Thermal Expansion and Polymorphism of Slawsonite SrAl 2 Si 2 O 8

: Slawsonite’s (SrAl 2 Si 2 O 8 ) structure evolutions depending on temperature (27–1000 ◦ C) have been studied by in situ single-crystal X-ray diffraction. The SrO 7 polyhedron expands regularly with the temperature increase. Silicon and aluminum cations are ordered in tetrahedral sites of the studied slawsonite; no signiﬁcant changes in their distribution as temperature increases were observed. Slawsonite demonstrates a relatively high volume thermal expansion ( α V = 23 × 10 − 6 ◦ C − 1 ) with high anisotropy, typical for framework feldspar-related minerals and synthetic compounds. It was found that, contrary to previously published data, the crystal structure of slawsonite is stable in the studied temperature range and no phase transitions occur up to 1000 ◦ C. The role of Ca and Ba substitution for Sr and Al/Si ordering on polymorphism of natural M Al 2 Si 2 O 8 ( M = Ca, Sr, Ba) is herein discussed.

As was mentioned above, slawsonite belongs to the feldspar group of minerals. According to a recent review on feldspars [14], all the feldspar-related crystal structures are divided into three groups: tetrahedral structures, octahedral structures, and structures with mixed coordination numbers. Wherein the first group is the most widespread in nature and the last one has not been found in nature at all. In turn, the tetrahedral structures are subdivided into four groups, based on the structural topology, i.e., feldspar, paracelsian, svyatoslavite (all three correspond to three-dimensional frameworks), and dmisteinbergite (layered). According to the classification [14], slawsonite together with paracelsian (BaAl 2 Si 2 O 8 ), danburite-(MB 2 Si 2 O 8 , M = Ca, Sr, Ba) and hurlbutite-group minerals (MBe 2 P 2 O 8 , M = Ca, Sr) belong to the minerals with the paracelsian topology.
Slawsonite is quite a rare mineral, which has been found only in several localities and is usually associated with metasomatic reactions at low temperatures and low-pressure metamorphism [15]. As a mineral, slawsonite has been first described in Martin Bridge formation, Oregon, the USA by Griffen et al. in 1977 [16], whereas its synthetic analog had already been known [17]. Moreover, according to the Inorganic Crystal Structure Database (ICSD) the synthetic compound with the general formula SrAl 2 Si 2 O 8 is known at least in four other polymorphic modifications: I2/c [18][19][20][21], P-3c [22], P6 3 /mcm [22,23] and Pm-3m [24]. It is interesting to note, that according to Bahat [25] the hexagonal modification of SrAl 2 Si 2 O 8 (as well as CaAl 2 Si 2 O 8 ) does not have any fields of stability under ambient pressure, whereas hexagonal BaAl 2 Si 2 O 8 can exist under ambient conditions, which is probably associated with the increase of the ionic size of the M 2+ (M = Ca, Sr, Ba) cation.
Due to the high geological importance and wide industrial implications, many minerals with the feldspar topology were studied in detail both under ambient and extreme conditions (high-and low temperatures and high pressures). Though feldspar minerals with the paracelsian topology are not very widespread in nature, almost all of them (except strontiohurlbutite) have been recently studied under high-pressure conditions [26][27][28][29][30], whereas only danburite-group minerals have been studied in detail under temperature increase up to the present time [31,32].
To date, it is known, that generally, the polymorphism of SrAl 2 Si 2 O 8 is similar to that of BaAl 2 Si 2 O 8 . However, as most of the published results have been obtained by transmission electron microscopy and powder X-ray diffraction, many inconsistencies occur. It is thought that slawsonite has a displacive phase transition at 320 • C from triclinic to monoclinic phase [33,34]. According to [35,36] at 500 or 600 • C slawsonite transforms into another monoclinic phase, namely Sr-celsian (i.e., compound with the feldspar topology), which melts at 1650 • C. Nevertheless, this phase transition was not mentioned by Tagai et al. [33], who performed differential thermal analysis (DTA) up to 1000 • C. Three hexagonal (or pseudohexagonal) SrAl 2 Si 2 O 8 polymorphs (P6/mmm, P6 3 /mcm, and Immm), similar to hexacelsians (BaAl 2 Si 2 O 8 ) [37] can be obtained by quenching melt. According to Toepel-Schadt et al. [36], two hexagonal phases coexist at room temperature and the transition to orthorhombic phase occurs at about 600 • C [38].
The present study aims to investigate the high-temperature behavior of slawsonite using in situ single-crystal X-ray diffraction data at a temperature range of 27-1000 • C in order to clarify whether there are phase transitions and compare the obtained data with the data on other isotypic minerals.

Materials and Methods
The natural sample of slawsonite, SrAl 2 Si 2 O 8 , originating from the Rendai, Kochi Prefecture, Shikoku Island, Japan, was obtained from the private systematic collection of AK. The chemical composition of this sample has stochiometry close to ideal (Sr 0.94 Ba 0.03 Al 1.97 Si 2.04 O 8 ; [29]).
Thermal behavior of slawsonite under heating in the air was studied in situ by hightemperature 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 40 mA and equipped with an HyPix-6000HE detector with a unique high-temperature FMB Oxford system. The sample heating is done using a gas blower (Figure 1a) up to 1000 (±1) • C, which consists of a hot air generator controlled by a Eurotherm regulator and gas flow controller. The temperature stability is about ±0.5 • C at the temperature above 250 • C, the heating rate is about 10 • C/min for the temperatures below 400 • C and 5 • C/min from 400 to 1000 • C. Before the experiments, the accuracy of this device has been verified using a single crystal of quartz, which has an α↔β transformation at 573 • C [39]. The temperature determination error is ±10 • C. After heating, the crystal was kept at each temperature for about 10 min and after that, the SCXRD data were collected (that took about 20 min at each temperature point). For this experiment, the single crystal with the approximate size of 30 µm × 30 µm × 10 µm was mounted on the glass fiber, which was placed into the quartz capillary ( Figure 1b) 300 µm in outer diameter and 10 µm of wall thickness, produced by Hampton Research (USA). Diffraction data were collected at different temperatures without changing the orientation of the crystal in the range of 27 to 1000 • C with a temperature step of 100 • C. The frame width was 1.0 • in ω and ϕ, the exposure of 3 s per frame. The data were integrated and corrected for background, Lorentz, and polarization effects. An empirical absorption correction based on the spherical harmonics implemented in the SCALE3 ABSPACK algorithm was applied in the CrysAlisPro program [40]. The unit-cell parameters were refined using the least-square techniques. The SHELXL program package [41] was used for all structural calculations. At temperatures below 300 • C, the crystal structures of slawsonite were refined in both possible space groups (P2 1 /c and P-1) (see below for details). The structure models of natural slawsonite by Griffen et al. [16] (P2 1 /c) and Tagai et al. [33] (P-1) were used as starting models for structure refinements. The crystal structures were refined at 11 temperature points (Tables 1 and 2), including room temperature after heating. Anisotropic displacement parameters were refined for all atoms. The temperature dependencies of the unit-cell parameters were described by quadratic and linear (for comparison with other minerals) polynomial functions in the whole temperature range (see below for more details).
Based on these data, the thermal expansion coefficients (α a ( • C −1 ) = (1/a)(da/dT) were determined using the TTT program package [42,43]. This program package has also been used for the thermal-expansion parameter tensor visualization.

Results and Discussion
3.1. Polymorphism of Natural SrAl 2 Si 2 O 8 in a Temperature Range 27-1000 • C Generally, the crystal structure of slawsonite is based on a three-dimensional tetrahedral framework that contains four-and eight-membered rings of SiO 4 and AlO 4 tetrahedra [16], framework channels are occupied by Sr atoms (Figure 2). However, if this crystal structure is compared with other feldspar-related minerals, it could be described as consisting of the crankshaft chains of tetrahedral (Figure 3a), formed by the successive polymerization of four-membered rings [44].
The temperature dependencies for the unit cell parameters, obtained during our SCXRD experiment, and the evolution of normalized unit cell parameters are shown in Figures 4 and 5, respectively. In the whole temperature range (up to 1000 • C) the crystal structure of slawsonite undergoes a continuous expansion of the unit-cell parameters. At the first glance, the changes of the unit-cell parameters are not linear and should be divided into three parts (from 27 to 200, from 300 to 700, and from 800 to 1000 • C), i.e., demonstrate two possible phase transformations (at about 300 and 700 • C). According to the SCXRD and DTA data by Tagai et al. [33] a phase transition should occur at 320 • C with the symmetry increasing from P-1 to P2 1 /c. The second phase transition has been previously mentioned at 500 or 600 • C [34,35] but has not been confirmed by Tagai et al. [33]. According to SCXRD data by Tagai et al. [33], collected below 320 • C, there are some weak but sharp reflections, which are excluded by the space group P2 1 /c and as a consequence, the real structure of slawsonite under ambient conditions has to be triclinic. Our SCXRD data at all temperature points contain about 150 very weak reflections (<3σ) and only 1-5 relatively low reflections (>3σ) (see supplementary crystallographic information data files). Our attempts to refine the crystal structure of slawsonite at temperatures below 220 • C in the triclinic symmetry did not demonstrate any improvements over the refinements in the monoclinic symmetry (Tables 1-4): R-factors were almost the same regardless of symmetry and even better for the P2 1 /c space group. Besides, the bond length errors are also the same for both space groups. According to both Tagai et al. [33] and our data (Tables 1-4), there is Si-Al order in tetrahedra. Moreover, our data shows that crystal structure is fully ordered in the whole temperature range (<Si-O> = 1.616-1.622, <Al-O> = 1.743-1.749 Å under ambient conditions), i.e., no disordering occur in tetrahedra ((Si-O) = 1.64, (Al-O) = 1.77 Å; [45]). Thus, the only opportunity for P2 1 /c → P-1 transition could be Sr/Ba ordering. As there are no strong crystal-chemical reasons to decrease the symmetry, we suppose the absence of the phase transition at 300 • C. A similar situation is with the second inflection of the unit cell parameters between 700 and 800 • C: according to the SCXRD data, the crystal structure does not undergo any phase transformation. The similar non-linear behavior of the unit cell parameters without any phase transformations has been previously observed for slawsonite [29] and some other isotypic minerals (danburite-group minerals MB 2 Si 2 O 8 (M = Ca, Sr, Ba) [26,28], paracelsian BaAl 2 Si 2 O 8 [27], hurlbutite CaBe 2 P 2 O 8 [30]) under high-pressure conditions. As a consequence, for further discussion, we will use the crystal structure of slawsonite refined in the monoclinic P2 1 /c space group at all temperatures. Though the crystals studied by Tagai et al. [33] and by us are from the same locality (Kochi Prefecture, Shikoku Island, Japan) and the chemical composition is very close (Sr/Ba ≈ 0.95/0.05), the different high-temperature behavior could be explained by the blockiness of the crystals: some domains could indeed have triclinic crystal structures, but the average crystal structure is better described by the monoclinic (P2 1 /c) space group in the whole temperature range. The other option could be that two polymorphs are present in the same deposit: low-slawsonite (Sp. gr. P-1, described by Tagai et al. [33]) and high-slawsonite (Sp. gr. P2 1 /c, described by us). Low-slawsonite could be a result of Sr/Ba ordering at the 7-coordinated site or there is slightly higher Ba content, which resulted in phase transition ( VII Sr-O 2.61, VII Ba-O 2.78 Å; [43]): compare 2.651 Å [33] and 2.644 Å (Tables 3 and 4).   Besides the suspected displacive phase transition [33], a reconstructive transformation into Sr-feldspar (namely in the crystal structure with the feldspar topology, isostructural to celsian) at the temperature of about 500-600 • C was also proposed [35,36]. Our data demonstrate the absence of both phase transitions. However, the chemical composition of the samples transformed to the 'Sr-feldspar' form has not been studied. Thus, it can be assumed that the presence of even small Sr to Ba substitution can play an important role in the stabilization process of the slawsonite form of the crystal structure and Sr/Ba/ (vacancy) ratio could result in unexpected phase transitions (e.g., [46]).

Slawsonite Thermal Expansion
Thermal expansion coefficients (TECs) calculated both with the linear and secondorder polynomial approximation of the unit-cell parameters are listed in Table 5 Regardless of the approximation method, the thermal expansion of slawsonite has quite an anisotropic character. The maximum and minimum expansion was observed along the c and b axes, correspondingly, i.e., in the plane of four-and eight-membered rings of TO 4 (T = Si, Al) tetrahedra ( Figure 2). Such behavior could be explained by the hinge deformation mechanism, which was previously noted for isotypic danburite-group minerals [32].
The linear approximation of the unit cell parameters has also been done for the three separate parts according to their inflections , 300-700, and 800-1000 • C), but the R 2 fitting values for all parameters were not reasonable (below 0.9 and for some parameters below 0.5). So, all the data were used together for TECs calculation. It is worth noting that the β angle fitting was not precise enough both in whole and in the three separate data sets, which is associated with the general problems of the β determination. Table 5. TECs (×10 6 • C −1 ) of slawsonite along the principal axes of the thermal expansion tensor and crystallographic axes. The observed bond lengths and polyhedral volumes for slawsonite at different temperatures are given in Table 3. The expansion of the three types of polyhedral (SrO 7 , SiO 4, and AlO 4 ) are significantly different, which is consistent with other studies of aluminoand borosilicate compounds (e.g., [19,31], etc).
In the whole temperature range, Sr atoms are coordinated by seven oxygen atoms. As it was mentioned above, Sr atoms are located in the cavities of eight-membered rings of TO 4 (T = Si, Al) tetrahedra. If we consider the crystal structure of slawsonite as consisting of the layers of four-and eight-membered rings of TO 4 tetrahedra, Sr atoms are located between these layers (Figure 3b). Wherein O1, O4, O5, and O5 atoms belong to the layer consisting of Al1O 4 and Si1O 4 tetrahedra, whereas O2, O3, and O7 atoms belong to the layer of Al2O 4 and Si2O 4 tetrahedra. All Sr-O bonds increase continuously with the increasing temperature ( Figure 6): three longest bonds (Sr-O2, Sr-O8 and Sr-O1), located parallel to the c axis, demonstrate the most intense increase (first two increase by 3.6%, whereas the last one by 2.2%). The elongation of four other bonds, located close to the b axis, is in the range of 0.5-1.5%. Such bond behavior agrees with the thermal expansion of the whole crystal structure, as the maximal and minimal thermal expansion is along the c and b axes, respectively (Table 5). Generally, in the whole studied temperature range, the volume of SrO 7 polyhedra increases up to 5.5%. The changes in the bond-lengths of AlO 4 and SiO 4 tetrahedra are not so intense (Table 3, Figure 6a,b), i.e., do not exceed 0.02 Å or 0.3%. This fact agrees with the general idea about the model of the rigid-unit mode [54][55][56] and the idea of cooperative motions of tetrahedral groups, such as titling and rotating [57]. It should also be noted that these changes are negative: only Si2-O7, Al1-O4, Al2-O1, and Al2-O3 bonds slightly increase, whereas all other bonds decrease.

Conclusions
Based on in situ high-temperature single-crystal X-ray diffraction data we can conclude, that (Sr 1-x Ba x )Al 2 Si 2 O 8 (x < 0.03) could exist only in one polymorph modification (Sp. gr. P2 1 /c) in a temperature up to 1000 • C despite non-linear behavior of the unit cell parameters. Natural compounds with stoichiometry close to slawsonite and high Ca/Ba content should be carefully checked by X-ray diffraction techniques. Our results compared with data of Tagai et al. [33] show that several polymorphs could potentially occur at the same deposit, but for their determination, single-crystal X-ray diffraction studies are necessary.
Generally, the thermal expansion of the unit-cell parameters of slawsonite is very close to that of danburite-group minerals MB 2 Si 2 O 8 (M = Ca, Sr, Ba) [32], i.e., the feldsparrelated minerals with the paracelsian topology. For all four compounds the directions of maximal and minimal thermal expansion are located on the plane of four-and eightmembered rings, which is associated with the hinge deformations of eight-membered rings. Gorelova et al. [32] mentioned that the increase of the extra framework cation size (from Ca to Ba) leads to the increase of the thermal expansion anisotropy and volume thermal expansion. This study showed that not only the extra framework cation, but also the framework forming cation (the replacement of B (0.11 Å) to Al (0.39 Å) [45]) causes similar changes: under ambient conditions α max /α min = 2 and 4 and α V = 15 and 17 × 10 6 • C −1 for pekovite (SrB 2 Si 2 O 8 ) and slawsonite, respectively.
The comparison of the behavior of slawsonite under high-temperature (up to 1000 • C) and high-pressure (up to 6 GPa [29] conditions ( Figure 3) demonstrate the similarity of deformations in the layer of four-and eight-membered rings (i.e., in the bc plane) and differences in other directions. Generally, in both cases, the deformations are quite anisotropic, but under temperature increase, the minimal changes are along the b axis, whereas under pressure increase the structure remains almost unchanged along the a-axis, i.e., parallel to the extension of the crankshaft chains.