A new hydrate of magnesium carbonate, MgCO3·6H2O

The formation of a previously unknown higher hydrated magnesium carbonate, MgCO3·6H2O, was confirmed. Its crystal structure differs from the other known magnesium carbonates significantly, but it exhibits similarities to NiCO3·5.5H2O.


Introduction
In the MgO-H 2 O-CO 2 system, besides the thermodynamically stable MgCO 3 (magnesite), a variety of hydrated magnesium carbonates are known, which can be divided in basic magnesium carbonates, containing OH À ions [Mg 5 (CO 3 ) 4 (OH) 2 ÁnH 2 O and Mg(CO 3 )(OH) 2 ÁnH 2 O], and neutral magnesium carbonates with the composition MgCO 3 ÁnH 2 O (Hopkinson et al., 2012;Jauffret et al., 2015). All these phases are of significant relevance in various technological processes, in geological explorations, mineral conversion in the sequestration of CO 2 and in biomineralization (Chaka & Felmy, 2014). Nevertheless, there are many open questions with respect to the conditions of formation, the characterization, the crystal structure and the stability of higher hydrated neutral magnesium carbonates (Hä nchen et al., 2008;Hopkinson et al., 2012;Rincke, 2018).
At lower temperatures, the pentahydrate, i.e. MgCO 3 Á5H 2 O (mineral name: lansfordite), is known. Its crystal structure (monoclinic space group P2 1 /m) was determined by Liu et al. (1990) from a synthetic sample and by Nestola et al. (2017) from a mineral. Several possibilities are described to synthesize lansfordite (Ming & Franklin, 1985;Liu et al., 1990). In order to obtain large prismatic crystals, CO 2 can be bubbled through an aqueous suspension of MgO and, subsequently, the crystallization can be carried out in the filtered solution at low temperature (Liu et al., 1990). However, the authors (Liu et al., 1990) did not provide information about the exact CO 2 pressure, the regime of temperature, the concentration of magnesium ions in the solution and the time needed for crystallization. According to Ming & Franklin (1985), these factors are important to avoid the formation of nesquehonite.
Furthermore, the solubility of lansfordite is highly dependent on temperature and on CO 2 pressure (Kö nigsberger et al., 1999;Takahashi & Hokoku, 1927;Rincke, 2018). Besides that, there are contradictory statements about the temperature and the rate of conversion of the pentahydrate to the trihydrate. Some research groups have recorded a transition temperature between 283.15 and 288.15 K (Takahashi & Hokoku, 1927;Gloss, 1937;Yanaťeva & Rassonskaya, 1961;Hill et al., 1982;Langmuir, 1965;Ming & Franklin, 1985), while others observed the stability of synthesized and natural samples of lansfordite at room temperature over a period of a few months (Liu et al., 1990;Nestola et al., 2017). Neutral magnesium carbonates with a water content greater than five units per formula are not known up to now. Such highly hydrated neutral carbonates of other bivalent metal ions have been found only for calcium, i.e. CaCO 3 Á6H 2 O (ikaite; Hesse et al., 1983), and nickel, i.e. NiCO 3 Á5.5H 2 O (hellyerite; Bette et al., 2016). Our own investigations should elucidate the conditions of formation of the magnesium carbonate hydrates.

Synthesis and crystallization
To obtain crystals of MgCO 3 Á6H 2 O suitable for singlecrystal diffraction analysis (see V11 in Table S1 of the supporting information), carbon dioxide was bubbled through a suspension of magnesium oxide in deionized water (3.1 g, Magnesia M2329, p.a.) for 22 h at 273.15 K. After that, the solution was filtered and stored without stirring at 273.15 K for 16 d until the product crystallized. The product was filtered off for characterization by powder X-ray diffraction and Raman spectroscopy. For intensity data collection, a prismatic crystal of MgCO 3 Á6H 2 O was recovered from a droplet of its mother liquor and mounted rapidly in the cold (200 K) stream of nitrogen gas of the diffractometer.

Powder X-ray diffraction
The powder X-ray diffraction (PXRD) patterns were taken for phase identification with a Bruker D8 Discover laboratory powder diffractometer in the Bragg-Brentano set-up (Cu K 1 radiation, Vantec 1 detector). The samples were prepared as
flat plates and measured at low temperatures (about 273.15 K) with a home-made cooling box (Rincke, 2018).

Raman spectroscopy
The Raman spectra were recorded shortly after synthesis with a Bruker RFS100/S FT spectrometer at room temperature (Nd/YAG laser, wavelength of the laser = 1064 nm).

Refinement
Crystal data, data collection and structure refinement details are given in Table 1. The positions of the H atoms could be located from residual electron-density maxima after further refinement and were refined isotropically.

Conditions of formation and characterization of magnesium carbonate hexahydrate
On the basis of the information of Liu et al. (1990) for the formation of lansfordite, CO 2 was bubbled through aqueous MgO suspensions with various concentrations. After filtration of the solution, the product crystallized at low temperature (273.15-278.15 K), while the CO 2 pressure was decreased by slow degassing of the CO 2 and the solubility product of the carbonate was exceeded. The detailed experimental conditions are given in the supporting information (see Table S1). Characterization of the product with PXRD revealed that nesquehonite is formed at low MgO concentrations, while an unknown phase crystallizes from the solutions at higher Mg 2+ concentrations, near the solubility of lansfordite at p( nigsberger et al., 1999;Rincke, 2018). Fig. 1 shows the PXRD pattern of the new product phase in comparison with the reference data for MgCO 3 Á3H 2 O and MgCO 3 Á5H 2 O. The composition of this unknown phase was determined by singlecrystal diffraction as MgCO 3 Á6H 2 O. The pentahydrate was not found in our investigations.
Large prismatic crystals of MgCO 3 Á6H 2 O were obtained while using a longer time of crystallization of 16 d (see V11 in Table S1 of the supporting information). These crystals, which are partly intergrown, convert in a few minutes at room temperature into the typical needles of MgCO 3 Á3H 2 O (Fig. 2). The process of phase transformation could also be seen by means of Raman spectroscopy (Fig. 3). The assignments of the band positions in comparison with the spectra of nesquehonite and lansfordite are given in the supporting information (Table S2).

Crystal structure of magnesium carbonate hexahydrate
Magnesium carbonate hexahydrate crystallizes in the orthorhombic space group Pbam (No. 55). The Mg1 atom is located on a twofold axis of symmetry. Atoms C1, O1 and O5, and the water molecule H6A-O6-H6B are positioned on a mirror plane.
Isolated pairs of edge-linked Mg(CO 3 ) 2 (H 2 O) 4 octahedra are the main building blocks in the crystal structure (Fig. 4)  Microscopic image of prismatic MgCO 3 Á6H 2 O crystals (framed in black) which are partly intergrown. The red-framed crystals are the typical needles of the transformation product MgCO 3 Á3H 2 O.

Figure 5
Projection of the crystal structure of MgCO 3 Á6H 2 O (a) in the a direction, (b) in the b direction and (c) in the c direction.  (Liu et al., 1990). The MgO octahedra in MgCO 3 Á6H 2 O are slightly distorted ( Table 2).
The carbonate units are linked in a monodentate manner to two magnesium ions across the O1 atom. They are planar and exhibit C 2v symmetry, because the C1-O1 bond is a little longer than the C1-O4 bond. Furthermore, the O1-C1-O4 angle is a little narrower than the O4-C1-O4 iv angle (see Table 2 for numerical data and symmetry code).
The main building blocks are arranged in a sheet-like pattern, perpendicular to the c axis (Figs. 5a and 5b). Within a sheet, every second main building unit is shifted along the [ 1 2 , 1 2 , 0] direction and rotated by 90 . Consequently, a zigzag-like stacking order results (Fig. 5c). The main building units in a sheet are linked by hydrogen bridging bonds ( Fig. 6 and Table 3). The sheets are separated by layers of noncoordinating water molecules in the (001) plane. All the atoms of the noncoordinating H6A-O6-H6B molecule are located in the (001) plane, whereas in the H5-O5-H5 i molecule, only the O5 atom is situated in this plane (Fig. 7). The (001) plane is also the mirror plane of this molecule. The noncoordinating water molecules are linked by hydrogen bridging bonds both in the (001) plane among themselves and with the MgO octahedra. Thus, the crystal structure is three-dimensional crosslinked (Fig. 7).

Comparison with crystal structures of other carbonate hydrates of bivalent metal ions
Other neutral carbonate hydrates of bivalent metal ions with a water content greater than five units per formula are only known for calcium (CaCO 3 Á6H 2 O) and nickel (NiCO 3 Á-5.5H 2 O). Like the title compound, these phases can be synthesized only at low temperatures of about 273.15 K and are transformed at room temperature to CaCO 3 (Coleyshaw et al., 2003) and amorphous nickel carbonate (Bette et al., 2016;Rincke, 2018), respectively.
The crystal structure of CaCO 3 Á6H 2 O is significantly different from that of MgCO 3 Á6H 2 O for the very reason that the coordination number of the cation in CaCO 3 Á6H 2 O is eight and not six as in MgCO 3 Á6H 2 O (Dickens & Brown, 1970;Hesse et al., 1983).
However, the radii of nickel and magnesium ions are very similar and actually the crystal structures of NiCO 3 Á5.5H 2 O and MgCO 3 Á6H 2 O exhibit similarities. Both crystal structures consist of isolated edge-linked pairs of M(CO 3 )(H 2 O) 4 (M = Mg or Ni), which are the main building units and are arranged in sheets, together with noncoordinating water molecules, perpendicular to the c axis. The symmetry of NiCO 3 Á5.5H 2 O is lower; it crystallizes in the monoclinic group P2/n. As a consequence, there are two crystallographically different Ni atoms. In contrast to MgCO 3 Á6H 2 O, in NiCO 3 Á5.5H 2 O, the NiO octahedra of Ni2 are not rotated by 90 (Bette et al., 2016).

Conclusion
A new neutral magnesium carbonate with the previously unknown high water content of six units per formula, i.e. MgCO 3 Á6H 2 O, was produced by passing gaseous CO 2 through an aqueous suspension of MgO and storing the filtered solution at 273.15 K. The X-ray diffraction pattern and Raman spectra confirmed the formation of the new phase and its transformation to MgCO 3 Á3H 2 O. MgCO 3 Á5H 2 O was not found in our study. Obviously, the formation conditions of magnesium carbonate hydrates depend on the concentration of the MgO suspension, the CO 2 pressure, the temperature regime and the time of storage. Therefore, it would be useful to carry out further systematic investigations on the chemical kinetics of the formation of the magnesium carbonate hydrates.   ; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2017); software used to prepare material for publication: publCIF (Westrip, 2010). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.