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Thermal stability and dehydration of armstrongite, a microporous zirconium silicate
Microporous and Mesoporous Materials 272 (2018) 137–142
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Thermal stability and dehydration of armstrongite, a microporous zirconium silicate
T
E. Schingaroa, M. Lacalamitab,∗, E. Mestoa, G. Della Venturac,d Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari “Aldo Moro”, via E. Orabona 4, I-70125, Bari, Italy Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, I-56100, Pisa, Italy c Dipartimento di Scienze, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146, Roma, Italy d INFN-Laboratori Nazionali di Frascati, Via E. Fermi 40, I-00044, Frascati, Roma, Italy a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Armstrongite Dehydration/rehydration Thermal analysis In situ HT-XRPD In situ HT-FTIR
The dehydration of armstrongite from Khan Bogdo (Mongolia) was investigated by combining thermal analysis, in situ HT X-ray powder diffraction (XRPD) and Fourier Transform InfraRed (FTIR) spectroscopy. The process starts at ∼380 °C and is completed within few tenths of degrees. It involves a mass loss of 6.1 wt% and a cell volume decrease of 7%. Armstrongite at RT has C2/m symmetry with (in Å) a = 14.010 (2), b = 14.115 (1), c = 7.838 (1), β = 109.387 (3)°, V = 1462.2 (2) Å3. XRPD data in the T-range 370–400 °C show a significant contraction of the cell volume without any symmetry change. At 400 °C the dehydrated phase has cell dimensions (in Å): a = 13.425 (2), b = 13.752 (1), c = 7.818 (1), β = 110.246 (3)°, V = 1354.2 (2) Å3. The patterns collected in the T-range from 800 to 30 °C show that armstrongite rehydrates quickly at T ∼320 °C; unit cell parameters and volume refined at the end of the heating/cooling cycle point to a complete reversibility of the dehydration process. Fast rehydration upon cooling is also evident in the FTIR spectra; a complete recovery of the OH-stretching and bending signals is observed at T ∼280–300 °C. Notably, this process can be monitored on single-crystals, while powders embedded in KBr pellets do not recover the structural water content. The thermal expansion of armstrongite is more pronounced along the b axis, with αa: αb: αc = 1.09 × 10−6: 1.69 × 10−5: 7.61 × 10−7 at 90 °C and 7.73 × 10−6: 8.94 × 10−6: 5.85 × 10−6 at 800 °C.
1. Introduction Armstrongite, CaZr [Si6O15]·2H2O, is a microporous Zr-silicate with a crystal structure based on a heteropolyhedral framework, consisting of SiO4 tetrahedra and ZrO6 octahedra, giving rise to cavities occupied by Ca-exchangeable cations. The crystal structure was studied by a number of authors [1–4] but only recently Mesto et al. [5] were successful in defining some debated structural details, such as the space group and the content and location of water molecules. These authors carried out a single crystal structure refinement on armstrongite from Khan Bogdo massif (Gobi, Mongolia) showing that the mineral crystallizes in the C2/m space group with lattice parameters: a = 14.0178 (7) Å, b = 14.1289 (6) Å, c = 7.8366 (3) Å, β = 109.436 (3)°, V = 1463.6 (1) Å3, Z = 4. In the structural model, silicate radicals, in the form of [Si6O15]6- sheets, are connected by vertices to ZrO6 octahedra. The latter are linked via edges to seven-fold coordinated CaO5(H2O)2 polyhedra to form columns running parallel to the crystallographic b axis. The occurrence of only two water groups per formula unit was inferred after the XRD refinement and supported by
∗
Corresponding author. E-mail address: [email protected] (M. Lacalamita).
https://doi.org/10.1016/j.micromeso.2018.06.030 Received 3 April 2018; Received in revised form 14 June 2018; Accepted 17 June 2018 Available online 18 June 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
infrared analysis. The final crystal chemical formula, based on X-ray and EMP data was (Ca0.96Ce0.01Yb0.01)Zr0.99Si6O14.97 • 2.02H2O. The calculated framework density of the mineral (21.86 per 1000 Å3) allowed it to be definitely classified as a zeolite-like phase. In the last years increasing attention has been addressed to zeolitelike materials because of their potential application in the fields of catalysis, ion exchange and sorption [e.g.6,7] and study on natural materials (minerals) are of interest prior to the production of synthetic counterparts [8]. On the other hand, most of the chemical and physical processes involving porous materials are indeed affected by structural water molecules. For example, the efficiency of the ion exchange process may depend on the hydration degree of the extraframework cations [9]. In particular, the water molecules play a role in the distribution and interaction of exchangeable cations with the structural framework, especially in relation to temperature changes [10]. The present study aims at characterizing the thermal behaviour of armstrongite that, to the best of our knowledge, has never been explored so far. To this purpose, we combine thermal techniques
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(Differential Thermal Analysis, DTA, Thermo Gravimetry, TG and Differential Thermo Gravimetry, DTG), in situ High Temperature X-Ray Powder Diffraction (HTXRPD) and in situ High Temperature Fourier Transform InfraRed spectroscopy (HT-FTIR) measurements, both from powders and single-crystal, on the same armstrongite specimen studied by Mesto et al. [5]. 2. Experimental The armstrongite specimen studied here has been separated from the same rock sample (608/19a) studied by Vladykin [11] and Vladykin and Kovalenko [12]. The geological setting is reviewed elsewhere [5]. Thermogravimetric analysis was carried out using a DTA-TG SEIKO 6300. A Pt crucible was filled with 11.5 mg of powdered armstrongite and heated at a rate of 10 °C/min, from room temperature up to 1000 °C. High-temperature X-ray powder diffraction data were collected in air using a Panalytical Empyrean X-ray diffractometer with BraggBrentano geometry, large beta filter-Nickel detector (PIXcel3D) and CuKα radiation, operating at 40 kV/40 mA. The instrument was equipped with an Anton Paar HTK 1200 N high-temperature chamber. The powder was deposited on a corundum sample holder. The X-ray data were collected in the 2θ range 10–65° (step size 0.0263°, step time 22.440s) from 30° to 800 °C and back. The temperature step on heating/ cooling rate was 10 °C/min and the equilibration time at every 10 °C temperature step was 10 min. Diffraction patterns were processed using the PANalytical B.V. software HIGHScore Plus version 3.0e. Structure refinements were carried out in the C2/m space group by the Rietveld method as implemented in GSASII [13], starting from the structure model of Mesto et al. [5]. The scale factor, lattice parameters and atomic coordinates were refined while the isotropic-displacement parameters were kept fixed. The zero shift was refined at each step; the background was modelled using a Chebychev polynomial approximation of 15 t h order. The peak profile was described by a pseudo-Voight. The orientation of the principal axes of the thermal-expansion tensor with respect to the crystallographic axes was determined using the TEV program [14]. The in situ dehydration of armstrongite was monitored via FTIR spectroscopy both on powders and single-crystals. In the former case, ∼0.5 mg of sample were embedded in 150 mg of KBr and pressed; data were collected with a Nicolet iS50 FTIR spectrometer equipped with a DTGS detector and a KBr beamsplitter; the nominal resolution was 4 cm−1 and 64 scans were averaged for both sample and background. In the latter case a very thin crystal fragment was used; data were acquired on a Bruker Hyperion 3000 IR microscope, equipped with a KBr beamsplitter and a N2-cooled MCT detector, with a beam diameter of 40 μm. Heating experiments were conducted using a Linkam T600 heating/freezing stage, fitted in the FTIR microscope (single-crystal); the cell is tightly closed to avoid any contact with the external atmosphere. Experiments with the powder pellet were done using a Specac high T/P call fitted in the optical bench. The heating ramps from 30 to 550 °C were done with a 10°/min T rate and spectra were collected every 20 or 10 °C, depending on the T segment, immediately after reaching the target T; reverse experiments were done by collecting spectra every 20 or 50 °C.
Fig. 1. TG (blue line), DTG (red line) and DTA (green line) curves of armstrongite measured in air. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. Armstrongite heteropolyhedral framework, modified after [5] as seen slightly off the c axis. W10 and W11 are water molecules coordinated by the Ca polyhedron (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
sample. It is associated to the release of the water molecules (W10 and W11) coordinated by the Ca extraframework cations (Fig. 2). This process is clearly visible in the DTA plot as a strong endothermic peak. According to the DTG curve, the weight-loss has a very steep increase in the 300–400 °C range and occurs abruptly at T ∼400 °C. The DTA curve shows a weak endothermic peak at 700 °C and an exothermic peak at 980 °C, whereas no changes are observed in the TG-DTG curves at these temperatures. These effects may be associated to structural modifications induced by heating. 3.2. In situ HT-XRPD
3. Results
Rietveld refinements were performed for all patterns and provided agreement factors, Rwp, in the range from 6.67 to 8.00%. Details are given in Table S1 (submitted as supplementary material). The room temperature XRPD pattern of the studied armstrongite matches exactly the expected pattern from the PDF database (01-0742684 PDF number). In Fig. S3 the Rietveld refinement of the RT data obtained using the structure model from Mesto et al. [5] is shown. Unit cell parameters very close to those derived from single crystal structure refinement were obtained (see Table S1). Peaks of quartz impurities are observed in the XRPD patterns, amounting to less than 4%, as
3.1. Thermogravimetric analysis The TG-DTA-DTG curves collected on the studied armstrongite are displayed in Fig. 1. The sample exhibits two steps of thermal degradation associated with desorption of water. The first step occurs up to ∼100 °C, with a mass loss of ∼0.5 wt%, and corresponds to the release of adsorbed water. The second step is observed in the range 100–400 °C and shows a significant mass loss (∼6.1 wt%) of the 138
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Fig. 3. Selected XRPD patterns of armstrongite showing the evolution of the diffraction patterns collected during the heating (from 30 to 800 °C) (a) and the cooling (b, submitted as supplementary material) experiment. In all patterns the 10–30° 2θ range is reported (see text).
calculated from the Rietveld refinement. On heating, no modifications are observed in the diffraction patterns collected from 30 to 360 °C, being the peaks positions and intensities unchanged, see Fig. 3(a). From 370 °C the reflections at 2θ ∼13.6° and 21.0°, blue arrows in Fig. 3(a), characteristic of the hydrated phase, start decreasing in intensity while two new peaks at 2θ ∼ 14.2° and 21.7°, red arrows in Fig. 3(a), due to the anhydrous phase, grow rapidly, indicating the progress of the dehydration. From 370 to 390 °C, diffraction peaks of both the hydrated and the dehydrated phases coexist. The completion of the process, as deduced by the total absence of the diffraction peaks of the hydrated phase, is observed at T = 400 °C. In the 400–800 °C interval, the XRD patterns are again similar, indicating that the anhydrous phase is stable in this T range. On cooling, no changes in the diffraction patterns are observed down to 330 °C. In the 320-290 °C range there is the progressive recover of peaks characteristic of the hydrated phase, at ∼ 2θ 13.5° and 21°, indicated by the blue arrows in Fig. S4(b). For T < 290° the pattern of the hydrous phase only is observed. The rehydration process thus occurs at temperatures lower than those observed for the H2O loss on heating. The evolution of the unit cell parameters and volume of armstrongite as a function of temperature is plotted in Fig. 4 for the direct experiment. The data set was normalized on the basis of the lattice parameters at room temperature. The dehydration is preceded by a slight increase in the cell volume up to 300 °C, Fig. 4(b), due to the thermal expansion of the structure, particularly visible along the b-axis direction, Fig. 4(a). In the 300–400 °C range a slight contraction of the structure is observed while for T > 375 °C there is a sudden collapse of the structure due to the dehydration. An abrupt discontinuity occurs in the T range 370–400 °C where a significant contraction of the cell volume is observed. This is also evident from the inspection of the refined cell parameters derived from Rietveld analysis of the pattern at 400 °C (Table S1). The dehydrated phase (at 400 °C) exhibits significantly shortened a and b cell dimensions, increased β angle, and smaller unit cell volume with respect to the hydrated phase (see Table S1). The full Rietveld refinement of the 400 °C pattern (Fig. S6) was carried out using the same space group as the hydrated structure (C2/m) and converged at Rwp = 6.68%, confirming that there is no symmetry change in the dehydrated structure. By increasing the temperature up to 800 °C, thermal expansion seems to affect again the lattice parameters of the dehydrated phase (Fig. 4). Scattered data in the b/b0 trend are observed
Fig. 4. Normalized unit cell parameters (a) and volume (b) of armstrongite versus temperature for the heating experiment. Symbols: solid for the hydrated phase; empty for the dehydrated phase; a/a0 (black squares); b/b0 (red circles); c/c0 (green upward triangle); β/β0 (blue downward triangle); V/V0 (black diamond). a0, b0, c0, β0 and V0 are lattice parameters and unit cell volume at room temperature, respectively. The size of the symbols is larger than the associated esd's.
at ∼700 °C (see Fig. 4). At this temperature, a slight endothermic peak has been observed in the DTA curve that is not accompanied by a weight loss in the TG-DTG curves (see Fig. 1). This feature is not associated to measurable variations of the relevant diffraction patterns. Unit cell parameters and cell volume measured from 800 °C down to room temperature (Fig. S7) closely resemble the trends obtained under heating-up conditions (Fig. 4). In addition, the values of the cell parameters refined at room temperature after the rehydration process are the same obtained before the direct experiment (see Table S1), indicating that the dehydration/rehydration process of armstrongite is completely reversible.
3.3. In situ HT-FTIR The RT FTIR powder spectrum of armstrongite (Fig. 5) shows two well resolved peaks in the principal water stretching region, H2Oν3, centered at 3558 and 3512 cm−1, and two peaks in the water bending region, H2Oν2, centered at 1638 and 1608 cm−1, respectively, in agreement with the presence of two water molecules in the structure [5]. In the low-frequency region (< 1200 cm−1) bands due to framework vibrations occur. In particular, the most intense peaks in the 1200-900 cm−1 range can be assigned to the Si-O and Si-O-Si stretching modes, while Zr-O vibrations are observed around 600-650 cm−1; Si-O139
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Fig. 5. (a) Powder FTIR spectrum of armstrongite from Khan Bogdo, collected on a pellet prepared with 0.5 mg of powdered sample in 150 mg KBr; (b) unpolarized single-crystal FTIR spectrum of armstrongite from Khan Bogdo, collected on a randomly oriented very thin (∼10 μm) crystal fragment.
Fig. 6. Evolution of (a) the integrated absorbance of the H2O stretching band collected on a KBr pellet vs T for armstrongite from Khan Bogdo, and (b) integrated absorbance relative to the RT absorbance (IT/IRT) of the H2O stretching (filled red squares) and bending (filled blue circles) modes vs T, collected in situ on a single-crystal (see text). Data collected on cooling are shown by the green triangles.
Si and Si-O-Zr deformation modes occur in the 400-500 cm−1 range [15]. In order to collect a single-crystal FTIR spectrum of armstrongite we tried to prepare a doubly-polished section; however even at thicknesses as low as 40 μm the signal in the principal water-stretching region was strongly saturated. Due to the tendency of the sample to split apart into fibrous crystallites, it was impossible to polish thinner sections, therefore we used a very small and thin fragment obtained by crushing a larger piece under a metallic die. The collected spectrum is given in Fig. 5(b) and shows a very intense peak in the H2O antisymmetric stretching region, at ∼ 3519 cm−1, and a well-defined doublet in the H2O bending region at 1638 and 1608 cm−1; it also exhibits a weak doublet at 5188-5153 cm−1, assigned to the H2Oν3 + H2Oν2 combination mode [16]. No bands are observed around 4500 cm−1, thus excluding the presence of hydroxyl groups in the sample. Note that the 3558 cm−1 peak that occurs in the powder spectrum, Fig. 5(a), is also present in the single-crystal spectrum as a shoulder on the higher-wavenumber side of the 3519 cm−1 main band. The difference in relative intensity between the 3558/3519 cm−1 components suggests that these modes are very sensitive to the crystal orientation. At the beginning we tried to monitor the dehydration process of armstrongite by collecting in situ transmission data on powder pellets. The evolution of the integrated absorbance (area) of the H2Oν3 mode obtained in this way is illustrated in Fig. 6(a); it shows an initial loss of moisture adsorbed on the KBr pellet, up to ∼ 160 °C; the dehydration of
armstrongite seems to occur in a first step in the range 160 < T < 370 °C, followed by a steeper loss in the range 400–500 °C. The sample is totally anhydrous for T ≥ 500 °C. Upon cooling, the pelletized powder does not recover any structured H2O signal, even after long time periods (months). It is immediately apparent from Fig. 6(a) that the dehydration behaviour provided by powder FTIR spectroscopy is different to what expected based on X-ray diffraction data (see above). Therefore we monitored the dehydration behaviour by using a single-crystal, comparing the absorption evolution of both the H2Oν3 stretching and the H2Oν3 bending modes. Note that, as explained before, the used crystal could not be doubly polished, but great care was paid in collecting the spectra always in the same point. Selected in situ spectra, collected both upon heating and during the cooling ramp, are given in Fig. 7; the integrated intensities data are plotted in Fig. 6(b) as a function of increasing temperature. Differently from Fig. 6(a), the plot of Fig. 6(b) shows that, although slight non linearity in the data points due to release of absorbed moisture and within possible errors in the data acquisition, up to T ∼370 °C there is no variation in the intensity of both modes; for T > ∼370 °C there is a sudden decrease of the intensity that drops out drastically in the 390–420 °C range. Armstrongite is totally anhydrous for T > ∼450 °C. On cooling down the sample after the heating ramp, the mineral completely recovers its water content, Figs. 6(b) and 7(b). Three successive heating ramps were repeated on the same crystal, and 140
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at the end of each experiment the sample was found to recover the same spectrum, indicating the dehydration to be completely, quickly and repeatedly reversible. 4. Discussion The number of water molecules and their position in the armstrongite structure has been long debated. Based on chemical analysis and density measurements, 2.5 water groups per formula unit were initially inferred for the Mongolian armstrongite [1], whereas Kabalov et al. [4] provided three structurally different water sites, two of which lying on the 2-fold axis within the voids of the zeolite-like framework. Three water groups per formula unit were also obtained for a Canadian specimen based EPMA data; in this case H2O wt% was calculated by difference from 100 wt% [3]. Mesto et al. [5], instead, recognized only two water molecules in the Khan Bogdo armstrongite on the basis of an accurate structure refinement supported by void analysis with Platon software [17] and by FTIR data. This inference is confirmed by the combined results here presented; in particular, the mass loss calculated from the TG analysis is 6.1 wt%, and this value is in agreement with the theoretical value of 6.3 wt% expected for the loss of two water molecules per formula unit. In turn, the loss of water molecules is expected to produce a shrinkage of the cell volume. Consistently, the XRPD data detect a cell volume decrease of the dehydrated phase of about 7% with respect to the room temperature phase. The in situ structural analysis shows that the dehydration process starts around 370 °C; this temperature is in good agreement with those determined by DTA (400 °C) and FTIR (380 °C). The most intriguing aspect that came out from our study is that the process is totally reversible, at least under the conditions used. XRPD data indicate that the rehydration of the mineral occurs at lower temperature (320 °C) with respect to the dehydration. Further details on this aspect are provided by the FTIR experiments. Fig. 7(b) shows that, on cooling, the single crystal begins to incorporate weak amounts of water at ∼ 380-360 °C; initially only the H2O molecule characterized by the higher-frequency (stretching) and lower frequency (bending) seems to be allotted into the structure. However, starting from T ∼300 °C the second H2O molecule shows up; the total water content increases strongly for 280 °C < T < 260 °C; in this range almost 50% of the total water amount is recovered. At the fixed T = 260 °C we collected three spectra after increasing time [3, 6 and 9 min, respectively (circled in Fig. 6(b)] to check for any effect of time on the water diffusion into the armstrongite structure. The results show that time has virtually no effect, while, at least at these conditions, it is the temperature that plays a major role. This point, i.e. definition of the exact kinetic of the water diffusion within the micropores of armstrongite needs however further investigation to be definitively characterized. At the end of the experiment the same signal as before the run is obtained. Notably, as discussed above, no rehydration occurs in the armstrongite powder pressed within the KBr pellet, suggesting that the lack of contact of the mineral with the atmospheric humidity prevents the capture of water molecules from the external environment. This point should be clarified by performing heating/cooling experiments in dry atmosphere. An additional notable feature that arises from Fig. 6 is that powder FTIR spectroscopy is not effective in monitoring the dehydration/rehydration process of armstrongite. This conclusion suggests that HT powder spectroscopy should be used with caution as a tool to study the thermal behaviour of zeolite-like minerals, and this is another issue that merits systematic work on other zeolite species. To get further insights into the thermal behaviour of armstrongite, the coefficients of the thermal expansion tensors with respect to the crystallographic axes have been calculated before and after the dehydration process (Table S2, submitted as supplementary material). In detail, a fourth- and third-order polynomial approximation of temperature dependencies for the cell parameters in the range 30–360 °C for the hydrated phase and 400–800 °C for the dehydrated one have
Fig. 7. Selected in situ single-crystal spectra collected during (a) the heating ramp and (b) the cooling ramp.
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rehydrate on cooling. On the contrary, armstrongite single crystal is anhydrous for T > ∼450 °C and completely recovers its water molecules upon cooling. Our study suggests that the kinetic of water diffusion into the micropores of zeolite-like materials deserves systematic investigation, possibly comparing the behaviour of powders vs. single crystals, either in inert vs. humid atmosphere.
been used, respectively. Axial expansion for hydrated and dehydrated armstrongite is anisotropic, with αa: αb: αc are −1.09 × 10−6: 1.69 × 10−5: 7.61 × 10−7 at 90 °C and 7.73 × 10−6: 8.94 × 10−6: 5.85 × 10−6 at 800 °C. The graphical representation of thermal expansion of armstrongite is displayed in Fig. S11. At T = 30 °C the mineral exhibits a positive expansion for all the directions, Fig. S11(a), while a shrinkage along the a direction occurs at T = 90 °C, Fig. S11(b). In the temperature range 280–360 °C armstrongite shows a strong negative thermal expansion, Fig. S11(c), in all directions. In the dehydrated armstrongite, the expansion along the b axis is associated to a shrinkage between b and c, Fig. S11(d). Starting from 520 °C, the mineral expands again preferentially along b, see Fig. S11(e) and S11(f). Therefore, the thermal evolution of armstrongite involves structural deformations mainly in the b crystallographic direction, i.e. parallel to the columns composed of the seven-fold coordinated, CaO5(H2O)2, Capolyhedra and ZrO6 octahedra. The thermal behaviour of armstrongite differs from that of elpidite, (Na,Ca)2Zr [Si6O15] • 2.8H2O, which is found in association with armstrongite in the Khan Bogdo alkaline granites massif [18]. The two minerals show a topologically similar heteropolyhedral framework but a different content and location of the water molecules. In the case of elpidite, the dehydration was studied under dry conditions and was found to occur in two steps and at lower temperature (∼100 °C and 250 °C [19]) with respect to the sample studied here. In addition, after the loss of one water molecule at 100 °C, elpidite undergoes a structural change from space group Pbcm to Cmca which is accompanied by doubling of the a parameter and of the cell volume. Our study shows that the dehydration of armstrongite is associated to significant variation of the cell volume but, seemingly, not to a change in the space group. The latter finding has also been confirmed by preliminary structure refinements on high temperature single crystal X-ray diffraction data (not reported). The dehydrated structure seems also to be affected by positional disorder of the extraframework Ca cations, but no significant changes of the heteropolyhedral framework occur. However, due to the worsening of the quality of the diffraction data as a consequence of the temperature increase, additional work is necessary to get accurate structural details of the dehydrated phase.
Conflicts of interest There are no conflicts to declare. Acknowledgments The TG facility at the Politecnico of Milan is gratefully acknowledged. XRPD laboratory at the Dipartimento di Scienze della Terra and Geoambientali, University of Bari “Aldo Moro”, was funded by Potenziamento Strutturale PONa3_00369 “Laboratorio per lo Sviluppo Integrato delle Scienze e delle TEcnologie dei Materiali Avanzati e per dispositivi innovativi (SISTEMA)”. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.micromeso.2018.06.030. References [1] N.V. Vladykin, V.I. Kovalenko, A.N. Kashaev, A.N. Sapozhnikov, V.A. Pisarskaya, Dokl. Akad. Nauk SSSR 209 (1973) 1185–1188 (in Russian). [2] A.A. Kashaev, A.N. Sapozhnikov, Sov. Phys. Crystallogr. 23 (1978) 956–961. [3] J.L. Jambor, A.L. Roberts, J.D. Grice, Powder Diffr. 2 (1987) 2–4. [4] YuK. Kabalov, N.V. Zubkova, D.Yu Pushcharovsky, J. Schneider, A.N. Sapozhnikov, Z. Kristallogr, 215 (2000) 757–761. [5] E. Mesto, E. Kaneva, E. Schingaro, N. Vladykin, M. Lacalamita, F. Scordari, Am. Mineral. 99 (2014) 2424–2432. [6] C.B. Lopes, J. Coimbra, M. Otero, E. Pereira, A.C. Duarte, Quim. Nova 31 (2) (2008) 321–325. [7] Y. Kuwahara, D. Kang, J.R. Copeland, N.A. Brunelli, S.A. Didas, P. Bollini, C. Sievers, T. Kamegawa, H. Yamashita, C.W. Jones, J. Am. Chem. Soc. 134 (2012) 10757–10760. [8] N.V. Chukanov, A.I. Kazakov, V.V. Nedelko, I.V. Pekov, N.V. Zubkova, D.A. Ksenofontov, Y.K. Kabalov, A.A. Grigorieva, D. Yu, Pushcharovsky, in: S.V. Krivovichev (Ed.), Minerals as Advanced Materials II, Springer, 2012, pp. 167–179. [9] Y. Kuwahara, D. Kang, J.R. Copeland, N.A. Brunelli, S.A. Didas, P. Bollini, C. Sievers, T. Kamegawa, H. Yamashita, C.W. Jones, J. Am. Chem. Soc. 134 (2012) 10757–10760. [10] F.M. Higgins, H. De Leeuw, S.C. Parker, J. Mater. Chem. 12 (2002) 124–131. [11] N.V. Vladykin, Mineralogical-geochemical Features of Rare-metal Granitoides of Mongolia, Nauka, Novosibirsk, 1983 (in Russian). [12] N.V. Vladykin, V.I. Kovalenko, M.I. Novgorodova (Ed.), Minerals of Mongolia, ЭКОСТ, 2006, pp. 250–256 (in Russian). [13] A.C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 2004, pp. 86–748. [14] T. Langreiter, V. Kahlenberg, Crystals 5 (2015) 143–153. [15] A.A. Grigor’eva, N.V. Zubkova, I.V. Pekov, U. Kolitsch, D.Yu Pushcharovsky, M.F. Vigasina, G. Giester, T. Dordevic, E. Tillmanns, N.V. Chukanov, Crystallogr. Rep. 56 (5) (2011) 832–841. [16] G. Della Ventura, D. Gatta, G. Redhammer, F. Bellatreccia, A. Loose, G.C. Parodi, Phys. Chem. Miner. 36 (2009) 193–206. [17] A.L. Spek, Acta Crystallogr. D65 (2) (2009) 148–155. [18] A.N. Sapozhnikov, A.A. Kashaev, Sov. Phys. Crystallogr. 23 (1978) 52–56. [19] G. Cametti, T. Armbruster, M. Nagashima, Microporous Mesoporous Mater. 227 (2016) 81–87.
5. Conclusions The study of the thermal behaviour of armstrongite evidences that this zeolite-like mineral undergoes a dehydration process that is complete, quick and repeatedly reversible under the condition of the experiment. In particular, thermal analysis indicates that there are two steps of thermal degradation (the first up to ∼100 °C, the second in the range 100–400 °C) associated with desorption of absorbed and structural water. The XRPD heating/cooling experiment indicates that the dehydration process is complete at T = 400 °C while the rehydration occurs at temperatures (∼320 °C) lower than those observed for the H2O loss on heating. An additional notable point emerged from this study is that there is a significant difference between the results obtained by FTIR on powders or on a single crystal. In particular, powdered armstrongite embedded within a KBr disk dehydrates totally for T ≥ 500 °C and does not
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Thermal stability and dehydration of armstrongite, a microporous zirconium silicate
T
E. Schingaroa, M. Lacalamitab,∗, E. Mestoa, G. Della Venturac,d Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari “Aldo Moro”, via E. Orabona 4, I-70125, Bari, Italy Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, I-56100, Pisa, Italy c Dipartimento di Scienze, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146, Roma, Italy d INFN-Laboratori Nazionali di Frascati, Via E. Fermi 40, I-00044, Frascati, Roma, Italy a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Armstrongite Dehydration/rehydration Thermal analysis In situ HT-XRPD In situ HT-FTIR
The dehydration of armstrongite from Khan Bogdo (Mongolia) was investigated by combining thermal analysis, in situ HT X-ray powder diffraction (XRPD) and Fourier Transform InfraRed (FTIR) spectroscopy. The process starts at ∼380 °C and is completed within few tenths of degrees. It involves a mass loss of 6.1 wt% and a cell volume decrease of 7%. Armstrongite at RT has C2/m symmetry with (in Å) a = 14.010 (2), b = 14.115 (1), c = 7.838 (1), β = 109.387 (3)°, V = 1462.2 (2) Å3. XRPD data in the T-range 370–400 °C show a significant contraction of the cell volume without any symmetry change. At 400 °C the dehydrated phase has cell dimensions (in Å): a = 13.425 (2), b = 13.752 (1), c = 7.818 (1), β = 110.246 (3)°, V = 1354.2 (2) Å3. The patterns collected in the T-range from 800 to 30 °C show that armstrongite rehydrates quickly at T ∼320 °C; unit cell parameters and volume refined at the end of the heating/cooling cycle point to a complete reversibility of the dehydration process. Fast rehydration upon cooling is also evident in the FTIR spectra; a complete recovery of the OH-stretching and bending signals is observed at T ∼280–300 °C. Notably, this process can be monitored on single-crystals, while powders embedded in KBr pellets do not recover the structural water content. The thermal expansion of armstrongite is more pronounced along the b axis, with αa: αb: αc = 1.09 × 10−6: 1.69 × 10−5: 7.61 × 10−7 at 90 °C and 7.73 × 10−6: 8.94 × 10−6: 5.85 × 10−6 at 800 °C.
1. Introduction Armstrongite, CaZr [Si6O15]·2H2O, is a microporous Zr-silicate with a crystal structure based on a heteropolyhedral framework, consisting of SiO4 tetrahedra and ZrO6 octahedra, giving rise to cavities occupied by Ca-exchangeable cations. The crystal structure was studied by a number of authors [1–4] but only recently Mesto et al. [5] were successful in defining some debated structural details, such as the space group and the content and location of water molecules. These authors carried out a single crystal structure refinement on armstrongite from Khan Bogdo massif (Gobi, Mongolia) showing that the mineral crystallizes in the C2/m space group with lattice parameters: a = 14.0178 (7) Å, b = 14.1289 (6) Å, c = 7.8366 (3) Å, β = 109.436 (3)°, V = 1463.6 (1) Å3, Z = 4. In the structural model, silicate radicals, in the form of [Si6O15]6- sheets, are connected by vertices to ZrO6 octahedra. The latter are linked via edges to seven-fold coordinated CaO5(H2O)2 polyhedra to form columns running parallel to the crystallographic b axis. The occurrence of only two water groups per formula unit was inferred after the XRD refinement and supported by
∗
Corresponding author. E-mail address: [email protected] (M. Lacalamita).
https://doi.org/10.1016/j.micromeso.2018.06.030 Received 3 April 2018; Received in revised form 14 June 2018; Accepted 17 June 2018 Available online 18 June 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
infrared analysis. The final crystal chemical formula, based on X-ray and EMP data was (Ca0.96Ce0.01Yb0.01)Zr0.99Si6O14.97 • 2.02H2O. The calculated framework density of the mineral (21.86 per 1000 Å3) allowed it to be definitely classified as a zeolite-like phase. In the last years increasing attention has been addressed to zeolitelike materials because of their potential application in the fields of catalysis, ion exchange and sorption [e.g.6,7] and study on natural materials (minerals) are of interest prior to the production of synthetic counterparts [8]. On the other hand, most of the chemical and physical processes involving porous materials are indeed affected by structural water molecules. For example, the efficiency of the ion exchange process may depend on the hydration degree of the extraframework cations [9]. In particular, the water molecules play a role in the distribution and interaction of exchangeable cations with the structural framework, especially in relation to temperature changes [10]. The present study aims at characterizing the thermal behaviour of armstrongite that, to the best of our knowledge, has never been explored so far. To this purpose, we combine thermal techniques
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(Differential Thermal Analysis, DTA, Thermo Gravimetry, TG and Differential Thermo Gravimetry, DTG), in situ High Temperature X-Ray Powder Diffraction (HTXRPD) and in situ High Temperature Fourier Transform InfraRed spectroscopy (HT-FTIR) measurements, both from powders and single-crystal, on the same armstrongite specimen studied by Mesto et al. [5]. 2. Experimental The armstrongite specimen studied here has been separated from the same rock sample (608/19a) studied by Vladykin [11] and Vladykin and Kovalenko [12]. The geological setting is reviewed elsewhere [5]. Thermogravimetric analysis was carried out using a DTA-TG SEIKO 6300. A Pt crucible was filled with 11.5 mg of powdered armstrongite and heated at a rate of 10 °C/min, from room temperature up to 1000 °C. High-temperature X-ray powder diffraction data were collected in air using a Panalytical Empyrean X-ray diffractometer with BraggBrentano geometry, large beta filter-Nickel detector (PIXcel3D) and CuKα radiation, operating at 40 kV/40 mA. The instrument was equipped with an Anton Paar HTK 1200 N high-temperature chamber. The powder was deposited on a corundum sample holder. The X-ray data were collected in the 2θ range 10–65° (step size 0.0263°, step time 22.440s) from 30° to 800 °C and back. The temperature step on heating/ cooling rate was 10 °C/min and the equilibration time at every 10 °C temperature step was 10 min. Diffraction patterns were processed using the PANalytical B.V. software HIGHScore Plus version 3.0e. Structure refinements were carried out in the C2/m space group by the Rietveld method as implemented in GSASII [13], starting from the structure model of Mesto et al. [5]. The scale factor, lattice parameters and atomic coordinates were refined while the isotropic-displacement parameters were kept fixed. The zero shift was refined at each step; the background was modelled using a Chebychev polynomial approximation of 15 t h order. The peak profile was described by a pseudo-Voight. The orientation of the principal axes of the thermal-expansion tensor with respect to the crystallographic axes was determined using the TEV program [14]. The in situ dehydration of armstrongite was monitored via FTIR spectroscopy both on powders and single-crystals. In the former case, ∼0.5 mg of sample were embedded in 150 mg of KBr and pressed; data were collected with a Nicolet iS50 FTIR spectrometer equipped with a DTGS detector and a KBr beamsplitter; the nominal resolution was 4 cm−1 and 64 scans were averaged for both sample and background. In the latter case a very thin crystal fragment was used; data were acquired on a Bruker Hyperion 3000 IR microscope, equipped with a KBr beamsplitter and a N2-cooled MCT detector, with a beam diameter of 40 μm. Heating experiments were conducted using a Linkam T600 heating/freezing stage, fitted in the FTIR microscope (single-crystal); the cell is tightly closed to avoid any contact with the external atmosphere. Experiments with the powder pellet were done using a Specac high T/P call fitted in the optical bench. The heating ramps from 30 to 550 °C were done with a 10°/min T rate and spectra were collected every 20 or 10 °C, depending on the T segment, immediately after reaching the target T; reverse experiments were done by collecting spectra every 20 or 50 °C.
Fig. 1. TG (blue line), DTG (red line) and DTA (green line) curves of armstrongite measured in air. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. Armstrongite heteropolyhedral framework, modified after [5] as seen slightly off the c axis. W10 and W11 are water molecules coordinated by the Ca polyhedron (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
sample. It is associated to the release of the water molecules (W10 and W11) coordinated by the Ca extraframework cations (Fig. 2). This process is clearly visible in the DTA plot as a strong endothermic peak. According to the DTG curve, the weight-loss has a very steep increase in the 300–400 °C range and occurs abruptly at T ∼400 °C. The DTA curve shows a weak endothermic peak at 700 °C and an exothermic peak at 980 °C, whereas no changes are observed in the TG-DTG curves at these temperatures. These effects may be associated to structural modifications induced by heating. 3.2. In situ HT-XRPD
3. Results
Rietveld refinements were performed for all patterns and provided agreement factors, Rwp, in the range from 6.67 to 8.00%. Details are given in Table S1 (submitted as supplementary material). The room temperature XRPD pattern of the studied armstrongite matches exactly the expected pattern from the PDF database (01-0742684 PDF number). In Fig. S3 the Rietveld refinement of the RT data obtained using the structure model from Mesto et al. [5] is shown. Unit cell parameters very close to those derived from single crystal structure refinement were obtained (see Table S1). Peaks of quartz impurities are observed in the XRPD patterns, amounting to less than 4%, as
3.1. Thermogravimetric analysis The TG-DTA-DTG curves collected on the studied armstrongite are displayed in Fig. 1. The sample exhibits two steps of thermal degradation associated with desorption of water. The first step occurs up to ∼100 °C, with a mass loss of ∼0.5 wt%, and corresponds to the release of adsorbed water. The second step is observed in the range 100–400 °C and shows a significant mass loss (∼6.1 wt%) of the 138
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Fig. 3. Selected XRPD patterns of armstrongite showing the evolution of the diffraction patterns collected during the heating (from 30 to 800 °C) (a) and the cooling (b, submitted as supplementary material) experiment. In all patterns the 10–30° 2θ range is reported (see text).
calculated from the Rietveld refinement. On heating, no modifications are observed in the diffraction patterns collected from 30 to 360 °C, being the peaks positions and intensities unchanged, see Fig. 3(a). From 370 °C the reflections at 2θ ∼13.6° and 21.0°, blue arrows in Fig. 3(a), characteristic of the hydrated phase, start decreasing in intensity while two new peaks at 2θ ∼ 14.2° and 21.7°, red arrows in Fig. 3(a), due to the anhydrous phase, grow rapidly, indicating the progress of the dehydration. From 370 to 390 °C, diffraction peaks of both the hydrated and the dehydrated phases coexist. The completion of the process, as deduced by the total absence of the diffraction peaks of the hydrated phase, is observed at T = 400 °C. In the 400–800 °C interval, the XRD patterns are again similar, indicating that the anhydrous phase is stable in this T range. On cooling, no changes in the diffraction patterns are observed down to 330 °C. In the 320-290 °C range there is the progressive recover of peaks characteristic of the hydrated phase, at ∼ 2θ 13.5° and 21°, indicated by the blue arrows in Fig. S4(b). For T < 290° the pattern of the hydrous phase only is observed. The rehydration process thus occurs at temperatures lower than those observed for the H2O loss on heating. The evolution of the unit cell parameters and volume of armstrongite as a function of temperature is plotted in Fig. 4 for the direct experiment. The data set was normalized on the basis of the lattice parameters at room temperature. The dehydration is preceded by a slight increase in the cell volume up to 300 °C, Fig. 4(b), due to the thermal expansion of the structure, particularly visible along the b-axis direction, Fig. 4(a). In the 300–400 °C range a slight contraction of the structure is observed while for T > 375 °C there is a sudden collapse of the structure due to the dehydration. An abrupt discontinuity occurs in the T range 370–400 °C where a significant contraction of the cell volume is observed. This is also evident from the inspection of the refined cell parameters derived from Rietveld analysis of the pattern at 400 °C (Table S1). The dehydrated phase (at 400 °C) exhibits significantly shortened a and b cell dimensions, increased β angle, and smaller unit cell volume with respect to the hydrated phase (see Table S1). The full Rietveld refinement of the 400 °C pattern (Fig. S6) was carried out using the same space group as the hydrated structure (C2/m) and converged at Rwp = 6.68%, confirming that there is no symmetry change in the dehydrated structure. By increasing the temperature up to 800 °C, thermal expansion seems to affect again the lattice parameters of the dehydrated phase (Fig. 4). Scattered data in the b/b0 trend are observed
Fig. 4. Normalized unit cell parameters (a) and volume (b) of armstrongite versus temperature for the heating experiment. Symbols: solid for the hydrated phase; empty for the dehydrated phase; a/a0 (black squares); b/b0 (red circles); c/c0 (green upward triangle); β/β0 (blue downward triangle); V/V0 (black diamond). a0, b0, c0, β0 and V0 are lattice parameters and unit cell volume at room temperature, respectively. The size of the symbols is larger than the associated esd's.
at ∼700 °C (see Fig. 4). At this temperature, a slight endothermic peak has been observed in the DTA curve that is not accompanied by a weight loss in the TG-DTG curves (see Fig. 1). This feature is not associated to measurable variations of the relevant diffraction patterns. Unit cell parameters and cell volume measured from 800 °C down to room temperature (Fig. S7) closely resemble the trends obtained under heating-up conditions (Fig. 4). In addition, the values of the cell parameters refined at room temperature after the rehydration process are the same obtained before the direct experiment (see Table S1), indicating that the dehydration/rehydration process of armstrongite is completely reversible.
3.3. In situ HT-FTIR The RT FTIR powder spectrum of armstrongite (Fig. 5) shows two well resolved peaks in the principal water stretching region, H2Oν3, centered at 3558 and 3512 cm−1, and two peaks in the water bending region, H2Oν2, centered at 1638 and 1608 cm−1, respectively, in agreement with the presence of two water molecules in the structure [5]. In the low-frequency region (< 1200 cm−1) bands due to framework vibrations occur. In particular, the most intense peaks in the 1200-900 cm−1 range can be assigned to the Si-O and Si-O-Si stretching modes, while Zr-O vibrations are observed around 600-650 cm−1; Si-O139
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Fig. 5. (a) Powder FTIR spectrum of armstrongite from Khan Bogdo, collected on a pellet prepared with 0.5 mg of powdered sample in 150 mg KBr; (b) unpolarized single-crystal FTIR spectrum of armstrongite from Khan Bogdo, collected on a randomly oriented very thin (∼10 μm) crystal fragment.
Fig. 6. Evolution of (a) the integrated absorbance of the H2O stretching band collected on a KBr pellet vs T for armstrongite from Khan Bogdo, and (b) integrated absorbance relative to the RT absorbance (IT/IRT) of the H2O stretching (filled red squares) and bending (filled blue circles) modes vs T, collected in situ on a single-crystal (see text). Data collected on cooling are shown by the green triangles.
Si and Si-O-Zr deformation modes occur in the 400-500 cm−1 range [15]. In order to collect a single-crystal FTIR spectrum of armstrongite we tried to prepare a doubly-polished section; however even at thicknesses as low as 40 μm the signal in the principal water-stretching region was strongly saturated. Due to the tendency of the sample to split apart into fibrous crystallites, it was impossible to polish thinner sections, therefore we used a very small and thin fragment obtained by crushing a larger piece under a metallic die. The collected spectrum is given in Fig. 5(b) and shows a very intense peak in the H2O antisymmetric stretching region, at ∼ 3519 cm−1, and a well-defined doublet in the H2O bending region at 1638 and 1608 cm−1; it also exhibits a weak doublet at 5188-5153 cm−1, assigned to the H2Oν3 + H2Oν2 combination mode [16]. No bands are observed around 4500 cm−1, thus excluding the presence of hydroxyl groups in the sample. Note that the 3558 cm−1 peak that occurs in the powder spectrum, Fig. 5(a), is also present in the single-crystal spectrum as a shoulder on the higher-wavenumber side of the 3519 cm−1 main band. The difference in relative intensity between the 3558/3519 cm−1 components suggests that these modes are very sensitive to the crystal orientation. At the beginning we tried to monitor the dehydration process of armstrongite by collecting in situ transmission data on powder pellets. The evolution of the integrated absorbance (area) of the H2Oν3 mode obtained in this way is illustrated in Fig. 6(a); it shows an initial loss of moisture adsorbed on the KBr pellet, up to ∼ 160 °C; the dehydration of
armstrongite seems to occur in a first step in the range 160 < T < 370 °C, followed by a steeper loss in the range 400–500 °C. The sample is totally anhydrous for T ≥ 500 °C. Upon cooling, the pelletized powder does not recover any structured H2O signal, even after long time periods (months). It is immediately apparent from Fig. 6(a) that the dehydration behaviour provided by powder FTIR spectroscopy is different to what expected based on X-ray diffraction data (see above). Therefore we monitored the dehydration behaviour by using a single-crystal, comparing the absorption evolution of both the H2Oν3 stretching and the H2Oν3 bending modes. Note that, as explained before, the used crystal could not be doubly polished, but great care was paid in collecting the spectra always in the same point. Selected in situ spectra, collected both upon heating and during the cooling ramp, are given in Fig. 7; the integrated intensities data are plotted in Fig. 6(b) as a function of increasing temperature. Differently from Fig. 6(a), the plot of Fig. 6(b) shows that, although slight non linearity in the data points due to release of absorbed moisture and within possible errors in the data acquisition, up to T ∼370 °C there is no variation in the intensity of both modes; for T > ∼370 °C there is a sudden decrease of the intensity that drops out drastically in the 390–420 °C range. Armstrongite is totally anhydrous for T > ∼450 °C. On cooling down the sample after the heating ramp, the mineral completely recovers its water content, Figs. 6(b) and 7(b). Three successive heating ramps were repeated on the same crystal, and 140
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at the end of each experiment the sample was found to recover the same spectrum, indicating the dehydration to be completely, quickly and repeatedly reversible. 4. Discussion The number of water molecules and their position in the armstrongite structure has been long debated. Based on chemical analysis and density measurements, 2.5 water groups per formula unit were initially inferred for the Mongolian armstrongite [1], whereas Kabalov et al. [4] provided three structurally different water sites, two of which lying on the 2-fold axis within the voids of the zeolite-like framework. Three water groups per formula unit were also obtained for a Canadian specimen based EPMA data; in this case H2O wt% was calculated by difference from 100 wt% [3]. Mesto et al. [5], instead, recognized only two water molecules in the Khan Bogdo armstrongite on the basis of an accurate structure refinement supported by void analysis with Platon software [17] and by FTIR data. This inference is confirmed by the combined results here presented; in particular, the mass loss calculated from the TG analysis is 6.1 wt%, and this value is in agreement with the theoretical value of 6.3 wt% expected for the loss of two water molecules per formula unit. In turn, the loss of water molecules is expected to produce a shrinkage of the cell volume. Consistently, the XRPD data detect a cell volume decrease of the dehydrated phase of about 7% with respect to the room temperature phase. The in situ structural analysis shows that the dehydration process starts around 370 °C; this temperature is in good agreement with those determined by DTA (400 °C) and FTIR (380 °C). The most intriguing aspect that came out from our study is that the process is totally reversible, at least under the conditions used. XRPD data indicate that the rehydration of the mineral occurs at lower temperature (320 °C) with respect to the dehydration. Further details on this aspect are provided by the FTIR experiments. Fig. 7(b) shows that, on cooling, the single crystal begins to incorporate weak amounts of water at ∼ 380-360 °C; initially only the H2O molecule characterized by the higher-frequency (stretching) and lower frequency (bending) seems to be allotted into the structure. However, starting from T ∼300 °C the second H2O molecule shows up; the total water content increases strongly for 280 °C < T < 260 °C; in this range almost 50% of the total water amount is recovered. At the fixed T = 260 °C we collected three spectra after increasing time [3, 6 and 9 min, respectively (circled in Fig. 6(b)] to check for any effect of time on the water diffusion into the armstrongite structure. The results show that time has virtually no effect, while, at least at these conditions, it is the temperature that plays a major role. This point, i.e. definition of the exact kinetic of the water diffusion within the micropores of armstrongite needs however further investigation to be definitively characterized. At the end of the experiment the same signal as before the run is obtained. Notably, as discussed above, no rehydration occurs in the armstrongite powder pressed within the KBr pellet, suggesting that the lack of contact of the mineral with the atmospheric humidity prevents the capture of water molecules from the external environment. This point should be clarified by performing heating/cooling experiments in dry atmosphere. An additional notable feature that arises from Fig. 6 is that powder FTIR spectroscopy is not effective in monitoring the dehydration/rehydration process of armstrongite. This conclusion suggests that HT powder spectroscopy should be used with caution as a tool to study the thermal behaviour of zeolite-like minerals, and this is another issue that merits systematic work on other zeolite species. To get further insights into the thermal behaviour of armstrongite, the coefficients of the thermal expansion tensors with respect to the crystallographic axes have been calculated before and after the dehydration process (Table S2, submitted as supplementary material). In detail, a fourth- and third-order polynomial approximation of temperature dependencies for the cell parameters in the range 30–360 °C for the hydrated phase and 400–800 °C for the dehydrated one have
Fig. 7. Selected in situ single-crystal spectra collected during (a) the heating ramp and (b) the cooling ramp.
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rehydrate on cooling. On the contrary, armstrongite single crystal is anhydrous for T > ∼450 °C and completely recovers its water molecules upon cooling. Our study suggests that the kinetic of water diffusion into the micropores of zeolite-like materials deserves systematic investigation, possibly comparing the behaviour of powders vs. single crystals, either in inert vs. humid atmosphere.
been used, respectively. Axial expansion for hydrated and dehydrated armstrongite is anisotropic, with αa: αb: αc are −1.09 × 10−6: 1.69 × 10−5: 7.61 × 10−7 at 90 °C and 7.73 × 10−6: 8.94 × 10−6: 5.85 × 10−6 at 800 °C. The graphical representation of thermal expansion of armstrongite is displayed in Fig. S11. At T = 30 °C the mineral exhibits a positive expansion for all the directions, Fig. S11(a), while a shrinkage along the a direction occurs at T = 90 °C, Fig. S11(b). In the temperature range 280–360 °C armstrongite shows a strong negative thermal expansion, Fig. S11(c), in all directions. In the dehydrated armstrongite, the expansion along the b axis is associated to a shrinkage between b and c, Fig. S11(d). Starting from 520 °C, the mineral expands again preferentially along b, see Fig. S11(e) and S11(f). Therefore, the thermal evolution of armstrongite involves structural deformations mainly in the b crystallographic direction, i.e. parallel to the columns composed of the seven-fold coordinated, CaO5(H2O)2, Capolyhedra and ZrO6 octahedra. The thermal behaviour of armstrongite differs from that of elpidite, (Na,Ca)2Zr [Si6O15] • 2.8H2O, which is found in association with armstrongite in the Khan Bogdo alkaline granites massif [18]. The two minerals show a topologically similar heteropolyhedral framework but a different content and location of the water molecules. In the case of elpidite, the dehydration was studied under dry conditions and was found to occur in two steps and at lower temperature (∼100 °C and 250 °C [19]) with respect to the sample studied here. In addition, after the loss of one water molecule at 100 °C, elpidite undergoes a structural change from space group Pbcm to Cmca which is accompanied by doubling of the a parameter and of the cell volume. Our study shows that the dehydration of armstrongite is associated to significant variation of the cell volume but, seemingly, not to a change in the space group. The latter finding has also been confirmed by preliminary structure refinements on high temperature single crystal X-ray diffraction data (not reported). The dehydrated structure seems also to be affected by positional disorder of the extraframework Ca cations, but no significant changes of the heteropolyhedral framework occur. However, due to the worsening of the quality of the diffraction data as a consequence of the temperature increase, additional work is necessary to get accurate structural details of the dehydrated phase.
Conflicts of interest There are no conflicts to declare. Acknowledgments The TG facility at the Politecnico of Milan is gratefully acknowledged. XRPD laboratory at the Dipartimento di Scienze della Terra and Geoambientali, University of Bari “Aldo Moro”, was funded by Potenziamento Strutturale PONa3_00369 “Laboratorio per lo Sviluppo Integrato delle Scienze e delle TEcnologie dei Materiali Avanzati e per dispositivi innovativi (SISTEMA)”. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.micromeso.2018.06.030. References [1] N.V. Vladykin, V.I. Kovalenko, A.N. Kashaev, A.N. Sapozhnikov, V.A. Pisarskaya, Dokl. Akad. Nauk SSSR 209 (1973) 1185–1188 (in Russian). [2] A.A. Kashaev, A.N. Sapozhnikov, Sov. Phys. Crystallogr. 23 (1978) 956–961. [3] J.L. Jambor, A.L. Roberts, J.D. Grice, Powder Diffr. 2 (1987) 2–4. [4] YuK. Kabalov, N.V. Zubkova, D.Yu Pushcharovsky, J. Schneider, A.N. Sapozhnikov, Z. Kristallogr, 215 (2000) 757–761. [5] E. Mesto, E. Kaneva, E. Schingaro, N. Vladykin, M. Lacalamita, F. Scordari, Am. Mineral. 99 (2014) 2424–2432. [6] C.B. Lopes, J. Coimbra, M. Otero, E. Pereira, A.C. Duarte, Quim. Nova 31 (2) (2008) 321–325. [7] Y. Kuwahara, D. Kang, J.R. Copeland, N.A. Brunelli, S.A. Didas, P. Bollini, C. Sievers, T. Kamegawa, H. Yamashita, C.W. Jones, J. Am. Chem. Soc. 134 (2012) 10757–10760. [8] N.V. Chukanov, A.I. Kazakov, V.V. Nedelko, I.V. Pekov, N.V. Zubkova, D.A. Ksenofontov, Y.K. Kabalov, A.A. Grigorieva, D. Yu, Pushcharovsky, in: S.V. Krivovichev (Ed.), Minerals as Advanced Materials II, Springer, 2012, pp. 167–179. [9] Y. Kuwahara, D. Kang, J.R. Copeland, N.A. Brunelli, S.A. Didas, P. Bollini, C. Sievers, T. Kamegawa, H. Yamashita, C.W. Jones, J. Am. Chem. Soc. 134 (2012) 10757–10760. [10] F.M. Higgins, H. De Leeuw, S.C. Parker, J. Mater. Chem. 12 (2002) 124–131. [11] N.V. Vladykin, Mineralogical-geochemical Features of Rare-metal Granitoides of Mongolia, Nauka, Novosibirsk, 1983 (in Russian). [12] N.V. Vladykin, V.I. Kovalenko, M.I. Novgorodova (Ed.), Minerals of Mongolia, ЭКОСТ, 2006, pp. 250–256 (in Russian). [13] A.C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 2004, pp. 86–748. [14] T. Langreiter, V. Kahlenberg, Crystals 5 (2015) 143–153. [15] A.A. Grigor’eva, N.V. Zubkova, I.V. Pekov, U. Kolitsch, D.Yu Pushcharovsky, M.F. Vigasina, G. Giester, T. Dordevic, E. Tillmanns, N.V. Chukanov, Crystallogr. Rep. 56 (5) (2011) 832–841. [16] G. Della Ventura, D. Gatta, G. Redhammer, F. Bellatreccia, A. Loose, G.C. Parodi, Phys. Chem. Miner. 36 (2009) 193–206. [17] A.L. Spek, Acta Crystallogr. D65 (2) (2009) 148–155. [18] A.N. Sapozhnikov, A.A. Kashaev, Sov. Phys. Crystallogr. 23 (1978) 52–56. [19] G. Cametti, T. Armbruster, M. Nagashima, Microporous Mesoporous Mater. 227 (2016) 81–87.
5. Conclusions The study of the thermal behaviour of armstrongite evidences that this zeolite-like mineral undergoes a dehydration process that is complete, quick and repeatedly reversible under the condition of the experiment. In particular, thermal analysis indicates that there are two steps of thermal degradation (the first up to ∼100 °C, the second in the range 100–400 °C) associated with desorption of absorbed and structural water. The XRPD heating/cooling experiment indicates that the dehydration process is complete at T = 400 °C while the rehydration occurs at temperatures (∼320 °C) lower than those observed for the H2O loss on heating. An additional notable point emerged from this study is that there is a significant difference between the results obtained by FTIR on powders or on a single crystal. In particular, powdered armstrongite embedded within a KBr disk dehydrates totally for T ≥ 500 °C and does not
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