Introduction
Feldspar-group minerals (Krivovichev, Reference Krivovichev2020) are among the most widespread minerals in the Earth's crust, and they have been studied extensively under various pressure–temperature conditions (e.g. Smith and Brown, Reference Smith and Brown1988; Parsons, Reference Parsons1994; Deer et al., Reference Deer, Howie and Zusmann2001; Bokii and Borutzkii, Reference Bokii and Borutskii2003; Henderson, Reference Henderson2021; Gorelova, Reference Gorelova2023). The feldspar group includes 29 mineral species belonging to the aluminosilicate (the most widespread), borosilicate, beryllophosphate, ferrisilicate and aluminoarsenate groups (Krivovichev, Reference Krivovichev2020). The aluminoarsenate feldspar filatovite was first described by Vergasova et al. (Reference Vergasova, Krivovichev, Britvin, Burns and Ananiev2004), with the simplified formula K(Al,Zn)2(As,Si)2O8, however this formula was later modified as follows: K(Al1+xM 2+1–x)Ʃ2(As5+2–xSix)Ʃ2O8 with M 2+ = Cu, Zn and x < 1 (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a).
The crystal structure of filatovite was determined as monoclinic (Filatov et al., Reference Filatov, Krivovichev, Burns and Vergasova2004) based upon the three-dimensional framework of TO4 tetrahedra (T = Si, Al and As5+). This type of topology (feldspar topology; Krivovichev, Reference Krivovichev2020) is inherent to a series of widespread minerals (albite, anorthite, sanidine, orthoclase and microcline) as well as some rare minerals (rubicline, buddingtonite, celsian, reedmergnerite and ferrisanidine). Feldspar topology is one of the five possible topologies in the feldspar group of minerals, namely feldspar, paracelsian, svyatoslavite, dmisteinbergite and hollandite (Krivovichev, Reference Krivovichev2020). The last two topologies differ from the other three significantly, as crystal structures with hollandite topologies are based upon TO6 octahedra, and crystal structures with dmisteinbergite topologies upon TO4 tetrahedra forming layers; whereas feldspar, paracelsian and svyatoslavite topologies are framework structures built of TO4 tetrahedra; they differ from each other only in the way the tetrahedra are connected.
It should be noted that filatovite has an almost unique chemical composition, containing Al, Si and As together as main constituents. Only eight minerals comprising meaningful amounts of these three elements are known to date (Table 1; more detailed description is provided in the Discussion). Due to the rarity of these minerals, they have not been studied under non ambient (extreme) conditions and the information about their thermal stability is limited to our knowledge of their formation conditions. Consequently, there are no data on the influence of the substitution of Si4+ for As5+ on the mineral stability. It is interesting that filatovite forms a continuous solid-solution series with sanidine (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a). Moreover, this is the only example of a wide-ranging solid solution between (alumino)silicate and (alumino)arsenate minerals (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a).
List of IMA approved minerals, containing Al, Si, As and O together as the main constituents.
*References: [1] von Lasaulx (Reference von Lasaulx1872); [2] Barresi et al. (Reference Barresi, Orlandi and Pasero2007); [3] Sarp et al. (Reference Sarp, Černý, Pushcharovsky, Schouwink, Teyssier, Williams, Babalik and Mari2014); [4] Hawthorne et al. (Reference Hawthorne, Abdu, Ball and Pinch2013); [5] Armbruster et al. (Reference Armbruster, Bühler, Graeser, Stalder and Amthauer1988); [6] Demartin et al. (Reference Demartin, Gramaccioli and Graeser2008); [7] Vergasova et al. (Reference Vergasova, Krivovichev, Britvin, Burns and Ananiev2004); [8] Filatov et al. (Reference Filatov, Krivovichev, Burns and Vergasova2004); [9] Raade et al. (Reference Raade, Johnsen, Erambert and Petersen2007); [10] Moore and Ito (Reference Moore and Ito1978); [11] Dunn and Nelen (Reference Dunn and Nelen1980); [12] Cooper and Hawthorne (Reference Cooper and Hawthorne2012); [13] Palache and Bauer (Reference Palache and Bauer1927).
To date, filatovite has been found in only one locality, namely in fumaroles that have appeared as a result of the Great Fissure Tolbachik Eruption, Tolbachik volcano, Kamchatka peninsula, Russia (Vergasova et al., Reference Filatov, Krivovichev, Burns and Vergasova2004; Shchipalkina et al., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020b). According to Vergasova et al. (Reference Vergasova, Krivovichev, Britvin, Burns and Ananiev2004), the temperature of gases in filatovite-bearing fumaroles was ~410–420°C, whereas Shchipalkina et al. (Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020b) assumed that the temperature formation was not lower than 500°C. The temperature of filatovite formation could be even higher, as the temperature of the fumarole fields during the Tolbachik eruption was ~700°C (Menyalov et al., Reference Menyalov, Nikitina and Shapar’1980).
The present study aims to investigate the high-temperature behaviour of filatovite using in situ high-temperature X-ray diffraction up to 800°C and in situ high-temperature (hot-stage) Raman spectroscopy up to 600°C in order to determine the stability temperatures of the mineral and the influence of As5+ on the stability of feldspars.
Materials and methods
The sample of filatovite was collected from the Arsenatnaya fumarole, Second scoria cone of the North Breakthrough of the Great Fissure Tolbachik Eruption, Kamchatka peninsula, Russia. Six filatovite crystals were studied by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). The crystal with the maximum arsenic content was epoxy-mounted, polished, carbon-coated and analysed using SEM with wavelength-dispersive X-ray spectroscopy (WDX). Next, the crystal (final size 30 × 10 × 10 μm) was extracted, polished by reactive ion etching with Ar+ ions using an Oxford Instruments IonFab-300 instrument (500 V and 2.4 mA cm–2 flow current), studied using Raman spectroscopy, and then using single-crystal X-ray diffraction.
The chemical composition was determined using a S-3400N (Hitachi, Japan) SEM equipped with an Aztec Energy 350 (Oxford instruments, UK) EDX (SSD detector, accelerating voltage 20 kV, beam current 1 nA, and 1 μm beam diameter at the sample surface) and an Inca 500 (Oxford instruments, UK) WDX (accelerating voltage 20 kV, beam current 15 nA, and 3 μm beam diameter at the sample surface), using natural and synthetic standards. The empirical formula of filatovite (Table 2) was calculated on the basis of eight O atoms per formula unit (apfu).
Chemical composition of filatovite (No 1; our data) and As-rich potassic feldspar from the Arsenatnaya fumarole (No 2; Shchipalkina et al., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020b).
Note: bdl – below detection limit
High-temperature Raman spectroscopy studies in air were conducted up to 600°C with a temperature step of 50°C. The heating rate was ~25°C/min. Raman spectra of the sample were recorded from a single crystal in arbitrary orientation using a LabRam HR 800 spectrometer (Horiba Jobin-Yvon, Japan) equipped with a BX-41 (Olympus, Japan) microscope and a high-temperature attachment THMS600 System (Linkam, UK) in a back-scattering geometry system using a 532 nm laser. The Raman spectra were recorded in the range of 70–1800 cm–1 at a resolution of 1 cm–1 and 20 s acquisition time and with the power at the sample of 10 mW. To improve the signal-to-noise ratio, the number of acquisitions was set to 50.
The thermal behaviour of filatovite under heating in air was also studied in situ by high-temperature single-crystal X-ray diffraction (SCXRD) using a XtaLAB Synergy-S diffractometer (Rigaku Oxford Diffraction, Japan) operated with monochromated MoKα radiation (λ[MoKα] = 0.71073 Å) at 50 kV and 1 mA and equipped with an HyPix-6000HE detector with a high-temperature FMB Oxford system (Oxford, UK). The sample was heated using a gas blower up to 800 (±10) °C. For the SCXRD study at all temperatures, the single crystal previously used for high-temperature Raman spectroscopy was used. First, this crystal was mounted on a polymer loop using paraton-n to collect diffraction data under ambient temperature. A hemisphere of diffraction data (with a frame width of 0.5°) was collected. After that this crystal was mounted on a quartz fibre placed in a quartz capillary and fixed between the fibre and the capillary wall (see Gorelova et al., Reference Gorelova, Vereshchagin and Kasatkin2021, for more details) to obtain SCXRD data under high-temperature conditions. High-temperature diffraction data were collected at 200, 400, 600 and 800°C. At each temperature the crystal was kept for ~10 minutes prior to data collection. It should be also noted, that at 800°C, the crystal turned out to be X-ray amorphous and the full SCXRD data were not collected at this temperature. For all other temperatures, a hemisphere of diffraction data (with a frame width of 0.5°) was collected by analogy with the ambient temperature experiment. However, due to the instrumental feature of high-temperature experiments, namely a longer distance to the detector, when using the strategy similar to that used previously, the number of collected reflections is noticeably smaller. To avoid this discrepancy, the counting time was increased from 6 s for each frame at ambient temperature to 15 s under non-ambient conditions. Though the difference in the collected reflection was still large (see Table 3), no further increasing was possible. The data were integrated and corrected for background, Lorentz, and polarisation effects. The empirical absorption correction based on spherical harmonics implemented in the SCALE3 ABSPACK algorithm was applied in the CrysAlisPro program (Agilent Technologies, 2012). The unit-cell parameters were refined using the least-square techniques. The SHELXL program package (Sheldrick, Reference Sheldrick2008) was used for all structural calculations. All bond lengths in crystal structures at high temperatures were corrected for thermal vibrations of atoms according to the procedure described by Downs (Reference Downs, Hazen and Downs2000) and considering the 8π2 difference between the U ij and B ij factors. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).
Crystallographic data and refinement parameters for filatovite at different temperatures.
Due to the small number of experimental points, the temperature dependencies of the unit-cell parameters were described by linear polynomial functions up to 600°C, as the sample decomposed at 800°C (see below for more details). Using the approximation coefficient, the tensor component was determined in the Cartesian crystal-physical coordinate system as a solution of the system of six equations of the following form (Bubnova et al., Reference Bubnova, Firsova and Filatov2013):
where αij are the tensor components and xd, yd and zd are the directional cosines of normal vectors with respect to the crystal-physical axes xyz. The thermal expansion along the normal vector to the (hkl) plane with the interplanar distance dhkl was calculated as follows:
where $d_{hkl}^2 =$
f (h, k, l, a, b, c, α, β, γ) is a function of indices and unit-cell parameters. The standard orientation of the crystallographic axes with respect to the crystal-physical axes was used.
This procedure was performed using the TTT program package (Bubnova et al., Reference Bubnova, Firsova and Filatov2013), which was also used for the thermal-expansion parameter tensor visualisation.
Results and discussion
Chemical composition and Raman spectroscopy under ambient conditions
The chemical composition of filatovite is quite simple (Table 2), the Al:As:Si atomic ratio is close to 2:1:1. The crystal under study does not contain impurities of divalent cations (e.g. Cu and Zn), which is in a good agreement with the previous studies of As-rich feldspar from the Arsenatnaya fumarole (Shchipalkina et al., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020b). Potassium is the main extra-framework cation (0.91 apfu), while sodium is present in very small amounts (0.03 apfu).
The Raman spectrum of filatovite under ambient conditions (Fig. 1a) is in a good agreement with the published data on the sanidine–filatovite solid-solution series (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a). The Raman band positions of the sample in question are very close to those of a sanidine–filatovite solid solution, whereas the intensity ratio is significantly different. Though the intensity of the Raman bands often depends strongly on the crystal orientation, the framework crystal structure of filatovite with different orientations of all structural units allows us to exclude the orientation influence.
Raman spectra of filatovite (a) under ambient conditions; (b) at different temperatures from 70 to 1500 cm–1; and (c) temperature evolution of 12 selected Raman bands. The errors are smaller than the size of the symbols.
In the sample studied, the three most intense bands with very close intensities are located in the region of 800–1000 cm–1, which is usually attributed to the symmetric stretching modes of TO4 tetrahedra. Based on literature data, the band at 863 cm–1 is usually attributed to AsO4 tetrahedra (e.g. Botto and Baran, Reference Botto and Baran1982; Vereshchagin et al., Reference Vereshchagin, Britvin, Perova, Brusnitsyn, Polekhovsky, Shilovskikh, Bocharov, van der Burgt, Cuchet and Meisser2019), whereas the bands at 985 and 994 cm–1 are related to SiO4 and AlO4 tetrahedra (e.g. Dowty, Reference Dowty1987). The close intensities of the bands between 800 and 1000 cm–1 can indicate a high amount of As in the sample being considered (e.g. ~Si:As ratio is 1:1), that is also confirmed by electron microprobe (see above) and SCXRD data (see below). In contrast, all the previously studied samples of the sanidine–filatovite solid-solution series with the maximum Si:Al ratio 1.61:0.63 have no intense band at ~863 cm–1 (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a). Weak bands in the region between 1000 and 1200 cm–1 were also attributed to the asymmetric stretching modes of TO4 tetrahedra, according to the calculations (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a). The band at 1448 cm–1 can be presumably assigned to an overtone or combination mode.
The next group of medium intense bands is located between 400 and 700 cm–1, which were attributed to the ring breathing modes of four-membered rings of TO4 tetrahedra (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a). It should be noted that the most intense band of the sample within this group of bands studied in this work is located at 456 cm–1, whereas in all the samples of sanidine–filatovite series studied previously, this band was less intense. The bands at 625 and 631 cm–1 are mainly bending deformation of the tetrahedra, where the contribution of the aluminium to the vibration is predominant (Aliatis et al., Reference Aliatis, Lambruschi, Mantovani, Bersani, Ando, Gatta, Gentile, Salvioli-Mariani, Prencipe, Tribauldino and Lottici2015).
Another group of bands with frequencies below 400 cm–1, responsible for the rotation–translation modes of four-membered rings and cage-shear modes, has no significant differences to the samples with a different Al:Si:As ratio (Shchipalkina et al., Reference Shchipalkina, Pekov, Britvin, Koshlyakova and Sidorov2020a).
Raman spectra evolution of filatovite upon heating
As the crystals of filatovite were small and the SCXRD study could not be performed at many temperatures, a high-temperature Raman spectroscopy study was undertaken for a quick evaluation of the high-temperature behaviour of filatovite, i.e. to determine the presence or absence of phase transformations. The evolution of 12 Raman bands upon heating was traced. Generally, all Raman bands moved to lower wavenumbers, i.e. underwent a red shift (Fig. 1c), which is typical for high-temperature conditions. Nevertheless, their shift speed was different: the bands in a low frequency number region (below 500 cm–1) hardly changed their position across the whole temperature range, whereas the bands in a higher frequency number region, i.e. related to the deformations and vibrational stretching modes of TO4 tetrahedra, moved significantly. At first glance, it could be seen that there were abrupt changes at ~200 and 300°C, but all these correspond to weak peaks and cannot be considered as structural changes. The disappearance of some peaks (ν5, ν10 and ν11) refers to a general gradual deterioration of the Raman spectra upon heating (Fig. 1b).
Crystal structure evolution and thermal expansion of filatovite upon heating
The crystal structure of filatovite was refined at four temperature points (Tables 3 and 4) including ambient conditions. According to SCXRD data, the M site is occupied by K and Na with a 9:1 ratio, two of four T sites are fully occupied by Al, whereas the other two T sites are occupied by Si and As with a ratio close to 1:1 (e.g. the Al:Si:As ratio is 2:1:1). The bond lengths obtained are in a good agreement with the previous study of filatovite [our work vs Filatov et al., Reference Filatov, Krivovichev, Burns and Vergasova2004]: <T1–O> = 1.657 vs 1.634 Å; <T2–O> = 1.654 vs 1.634 Å; <Al1–O> = 1.753 vs 1.753 Å; <Al2–O> = 1.754 vs 1.754 Å; and <M–O> = 3.012 and 3.020 Å. Small differences in the tetrahedral bond length show there are slightly different Al:As:Si ratios in the crystals, which were studied by Filatov et al. (Reference Filatov, Krivovichev, Burns and Vergasova2004) (1.8:1.2:0.70) and in this work (~2:1:1), and that [4]Al > [4]As > [4]Si (ionic radii are 0.39, 0.34 and 0.26 Å, respectively; Shannon, Reference Shannon1976). The shorter Al–O bonds in the crystal structure we studied are explained by the absence of Zn (ionic radii are 0.39 and 0.6 Å for Al and Zn, respectively; Shannon, Reference Shannon1976).
Bond distances in filatovite at different temperatures.
Attempts to refine the cation distribution by the sites at different temperatures lead to similar results at all temperatures, therefore they were fixed. This fact indicates the absence of the order–disorder process in the temperature range under consideration. Anisotropic displacement parameters were refined for all atoms at all temperatures.
As detailed above, filatovite belongs to minerals with feldspar topology (Krivovichev, Reference Krivovichev2020). Such crystal structures are described traditionally as consisting of so-called crankshaft chains of TO4 tetrahedra (in our case – alternate corner-sharing AlO4 and (Si,As)O4 tetrahedra), formed by successive polymerisation of four-membered rings (Smith and Rinaldi, Reference Smith and Rinaldi1962; Smith, Reference Smith1978). Such crankshaft chains in the crystal structure of filatovite are elongated along the a axis, and join together to form a three-dimensional framework (Fig. 2).
Crystal structure of filatovite in different projections with averaged thermal expansion section (black parts of the sections demonstrate the negative thermal expansion). AlO4 and (Si,As)O4 tetrahedra are given in orange and yellow, respectively; M (M = K and Na) atoms are shown as black displacement ellipsoids. The program package Vesta (Momma and Izumi, Reference Momma and Izumi2011) was used for crystal structure visualisation.
The temperature dependencies of the unit cell parameters are shown in Fig. 3. As there are only four experimental points, it is not possible to determine the existence of any phase transformation based on this graph only. Nevertheless, the crystal structure refinement at all temperatures clearly indicates the absence of phase transitions.
Unit-cell parameters of filatovite at different temperatures. Errors are smaller than symbols.
Due to the small number of experimental points, the thermal expansion coefficients were only calculated with a linear approximation of the temperature dependencies of the unit-cell parameters: α11 = 17.7(1), α22 = –1.2(1), α33 = 0(1), μ(α11^a) = 19.7 (5), μ(α33^c) = 6.2(5), αa = 15.70(4), αb = –1.2(1), αc = 0(1), αβ = –1.8(5) and αV = 16(1) × 10–6 °C–1. In other words, the thermal expansion of filatovite has an extremely anisotropic character up to negative, and close to zero, expansion along the b and c axes (Fig. 2). The direction of the maximal thermal expansion is close to the a axis, i.e. along the crankshaft chain of TO4 tetrahedra. The TO4 (T = Si and As) and AlO4 tetrahedra remain rigid upon heating: the bond lengths vary within 3 standard error(s) upon heating up to 600°C (Table 4). This is consistent with the data of Dove et al. (Reference Dove, Cool, Palmer, Putnis, Salje and Winkler1993, Reference Dove, Pride and Keen2000) and Palmer et al. (Reference Palmer, Dove, Ibberson and Powell1997), which indicates the thermal stability of TO4 (T = Si and Al) tetrahedra.
Though the crystal structure of filatovite does not undergo any polymorphic transformation, its deformation decreases. The two longest bonds (M–O1 and M–O3) in a MO9 polyhedron decrease upon heating, whereas all other bond lengths increase as temperature increases (Table 4, Fig. 4). The (M–O) bond lengths change in the range of 0.013–0.078 Å (Table 4). In other words, the MO9 polyhedron becomes more regular (less distorted) upon heating, which is also confirmed by the decrease of the polyhedron distortion index from 0.033 to 0.027 (calculated using the Vesta program package, Momma and Izumi, Reference Momma and Izumi2011).
Changes of the M–O bonds upon heating in (a) MO9 polyhedra and (b) the MO9 polyhedron in the ball-and-stick representation at 27°C and (c) 600°C. Red and purple ellipsoids show oxygen and M atoms, respectively.
It should also be noted that the direction of the maximal thermal expansion (α11) is close to the direction of the minimal thermal vibration of the M cation regardless of temperature (Fig. 4b,c). A similar result was obtained previously for other alkaline feldspars (Filatov, Reference Filatov1990). This seemingly contradictory behaviour has been explained by the different nature of the factors determining thermal vibrations of atoms and thermal deformations of crystal structures (Filatov, Reference Filatov1990). Therefore, as it was mentioned above, the main factor determining the nature of thermal expansion of feldspar-related minerals is the crankshaft chain of TO4 tetrahedra and the framework topology, and not the extra-framework cation.
As mentioned above, the filatovite heating products were X-ray amorphous at 800°C, whereas at 600°C the mineral preserved crystallinity. Feldspar-bearing mineral assemblages in the Arsenatnaya and Yadovitaya fumaroles (including the one where filatovite was found) are thought to form at temperatures above 500°C (Pekov et al., Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018). The synthetic analogue of filatovite was crystallised from the amorphous stoichiometric phase K(Al2AsSiO8) at 650°C under atmospheric pressure (Kotelnikov et al., Reference Kotelnikov, Shchipalkina, Suk and Ananiev2019), which is consistent with our findings on the high temperature stability of filatovite. Thus, according to our investigation, we can conclude that the formation temperatures of this mineral is ~700 ± 50°C.
Discussion
Crystal chemistry of natural aluminoarsenosilicates
The chemical composition of filatovite is very specific, which is confirmed by the fact that, according to the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC, Pasero, Reference Pasero2024), only eight minerals containing significant amounts of Al, Si, As and O together are known to date (Table 1). All these minerals, except ardennite-(As), are extremely rare and are known from one to three localities each. Most of them are formed in very specific geological settings and originate in famous and geochemically unique deposits such as Långban in Sweden (Moore, Reference Moore1970; Holtstam and Langhof, Reference Holtstam and Langhof1999; Christy and Gatedal, Reference Christy and Gatedal2005) and Franklin and Sterling Hill in New Jersey, USA (Palache, Reference Palache1941; Cook, Reference Cook1973; Dunn, Reference Dunn1995).
From the crystal chemical point of view, all the minerals mentioned above differ significantly from filatovite, as only filatovite has Al, Si and As in tetrahedral coordination simultaneously. Ardennite-(As) and barrotite have Al in octahedral coordination (Donnay and Allmann, Reference Donnay and Allmann1968 and Sarp et al., Reference Sarp, Černý, Pushcharovsky, Schouwink, Teyssier, Williams, Babalik and Mari2014, respectively). Carlfrancisite, mcgovernite, hundholmenite-(Y), and kraisslite, have even more differences, because, in addition to AlO6 octahedra, they all also contain As3+ in triangular pyramidal coordination (Hawthorne, Reference Hawthorne2018; Raade et al., Reference Raade, Johnsen, Erambert and Petersen2007; Cooper and Hawthorne, Reference Cooper and Hawthorne2012). Cervandonite-(Ce) has only silicon in tetrahedral coordination, whereas Al is in octahedra and As3+ in trigonal pyramidal coordination (Demartin et al., Reference Demartin, Gramaccioli and Graeser2008). In other words, filatovite is the only mineral, containing Al, Si and As in tetrahedral coordination.
Thermal behaviour of feldspars isotypical to filatovite
As mentioned above, feldspar-group minerals, and especially those with feldspar topology, were studied in detail both under ambient and extreme (high-temperature and / or high-pressure) conditions (Parsons, Reference Parsons1994; Hovis et al., Reference Hovis, Morabito, Spooner, Mott, Person, Henderson, Roux and Harlov2008, Reference Hovis, Medford, Conlon, Tether and Romanoski2010; Angel et al., Reference Angel, Sochalski-Kolbus and Tribaudino2012; Pakhomova et al., Reference Pakhomova, Simonova, Koemets, Koemets, Aprilis, Bykov, Gorelova, Fedotenko, Prakapenka and Dubrovinsky2020; Henderson, Reference Henderson2021). According to Hovis et al. (Reference Hovis, Morabito, Spooner, Mott, Person, Henderson, Roux and Harlov2008, Reference Hovis, Medford, Conlon, Tether and Romanoski2010), the thermal expansion behaviour of any feldspar (with feldspar topology) can be predicted if its chemical composition and unit cell volume under ambient conditions are known. Their conclusion was based on the idea, that all feldspars can be generally divided into two groups: (1) A +AlSi3O8 (‘AlSi3’), where A are univalent alkali extra-framework cations and (2) B 2+Al2Si2O8 (‘Al2Si2’), where B are divalent alkali earth extra-framework cations. In terms of the framework architecture, filatovite is closer to the ‘Al2Si2’ type, whereas extra-framework cations are univalent alkali metals, so the prediction of its thermal expansion is a bit more difficult. Calculation of the volume thermal expansion coefficients for filatovite using the formulae for ‘AlSi3’ and ‘Al2Si2’ feldspars, suggested by Hovis et al. (Reference Hovis, Morabito, Spooner, Mott, Person, Henderson, Roux and Harlov2008, Reference Hovis, Medford, Conlon, Tether and Romanoski2010), gives a value of 9.7 and 12.8 × 10–6 °C–1, respectively. As mentioned above, the experimental αV = 16(1) × 10–6 °C–1, i.e. the formula for ‘Al2Si2’ feldspar is more suitable for filatovite but it does not take into account all the crystal chemical features of filatovite. In order to find a more appropriate equation, it is necessary to study the thermal expansion of intermediate members of the sanidine–filatovite series.
We can conclude that the chemical composition of the framework has almost no influence on the volume thermal expansion orthoclase (αV = ~17 × 10–6 °C–1; Henderson, Reference Henderson2021), microcline (αV = ~17 × 10–6 °C–1; Openshaw et al., Reference Openshaw, Henderson and Brown1979), sanidine (αV = ~21 × 10–6 °C–1; Filatov, Reference Filatov1990) and filatovite (αV = 16(1) × 10–6 °C–1), which have almost the same thermal expansion coefficients. This is in a good agreement with Hovis et al. (Reference Hovis, Medford, Conlon, Tether and Romanoski2010), who stated that the thermal expansion of framework feldspars with feldspar topology is determined primarily by the size of extra-framework cations.
The strong anisotropy of thermal expansion demonstrated by filatovite is typical for feldspar family minerals and synthetic compounds with alkali extra-framework cations (Henderson, Reference Henderson2021). The most probable reason for the sharp anisotropy of filatovite thermal deformation is shear deformations caused by the intense increase of the M–O bond lengths, proposed by Filatov (Reference Filatov1990) for feldspar-related minerals with feldspar topology. It should be noted that framework-type feldspar minerals with alkaline earth extra-framework cations demonstrated a much lower anisotropy degree (Henderson, Reference Henderson2021; Gorelova, Reference Gorelova2023) compared to alkaline feldspars, regardless of framework topology.
Acknowledgments
The authors thank the X-ray Diffraction Centre, Centre for Geo-Environmental Research and Modelling (Geomodel) and Nanophotonics and Nanotechnology Resource Center of Saint Petersburg State University for providing instrumental and computational resources. This research was funded by the Russian Science Foundation, grant number 22-77-10033 (to L.A.G. and O.S.V.).
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2024.10.
Competing interests
The authors declare none.
