Hydrothermal Synthesis and Structure Solution of Na2Ca(CO3)2

Article pubs.acs.org/crystal

Hydrothermal Synthesis and Structure Solution of Na2Ca(CO3)2: “Synthetic Analogue” of Mineral Nyerereite Pavel N. Gavryushkin,*,†,‡ Victor G. Thomas,† Nadezhda B. Bolotina,§ Vladimir V. Bakakin,∥ Alexander V. Golovin,†,‡ Yurii V. Seryotkin,†,‡ Dmitry A. Fursenko,† and Konstantin D. Litasov†,‡ †

V. S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Science, Siberian Branch, Novosibirsk 630090, Russia Novosibirsk State University, Novosibirsk 630090, Russia § Shubnikov Institute of Crystallography RAS, Russian Academy of Science, Moscow 119333, Russia ∥ Nikolaev Institute of Inorganic Chemistry, Russian Academy of Science, Siberian Branch, Novosibirsk 630090, Russia ‡

S Supporting Information *

ABSTRACT: Crystals of Na2Ca(CO3)2, the structural analogues of mineral nyerereite, were synthesized using hydrothermal technique at 1 kbar and 450 °C. The crystals are transformational twins formed at the transition from the high-temperature hexagonal modification to the lowtemperature orthorhombic modification. The structure was solved and refined to R = 0.059 in P21ca (No. 29) space group with a = 10.0713(5) Å, b = 8.7220(2) Å, and c = 12.2460(4) Å. The only structural analogue of the synthesized crystal is the high-temperature modification of K2Ca(CO3)2, which can be considered as a disordered analogue of Na2Ca(CO3)2. Structural analogues among borates and other classes of compounds have not been found. Based on group−subgroup analysis, we propose the structures of high- and intermediate-temperature modifications of Na2Ca(CO3)2. The relations of the determined structure with other polymorphs of Na2Ca(CO3)2 have also been considered.

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(2) thermally induced solid-state reaction Na2CO3 + CaCO3 Na2Ca(CO3)2,10 and (3) by dehydration of mineral gaylussite, Na2CO3·CaCO3·5H2O.11,12 We choose the hydrothermal technique as the most promising for obtaining single crystals of necessary size and quality. We leave it for future investigations to determine whether or not all these techniques produce the same polymorph. The polar space group, together with the relatively low cost and availability of initial reagents (Na2CO3 and CaCO3), makes Na2Ca(CO3)2 attractive for practical applications, for example, in laser optics. Additionally, Na2Ca(CO3)2 can be used as a standard material for thermal analysis due to a series of phase transitions to the high-temperature (β- and γ-) phases.10,13 Another important application of nyerereite investigations is related to Earth. In the next section we provide a brief review with examples of geological occurrence of nyerereite. This review is important for understanding the factors that affect stabilization of different Na2Ca(CO3)2 polymorphs in natural and artificial processes. We discuss these factors providing an insight for future research.

INTRODUCTION Until recently, only two intermediate phases were known in the system Na2CO3−CaCO3: Na2Ca(CO3)2 and Na2Ca2(CO3)3.1−3 The first phase corresponds to the mineral nyerereite and the secondto the mineral shortite. In our recent high-pressure experiments another three phases were discovered in this system: Na2Ca3(CO3)4, Na2Ca4(CO3)5, and Na4Ca(CO3)3.4 Currently structures of only two intermediate compounds are fully solved: Na 2 Ca 2 (CO 3 ) 3 5 and Na2Ca3(CO3)4.6,7 The crystal structure of Na2Ca(CO3)2, which corresponds to mineral nyerereite, is solved only partially,8 positions of some oxygen atoms are not determined and positions of cations are determined only approximately. The main obstacle in solving nyerereite crystal structure is the presence of incommensurate modulations together with inevitable twinning of natural samples.8 High hygroscopicity of crystals makes it difficult to obtain good quality samples. Our approach to this complex problem of determination of incommensurately modulated nyerereite crystal structure is to start from the structure solution of nyerereite’s synthetic analogue.9 Being homeotypic to the natural sample, synthetic nyerereite is not incommensurately modulated and can be relatively easily synthesized in the form of crystals appropriate for single-crystal X-ray diffraction experiments. Three techniques of Na2Ca(CO3)2 synthesis are known: (1) hydrothermal,9 © 2016 American Chemical Society

Received: September 29, 2015 Revised: February 1, 2016 Published: February 3, 2016 1893

DOI: 10.1021/acs.cgd.5b01398 Cryst. Growth Des. 2016, 16, 1893−1902

Crystal Growth & Design

Article

Figure 1. Backscattered electron images showing products of experiments I, II, and III, presented by Na2Ca(CO3)2 (N) and CaCO3 (C) crystals.

pipe. 51 Zemkorite has empirical formula (Na 1.8 K 0.29 )Ca1.1(CO3)1.93, although the concentrations of S, P, F and Cl are not determined.51 Currently only two findings of zemkorite are known. One of them is mentioned previously,51 and the other instance was in kimberlite groundmass of Pipe-7 (India).52 In both cases zemkorite presents in the groundmass of the kimberlite rocks. The origin of zemkorite is unclear. Initially, the mineral was identified as hydrothermal in the rocks of the Udachnaya East pipe,51 while in kimberlite of Pipe-7 zemkorite was interpreted as of metasomatic or subsolidus origin.52 Findings of carbonate with composition (Na,K)2Ca(CO3)2 among daughter phases of melt inclusions in olivine of Udachnaya kimberlites at the same depths and the same assemblage as in ref 51 suggest the domination of the magmatic hypothesis of zemkorite origin over the hydrothermal or metasomatic hypotheses.39,40

Origin and Occurrence of Nyerereite in Geological Settings. The idealized chemical formula of nyerereite is Na2Ca(CO3)2,14,15 although pure end-member Na2Ca(CO3)2 was not identified in nature. Nyerereite always contains some impurities of K, Sr, Ba, S, P, F, and Cl (e.g.,16−24 there also are significant variations in the chemical composition of nyerereite from different localities). The only case when nyerereite is known as one of the two major rock-forming minerals (see for example, refs 8 and 16−24) is in the single and unique composition (among carbonatite clan of rocks) natrocarbonatites of Oldoinyo-Lengai volcano (Tanzania).17,25−27 No other finding of nyerereite among the mineral matrix of any rocks is known. However, nyerereite was identified as a monomineralic inclusion, as a part of multiphase crystalline inclusions in the minerals from carbonatites,28−33 and as the daughter phase in melt inclusions of different rocks’ minerals.34−37 Recently, nyerereite has been identified as micro- and nanophase in polymineralic solid inclusions within diamonds from the Juina area, Brazil.38 The phase of (Na,K)2Ca(CO3)2 composition has also been identified in the minerals melt inclusions of kimberlite multiple times worldwide.39−44 In the opinion of many authors,21,28−30,35,37 findings of nyerereite as the inclusion phase in minerals from different carbonatite localities worldwide (mainly of Na−Ca one) provide evidence that some parts of carbonatites initially contain substantial amounts of alkaline carbonate(s); i.e., they are close by composition to the rocks of Oldoinyo-Lengai volcano, the lavas of which presently erupt and crystallize on the Earth’s surface. Thus, currently unique natrocarbonatites of Oldoinyo-Lengai volcano could not be exceptional through the history of Earth. Their “unicity” is explained by the fact that both nyerereite and gregoryite (Na2CO3), which is the second rock-forming mineral of this carbonatite, are not stable under contact with the atmosphere and meteoric water. The transformation of natrocarbonatite of Oldoinyo-Lengai at the Earth’s surface through the intermediate stages into the endmembers Ca-carbonatite rocks is typically very fast and takes only about a few years or even months (e.g., see refs 21, 28, and 45−50). The findings of nyerereite as an inclusion in magmatic minerals and as the daughter phase of melt inclusions in minerals indicate its magmatic origin. The findings of nyerereite of hydrothermal, metamorphic, and metasomatic origin are not yet described. Another carbonate of the (Na,K)2Ca(CO3)2 composition zemkoritehas initially been found as a mineral of groundmass of deep horizons (400−450 m) of the East body of Udachnaya

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EXPERIMENTAL SECTION

Instruments. Single-Crystal X-ray Diffraction. The Oxford Diffraction Gemini R Ultra single-crystal diffractometer (CCD detector, graphite-monochromatized Mo Kα radiation) was used for X-ray diffraction data collection; the Oxford Diffraction CrysAlisPro softwarefor data reduction. Powder X-ray Diffraction. Diffractometer ARL X-TRA (ThermoScientific) was used, with Bragg−Brentano geometry, Cu Kα radiation, Si(Li) detector. Scanning Electron Microscopy. A Tescan MYRA 3 LMU scanning electron microscope (Tescan) coupled with an INCA Energy dispersive X-ray microanalysis system 350 was utilized with the liquid-nitrogen-free large-area EDS X-Max-80 silicon drift detector (Oxford Instruments). Synthesis. The synthesis of nyerereite single crystals is based on data from ref 9, which shows that in the temperature−pressure ranges of 200−500 °C and 500−1500 bar in the system Na2CO3−CaCO3− H2O two double carbonates are formed: shortite (Na2CO3·2CaCO3) and nyerereite (Na2CO3·CaCO3). Nyerereite is the higher-temperature and lower-pressure phase. Being metastable (transforming to shortite via partial elimination of Na2CO3) under ambient conditions, nyerereite crystallizes at all ranges of compositions Na2CO3−CaCO3 at 1000 bar and temperature above ∼400 °C. Based on these data we choose the conditions of hydrothermal experiment as 1000 bar and 450 °C. Three experiments with different initial charges were carried out (for all three experiments molar ratio was set to Na/Ca = 2). In experiment I the initial loading of the autoclave was 0.53 g of Na2CO3 + 0.5 g of CaCO3 + 0.3 g of H2O. In experiments II and III 10 mol % Na2CO3 was replaced by NaHCO3 and by NaOH, respectively. The motivation for carrying out the two latter experiments was aimed at increasing the CaCO3 solubility (and possibly that of Na2Ca(CO3)2) in the water solution because of the formation of more soluble acidic (experiment II) and alkaline 1894



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(experiment III) carbonate forms. Based on our argument, we expect this property to lead to crystallization of larger spontaneous nyerereite crystals suitable for high-quality single-crystal XRD experiments. Three listed compositions were loaded in 5 mL gold ampules. The ampules were sealed by electric welding and placed in a stainless steel 230 mL autoclave. The autoclave with the golden ampules was filled with distilled water with the filling factor of 0.62, sealed, and heated to 450 °C during 5 h in a shaft furnace. The temperature of the experiment was controlled by two chromel−alumel thermocouples, fixed in the bottom and the cover of the autoclave. We estimate the experimental pressure to be 1000 bar based on the isochores of pure water.53 After the 2 week exposition at 450 °C, autoclave rapidly (over about 5 min) cooled to room temperature in cold water. This quenching was necessary to prevent possible hydrolysis of synthesized nyerereite crystals and their transformation to shortite, which became stable under cooling. The products of the experiments were rapidly washed in water three times, then washed in 96% ethanol, and then dried under atmosphere. The products of the experiments were analyzed by the means of powder XRD and scanning electron microscopy. The main amount of the experimental products (nearly 80%) in all three cases was presented by nyerereite with negligible amount of calcite (CaCO3) and γ-Na2CO3. In experiment I, the nyerereite crystals were strongly flattened and could not be used for single-crystal X-ray investigation, whereas in experiments II and III the isometric crystals of size up to 200 μm were formed (Figure 1). One of such crystals from experiment III was used for the data collection and structure solution. Data Collection and Structure Solution. For X-ray structure analysis a transparent crystal without cracks and visual defects was picked and studied under a polarizing microscope. The size of the crystal was 0.09 × 0.07 × 0.09 mm3. Other details of data collection are summarized in Table 1. The hexagonal unit cell with parameters a = 20.1426(7) Å and c = 12.2460(4) Å were determined from diffraction patterns. The determination of the space group by systematic absences in such a

hexagonal unit cell was ambiguous, and the structure was solved in the P1 space group with 32 formula units by charge flipping method with Superflip54 and Jana2006.55 The determined atomic coordinates could not be divided into groups of three with equal z-coordinate, which excluded the possibility of the presence of the 3- or 6-fold axis in the structure. It would be reasonable to assume that the crystal was the three-component twin and the diffraction pattern consisted of three orthorhombic domains, which could be transformed into each other via the 3-fold axis about [001] (Figure 2). The unit cell parameters of

Figure 2. (001) projection of the unit cells forming a three-component twin. the orthorhombic components in this case are the following: a = 10.0713(5) Å, b = 8.7220(2) Å, and c = 12.2460(4) Å. The transformation from the hexagonal basis (A, B, and C) to orthorhombic basis of each of the components (ai, bi, ci) is the following:

a1 = A/2 + B /2;

crystal data

instrument radiation type measd reflecns reflecns with I > 3σ(I) indepen reflecns hkl limits

Rint refinement R factors (I > 3 σ(I)) R factors (all data) refined params residual electron density/(e Å−3)

a 2 = − A/2;

b2 = − A/4 − B /2;

a3 = − B /2;

b3 = A/2 + B /4;

c1 = C

c2 = C

c3 = C

Although the number of symmetry-independent reflections as well as Rint are given in Table 1, the twin structure was refined using nonaveraged data to avoid the distortion of relative volumes of the twin components. The refined ratio of the twin components shows that they are present in nearly identical amounts: 0.3363(4):0.3446(4):0.3191(4). It might appear that the ideal hexagonal a/b ratio of twinned Na2Ca(CO3)2 crystals suggests the presence of the doubled a-axis. However, this appearance is misleading and might become a source of errors. Space group P21ca (No. 29) was determined manually by analysis of atomic coordinates. The correctness is confirmed by the low value of the R-factor (Table 1). The space group is given in the nonstandard setting to provide the direct correspondence with the crystallographic axis of the natural sample8 and make group−subgroup relations with high-temperature modification clear. The atomic coordinates and isotropic displacement parameters are presented in Table 2, while the bond lengths are in the Supporting Information. All atomic displacement parameters are characterized by reasonable values except for that of carbon atoms. They are negative for C1 and C4 atoms and too high for C2 and C3. The (010) projection of the solved structure is shown in Figure 3; the structure model proposed in ref 8 for natural nyerereite is shown for comparison. The 2 × 1 × 1 supercell is shown to highlight the modulation along the a-axis. For detailed comparison of the structures the incommensurate model of the natural nyerereite is necessary. The structure solution and construction of such a model is a subject of future research. As to the moment only average structure of the natural nyerereite and its modulation vector are known, we can only conclude that synthetic and natural nyerereites are isostructural but differ in the length of the modulation vector. An expanded set of 88 atoms previously refined in P21ca space group was analyzed “by hand”, and the center of pseudosymmetry was found. This provides a possibility of using the centrosymmetric Pbca

Table 1. Parameters of Single-Crystal Data Collection and Structure Refinement

formula fw space group a/Å b/Å c/Å V/Å3 calcd density/(g cm−3) F(000) μ/mm−1 data collection

b1 = − A/4 + B /4;

Na2CaC2O6 (Z = 8) 206.04 P21ca (No. 29; cell choice 4) 10.0713(5) 8.7220(2) 12.2460(4) 1075.71(7) 2.54 816 1.294 Xcalibur, Ruby, Gemini R Ultra Mo Kα radiation (l = 0.7107 Å) 51251 26332 10891 −29 ≤ h ≤ 29 −29 ≤ k ≤ 29 −17 ≤ l ≤ 18 0.048 based on F R = 0.059; Rw = 0.061 R = 0.127; Rw = 0.075 121 max, 9.65; min, −6.53 1895



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domains with slightly different structures in particular. Such hypothesis is experimentally approved for example for vaterite, one of the CaCO3 polymorphs.57

Table 2. Fractional Atomic Coordinates and Isotropic (Equivalent) Displacement Parameters (Å2) of Na2Ca(CO3)2a Ca1 Ca2 Na1 Na2 Na3 Na4 C1 C2 C3 C4 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 a

x

y

z

Uiso

0.75132(6) 0.49406(6) 0.48121(14) 0.02484(12) 0.77039(12) 0.22821(13) 0.50133(19) 0.7521(3) 0.9793(4) 0.2577(2) 0.85862(18) 0.6340(2) 0.4959(2) 0.61215(19) 0.69705(17) 0.3949(2) 0.7461(3) 0.0228(2) 0.5644(2) 0.3960(2) 0.23860(17) 0.8597(2)

0.45301(5) 0.95740(5) 0.30651(13) 0.30344(17) 0.81255(12) 0.81429(12) 0.62496(16) 0.1164(3) 0.9943(4) 0.4981(2) 0.1983(2) 0.1951(2) 0.47026(15) 0.6909(2) 0.38014(17) 0.6951(2) 0.98090(16) 0.9304(2) 0.8993(2) 0.0476(2) 0.41017(14) 0.5446(2)

0.22308(4) 0.22261(5) 0.07950(10) 0.10670(11) 0.08688(10) 0.11613(9) 0.22683(13) 0.2151(2) 0.0042(3) 0.00554(15) 0.22208(15) 0.21183(15) 0.23090(13) 0.22699(14) 0.02480(13) 0.21315(17) 0.22933(14) 0.09174(18) 0.01790(16) 0.05619(16) 0.09027(11) 0.05733(16)

0.01294(12) 0.01878(15) 0.0284(4) 0.0337(4) 0.0231(4) 0.0212(3) −0.0048(3) 0.0351(7) 0.0355(7) −0.0012(3) 0.0125(4) 0.0240(5) 0.0118(3) 0.0140(4) 0.0144(3) 0.0263(5) 0.0207(4) 0.0421(6) 0.0377(5) 0.0269(5) 0.0084(3) 0.0245(5)

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RESULTS AND DISCUSSION Traditional structure description based on cation polyhedrons does not work well for such complex carbonates as Na2Ca(CO3)2 with diverse orientations of CO3 groups.6 The approach that represents the structure as an anion-stuffed cation array58,59 is more fruitful. Crystal chemical formula of the determined structure is Ca[9]Na[8]Na[6]C2[3]O5(5)O(4), where the superscripts in square brackets and in parentheses indicate the coordination numbers of cations and anions, respectively. Crystal chemical formulas of other compounds, which will be discussed later, are presented in Supporting Information. The structure of Na2Ca(CO3)2 can be presented as a stacking of close-packed monocation Na- and Ca-layers in 6layered close-packed structure by the principle ...ABCBAC... (Figure 4a); layers have (001) orientation. Ca-layers are slightly puckered and placed between two Na-layers forming Na−Ca− Na quasi mirror symmetrical blocks. In the presented sequence of layers such blocks correspond to the ACA and BCB combinations of letters. Considering such a block as a single building unit, the structure can be described as the hexagonal close-packed one. CO3 groups are placed in interpackage cation cavities of the two different types: (1) within Ca-layers and (2) in Na−Na interpackages spaces. CO3 groups of different orientations are related to the cavities of different topologies. The cavities of the first type are centered by horizontal (nearly parallel to (001)) CO3 groups; the second typeby inclined CO 3 groups, which are presented in two nonparallel orientations both forming the angle of 60−70° with (001) plane. We consider the cation environment of the horizontal and inclined CO3 triangles in more detail. Horizontal CO3 triangles lie in the plane of the Ca-layer in trigonal Ca3 loops (Figure 5a). Each O2− anion of such triangles has two O−Ca bonds. According to suggested crystal chemical formula, another two O−Na bonds are necessary for the optimal sum of bond valences. One of these Na atoms lies in the above- and one in the below-layer. Thus, the coordination polyhedron is the threecapped trigonal prism Na6Ca3 (Figure 6a,b). Inclined CO3 groups are placed between two close-packed Na-layers in all octahedral interstices of such a package (Figure 5b, Figure 6c).

Site occupancies of all atoms are equal to 1.

group with tolerance equal to 0.1 Å. The value of tolerance is quite high and cannot be explained by experimental or refinement errors. The analysis of reflection conditions shows that out of about 26000 reflections with I > 3σ, nearly 400 reflections break the reflection conditions of Pbca space group, while only 5 reflections close to 3σ break the reflection conditions of P21ca space group. On the other hand, the refinement in Pbca space group (R = 0.062) gives positively determined anisotropic displacement parameters (ADPs) for all of the atoms, while the isotropic ADPs are negative for the two carbon atoms and insensibly high for the other two carbon atoms in P21ca space group. Besides, the refinement in Pbca space group gives reasonable values of residual electron density with maximum and minimum values equal to 1.12 and −0.79 e/Å3, respectively, while for P21ca space group these values are too high. This interplay of P21ca and Pbca space groups, which results in the high residual electron density and negative isotropic displacement parameters, can be explained by the microstructure of the sample at the nanolevel and by the intergrowth of two

Figure 3. (010) projections of (a) the average structure of the natural (structural data from ref 8) and (b) modulated structure of hydrothermal modifications of Na2Ca(CO3)2. Na atoms are shown in dark blue; Ca, in green; O − in red; and C, in light blue. The dashed line outlines the 2 × 1 × 1 supercell of hydrothermal Na2Ca(CO3)2. Produced with VESTA software.56 1896



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Figure 4. Representations of structures: (a) Na2Ca(CO3)2, (b) fairchildite K2Ca(CO3)2, and (c) buetschliite K2Ca(CO3)2. Oxygen atoms are shown in red; Na, K, in green; and Ca, in blue, respectively.

Figure 5. (a) Na−Ca−Na package with horizontal CO3 groups in Ca layer. (b) Na−Na slab formed by close-packed Na layers with inclined CO3 groups in each octahedron.

Figure 6. (a, b) Horizontal CO3 triangle in the three-capped trigonal prism Na6Ca3. (c) Na6 octahedron centered by CO3 triangle. (d) Two-capped Na6Ca2 octahedron centered by the inclined CO3 triangle.

packages of two-capped Na6Ca2 octahedrons with inclined CO3 groups (Figure 7). Two different orientations of CO3 triangles in Na2Ca(CO3)2 are a common feature for all known Na−Ca carbonates. This property is unusual for other simple and double carbonate structures, where all CO3 groups are (nearly) parallel. Another unusual structural feature of Na2Ca(CO3)2 is the relatively high deviation of CO3 groups from planarity, varying in the range 1°−8.23° (dihedral O−C−O−O angle). Another Na−Ca double carbonate Na2Ca3(CO3)4 is characterized by nearly the same values.6

The tilt of the triangle inside the prism is caused by the necessity (for compensation of residual charge) for each oxygen atom to form one more O−Ca bond with bond valence equal to 2/9 in addition to two or three O−Na bonds with bond valences equal to 1/6 or 1/8. Adding of these two Ca−O bonds transforms the Na6 octahedron into two-capped octahedron Na6Ca2 (Figure 6d). Summarizing the preceding description, the Na2Ca(CO3)2 structure can be described as a stacking of packages of threecaped Na6Ca3 trigonal prisms with horizontal CO3 groups and 1897



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Figure 7. Na2Ca(CO3)2 structure (projection on (010)) represented as a stacking of the cation polyhedrons around CO3 groups of two orientations: on the left, only packages of trigonal prisms Na6Ca3; on the right, only packages of octahedrons Na6Ca2; in the center, their superposition.

Pseudosymmetry and Relations of Na 2 Ca(CO 3 ) 2 Polymorphs. Previously we discussed one polymorph of Na2Ca(CO3)2 synthesized in our experiments. In this section we will discuss relations of different nyerereite polymorphs synthesized under different conditions. Unfortunately no uniform nomenclature for these polymorphs exists, and different authors use different notations. The absence or incompleteness of structural information on these polymorphs makes the situation more complicated. We propose the following notations. Since the high-temperature polymorphs synthesized by heating of natural crystals differ from synthetic samples, it should be reflected in the name of the polymorph from what sample the polymorph is formed, for example synthetic β-Na2Ca(CO3)2, or natural β-Na2Ca(CO3)2. For the sake of brevity we denote Na2Ca(CO3)2 as N. Moreover, the different synthetic phases also provide different polymorphs. Later we compare high- and low-temperature polymorphs of a hydrothermal sample with polymorphs formed under decomposition of gaylussite. In previous works12,13 the latter were designated as α-, β-, γ-Na2Ca(CO3)2. Now we will designate them as decompositional α-, β-, and γ-N to distinguish them from hydrothermal α-, β-, and γ-N. If phases formed from different types of synthetic samples are the same, they will be designated with the word synthetic, for example, synthetic β-N. If some phases are the same for both natural and synthetic samples, no word will be added, for example γ-N. The twinning of synthesized crystals is explained by the phase transition from high-temperature to low-temperature modification, similarly to the natural sample. The presence of transformational twins indicates the structural similarity of the high- and low-temperature polymorphs of Na2Ca(CO3)2 and the presence of a pseudosymmetry in the low-temperature structure. The analysis of this pseudosymmetry allows one to determine the space groups and structures of high-temperature polymorphs. In later discussion we present this analysis, carried out with instruments of the Bilbao Crystallographic server.70−73 The displacements of atoms at the distance of not more than 0.1 Å allow representation of the structure in the highersymmetric space group Pbca with the same unit cell parameters. The transition to the next group Pbcm with the halved a-axis needs the displacements on 0.8 Å. However, such significant

We consider the relation of the solved structure with the structures of other carbonates and borates of stoichiometry (M,M′)3(TO3)2. In the ICSD database we found only one structure homeotypic with Na2Ca(CO3)2. The structure is the high-temperature modification of K2Ca(CO3)2 (K2Ca(CO3)2HT) that corresponds to mineral fairchildite.60 The difference of the K2Ca(CO3)2-HT structure from that of Na2Ca(CO3)2 is that oxygen atoms of the inclined triangles are in disordered state and not in the fixed state (Figure 4a,b). The angle (locally fixed) of these triangles with (001) plane is almost the same for both structures: 69° for K2Ca(CO3)2-HT and 60−70° for Na2Ca(CO3)2. Thus, Na2Ca(CO3)2 can be considered as a sodium analogue of ordered K2Ca(CO3)2-HT. The ordered fairchildite has not been found yet, although the possibility of its synthesis has been pointed out.60 Finding of fairchildite crystals with four times longer a-axis than that of the disordered structure61 is an additional argument in favor of this hypothesis. It is interesting to compare structural changes of Na2Ca(CO3)2 and K2Ca(CO3)2 with increasing temperature. The described structure of Na2Ca(CO3)2 corresponds to the lowtemperature modification. The structure of the high-temperature modification of Na2Ca(CO3)2 has not yet been solved. Below we give arguments to support the hypothesis that this structure is identical to the high-temperature modification of K2Ca(CO3)2. Accordingly, upon transition from a high- to a low-temperature structure of Na2Ca(CO3)2 the overall structural features are preserved and the only differences are related to the disordered state of some oxygen atoms. In contrast, the high-temperature (fairchildite) and the lowtemperature (butcheliite) modifications of K2Ca(CO3)2 are fundamentally different. In the low-temperature modification K and Ca cations form 9-layered close-packed structure by the principle ...ABCBCACAB... (Figure 4c), in which all CO3 groups are parallel to each other, whereas the high-temperature modification described earlier has a 6-layered close-packed structure. The structure of the low-temperature modification, as opposed to the high-temperature one, is richly represented among compounds of other classes. The other classes include carbonates K2Mg(CO3)262 and Na2Mg(CO3)2,63 borates Ba2Ca(BO3)2,64 KBaY(BO3)2,65 Sr2Mg(BO3)2,66 and BaNaY(BO3)2,67 and selenates K2Co(SeO3)268 and K2Mn(SeO3)2.69 1898



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displacements are related exclusively to oxygen atoms of the inclined CO3 groups, which, analogously to K2Ca(CO3)2, should be disordered in the high-temperature form. Excluding these atoms from consideration allows climbing to space group Cmcm and then to P63/mmc (Figure 8) with maximum

Table 3. Atomic Coordinates and Space Groups of Hydrothermal β-N and γ-N Polymorphs Determined Based on Analysis of Pseudosymmetry x

y

z

Occ

Name, β-N; Space Group, Cmcm (No. 63); a = 5.0356 Å; b = 8.722 Å; c = 12.246 Å C1 0.5 0.12068 0.25 1 C3 0.5 0.5 0 1 Ca1 0 0.95521 0.25 1 Na1 0.5 0.80919 0.09731 1 O1 0.22096 0.69485 0.25 1 O3 0.5 0.97558 0.25 1 Name, γ-N; Space Group, P63/mmc (No. 194); a = 5.0356 Å; c = 12.246 Å Na1 0.33333 0.66667 0.09731 1 Ca1 0 0 0.25 1 C1 0.66667 0.33333 0.25 1 C3 0 0 0 1 O1 0.51992 0.03985 0.25 1 O2 0.0865 0.173 0.9224 0.333 O3 0.238 0 0 0.167

corresponds to γ-N. Although the group of natural β-N is hexagonal, it has the a-axis twice longer than that of the γ-N. Hence the symmetry of natural β-N is not presented in the pseudosymmetry of our sample. This suggests a hypothesis that if β-N were synthesized in a hydrothermal experiment, then it would be different from the natural β-N and would be characterized by one of the orthorhombic space groups: Cmcm, Pbcm, or Pbca. This conclusion is confirmed by results of ref 11 on heating and quenching of Na2Ca(CO3)2 formed under decomposition of gaylussite. Based on powder diffraction data it has been determined that under quenching of hexagonal γphase first orthorhombic C-centered β-N and then orthorhombic C-centered α-N are formed. Among space groups which are proposed for hydrothermal β-N the C-group is Cmcm. Based on the same type of centring and cell parameters, we can assume equivalency of hydrothermal and decompositional β-N and designate this phase as synthetic β-N. Structural data of this phase inferred from our pseudosymmetry analysis are presented in Table 3. The C-type of centring of decompositional α-N indicates that this polymorph is different from hydrothermal α-N. Taking into account group−subgroup relations, we can suggest Cmc21 space group for decompositional α-N (Figure 8). The temperature of the phase transitions determined in ref 11 is confirmed by ref 12. In addition, in this work the irreversible transition from decompositional α-N to a lower-temperature phase was found, which we designate as α′dec (Figure 9). Unfortunately, the only information on α′-dec is the temperature of the phase transition. Due to irreversible character of this transition we cannot make at this stage any conclusion about its structure or space group. Even less is known about mineral zemkorite than about natural β-N. For zemkorite based on powder diffraction X-ray data, the same unit cell parameters and diffraction symbol as that for natural β-N were determined (Table 4). Four variants of relations with other modifications of Na2Ca(CO3)2 can be proposed: (1) zemkorite is the analogue of the synthetic β-N, (2) zemkorite is the twinned natural α-N (as it was shown previously, the twinning in this case gives seeming effect of aaxis doubling), (3) zemkorite is the analogue of quenched natural β-N, and (4) zemkorite does not correspond to any of the known polymorphs of Na2Ca(CO3)2. The absence of

Figure 8. Partial group−subgroup graph showing the relations of different Na2Ca(CO3)2 polymorphs. The phases circumscribed by solid rectangles are experimentally fixed phases; with dashed rectangles, experimentally fixed but with incompletely determined space groups. Among two variants of space groups of natural β-phase, P63mc and P63/mmc, only the first one is shown; the second one is not considered to make the graph more clear and illustrative, although that space group is also possible.

displacement of less than 0.4 Å. The derived structure of the hexagonal aristotype may be used as the model for γ-N, which is identical for both synthetic and natural samples. The correspondence of the hexagonal P63/mmc aristotype and γN is confirmed by the following facts: (1) the identity of hexagonal aristotype and the high-temperature modification of K2Ca(CO3)2 (the atomic coordinates in these two structures are different by less than 1%); (2) the identity of the experimental diffraction symbol and the similarity of the unit cell parameters of hexagonal aristotype and natural γ-phase (currently this is the only structural information on this polymorph). To complete the model of γ-phase, oxygen atoms of the inclined triangles should be disordered in the same way as in the high-temperature polymorph of K2Ca(CO3)2 (Table 3). For natural β-N only lattice parameters and diffraction symbols have been determined (Table 4). Hydrothermal α-N is characterized by the ideal hexagonal a/b ratio, and in hexagonal setting its parameters are very close to that of natural β-N. This fact strongly supports the conclusion about the identity of these two polymorphs. Nevertheless, based on the analysis of the systematic absences of untwinned single-crystal McKie and coauthor8 came to the unambiguous conclusion about hexagonal symmetry of natural β-N and about its nonidentity to the hydrothermal α-N. The next question isis the phase, which is the same as the natural β-N, realized in our experiment on quenching? The aforementioned analysis of pseudosymmetry gives a negative answer on this question. Among the supergroups presented in the pseudosymmetry group of the sample, only one is hexagonal. This hexagonal group 1899



Article

Table 4. Symmetry and Unit Cell Parameters of Na2Ca(CO3)2 Polymorphsa composn, name Na1.64K0.36Ca(CO3)2, (low-nyerereite) α-nat Na1.64K0.36Ca(CO3)2, (intermediate-nyerereite) β-nat Na1.64K0.36Ca(CO3)2, (high-nyerereite) γ-nat (Na1.8K0.29)Ca1.1(CO3)1.93, zemkorite Na2Ca(CO3)2, (synthetic nyerereite) α-hyd Na2Ca(CO3)2, α-hyd ???, (α-N) α-dec ???, (β-N) β-dec ???, (γ-N) γ-dec a

space group or diffraction symbol Natural Sample Cmc21 6/mmmP--c 6/mmmP--c 6/mmmP--c Hydrothermal Sample P21ca mmmPbca Decompositional Sample mmmC??? mmmC??? 6/mmm????

a, Å

b, Å

c, Å

5.044 10.1 5.05 10.06

8.809 10.1 5.05 10.06

12.743 12.75 12.85 12.72

8 8 8 51

10.0713 10.12

8.722 8.76

12.246 12.25

this work 9

5.016 5.034 5.079

8.746 8.865 5.079

12.234 12.644 12.755

11 11 11

ref

Names of the phases as in the reference are shown in brackets; names as they are in this work are shown without brackets.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The research was supported by the Russian Foundation for Basic Research through Grant Nos. 14-05-31051 and 130312158 and the Ministry of Education and Science of Russian Federation (Grant Nos. 14.B25.31.0032 and MK3766.2015.15).

Figure 9. Summary on high-temperature phase transitions of Na2Ca(CO3)2. The numbers under arrows are temperature (°C); syn, dec, and nat, abbreviations from synthetic, decompositional, and natural, respectively.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Alex Gavryushkin for navigating us through modern group theory and help with group−subgroup analysis.

single-crystal or Raman spectroscopy data for zemkorite does not allow choosing one of these hypotheses. Regarding hypothesis 3, it should be noted that no known experiment allows quenching of the high-temperature hexagonal (β- or γN) phase, even those with the highest rate of quenching and high content of impurities. Other counterarguments to this hypothesis are suggested in ref 24. In summary we emphasize that in high-temperature hexagonal phase γ-Na2Ca(CO3)2 the oxygen atoms of the inclined CO3 groups are in the disordered state, so are they in the hightemperature polymorph of K2Ca(CO3)2. The ordering takes place under quenching of γ-N. The dif ferent polymorphs of Na2Ca(CO3)2 are produced by dif ferent principles of this ordering. The solved structure of hydrothermal α-Na2Ca(CO3)2 provides one of the examples of such an ordering.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01398. Interatomic distances of the hydrothermal α-Na2Ca(CO3)2 in the range up to 3 Å and the relation of structural data of some of the compounds, mentioned in this work (PDF) Accession Codes

CCDC 1451297 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 1900



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Hydrothermal Synthesis and Structure Solution of Na2Ca(CO3)2

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