Selected bond distances (Å) derived from the structure refinement of the fluorcarletonite crystals studied at 25 and 450°C (Fcarl_1) and at 550°C (Fcarl_2).
Table 3. Selected anion bond distances (Å) and angles (°) derived from the structure refinement of the fluorcarletonite crystals studied at 25, 100, 200, 300 and 450°C (Fcarl_1), and at 550°C (Fcarl_2).
Ex situ HT FTIR
Fourier-transform infrared spectra were measured using an FT−801 spectrophotometer (Simex, Russia). Pelletised powdered samples were analysed at a resolution of 2 cm–1 by collecting a total of 32 scans for each spectrum in the T range 25–700°C. The ex situ experiment was performed by using the procedure described in previous publications (Kaneva et al., Reference Kaneva, Bogdanov and Shendrik2020b; Kaneva and Shendrik, Reference Kaneva and Shendrik2022) and briefly summarised as follows: (1) a mixture of fluorcarletonite and preliminarily dried pure KBr powder was pressed into a transparent tablet and heated to a target temperature for 5 min; (2) the same heating temperature and heating time were applied for pure dried KBr pellets, used as reference; (3) both the mixture and pure KBr pellets were cooled down to room temperature and the IR absorption spectra were measured again. These steps were repeated during the heating from 100°C up to 700°C.
Ab initio calculation
Ab initio modelling of IR spectra was carried out using the “VASP” ab initio code (Kresse and Hafner, Reference Kresse and Hafner1993), employing a pseudopotential method and plane wave basis sets. Exchange and correlation were expressed in terms of the PBEsol function (Perdew et al., Reference Perdew, Ruzsinszky, Csonka, Vydrov, Scuseria, Constantin, Zhou and Burke2008), with an energy cutoff for plane wave basis sets of 400 eV. The Brillouin zone sampling was performed using the gamma point only.
The geometry was relaxed until the maximal force acting on an atom was <0.001 eV/Å, followed by phonon calculation using the ‘Phonopy' code (harmonic approximation), Togo and Tanaka (Reference Togo and Tanaka2015). The infrared spectra were simulated with the ‘Phonopy−Spectroscopy' tool, with a single model containing fully populated O11w and O12w (six H2O molecules) being calculated. The methodology of assigning the calculated modes has been described in detail in Bogdanov et al. (Reference Bogdanov, Kaneva and Shendrik2021).
The presence of hydrogens in the structure can involve a significant anharmonicity, therefore the above simulations have been coupled to ab initio molecular dynamics runs, carried out by means of the ‘Abinit' code (Gonze et al., Reference Gonze, Jollet, Abreu Araujo, Adams, Amadon, Applencourt, Audouze, Beuken, Bieder, Bokhanchuk, Bousquet, Bruneval, Caliste, Côté, Dahm, Da Pieve, Delaveau, Di Gennaro, Dorado, Espejo, Geneste, Genovese, Gerossier, Giantomassi, Gillet, Hamann, He, Jomard, Laflamme Janssen, Le Roux, Levitt, Lherbier, Liu, Lukačević, Martin, Martins, Oliveira, Poncé, Pouillon, Rangel, Rignanese, Romero, Rousseau, Rubel, Shukri, Stankovski, Torrent, Van Setten, Van Troeye, Verstraete, Waroquiers, Wiktor, Xu, Zhou and Zwanziger2016), with PBEsol for exchange and correlation functions, and energy cut off converged to 90 eV. NVT ensemble molecular dynamics calculations have been carried out at 27, 327 and 627°C. For all the runs, the same (idealised) starting P1 geometry obtained from ab initio modelling in Kaneva et al. (Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) has been adopted (with a = 13.2256 Å, c = 16.7329 Å, V = 2926.9 Å3 and 226 atoms). Given the large unit cell and the consequent great number of atoms, no supercell has been adopted in the molecular dynamics simulations. The systems were equilibrated after 103 steps, with a time step of 2.5 fs.
Results
Crystal structure description at room and high temperature
Room-temperature refinement in space group P4/mbm converged to R 1 = 3.34% (for Fcarl_1) and 2.83% (for Fcarl_2), wR 2 = 4.30% (for Fcarl_1) and 3.14% (for Fcarl_2), see Table 1. The values of the refined lattice parameters and cell volume were very close for the two crystals studied (a = 13.2077(4) Å, c = 16.7234(5) Å, V = 2917.3(2) Å3 for Fcarl_1 and a = 13.2082(3) Å, c = 16.7219(4) Å, V = 2917.25(15) Å3 for Fcarl_2, Table 1) and similar to those in Kaneva et al. (Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023).
A representation of the complete crystal structure of fluorcarletonite, plotted down the b axis, is given in Fig 1. It consists of two tetrahedral sites (Si1 and Si2), two octahedral independent sites (Na1 and Na2), two 8-coordinated sites (Na3 and Ca1), one 10-coordinated site (K1), two 3-coordinated sites (C1 and C2), ten oxygen sites, one F site and two sites occupied by oxygens of H2O (O11w and O12w sites), Fig. 1 and Table S1. Figure 2, instead, displays a fragment of the fluorcarletonite crystal structure with details on the local environment of the oxygen at the O11w and O12w sites.
Figure 1. Crystal structure of fluorcarletonite from Murun Massif as seen along b. Si-tetrahedra (orange), Na-polyhedra (blue), Ca-polyhedron (pink), K+ (brown) and C (grey) atoms are represented. Oxygen and fluorine atoms are illustrated in red and green, respectively. Crystal drawings made using Diamond software. (Diamond – Crystal and Molecular Structure Visualization, Crystal Impact – Dr. H. Putz & Dr. K. Brandenburg GbR, Kreuzherrenstr. 102, 53227 Bonn, Germany, https://www.crystalimpact.de/diamond).
Figure 2. Detail of the crystal structure of fluorcarletonite showing the dimensions of the tetrahedral rings in the a-c plane at (a) T = 25°C and (b) T = 550°C. Colours as in Fig. 1.
The refined occupancies for the oxygens at the O11w and O12w sites at RT were, respectively, 0.93(2) and 0.62(3) for Fcarl_1, and 1.016(17) and 0.650(19) for Fcarl_2 (Table S1). Similar values were found in previous investigations for fluorcarletonite (O11w = 0.833(9) and O12w = 0.535(9) in Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a; O11w = 0.87(1) and O12w = 0.55(2) in Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023) as well as for carletonite (Chao, Reference Chao1972; Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023). The U iso/equivalent values of oxygens at the O11w and O12w sites are larger than those of all other atoms (Table S1), consistently with their low bond valence sums (BVS) (Table S2). The thermal motion for these oxygens is particularly pronounced already at room temperature. A similar behaviour was also observed in the case of other hydrated sheet silicates with complex bond topology (Lacalamita et al., Reference Lacalamita, Mesto, Kaneva, Shendrik, Radomskaya and Schingaro2023).
In addition, the short O12w–O12w distance (1.95(4) Å, Table 3) confirms the occurrence of mutually exclusive positions of nearest oxygens at the O12w site (Kaneva et al., Reference Kaneva, Bogdanov, Radomskaya, Belozerova and Shendrik2023). The values of individual and average bond length distances for tetrahedra, and Na- and Ca-polyhedra of the crystals studied (Table 2) were also very similar to those reported in the literature.
Given the close similarities in the geometrical features of the two crystals, in the text below and in the figures, we will consider the data collected at 550°C for Fcarl_2 in addition to those from 25 to 450°C for Fcarl_1.
With increasing temperature, fluorcarletonite shows no change in symmetry (Table 1). However, the evolution of the normalised unit cell parameters and volume as a function of the temperature (Figs 3 and S1) provides evidence that the unit cell dimensions remain almost constant until ~150–200°C when they start to increase.
Figure 3. Normalised unit cell parameters and volume of the Fcarl_1 crystal versus temperature. Symbols: a/a 0 and b/b 0 (light grey); c/c 0 (dark grey); V/V 0 (black). a 0, b 0, c 0, and V 0 are lattice parameters and unit cell volume at RT, respectively. The size of the symbols is larger than the associated esds.
The mean atomic number of the oxygen at O12w and especially at the O11w site progressively decreases as the temperature increases (Fig. 4). Considering that the correlation coefficient between occupancies and Uij's of O11w and O12w are < 0.6 both for RT and 550°C measurements, these results point to an overall dehydration of fluorcarletonite at 550°C of ~40%.
Figure 4. Mean atomic numbers (m.a.n., e –) for the oxygens at the O11w (black line and symbol) and O12w sites (light grey line and symbol) in fluorcarletonite versus temperature.
The oxygen at the O11w site coordinates the Na1 octahedron together with four O3 atoms of the silicate layer and one F. It points towards an equivalent O11w oxygen, both lying on the c axis (Fig. 2). During heating, the Na1–O11w bond weakens (0.229 vu and 0.119 vu at 25 and 550°C, respectively, Table S2). In detail, the oxygen moves away from the Na1 neighbouring cation (as indicated by the lengthening of the Na1–O11w distance from 2.349(7) to 2.59(3) Å, Table 2) and approaches the equivalent O11w oxygen (as testified by the shortening of the O11w–O11w’ distance from 2.822(13) to 2.26(6) Å, Table 3).
The O12w is shared by two Na2 octahedra which are also bonded to one O4 atom of the silicate layer, two O7 and two O10 atoms (Fig. 2). The oxygen at O12w receives 0.054 vu at RT and 0.026 vu at 550°C (Table S2). The water content at the O12w site decreases slightly (from 4.96 to 3.84 m.a.n.'s, e – at RT and 550°C, respectively, Fig. 4) while the distance of the oxygen at O12w site from the Na2 neighbouring cation increases strongly (from 2.886(3) Å at RT to 3.16(2) Å at 550°C, respectively, Table 2). At the same time, the O12w–O12w’ distance decreases (from 1.95(4) Å at RT to 1.28(7) Å at 550°C, respectively, Table 3, Fig. 2).
No significant modifications of the geometrical parameters of the Na3-, Ca1-, and K1-polyhedra as well as of the CO3 groups at 550°C with respect to RT are observed (Table 2).
As concerns the tetrahedral framework, Fig. 2 shows that the linkage between two single tetrahedral layers via the oxygen at the O6 site defines, in the a–c plane, four- and six-membered tetrahedral rings alternated along a and with a mean area 4.0047(37) × 3.2213(38) Å2 and 5.1720(28) × 4.4858(37) Å2, respectively, at RT (Fig. 2a). As the temperature increases, a distortion of these rings is observed. In particular, the stretching of the Si1 tetrahedra along the c direction causes an increment of the O5–O6–O5 angle (from 75.188(63) to 81.029(86)° at RT and 550°C, respectively, Table 3) and, consequently, an elongation of the four-membered tetrahedral rings along the c direction together with its compression along the a direction. On the contrary, the adjacent six-membered tetrahedral ring expands in the a direction (compare Fig. 2a and b). A tilting of the Si2 tetrahedra has also been observed as revealed by the O2–O1–O3 angle increment from 100.429(87) at 25°C to 102.028(116) at 550°C (Table 3).
High-temperature infrared data
The infrared spectra of fluorcarletonite heated from RT to 700°C are presented in Figs 5 and 6 for the Si–O framework and the O–H stretching vibration regions, respectively.
Figure 5. IR absorption spectra of fluorcarletonite annealed at different temperatures in the 500–1700 cm–1 range. The temperatures are given in degrees Celsius in the right side of the figure.
Figure 6. IR absorption spectra of fluorcarletonite annealed at different temperatures in the OH stretching vibration region. The temperatures are given in degrees Celsius in the right side of the figure.
At low frequencies, weak bands between 500 and 900 cm–1 are observed in the spectrum collected at RT (Fig. 5). By increasing the temperature, the bands at 526, 591 and 661 cm−1 shift and at T = 700°C are found centred at 519, 586 and 655 cm−1, respectively (Fig. S2). The intensity of the bands at 591 and 693 cm–1 decreases; the latter trend is also observed for the absorption band in the 802–815 cm−1 range which at T > 400°C shows a maximum at 808 cm−1. No changes affect the bands at 701, 729, 785 and 875 cm−1.
At higher frequencies, a strong band occurs at 1050 cm−1 as also reported elsewhere (Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a) but it is not shown in Fig. 5. The band at 1196 cm−1 splits into two bands at 1170 and 1220 cm−1 starting from T = 500°C. Bands peaking at 1397, 1418, 1451, 1480 and 1525 cm−1 with shoulders at 1371 and 1550 cm−1, change their shape during heating (Fig. 5). In particular, the bands at 1418 and 1480 cm−1 become less resolved at T = 600°C, and the intensities of the 1397 and 1451 cm−1 bands decrease slightly.
In the O–H stretching region three main peaks at 3557, 3582 and 3645 cm−1 (with a shoulder at 3595 cm–1) are observed at RT (Fig. 6). In the spectra of the annealed fluorcarletonite, the intensity of the peaks at 3557 and 3645 cm−1 progressively decreases until they vanish at T > 200 and 500°C, respectively. This evolution makes a peak centred at 3690 cm−1 more evident starting from T = 200°C. At the same temperature, a band centred at 3595 cm−1 becomes evident. This band together with that at 3582 cm–1 show only minimal changes during heating. Overall, the OH stretching bands are hardly evident at T = 600°C. The temperature dependences of these band intensities are given in Fig. S3.
In Fig. 6 very low intensity bands at 2950, 3015 and 3420 cm–1 are also shown. The wide absorption at 3420 cm−1 disappears after T = 100°C while the intensity and position of the bands at 2950 and 3015 cm−1 remains unchanged across the whole explored temperature range.
Discussion
Structural evolution under high temperature
The in situ high-temperature X-ray diffraction experiment from 25 to 550°C on fluorcarletonite from the Murun massif, Russia provides evidence that the mineral undergoes a thermal expansion of the unit cell volume accompanied by a progressive dehydration. In layered minerals the removal of interlayer H2O molecules and the reorganisation of the interlayer space usually results in a decrease in the d-spacing and, as a consequence, in the shrinking of the unit cell parameters and volume contraction (Bray et al., Reference Bray, Redfern and Clark1998; Zema et al., Reference Zema, Ventruti, Lacalamita and Scordari2010; Post et al., Reference Post, Bish and Heaney2015). However, in some hydrated sheet silicates with complex bond topology, the tetrahedral framework defines interlayer cavities hosting zeolitic H2O molecules and/or alkaline cations (e.g. McDonald and Chao Reference McDonald and Chao2009; Lacalamita et al., Reference Lacalamita, Mesto, Kaneva, Shendrik, Radomskaya and Schingaro2023). In particular, in fedorite, (K,Na)2.5(Ca,Na)7Si16O38(OH,F)2⋅3.5H2O, the tetrahedral rings of the [Si16O38]12– unit intrude the interlayer space thus hampering, during heating, the release of H2O molecules (Lacalamita et al., Reference Lacalamita, Mesto, Kaneva, Shendrik, Radomskaya and Schingaro2023). As a consequence, the partial dehydration of the mineral involves only a slight cell volume contraction (Lacalamita et al., Reference Lacalamita, Mesto, Kaneva, Shendrik, Radomskaya and Schingaro2023). The crystal structure of fluorcarletonite may be related to that of fedorite as in both silicates two tetrahedral single layers are linked by an apical oxygen atom (see figure 2 in Lacalamita et al., Reference Lacalamita, Mesto, Kaneva, Shendrik, Radomskaya and Schingaro2023 and Fig. 1, this study). However, fluorcarletonite does not contain zeolitic water. The mineral has two independent crystallographic sites occupied by the oxygens atoms of H2O molecules coordinating Na-centred octahedra and showing very different local environments (Fig. S4). In detail, the oxygen at the O11w site is located in a cavity defined by two overlapped eight-member rings whereas the oxygen at the O12w site is shared by two octahedra and statistically distributed on two symmetrically equivalent sites (O12w and O12w’), as stated above. The oxygen at the O11w site points towards the double tetrahedral layers and is bonded only to the cation at Na1 (Table S2). This explains its capability to shift along the c axis by moving away from the Na atom and to leave the crystal structure under heating conditions. Almost one half of the water content at the O11w site was, indeed, observed at 550°C with respect to that found at RT (Fig. 4; Table S1).
The oxygen at the O12w site moves toward the equivalent position at O12w’ (Table 3) and, as a consequence, it approaches the two symmetrically equivalent Na2’-octahedra (Fig. 2). The bonding of the oxygen at the O12w site may be also affected by the cations at the Na2’ sites. Despite a reduction of the BVS at the O12w site that was observed from 25 to 550°C (Table S2), negligible changes in BVS actually affect the O12w site if the contribution of the Na2’ ion bond strengths is considered. This hypothesis is also in keeping with the slight decrease of the occupancy observed at the O12w site (Fig. 4; Table S1).
Therefore, fluorcarletonite upon heating to 550°C undergoes dehydration which is accompanied by a substantially isotropic enlargement of the unit cell parameters (Δa = 0.53% and Δc = 0.55%) and expansion of the unit cell volume (ΔV = 1.6%). Its behaviour resembles that of framework silicates, specifically, of zeolites where the dehydration is not accompanied by significant changes in the framework structure and unit cell volume (Arletti et al., Reference Arletti, Fantini, Giacobbe, Gieré, Vezzalini, Vigliaturo and Quartieri2018).
Vibrational features upon heating
The assignment of the infrared bands in Figs 5 and 6 was performed using ab initio calculations (Table 4).
Table 4. Comparison between calculated and experimental values of infrared absorption bands of fluorcarletonite at room temperature and after heating at 700°C. The band assignment is given basing on ab initio calculation.
Focusing on the tetrahedral framework, the prominent band at 1050 cm–1 (not shown in Fig. 5) and the less intense one at 1196 cm−1 are associated with Si−O stretching vibrations. The splitting of the latter band is ascribed to the thermal expansion of the tetrahedral framework during annealing. The SiO4 bending vibrations are affected by H2O libration modes. In detail, the shifts detected for the 526, 591 and 661 cm–1 bands correlate well with temperature changes and are due to the dehydration (Fig. S2). The same reason explains the variation in shape of the band in the 802–815 cm−1 range, probably due to the change in the position of the O11w and O12w atoms during heating.
Regarding the (CO3)2− anions, the less intense band at 875 cm−1 may be associated with the out-of-plane bending of (CO3)2−. The two non-equivalent groups of (CO3)2− anions have a distorted D3h point group. Therefore, at least five bands (1371–1527 cm−1, Fig. 5) correspond to asymmetric stretching modes of the (CO3)2− anions. In detail, ab initio calculations indicate that the bands at 1397 and 1480 cm−1 are attributed to asymmetric stretching modes of (C2O3)2−, whereas the bands at 1418 and 1451 cm−1 are related to (C1O3)2−. At 1525 cm−1 the separate closed asymmetric vibrational modes of (C1O3)2− and (C2O3)2− anions are not well resolved. However, the change in the angle between (C1O3)2− and the b–c plane leads to a decrease in the oscillator strength of this band. This rotation and subsequent disorder of (C1O3)2− anions could also explain the widening of (C1O3)2− related bands. In addition, the (C2O3)2− disorder associated with the shoulder at 1371 cm–1 results from the dehydration process. In contrast, the C atoms trajectories (Figs S5 and S6) obtained from the molecular dynamic simulation suggest the usual direction of vibration of C atoms, typically perpendicular to the CO3 plane, for both C1 and C2 atoms.
Finally, the stretching vibration of the OH groups replacing fluorine provide the band at 3690 cm–1 that becomes detectable at T = 200°C and disappears at T > 500°C (Fig. 6; Fig. S3, curve 4) whereas the Si−O−H silanol groups give rise to the bands at 2950 and 3015 cm–1 that are unaffected by heating.
The adsorbed surface H2O molecules are lost at low annealing temperatures (see the evolution of the band at 3420 cm−1, in Fig. 6) whereas the structural H2O molecules (associated with O11w and O12w) are responsible for the band at 591 cm−1 (libration mode) and for the shoulder at 1550 cm−1 (bending mode), see Fig. 5. The trajectories of the H atoms during annealing (see the deposited netCDF files) corroborate the occurrence of strong libration of the O11w and O12w.
The O11w−H groups (stretching vibration at 3557 cm−1, Fig. 5) undergoes deprotonation at T > 200°C (Fig. S3, curve 1). The released protons interact with other oxygen atoms, mainly O6, forming new O–H bonds in which the vibrations entail the appearance of the band at 3595 cm–1 at T = 200°C that remains stable up to 500°C (Fig. S3, curve 3). The temperature behaviour of the 3595 cm–1 band (Fig. S3, curve 3) is similar to that of the band at 3690 cm–1 (Fig. S3, curve 4).
The O12w−H (stretching vibration at 3645 cm−1) bonds deprotonate at T > 600°C (Fig. S3, curve 2).
The molecular dynamics results confirm the partial dehydration of the structure as indicated by the H atoms trajectories (Fig. S7), as well as by the Pair Distribution Function g(r) of the H atoms (Fig. S8). Indeed, the g(r) pattern at 27°C actually differs from that at 327 or 627°C when the dehydration is taking place. The first peak for the T = 27°C case, centred around ≈1.2 Å, corresponds to the nearest neighbour shell for the O–H pair. This isolated peak (the probability g(r) drops to zero in the r range of ≈1.3–1.6 Å) for the T = 27 °C simulation is centred around the preferential distance between O and H atoms, that actually bind covalently, with small-amplitude vibrations around the equilibrium positions and with diffusive movements confined over small local regions, as suggested by the small spread of the peak. At higher temperatures, this Pair Distribution Function feature fails, and g(r) in the 1.3 < r < 1.6 Å range is significantly different from zero, suggesting that random collisions between particles and great positional disorder take place. This fact is also compatible with the trends shown by the values of the mean squared displacement (msd) of the H atoms vs. time at 27, 327 and 627°C as depicted in Fig. S7: here, the high-temperature patterns are those typically determined by significant self-diffusivity, whereas the T = 27°C case clearly shows no diffusion at all for the H atoms.
The molecular dynamics also suggest that at 327°C some of the released H atoms (coming from the O11w–H couples) form new bonds with oxygen atoms, mainly the O6 atoms, thus producing a kind of ‘isomerism’, i.e. rapidly interconverting isomers. The dehydrated O11w can re-interact with an H to form a new temporary OH bond. Two snapshots of the structures at 27 and 327°C during the molecular dynamics runs after 1.8 ps and 1 ps respectively are shown in Figs S9 and S10, respectively, whereas Table S3 can be used for better understanding of the bond setup evolution with increasing temperature from 27 to 327°C.
Conclusions
At elevated temperatures, the crystal structure of fluorcarletonite undergoes dehydration and structural modifications, knowledge of which may contribute to comprehending the stability of the mineral.
Under the adopted experimental conditions, the crystal structure of fluorcarletonite is stable from RT up to ~ 150–200°C when the mineral starts to progressively dehydrate until ~550°C. The present study allows us to elucidate the mechanism of dehydration that takes place in two stages since it firstly involves the oxygen at the O11w site (at ~150 ≤ T ≤ 550°C) and, successively, involves the oxygen at the O12w site (mainly at T > 500°C). In addition, ab initio calculations highlight a surprising behaviour of the protons which may depart from the O11w–H groups linking with nearest oxygen atoms thus leading to perturbations of the Si–O bonds in the double tetrahedral layer.
At a temperature above 500°C the OH groups substituting the F atoms leave the crystal structure. Release of CO2 at T > 630°C and defluorination at T > 1136°C (TG–DSC data by Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a) lead to the structure breakdown of fluorcarletonite as testified by the amorphisation of the single crystal during the in situ HTXRD experiment. Chao (Reference Chao1972) also observed amorphisation of carletonite quenched from 708°C by associating the collapse of the crystal structure to the loss of H2O and CO2. Defluorination of carletonite was reported to occur at much lower temperatures (300°C, see Chao, Reference Chao1972) with respect to that found for fluorcarletonite (Kaneva et al., Reference Kaneva, Radomskaya, Suvorova, Sterkhova and Mitichkin2020a).
The diffraction pattern of carletonite quenched from 775°C revealed the characteristic peaks of wollastonite (CaSiO3), albite (NaAlSi3O8), Na2Ca2Si3O9 and Na2CaSi3O8 (Chao, Reference Chao1972). Wollastonite was also recognised as an alteration product of primary minerals in the fluorcarletonite-hosted rock under supergene conditions (Kaneva et al., Reference Kaneva, Radomskaya and Shendrik2022).
The data of the present study can be integrated into a more general framework of investigations into the response to heating of a set of silicates with complex bond topology. The latter include, for instance, double-layer sheet silicates, i.e. the one based on 63 and 4.82 net, according to the silicate minerals hierarchy of Hawthorne et al. (Reference Hawthorne, Uvarova and Sokolova2019). Indeed, fedorite (63 net, Hawthorne et al., Reference Hawthorne, Uvarova and Sokolova2019) was recently examined (Lacalamita et al., Reference Lacalamita, Mesto, Kaneva, Shendrik, Radomskaya and Schingaro2023) and found to exhibit a thermal behaviour very similar to that of fluorcarletonite (4.82 net, Hawthorne et al., Reference Hawthorne, Uvarova and Sokolova2019). Therefore, it is reasonable to expect that the tetrahedral framework topology also prevents the total H2O migration in other hydrated phases e.g. lalondeite (Na,Ca)6(Ca,Na)3[Si16O38]F2(H2O), macdonaldite BaCa4[Si16O36(OH)2](H2O)10, monteregianite-(Y) KNa2Y[Si8O19](H2O)5 and seidite-(Ce) Na4Ce2Ti[Si8O22](OH)(H2O)5 members of the group.
Further implications of our study concern technological applications. Indeed, changes in the crystal structure of fluorcarletonite under high temperature can lead to alterations in the absorption spectra, as well as other optical characteristics. These modifications are of utmost importance for the development of advanced optical materials, including light-emitting diodes, lasers and optical sensors, with tailored properties.
Acknowledgements
This work was supported by a M. Lacalamita grant (SIMP 2020 Research Grant in Crystal-chemistry, in memory of Prof. Fiorenzo Mazzi). The Structures Editor Peter Leverett and two anonymous referees are gratefully acknowledged for their insightful suggestions.
Supplementary material
Figures S1–S10, Tables S1–S3 and Crystallography Information Files have been deposited with the Principal Editors of Mineralogical Magazine and can be found at https://doi.org/10.1180/mgm.2024.50.
Competing interests
The authors declare none.
