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– 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: 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 (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 (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 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 (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, , and ; 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 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 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. Anisotropic thermal ex- shows a significant decrease in intensity of the 112 reflec- pansion behavior in ikaite has already been observed in tions, which suggests the dehydration–induced decompo- the range of −159 to 20 °C (Lennie et al., 2004). The sition of ikaite into vaterite. Therefore, Mikkelsen et al. largest thermal expansion occurs along the crystallo- (1999) proposed that the relative humidity might be iden- graphic a–axis, and the smallest along the b–axis. In this tified as the most important factor controlling the rate of study, the lattice parameter variations normalized to the decomposition of ikaite. In our study, ikaite single crys- values at T = −50 °C (Fig. 1f) show thermal expansion tals were exposed directly to the stream of N2–gas flow anisotropy that are identical to the previous study (Lennie throughout the measurements. Because the dehydration et al., 2004). The largest thermal expansion is observed process involves the movement of H2O molecules on along the a–axis, followed by the c and b axes (Lennie et the interface, the N2–gas stream can accelerate the release al., 2004). The expansion rates in the a, b, and c direc- of H2O molecules from the ikaite crystal surface. Because tions at temperatures of −50 to −20 °C (Fig. 1f) are ap- of this, the loss of ikaite structures was observed at a proximately similar to those reported by Lennie et al. much lower temperature than the previously reported val- (2004). The ikaite structure was lost at the measurement ues (Lennie et al., 2004). Consequently, decomposition of temperature of 0 °C immediately after the anomalous ikaite can be induced not only by increasing temperature large thermal expansion at −10 °C. According to an ear- but also by decreasing humidity. lier study (Lennie et al., 2004), ikaite is retained at least The most noticeable feature of the results presented up to 20 °C. A crucial difference exists between the here is that an extremely strong lattice expansion occurs decomposition temperatures found in our results and in immediately before the ikaite structure is lost (Fig. 1e). previously reported results. In that earlier experiment We regarded the large increase in the unit cell volume of (Lennie et al., 2004), a synthesized ikaite sample was about 13 Å3 to be the result of a structural change, which transferred and sealed into a capillary tube. Consequently, engenders the H2O movement followed by the ikaite de- it was maintained at a given relative humidity under an hydration. The variations in intramolecular bond lengths air stream in the sealed capillary. In an experiment de- are shown in Figure 2. The molecular configuration of signed to investigate the stability of ikaite at atmospheric CaCO3·6H2O in ikaite analyzed in this study is presented 160 N. Tateno and A. Kyono

Table 2. Fractional atomic coordinates and isotropic and anisotropic ADPs (Å2) Structural change induced by dehydration in ikaite 161

Table 2. (Continued)

in Figure 3. Only the C–O2 bond length remains almost in Figure 4. Schematic drawings of the three–dimensional constant in the whole range of temperatures (Fig. 2e), and intermolecular hydrogen–bonding network linking the all Ca–O and C–O1 bond lengths confronted with intra- CaCO3·6H2O molecules are shown in Figures 5 and 6. molecular hydrogen bondings clearly increase immedi- The changes in the O–O distances provide more reliable ately before the decomposition of ikaite structure (Figs. evidence related to the anisotropic thermal expansion of 2a–2e). Particularly, the Ca–O2 and Ca–O5 bonds respec- the unit cell than that in the hydrogen bond lengths. Len- tively show marked elongations from 2.443 to 2.470 Å nie et al. (2004) reported that the hydrogen bonding link- and from 2.523 to 2.543 Å in the interval between −20 ing O2 to O5 aligned nearly parallel to the a–axis (Fig. 5) and −10 (Figs. 2a and 2d; Table 3). The C–O1 bond is contributes to thermal expansion in the a direction. The also greatly elongated from 1.277 to 1.289 Å (Fig. 2e; O2–O5 distance is, however, almost unchanged. It finally Table 3). These intramolecular bond length elongations decreases immediately before decomposition (Fig. 4d). toward the b–axis (Fig. 3) are directly responsible for The largest and second largest expansion rate between expansion of the b lattice parameter. The variations in the oxygen atoms are observed at the O3–O4 and O1– interatomic distances between oxygen atoms are given O4 distances (Figs. 4a and 4e). They are gradually elon- 162 N. Tateno and A. Kyono

(a) (b) (c)

(d) (e) (f)

Figure 1. Variations in lattice parameters and normalized lattice parameters of ikaite as determined from single–crystal structure refinements.

(a) (b) (c)

(d) (e)

Figure 2. Variations in intramolecular bond lengths in ikaite as determined from sin- gle–crystal structure refinements. (e) Be- cause the C–O1 and C–O2 bond lengths become equal at −10 °C, the error bars are completely overlapped. The wider bar corresponds to the C–O1. Structural change induced by dehydration in ikaite 163

H6 H6 gated at temperatures of −50 and −20 °C, but expand drastically at −10 °C from 2.913 to 2.933 Å and 2.863 H5 to 2.887 Å, respectively (Figs. 4a and 4e; Table 3). The H5 – H2 H2 expansion between O3 O4 lying nearly along the [010] direction (Fig. 5) contributes to that of the b–axis. Be- cause the O1–O4 direction is aligned perfectly parallel Ca to the (101) plane (Fig. 6), the expansions of the a– H1 H1 axis and c–axis are caused mainly by the substantial var- H4 H4 iation of hydrogen bonding between O1 and O4 atoms. H3 H3 The two–dimensional molecular sheets composed of the CaCO3·6H2O molecules are stacked with hydrogen bond- ings along the c–axis (Fig. 6). The thermal expansion along the c–axis is therefore affected directly by varia- tions in hydrogen bondings between O3–O2 and O4– C O5. The increase in intermolecular distance between b O1 and O5 atoms oriented in the b direction contributes to the thermal expansion in the b–axis. Although not only the intermolecular O1–O5 distance but also the intramo- lecular Ca–O2 and Ca–O5 bond distances are increased until the decomposition of ikaite, the thermal expansion rate exhibits the smallest increase along the b–axis. Fig- a c ure 7 shows variations in intramolecular bond angles re- Figure 3. Molecular configuration of CaCO3·6H2O complex, lated to CO3 geometry, which is aligned perfectly parallel which is a fundamental building unit in ikaite. Atoms are shown b– as a large gray sphere for oxygen, a small gray sphere for hydro- to the axis. Variations of less than 1° were observed gen, a large black sphere for calcium, and small black spheres at most of the intramolecular bond angles within the for carbon. The graphic is presented using the software Crystal- CaCO3·6H2O molecule at temperatures between −50 maker ® (CrystalMaker Software Ltd., UK). and −10 °C. Only the O2–C–O2 bond angle within the

Table 3. Intramolecular bond lengths (Å) and selected intermolecular distances (Å) and angles (°) 164 N. Tateno and A. Kyono

(a) (b) (c)

(d) (e) (f)

Figure 4. Variations in selected intermolecular distances in ikaite as determined from single–crystal structure refinements.

CO3 triangle is increased significantly by nearly 2° from are maintained almost constant until the H2O molecules 118.0 to 119.9° (Fig. 7a). With the increase in the O2–C– start to move. To investigate the molecular rotation of O2 bond angle, the Ca–O2–C angle is decreased from CaCO3·6H2O, the variation in angle between the direc- 94.1° to 93.2° (Fig. 7b). These motions negatively influ- tions of the intramolecular O3–O3 segment and the a–ax- ence the thermal expansion along the b–axis. In the ‘kite’ is was examined directly in the study. Results show that quadrilateral geometry composed of the Ca–O2–C–O2 the molecular rotation is only within 0.2° in the range of atoms, that is, the diagonal distance between Ca and C 45.4° and 45.5° (Fig. 9), which suggests that the CaCO3· atoms aligned to the b–axis, increases very slightly from 6H2O molecules are rotated only slightly until the H2O 2.842 to 2.848 Å, while another diagonal distance largely molecules start to move. It suggests, therefore, that the increases from 2.210 to 2.233 Å. The reason the b lattice dehydration of H2O molecules from the crystal lattice parameter exhibits the smallest thermal expansion is that occurs simultaneously because the CaCO3·6H2O mole- the increases in interatomic distance along the b–axis are cules stabilized by the hydrogen–bonding network are offset by variations in bond angles in the CO3 geometry. never moved until the decomposition. The phase trans- The ikaite decomposition mechanism has been proposed formation model from ikaite to vaterite proposed by Tang using differential scanning calorimetry (DSC) analyses as et al. (2009) is thought to be based on the structural sim- follows: in the first step, the crystal rearranges, and H2O ilarity between ikaite and vaterite. They reported that, molecules move from the crystal lattice to the crystal sur- with the loss of all H2O molecules from ikaite, Ca atoms face. In the second step, the H2O molecules evaporate coordinated by two oxygen atoms from CO3 are real- (Mikkelsen et al. 1999). The variations in intermolecular igned along the [001] direction (Tang et al., 2009). The angles are given in Figure 8. Although hydrogen bond- Ca atom position is moved markedly with residual hydro- ings linking the CaCO3·6H2O molecules are partially lost gen bondings if the loss of the H2O molecules occurs step immediately before dehydration, variations in the entire by step. In their model (Tang et al., 2009), therefore, all intramolecular angles are less than 1° in the observed the H2O molecules must be removed simultaneously in temperature range (Fig. 8): all the intermolecular angles the ikaite structure. In addition, some small translation of Structural change induced by dehydration in ikaite 165

C

b Ca Hydrogen bonding

Figure 5. Crystal structure of ikaite projected along the c–axis. White and gray colored bonds represent hydrogen bonding networks in a ikaite.

CO3 ions and volume contraction are necessary to accom- ikaite can be induced by not only increasing temperature plish the transformation to vaterite. Results reported here- but also decreasing humidity. With increasing tempera- in show that the hydrogen bonding network rigidly main- ture, the unit cell expands anisotropically along the a– tains the ikaite structure until dehydration. Then, H2O axis followed by the c–axis. An extremely strong lattice molecules start to move simultaneously with the collapse expansion occurs immediately before the ikaite structure of the hydrogen bonding network. This evidence supports is lost, which engenders the H2O movement followed by the proposed phase–transformation model (Tang et al., the ikaite dehydration. Then, H2O molecules in the ikaite 2009), but additional experiments must be conducted to structure start to move simultaneously with the collapse establish the exact phase transition mechanism involved of the hydrogen bonding network. in vaterite formation. ACKNOWLEDGMENTS CONCLUSION This work was supported by a Grant–in–Aid for Young Low–temperature single–crystal X–ray diffraction meas- Scientists (B) from the Japan Society for the Promotion urement of ikaite (CaCO3·6H2O) was performed to ex- of Science (project no. 24740352). Editorial comments plore the characteristics of structure change from ikaite from Dr. Okudera and detailed and constructive reviews to vaterite near dehydration. The present study confirms by Dr. Ohfuji and an anonymous referee helped improve that ikaite starts to decompose below the temperature of the manuscript and are greatly appreciated. 0 °C due to the aridity of the sample. Decomposition of O2-C-O2 angle (°) 166 117.5 118.5 119.5 120.5 c a (b) (a) 5 4 3 2 -10 -20 -30 -40 -50 Temperature ( iue7. Figure iue6. Figure aitosi O2 in Variations ° rsa tutr fiat rjce rmthe from projected ikaite of structure Crystal C) .Ttn n .Kyono A. and Tateno N. a – C – 2adCa and O2 – O2 Ca-O2-C angle (°) – nlsi ikaite. in angles C 92.5 93.0 93.5 94.0 94.5 b 5 4 3 2 -10 -20 -30 -40 -50 – axis. Temperature ( ° C) Structural change induced by dehydration in ikaite 167

(a) (b) (c)

(d) (e) (f)

Figure 8. Variations in selected intermolecular angles in ikaite.

46.0 Bischoff, J.L., Fitzpatrick, J.A. and Rosenbauer, R.J. (1993) The solubility and stabilization of ikaite (CaCO3.6H2O) from 0° to

) 25 °C: environmental and paleoclimatic implications for thi- ° 45.8 nolite . Journal of Geology, 101, 21–33. Bodnar, R.J., Steele–MacInnis, M., Capobianco, R.M., Rimstidt, 45.6 J.D., Dilmore, R., Goodman, A. and Guthrie, G. (2013) PVTX Properties of H2O–CO2–“salt” at PTX Conditions 45.4 Applicable to Carbon Sequestration in Saline Formations. In Geochemistry of geologic CO2 sequestration 77 (DePaolo, D.J., Cole, D.R., Navrotsky, A. and Bourg, I.C. Eds.). Re- 45.2 Rotation angle ( views in and Geochemistry, Mineralogical Soci- ety of America, Virginia, 123–152. 45.0 Carruthers, J.R., Rollett, J.S., Betteridge, P.W., Kinna, D., Pearce, -50 -40 -30 -20 -10 L., Larsen, A. and Gabe, E. (1999) CRYSTALS No. 11. Temperature (°C) Chemical Crystallography Laboratory, Oxford, U.K. Council, T.C. and Bennett, P.C. (1993) Geochemistry of ikaite for- Figure 9. Variations in angle between the directions of intramo- mation at , California: Implications for the origin lecular O3–O3 segment and the a–axis. of tufa mounds. Geology, 21, 971–974. Dickens, B. and Brown, W.E. (1970) Crystal structure of calcium carbonate hexahydrate at ~ −120. Inorganic Chemistry, 9, REFERENCES 480–486. Dieckmann, G.S., Nehrke, G., Papadimitriou, S., Gottlicher, J., Altomare, A., Burla, M.C., Camalli, M., Cascarano, G.L., Giaco- Steininger, R., Kennedy, H., Wolf–Gladrow, D. and Thomas, vazzo, C., Guagliardi, A., Moliterni, A.G.G., Polidori, G. and D.N. (2008) Calcium carbonate as ikaite crystals in Antarctic Spagna, R. (1999) SIR97: a new tool for crystal structure de- sea ice. Geophysical Research Letters, 35, L08501. termination and refinement. Journal of Applied Crystallogra- Greinert, J. and Derkachev, A. (2004) Glendonites and – phy, 32, 115–119. derived Mg–calcites in the , Eastern : Archer, D. and Maier–Reimer, E. (1994) Effect of deep–sea sedi- implications of a venting–related ikaite/glendonite formation. – mentary calcite preservation on atmospheric CO2 concentra- Marine Geology, 204, 129 144. tion. Nature, 367, 260–264. Higashi, T. (1995) ABSCOR—Empirical absorption correction 168 N. Tateno and A. Kyono

based on Fourier series approximation. Rigaku Corp., Tokyo. plications for palaeoclimatic studies using carbonates from Ito, T. (1998) Factors controlling the transformation of natural Laguna Potrok Aike (Patagonia, Argentina). Quaternary Sci- ikaite from Shiowakka, Japan. Geochemical Journal, 32, ence Reviews, 71, 46–53. 267–273. Riebesell, U., Zondervan, I., Rost, B., Tortell, P.D., Zeebe, R.E. Kaszuba, J., Yardley, B. and Andreani, M. (2013) Experimental and Morel, F.M.M. (2000) Reduced calcification of marine Perspectives of Mineral Dissolution and Precipitation due to plankton in response to increased atmospheric CO2. Nature, Carbon Dioxide–Water–Rock Interactions Reviews in Miner- 407, 364–367. alogy and Geochemistry. In Geochemistry of geologic CO2 Ries, J.B., Cohen, A.L. and McCorkle, D.C. (2009) Marine calci- sequestration, 77 (DePaolo, D.J., Cole, D.R., Navrotsky, A. fiers exhibit mixed responses to CO2–induced ocean acidifica- and Bourg, I.C. Eds.). Reviews in Mineralogy and Geochem- tion. Geology, 37, 1131–1134. istry, Mineralogical Society of America, Virginia 153–188. Rysgaard, S., Sogaard, D.H., Cooper, M., Pucko, M., Lennert, K., Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J.P., Lang- Papakyriakou, T.N., Wang, F., Geilfus, N.X., Glud, R.N. and don, C. and Opdyke, B.N. (1999) Geochemical consequences Ehn, J. (2013) Ikaite crystal distribution in winter sea ice and of increased atmospheric carbon dioxide on coral reefs. Sci- implications for CO2 system dynamics. Cryosphere, 7, 707– ence, 284, 118–120. 718. Larsen, D. (1994) Origin and paleoenvironmental significance of Selleck, B.W., Carr, P.F. and Jones, B.G. (2007) A review and syn- calcite pseudomorphs after ikaite in the Oligocene Creede thesis of glendonites (pseudomorphs after ikaite) with new Formation, Colorado. Journal of Sedimentary Research, 64, data: Assessing applicability as recorders of ancient coldwater 593–603. conditions. Journal of Sedimentary Research, 77, 980–991. Last, F.M., Last, W.M., Fayek, M. and Halden, N.M. (2013) Oc- Shahar, A., Bassett, W.A., Mao, H.K., Chou, I.M. and Mao, W. currence and significance of a cold–water carbonate pseudo- (2005) The stability and Raman spectra of ikaite, CaCO3· morph in microbialites from a saline lake. Journal of Paleo- 6H2O, at high pressure and temperature. American Mineralo- limnology, 50, 505–517. gist, 90, 1835–1839. Levi–Kalisman, Y., Raz, S., Weiner, S., Addadi, L. and Sagi, I. Shearman, D.J. and Smith, A.J. (1985) Ikaite, the parent mineral of (2002) Structural differences between biogenic amorphous jarrowite–type pseudomorphs. Proceedings of the Geological calcium carbonate phases using X–ray absorption spectrosco- Association, London, 96, 305–314. py. Advanced Functional Materials, 12, 43–48. Swainson, I.P. and Hammond, R.P. (2003) Hydrogen bonding in Lennie, A.R., Tang, C.C. and Thompson, S.P. (2004) The structure ikaite, CaCO3·6H2O. Mineralogical Magazine, 67, 555–562. and thermal expansion behaviour of ikaite, CaCO3·6H2O, Tang, C.C., Thompson, S.P., Parker, J.E., Lennie, A.R., Azough, F. from T=114 to T=293 K. Mineralogical Magazine, 68, 135– and Kato, K. (2009) The ikaite–to–vaterite transformation: 146. new evidence from diffraction and imaging. Journal of Ap- Lennie, A.R. (2005) Ikaite (CaCO3·6H2O) compressibility at high plied Crystallography, 42, 225–233. water pressure: a synchrotron X–ray diffraction study. Minera- Zabel, M. and Schulz, H.D. (2001) Importance of submarine land- logical Magazine, 69, 325–336. slides for non–steady state conditions in pore water systems – Mikkelsen, A., Andersen, A.B., Engelsen, S.B., Hansen, H.C.B., lower Zaire (Congo) deep–sea fan. Marine Geology, 176, 87– Larsen, O. and Skibsted, L.H. (1999) Presence and dehydra- 99. tion of ikaite, calcium carbonate hexahydrate, in frozen shrimp shell. Journal of Agricultural and Food Chemistry, 47, 911–917. Manuscript received March 20, 2014 Oehlerich, M., Mayr, C., Griesshaber, E., Lücke, A., Oeckler, Manuscript accepted June 16, 2014 O.M., Ohlendorf, C., Schmahl, W.W. and Zolitschka, B. Published online July 29, 2014 (2013) Ikaite precipitation in a lacustrine environment – im- Manuscript handled by Hiroki Okudera