A combination of X-ray diffraction, infrared, and chemical data has established that the ion exchange of vermiculite with singly charged benzidine cations in an aqueous solution at pH 1.6 results in a black, highly ordered benzidine-vermiculite intercalate. The intercalate has a basal spacing of 19.25 Å and a primitive unit cell with “a” and “b” edges parallel and equal to those of vermiculite. The number of benzidine molecules per cell is equal to its electric charge. In this structure the benzidine molecules are steeply inclined to the silicate surfaces and close-packed within domains. The domains contain alternating rows of benzidine cations; from row to row the planes are either approximately parallel or perpendicular to the (120) plane, but along any one row the planes of the aromatic rings are parallel to each other. Hydrogen bonding operates between amine nitrogens and surface oxygens.

A monoclinic IIb-2 clinochlore from the Achmatov mine, Ural Mountains, U.S.S.R., was investigated by neutron diffraction. The formula, based on electron microprobe and wet chemical analyses, is (Mg4.54Al0.97Fe2+ 0.28 Fe3+ 0.18Mn0.01)(Si2.85Al1.15)O10(OH)8. A refinement based on 512 unique reflections converged in space group C2/m to a final R =.066. Cation disorder was found in the two octahedral positions of the 2:1 layer, whereas partial Mg and Al ordering occurs in the interlayer sheet. The two hydroxyl dipoles are roughly perpendicular to the interlayer sheet, forming weak to medium hydrogen bonds with O…O distances of 2.859 and 2.881 Å. The OH-dipole of the 2:1 layer is perpendicular to the (001) plane.

The deintercalation of a low-defect kaolinite intercalated with hydrazine was studied by X-ray diffraction, diffuse reflectance infrared spectroscopy (DRIFT), and Raman microscopy over 30 d. X-ray diffraction showed that the kaolinite was fully intercalated. More than 120 h were required for the hydrazine-intercalate to decompose. The Raman spectra of the hydrazine intercalate showed only a single band at 3620 cm−1, which was attributed to the innerhydroxyl group. Upon deintercalation, additional Raman bands were observed at 3626 and 3613 cm−1. These bands decreased in intensity with further deintercalation. As deintercalation proceeded, the bands assigned to the inner-surface hydroxyl groups at 3695, 3682, 3670, and 3650 cm−1 occurred and increased in intensity. DRIFT spectra showed two bands at 3620 and 3626 cm−1 for the fully intercalated kaolinite only. Upon deintercalation, an additional band assigned to intercalated water was observed at 3599 cm−1 and increased in intensity at the expense of the 3626-cm−1 band. Bands attributed to the innersurface hydroxyl groups increased in intensity with deintercalation. Both the Raman and DRIFT spectra showed complexity in the NH-stretching region with two sets of NH-symmetric and antisymmetric stretching bands. Deintercalation was followed by the loss of intensity of these bands. Significant changes were also observed in the hydroxyl deformation and water-bending modes as a result of deintercalation. A model of hydrazine intercalation of kaolinite based on the insertion of a hydrazine-water unit is proposed. The hydrated end of the hydrazine molecule hydrogen bonds with the innersurface hydroxyl groups resulting in the formation of a band at 3626 cm−1 in the DRIFT spectra.

Silanol groups in protonated magadiite (H-magadiite) were characterized by 1H and 2H solid-state nuclear magnetic resonance (NMR). H-magadiite and deuterated (D) magadiite were synthesized by the treatment of Na-rich magadiite with 0.2 N HC1 and 0.2 N DC1, respectively. In the 1H NMR spectrum measured at room temperature, silanol groups of H-magadiite showed two signals at 3.75 and 5.70 ppm, indicating that two types of silanol groups were present. The ratio of silanol groups associated with strong hydrogen bonding (5.70 ppm) to those with weaker hydrogen bonding (3.75 ppm) was 2 to 1. The 2H NMR spectra of deuterated magadiite were measured in the temperature range from 150 to 440 K. In the spectra measured at temperatures below 294 K, silanol groups showed Pake doublet patterns. These patterns were composed of two components corresponding to the two types of silanol groups shown in the 1H NMR analysis. Both silanol groups produced wobbling motions with increasing temperature. Above 294 K, the profile of the Pake doublet pattern was transformed gradually to a near triangular pattern, indicating that the silanol groups underwent other motions also, such as a two-site jump.

Humic acid (HA) can cause environmental pollution, due to which, its removal from aqueous solutions has become an increasingly important issue. Although bentonite has an affinity for HA, the adsorption capacity of raw bentonite is still poor. As a commonly used organic modifier, 3-aminopropyltriethoxyorganosilane (APTES) exhibits excellent flocculation capability for HA. Therefore, the objective of the present study was to investigate the effectiveness of the addition of 3-aminopropyltriethoxyorganosilane (APTES) to raw bentonite to increase the adsorption of HA from aqueous solution. The experimental results showed that, when the solid-to-liquid ratio was 1:1, the amino-modified bentonite exhibited the highest adsorption capacity (qmax = 272.23 mg g-1). The adsorption affinity of amino-modified bentonite was mainly determined by the number of amino groups loaded onto its surface. The adsorption of HA on amino-modified bentonite occurred through electrostatic interactions and hydrogen bonding. These findings demonstrate the excellent potential of amino-modified bentonite in effectively remediating HA pollution.

Synthetic samples of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O, were used for calorimetric experiments. For krautite and koritnigite, single-crystal X-ray diffraction was used to determine positions of all atoms, including the H atoms. These data allowed the hydrogen-bond network and the function of H2O molecules in these structures to be determined. The structural formulae are Mn4(H2[3]O)4[AsO3(OH)]4 and Zn4(H2[3]O)2[AsO3(OH)]4(H2[4]O)2, where [3]H2O and [4]H2O are the ‘transformer’ and ‘non-transformer’ H2O groups, respectively. Even though the principal features of these structures are identical, the details, especially those regarding the H2O groups, differ from one structure to another structure in this group. The solubility products (log Ksp) were determined from calorimetric data, that is, from the experimentally measured enthalpies of formation and entropies. They relate to the reaction M[AsO3(OH)]⋅H2O → M2+ + HAsO42– + H2O and are –6.10 for krautite, –6.88 for koritnigite and –6.83 for cobaltkoritnigite. We also estimated the log Ksp for magnesiokoritnigite as –2.0. Calculation of phase diagrams shows that all these phases originate under acidic conditions from solutions with high metal and arsenate concentration. They are restricted to local environments, to pockets that maintain such high concentrations over the time necessary for crystallisation of the krautite-group phases.

Kainite, KMg(SO4)Cl⋅2.75H2O, is one of the most common hydrated sulfate minerals, and it plays an important role as a source of potassium. However, its properties and structure have, to date, been insufficiently studied. In our present work, kainite was investigated using multiple techniques, including single-crystal and powder X-ray diffraction, thermogravimetry, differential scanning calorimetry (DSC), and infrared spectroscopy (IR). The mineral is monoclinic, C2/m, a = 19.6742(2), b = 16.18240(10), c = 9.49140(10) Å, β = 94.8840(10)°, V = 3010.86(5) Å3 and Z = 16. The structure was refined to R1 = 0.0230 for 3080 unique observed reflections with |Fo| ≥ 4σF. The complex hydrogen bonding system for kainite is described for the first time. The structure of kainite contains seven symmetrically independent sites occupied by water molecules, six of which are strongly bonded to Mg2+ cations while the seventh resides in the framework cavities. The acceptors of the hydrogen bonds are either chloride anions, neighbouring water molecules or oxygens atoms of sulfate groups. A bifurcated hydrogen bond was described for one of the water molecules. Based on the analysis of the crystal structure, we have confirmed and propose the correct formula for kainite as KMg(SO4)Cl⋅2.75H2O. The thermal studies of kainite in the temperature range of –150°C to +600°C indicate its stability to 190°C. The decomposition products are K2Mg2(SO4)3, KCl and K2SO4. The thermal expansion was calculated, which for kainite has a character typical for monoclinic crystals and similar to the compressibility tensor described earlier.

Desoxyribosenucleic acid, DNA, and cellulose molecules self-assemble in aqueous systems. This aggregation is the basis of the important functions of these biological macromolecules. Both DNA and cellulose have significant polar and nonpolar parts and there is a delicate balance between hydrophilic and hydrophobic interactions. The hydrophilic interactions related to net charges have been thoroughly studied and are well understood. On the other hand, the detailed roles of hydrogen bonding and hydrophobic interactions have remained controversial. It is found that the contributions of hydrophobic interactions in driving important processes, like the double-helix formation of DNA and the aqueous dissolution of cellulose, are dominating whereas the net contribution from hydrogen bonding is small. In reviewing the roles of different interactions for DNA and cellulose it is useful to compare with the self-assembly features of surfactants, the simplest case of amphiphilic molecules. Pertinent information on the amphiphilic character of cellulose and DNA can be obtained from the association with surfactants, as well as on modifying the hydrophobic interactions by additives.

Davidbrownite-(NH4), (NH4,K)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O, is a new mineral species from the Rowley mine, Maricopa County, Arizona, USA. It occurs in an unusual bat-guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates and chlorides, some containing NH4+. Other secondary minerals found in association with davidbrownite-(NH4) are antipinite, fluorite, mimetite, mottramite, quartz, rowleyite, salammoniac, struvite, vanadinite, willemite and wulfenite. Crystals of davidbrownite-(NH4) are light green–blue needles or narrow blades up to ~0.2 mm long. The streak is white, the lustre is vitreous, Mohs hardness is ca. 2, tenacity is brittle and fracture is splintery. There are two good cleavages in the [010] zone, probably {100} and {001}. The measured density is 2.12(2) g cm–3. Davidbrownite-(NH4) is optically biaxial (+) with α = 1.540(2), β = 1.550(5) and γ = 1.582(2) (white light); 2V = 58.5(5)°; moderate r > v dispersion; and orientation Z = b and Y ≈ a. Pleochroism: X = pale blue, Y = nearly colourless, Z = light blue; and Y < X < Z. Electron microprobe analysis gave the empirical formula [(NH4)3.11K1.73Na0.09]Σ4.93[(V4+1.92Mg0.01Al0.02)Σ1.95O2](C2O4) [(P3.94As0.12)Σ4.06O10.94(OH)5.06]·3H2O, with the C and H content provided by the crystal structure. Raman and infrared spectroscopy confirmed the presence of NH4 and C2O4. Davidbrownite-(NH4) is monoclinic, P21/c, with a = 10.356(6), b = 8.923(5), c = 13.486(7) Å, β = 92.618(9)°, V = 1244.9(12) Å3 and Z = 2. The crystal structure of davidbrownite-(NH4) (R1 = 0.0524 for 2062 Io > 2σI reflections) consists of a chain structural unit with the formula {(V4+O)2(C2O4)[PO2.75(OH)1.25]4}5–, and a disordered interstitial complex containing five large monovalent cations (NH4+ and K+) and three H2O groups pfu. Strong hydrogen bonds form links within and between the chains.

Kaolinites were modified by controlled hydrothermal deuteration. Both infrared and near infrared vibration spectra of deuterated kaolinite were investigated by Diffuse Reflectance Infrared Fourier Transform spectroscopy. The stretching vibrations (v) of deuteroxyl groups of the kaolinite were observed at 2726, 2709, 2697 and 2667 cm-1, respectively. The overtone bands (2n) of the deuteroxyl groups were visible in NIR regions at 5454, 5419, 5388 and 5342 cm-1, respectively. The lattice vibrations of the kaolinite taking part in deuteration included 937, 915, 792, 751 and 690 cm-1 which are assigned to hydroxyl group-related deformation or translation-vibration mode. The hydroxyl band at 3435 cm-1 with its deuteroxyl band at 2542 cm-1 resulted from unevenly distributed structural defects, either due to the isomorphous substitution of foreign cations or the imperfections of the kaolinite lattice structure.

In recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals. Although diverse and complex, the surface chemistry of all clay minerals is defined spatially on a common scale of nanometres. This review is organized around the nanoscale architecture of clay minerals examined at several different length scales. The first, and perhaps most important, is the length scale associated with Hbonding in clay minerals. Hbonding interactions define the size and shape of 1:1 phyllosilicates and dominate the surface chemistry of many clay minerals. Structural and surface OHgroups contained within and on the surface of clay minerals provide a type of ‘molecular reporter group’ and are sensitive to subtle changes in their local environment. Examples of OH-reporter group studies in clay minerals, and the spatial scales at which they provide diagnostic information, are examined. The second length scale considered here is that associated with clay–water and clay–organic interactions. Inorganic and organic solutes can be used to explore the surface chemistry of clay minerals. Similar to the use of reporter groups, molecular probes have diagnostic properties that are sensitive to changes in their molecular environment. Clay–water interactions occur at a length scale that extends from the size of the H2O molecule (~0.3 nm) to the larger scales associated with clay-swelling (>10 nm). Similarly, clay–organic interactions are also defined, in part, on the basis of their molecular size, in addition to the type of chemical bonding interactions that take place between the organic solute and the clay surface. Examples illustrating the use of clay–water and clay–organic solute interactions as molecular probes are presented. The largest scale to be considered is that of the particles themselves, with scales that approach micrometres. Recent developments in the synthesis and characterization of ultrathin hybrid films of clay minerals provide complementary information about the nature and distribution of active sites on clay minerals, as well as providing new opportunities to exploit the surface chemistry of clay minerals in the design of functional materials.

The crystal structure of franconite, NaNb2O5(OH)·3H2O, has been characterized by single-crystal X-ray diffraction using material from Mont Saint-Hilaire, Québec, Canada. Results give a = 10.119(2), b = 6.436(1), c = 12.682(2) Å and β = 99.91(3)° and confirm the correct space group as P21/c. The crystal structure, refined to R = 4.63% and wR2 =11.95%, contains one Na site, two distorted octahedral Nb sites and nine O sites. It consists of clusters of four edge-sharing Nb(O,OH)6 octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH)6 octahedra. These alternate along [100] with layers composed of NaO(H2O)4 polyhedra, the two being linked together by well defined H bonding. The predominance of H bonding, essential to the mineral, results in a perfect {100} cleavage. Chemical analyses (n = 7) of four crystals give the empirical formula (Na0.73Ca0.13☐0.14)∑=1.00(Nb1.96Ti0.02Si0.02Al0.01)∑=2.01O5(OH)·3H2O (based on nine oxygens) or ideally NaNb2O5(OH)·3H2O. Franconite is crystallo-chemically related to SOMS [Sandia Octahedral Molecular Sieves; Na2Nb2−xMxO6−x(OH)x·H2O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion-exchange capabilities. Both hochelagaite (CaNb4O11·nH2O) and ternovite (MgNb4O11·nH2O) have X-ray powder diffraction patterns and cation ratios similar to those of franconite indicating that these minerals probably have similar structures.

The hydroxy-sulphide yushkinite from the type deposit has been re-examined by electron probe microanalysis and reflectance measurement. The EPMA indicates minor Cu (up to 1.5 wt.%) and confirms an excess of V. Taking into account its composite structure (brucite-like hydroxide layer alternating with a berndtite-type sulphide layer) the proposed structural formula is, with the addition of minor intercalated Cu: [(Mg0.71Al0.36V0.03)Σ1.10(OH)2.18O0.02]Cu0.02 [VS2]. At the crystal scale, some crystal edges give a variable excess of Si and Al (in the 1/1 ratio), indicating a late topotactic intergrowth with a layered alumino-silicate (or a composite silicate-sulphide). New specular reflectance data are given in air and oil from 400 to 800 nm, with the polarization plane parallel or perpendicular to the elongation. The refractive index n and absorption coefficient k are given; no absorption band exists in the perpendicular position, with constant n close to 1.9, and k to 0.3. In a parallel position, a pseudo-transparency window is centred around 500–520 nm (∼2.4 eV). The main absorption band, centred around 730 nm (∼1.7 eV), corresponds probably to a transition t2g → eg related to intra-layer V-V bonding. Charge transfer from the brucite-type layer to the VS2 one, strictly controlled through H-bonding, induces a mean formal valency of V close to 3.6. Yushkinite is compared with other synthetic layered hydroxy-sulphides, as well as brucite-type intercalated compounds.

The crystal structure of sarmientite, Fe2 3+ (AsO4)(SO4)(OH)·5H2O, from the type locality (Santa Elena mine, San Juan Province, Argentina), was solved and refined from in-house powder diffraction data (CuKα1,2 radiation). It is monoclinic, space group P21/n, with unit-cell dimensions a = 6.5298(1), b = 18.5228(4), c = 9.6344(3) Å, β = 97.444(2)º, V = 1155.5(5) Å3, and Z = 4. The structure model was derived from cluster-based Patterson-function direct methods and refined by means of the Rietveld method to R wp = 0.0733 (X2 = 2.20). The structure consists of pairs of octahedral-tetrahedral (Fe−As) chains at (y,z) = (0,0) and (½,½), running along a. There are two symmetry-independent octahedral Fe sites. The Fe1 octahedra share two corners with the neighbouring arsenate groups. Both individual chains are related by a symmetry centre and joined by two symmetry-related Fe2 octahedra. Each Fe2 octahedron shares three corners with double-chain polyhedra (O3, O4 with arsenate groups; the O8 hydroxyl group with the Fe1 octahedron) and one corner (O11) with the monodentate sulfate group. The coordination of the Fe2 octahedron is completed by two H2O molecules (O9 and O10). There is also a complex network of H bonds that connects polyhedra within and among chains. Raman and infrared spectra show that (SO4)2− tetrahedra are strongly distorted.

The crystal structure of zoned philipsbornite – hidalgoite, hexagonal (rhombohedral), Rm , Z = 3: a = 7.1142(4), c = 17.0973(9) A ˚ , V = 749.4(1) Å3, from the Tsumeb mine, Namibia, has been refined to R1 = 1.68% for 301 unique reflections collected on a Bruker D8 three-circle diffractometer equipped with a rotating-anode generator, multilayer optics and an APEX-II CCD detector. Chemical analysis by electron microprobe showed zoned crystals with a rim enriched in S and Fe relative to the core. The core composition is SO3 3.31, As2O5 30.57, Al2O3 23.05, FeO 1.44, PbO 33.94, H2Ocalc 9.58, total 101.79 wt.%, corresponding to Pb0.982+(Al2.92Fe0.132+)(AsO4)[(As0.72S0.27)O3.14(OH)0.85](OH)6; and the rim composition is SO3 8.88, As2O5 22.63, Al2O3 22.90, FeO 2.57, PbO 34.91, H2Ocalc 9.27, total 101.16 wt.%, corresponding to Pb0.992+(Al2.85Fe0.232+)(AsO4)[(As0.25S0.70)O3.30(OH)0.50](OH)6. Philipsbornite – hidalgoite has the alunite-type structure, sheets of corner-sharing octahedra, decorated on top and bottom by [(As,S)O4] and (AsO3OH) tetrahedra, that are linked into a three-dimensional structure by [12]-coordinated Pb2+ cations and hydrogen bonds. A new hydrogen-bonding scheme for the D2+G33+(T5+O4)(TO3OH)(OH)6 minerals is proposed.

The atomic structure of synthetic, deuterated goslarite (ZnSO4·7D2O), a = 11.8176(6) Å, b = 12.0755(7) Å, c = 6.8270(4)Å, space group P212121, Z = 4, has been refined in a combined neutron powder diffraction and X-ray single-crystal data refinement to wRp 1.92%, Rp 1.45% and R(F2) 12.66% for the neutron powder data contribution and R(F2) 8.72% for the X-ray single-crystal data contribution. Both data sets were necessary to achieve the best overall fit agreement in the Rietveld refinement and reasonable geometry within structural units. The results of this study confirm that the H-bonding scheme for goslarite is the same as that of the other epsomite group minerals. Small but significant variations of the Zn–O bond lengths can be attributed to details of the H bonds to the O atoms of the Zn octahedra. This investigation of the atomic structure and hydrogen bonding of goslarite is groundwork for future studies into phase relationships and the mechanisms of hydration and dehydration in the ZnSO4–H2O system.

The equation of state of synthetic deuterated burtite, CaSn(OD)6, has been determined to 7.25 GPa at 298 K by synchrotron X-ray powder diffraction. Fitting to a third-order Birch-Murnaghan equation of state gives K0 = 44.7(9) GPa and K0′ = 5.3(4). A second-order fit gives K0 = 47.4(4) GPa. Within experimental error the two fits are indistinguishable over the pressure range studied. The decrease in the a parameter with pressure is smooth and no phase transitions were observed. Burtite is much more compressible (by a factor of three or four) than CaSnO3 and CdSnO3 perovskites, indicating that the absence of a cavity cation has a major effect upon the compressibility of the octahedral framework. Burtite is also markedly more compressible than the closely-related mineral stottite FeGe(OH)6 (K0 = 78 GPa). Their different compressibilities correlate with the relative compressibilities of stannate and germanate perovskites. Although different octahedral compressions are likely to be the primary reason for the different compressibilities of burtite and stottite, we also consider the possible secondary role of hydrogen-bonding topology in affecting the compressibilities of protonated octahedral frameworks. Burtite and stottite have different hydrogen-bonding topologies due to their different octahedral-tilt system. Burtite, space group Pn and tilt system a+a+a+, has a hydrogen-bonded network of linked four-membered rings of O-H…O linkages, whereas stottite, space group P42/n and tilt system a+a+c−, has <100> O-H…O crankshafts and isolated four-membered rings. These different hydrogen-bonded configurations lead to different bracing of the empty cavity sites by the O-H…O linkages and very different hydrogen-bonding connectivities in these two minerals that may also enhance the difference between their compressibilities.

A single-crystal neutron diffraction study of oxy-dravite from Osarara (Narok district, Kenya) was performed. Intensity data were collected in Laue geometry at 10 K and anisotropic-structure refinement was undertaken. For the first time, two independent H sites were refined unambiguously for a mineral belonging to the tourmaline supergroup and located at 0.26, 0.13, 0.38 (labelled as H3, site occupancy ∼98%) and at 0, 0, 0.9 (labelled as H1, site occupancy ∼25%). The H-bonding scheme can thus be defined as follows: (1) the O at the O3 site acts as a ‘donor’ and the O at the O5 site as ‘acceptor’, the refined O3–H3 bond distance is 0.972(2) Å (and 0.9946 Å corrected for “riding motion”), H3⋯O5 = 2.263(2) Å, O3⋯O5 = 3.179(1) Å and O3–H3⋯O5 = 156.6(1)°; (2) the oxygen at the O1 site acts as a ‘donor’ and the O atoms at O4 and O5 as ‘acceptors’, the refined O1–H1 bond distance is 0.958(8) Å (and 0.9833 Å corrected for “riding motion”), H1⋯O4 = 2.858(6) Å, O1⋯O4 = 3.378(1) Å and O1–H1⋯O4 = 115.12(1)°, whereas H1⋯O5 = 2.886(6) Å, O1⋯O5 = 3.444(1) Å and O1–H1⋯O5 = 118.23(1)°. A further test refinement was performed with the H1 site out of the three-fold axis (at 0.02, 0.01, 0.90); this leads to O1–H1 = 0.995(8) Å (and 1.0112 Å corrected for “riding motion”), H1⋯O4 = 2.747(6) Å and O1–H1⋯O4 = 121.7(4)°, whereas H1⋯O5 = 2.654(9) Å and O1–H1⋯O5 = 136.5(6)°. Bond-valence analysis shows that the H-bonding strength involving O3 is stronger than that involving O1: ∼0.11 and <0.05 valence units, respectively.

The refined angle between the O3–H3 vector and [0001] is 3.40(9)°. Such a small angle is in line with a pleochroic scheme for the OH-stretching absorption bands measured by infrared spectroscopy.

Ikaite is a metastable hexahydrate of calcium carbonate, forming in aqueous conditions near freezing conditions. Neutron powder diffraction data of synthetic deuterated ikaite, collected at seven temperatures from 4 to 270 K, were refined. The linear thermal expansion coefficients are quite anisotropic, being smaller in the direction of the C—O bond. A review of the hydrogen-bonding scheme around the (CaCO03) ion pair is given and an additional weak potential hydrogen bond is suggested. Temperature-dependent growth of the atomic displacement factors of the carbonate O1 atom agrees with the previous suggestion of a possible low-frequency, hindered librational mode of the carbonate group.

The hydrous calcium carbonate mineral ikaite (CaCO3.6H2O) forms in nature at low temperature in carbonate- and Ca-rich waters. Ikaite crystallizes in the spacegroup C2/c, and consists of CaCO3.6H2O units with Ca ions coordinated by eight oxygens, six from H2O and two from the carbonate group. Hydrogen bonding links CaCO3.6H2O moieties to form the crystal structure.

We have used synchrotron X-ray powder diffraction at T = 243 K to refine the monoclinic structure of ikaite, and have measured unit-cell parameters of ikaite between T = 114 K and T = 293 K. Anisotropic thermal expansion in ikaite is evident, with the smallest relative increase occurring along the b direction parallel to 2-fold axes. The contribution of hydrogen bonding to thermal expansion is assessed by comparison of our data with previously published data for deuterated ikaite, ice and gypsum. Ikaite exhibits a coefficient of volume expansion intermediate between that of ice (Ih) and of deuterated gypsum (CaSO4.2D2O) between T = 114 K and T = 293 K.