Dehydration-induced structural change in ikaite, CaCO 3 ·6H 2 O, is investigated using a low-temperature singlecrystal 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 measurements were performed in 10°C steps, revealing a monotonous increase of unit cell volume from 756.3 to 758.0 Å 3 , up to −20°C. The unit cell volume then jumps to 771.0 Å 3 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 attributable 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 composed of the CaCO 3 ·6H 2 O 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 CO 3 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 H 2 O molecules from the crystal lattice occurs simultaneously because the CaCO 3 ·6H 2 O 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 INTRODUCTIONCalcium carbonates are abundant materials found in sedimentary rock throughout Earth's surface. During the past two decades, much research has been conducted on calcium carbonate nucleation, given that abiotic precipitation of calcium carbonate minerals constitutes a huge sink of CO 2 gas from the atmosphere, which directly affects the ocean chemistry, Earth's atmosphere, and climate (e.g., Archer and Maier-Reimer, 1994;Kleypas et al., 1999;Riebesell et al., 2000;Ries et al., 2009;Bodnar et al., 2013;Kaszuba et al., 2013). Calcium carbonate minerals occur naturally in six different modifications: three anhydrous crystalline polymorphs (CaCO 3 ) as calcite, aragonite, and vaterite; two hydrate phases as monohydrocalcite (CaCO 3 ·H 2 O) and ikaite (CaCO 3 ·6H 2 O); and amorphous calcium carbonate (ACC), which has similar stoichiometry to that of monohydrocalcite (LeviKalisman et al., 2002). Ikaite is a metastable mineral in the sedimentary realm formed naturally from aqueous solutions in near-freezing anoxic marine sediments (Zabel and Schulz, 2001;Greinert and Derkachev, 2004;Selleck et al.,...