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Structural Change Induced by Dehydration in Ikaite (Caco3·6H2O)

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Structural Change Induced by Dehydration in Ikaite (Caco3·6H2O) Journal of Mineralogical and Petrological Sciences, Volume 109, page 157–168, 2014 Structural change induced by dehydration in ikaite (CaCO3·6H2O) Natsuki TATENO and Atsushi KYONO Division of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1–1–1 Tennodai, Tsukuba, Ibaraki 305–8572, Japan Dehydration–induced structural change in ikaite, CaCO3·6H2O, is investigated using a low–temperature single– crystal X–ray diffraction study. At −50 °C, the crystal structure of ikaite is monoclinic, of space group C2/c with the unit cell parameters a = 8.8134 (1), b = 8.3108 (1), c = 11.0183 (1) Å, and β = 110.418 (1)°. The measure- ments were performed in 10 °C steps, revealing a monotonous increase of unit cell volume from 756.3 to 758.0 Å3,upto−20 °C. The unit cell volume then jumps to 771.0 Å3 at −10 °C. The unit cell expands anisotropically along the a–axis followed by the c–axis. The ikaite structure is finally lost at 0 °C, which is a much lower temperature for decomposition than previously reported values. The low temperature decomposition is attrib- utable to the aridity of the sample. The elongation of the O1–O4 intermolecular distance parallel to the (101) plane engenders the substantial increase in the a–axis and c–axis. The two–dimensional molecular sheets com- posed of the CaCO3·6H2O molecules are stacked with hydrogen bondings along the c–axis. The expansion of the c–axis is affected by variations in the hydrogen bondings between the sheets. The intramolecular Ca–O2 and Ca–O5 bond lengths and the intermolecular O1–O5 distance are greatly elongated immediately before the decomposition of ikaite structure. These expansions along the b–axis, however, are offset by the increase in the O2–C–O2 bond angle in the CO3 geometry, aligned perfectly parallel to the b–axis. The intermolecular angles are maintained as almost constant until the ikaite structure is lost. It can be concluded therefore that the movement of H2O molecules from the crystal lattice occurs simultaneously because the CaCO3·6H2O molecules are stabilized by the hydrogen–bonding network immediately before dehydration. Keywords: Calcium carbonate hydrate, Decomposition, Phase transition, Low–temperature single–crystal X–ray diffraction INTRODUCTION hydrocalcite (CaCO3·H2O) and ikaite (CaCO3·6H2O); and amorphous calcium carbonate (ACC), which has Calcium carbonates are abundant materials found in sedi- similar stoichiometry to that of monohydrocalcite (Levi– mentary rock throughout Earth’s surface. During the past Kalisman et al., 2002). Ikaite is a metastable mineral in two decades, much research has been conducted on cal- the sedimentary realm formed naturally from aqueous so- cium carbonate nucleation, given that abiotic precipita- lutions in near–freezing anoxic marine sediments (Zabel tion of calcium carbonate minerals constitutes a huge and Schulz, 2001; Greinert and Derkachev, 2004; Selleck sink of CO2 gas from the atmosphere, which directly af- et al., 2007). In recent years, a variety of different occur- fects the ocean chemistry, Earth’s atmosphere, and cli- rences have been reported, including in high salinity and mate (e.g., Archer and Maier–Reimer, 1994; Kleypas et alkaline lakes (Last et al., 2013; Oehlerich et al., 2013) al., 1999; Riebesell et al., 2000; Ries et al., 2009; Bodnar and polar sea ice (Dieckmann et al., 2008; Rysgaard et et al., 2013; Kaszuba et al., 2013). Calcium carbonate al., 2013). Both in nature and in the laboratory, ikaite minerals occur naturally in six different modifications: readily crystallizes from solution at temperatures near 0 three anhydrous crystalline polymorphs (CaCO3) as cal- °C, but rapidly decomposes into a mush of anhydrous cite, aragonite, and vaterite; two hydrate phases as mono- CaCO3 and water at warmer temperatures (Bischoff et al., 1993; Council and Bennett, 1993; Mikkelsen et al., doi:10.2465/jmps.140320 1999; Tang et al., 2009). The ease of breakdown of ikaite A. Kyono, [email protected] Corresponding author means that calcite pseudomorphs after ikaite are pre- 158 N. Tateno and A. Kyono served at the same time in the sedimentary record (Lars- other 10 min. Then, the solution was kept at 3–5 °C for en, 1994). Glendonite, thinolite, jarrowite, fundylite, gen- three months. Finally, large well–formed prismatic crys- noishi, gersternkõrner, and White Sea hornlets are local tals up to 3.0 mm long were obtained. The single crystals names attributed to calcite pseudomorph after metastable were extracted and placed on the cooling stage under a ikaite (Shearman and Smith, 1985). stereo microscope. The temperature of the cooling stage Ikaite crystallizes in the monoclinic space group C2/ operated with liquid nitrogen was maintained below −15 c with a = 8.8053(2), b = 8.3138(1), c = 11.0328(3) Å, °C. A crystal with approximate dimensions of 0.3 × 0.2 × β = 110.598(3)°, Z = 4, and V = 756.03(5) Å3 at a tem- 0.2 mm was selected and attached to a 0.1–mm–diameter perature of −30 °C (Lennie et al., 2004). The ikaite struc- glass fiber using a cyanoacrylate adhesive (Aron Alpha; ture comprises discrete CaCO3·6H2O molecules linked Toagosei Co. Ltd.). It was then immediately mounted on- together by hydrogen bonding in a unit cell (Lennie et to the goniometer under a nitrogen coldstream at −50 °C. al., 2004). The calcium is coordinated by two oxygen Low–temperature single–crystal X–ray diffraction atoms from carbonate, forming CaCO3, and by six oxy- measurements were taken using an imaging plate diffrac- gen atoms from water molecules (Lennie et al., 2004). tometer (Raxis–Rapid; Rigaku Corp.) equipped with a The phase transformation of ikaite has been studied ex- continuous N2–gas flow type cryostat (Rigaku Corp.). tensively using X–ray powder diffraction and Raman The sample temperature was calibrated using a K–type spectroscopy as a function of temperature, pressure, and thermocouple attached to the sample position of the go- time (Ito, 1998; Mikkelsen et al., 1999; Swainson and niometer head. The accuracy of the temperature control Hammond, 2003; Lennie et al., 2004; Lennie, 2005; Sha- was less than ±1.0 °C. To stabilize the crystal tempera- har et al., 2005; Tang et al., 2009). Ito (1998) investigated ture, the sample was kept in the N2–gas flow with con- the transformation of natural ikaite to elucidate the envi- trolled temperature for one hour before beginning each ronmental factors controlling the transformation kinetics. measurement. Intensity data were collected using graph- The quite anisotropic thermal expansion behavior of ite monochromatized MoKα radiation (λ = 0.71069 Å) at ikaite was observed clearly in neutrons and synchrotron temperatures of −50 to 0 °C in steps of 10 °C. All dif- X–ray powder diffraction studies (Swainson and Ham- fraction data were collected using an ω–oscillation meth- mond, 2003; Lennie et al., 2004). The largest thermal od with an oscillation width of 5.0° between 130 and expansion occurs along the crystallographic a–axis, 190° (χ = 45°, φ = 0°) and between 0 and 160° (χ = whereas the smallest occurs along the b–axis (Lennie et 45°, φ = 180°). The exposure rate was 180 s per degree al., 2004). Tang et al. (2009) investigated the phase trans- of oscillation. In all, 44 images were collected in each formation mechanism from ikaite to vaterite. Although a measurement. Intensities were corrected for Lorentz and model was proposed based on the structural similarity polarization effects. Absorption correction was applied between ikaite and vaterite (Tang et al., 2009), the fun- from the symmetry–equivalent reflections using the damental understanding of the phase transformation ABSCOR program (Higashi, 1995). The structure was mechanisms remains incomplete. Furthermore, no previ- solved using a direct method with the SIR97 program ously reported study has examined local structural package (Altomare et al., 1999). Only reflections with changes in ikaite induced by dehydration. jFoj > 4ðFoÞ were used for structural refinements in In this work, we explored the characteristics of the space group C2/c, performed using full–matrix least structure change near dehydration using a low–tempera- squares on F 2 with the CRYSTALS program (Carruthers ture single–crystal X–ray diffraction technique. We ob- et al., 1999) (Table 1). Atomic coordinates for all atoms tained reliable X–ray diffraction data up to the decompo- and anisotropic atomic displacement parameters (ADPs) sition. Herein, we present the results of crystal structure for non–H atoms were refined (Table 2). For the intensity refinements and the structural characteristics in ikaite data measured at T = −10 °C, however, the least–squares with dehydration. refinement was performed with isotropic ADPs for H atoms constrained to the values determined in these EXPERIMENTAL METHODS measurements. Single crystals of ikaite were prepared using the pro- RESULTS AND DISCUSSION cedures described by Dickens and Brown (1970). First, 4.6 g of Na2CO3 and 1.0 g of sodium polyphosphate The single crystal X–ray diffraction data were collected (NaPO3)6 were dissolved in 150 ml of water. After stir- successively from −50 °C to −10 °C. At the temperature ring for 10 min, a solution of 2.4 g of CaCl2·2H2O dis- of 0 °C, however, the observed X–ray diffraction pattern solved in 100 ml of water was added and stirred for an- suddenly lost the symmetry of the ikaite single crystal. Structural change induced by dehydration in ikaite 159 Table 1. Crystallographic data and refinement parameters for ikaite The experiment was, therefore, terminated at 0 °C. The conditions at 5 °C in open containers (Mikkelsen et al., variations in the lattice parameters as a function of tem- 1999), the X–ray diffraction profile obtained after 30 days perature are shown in Figure 1.
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