Crystal structure and characterization of magnesium carbonate chloride hepta­hydrate

Rincke, C.; Schmidt, H.; Buth, G.; Voigt, W.

doi:10.1107/S2053229620008153

research papers

STRUCTURAL
CHEMISTRY

ISSN: 2053-2296

Volume 76| Part 8| August 2020| Pages 741-745

https://doi.org/10.1107/S2053229620008153

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Crystal structure and characterization of magnesium carbonate chloride heptahydrate

Christine Rincke,^a ^* Horst Schmidt,^a Gernot Buth ^b and Wolfgang Voigt ^a

^aInstitute of Inorganic Chemistry, TU Bergakademie Freiberg, Leipziger Strasse 29, D-09599 Freiberg, Germany, and ^bInstitute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
^*Correspondence e-mail: christine.rincke@chemie.tu-freiberg.de

Edited by H. Uekusa, Tokyo Institute of Technology, Japan (Received 24 March 2020; accepted 19 June 2020; online 8 July 2020)

MgCO₃·MgCl₂·7H₂O is the only known neutral magnesium carbonate containing chloride ions at ambient conditions. According to the literature, only small and twinned crystals of this double salt could be synthesised in a concentrated solution of MgCl₂. For the crystal structure solution, single-crystal diffraction was carried out at a synchrotron radiation source. The monoclinic crystal structure (space group Cc) exhibits double chains of MgO octahedra linked by corners, connected by carbonate units and water molecules. The chloride ions are positioned between these double chains parallel to the (100) plane. Dry MgCO₃·MgCl₂·7H₂O decomposes in the air to chlorartinite, Mg₂(OH)Cl(CO₃)·nH₂O (n = 2 or 3). This work includes an extensive characterization of the title compound by powder X-ray diffraction, thermal analysis, SEM and vibrational spectroscopy.

Keywords: magnesium; carbonate; chloride; hydrate; synchrotron; twinning; crystal structure.

CCDC reference: 2010753

1. Introduction

In the context of CO₂ research, the interactions of CO₂ with salts and brine solutions are of great interest. Therefore, the system MgCl₂–MgCO₃–H₂O–CO₂ has been investigated. The only nonbasic salt containing carbonate and chloride ions is MgCO₃·MgCl₂·7H₂O (Rincke, 2018 ).

The formation conditions of MgCO₃·MgCl₂·7H₂O were described for the first time by Gloss (1937 ) and Walter-Levy (1937 ). It can be synthesized at room temperature by adding MgCO₃·3H₂O to a highly concentrated solution of magnesium chloride saturated with CO₂ (Gloss, 1937; Schmidt, 1960 ).

Within the scope of outbursts of CO₂ in potash mines, MgCO₃·MgCl₂·7H₂O was discussed as a storage compound for CO₂ in the 1960s (Schmidt, 1960; Serowy, 1963 ; Serowy & Liebmann, 1964 ; Schmittler, 1964 ; D'Ans, 1967 ). This salt forms needle-like crystals, which are only stable in concentrated MgCl₂ solution (Moshkina & Yaroslavtseva, 1970 ). It decomposes immediately when it is washed with water. When it was stored in air, basic carbonate was formed (Gloss, 1937).

Schmittler (1964) concluded from a powder X-ray diffraction (PXRD) pattern of MgCO₃·MgCl₂·7H₂O that its crystal structure exhibits a C-centred monoclinic lattice with parameters a = 13.27 (0), b = 11.30 (8), c = 9.22 (7) Å and β = 118.2 (6)°. Due to the low scattering power and the small size of the crystals, a crystal structure analysis of single crystals was not possible until now. Our own investigations should provide a better comprehension of the synthesis of MgCO₃·MgCl₂·7H₂O and provide a more detailed characterization, including a crystal structure analysis.

2. Experimental

2.1. Synthesis and crystallization

The synthesis of MgCO₃·MgCl₂·7H₂O is based on the information of Schmidt (1960). MgO (1 g, Magnesia M2329, p.a.) was added to 200 g of an aqueous solution of MgCl₂ (5.5 molal, Fluka, ≥98%). The suspension was stirred for 30 min. Afterwards, the undissolved MgO was filtered off. CO₂ was bubbled through the stirred solution for 24 h at room temperature. The product was filtered off for further characterization.

2.2. Single-crystal diffraction

Data were collected on beamline SCD at the KIT Synchrotron Radiation Source using a Stoe IPDS diffractometer with monochromated radiation of λ = 0.8000 Å. A crystal of MgCO₃·MgCl₂·7H₂O was recovered from a droplet of its mother liquor and mounted rapidly in the cold (150 K) stream of nitrogen gas of the diffractometer.

2.3. Powder X-ray diffraction (PXRD)

PXRD patterns were taken for phase identification with a laboratory Bruker D8 Discover powder diffractometer in Bragg–Brentano set up (Cu Kα₁ radiation, Vantec 1 detector). The samples were prepared as flat plates and measured at room temperature.

2.4. Thermal analysis

The thermal analysis was performed with a TG/DTA 220 instrument from Seiko Instruments (reference substance: Al₂O₃, open platinum crucible; argon flow: 300 ml min⁻¹; heating rate: 5 K min⁻¹, prior period 30 min at 298.15 K in an argon flow).

2.5. Scanning electron microscopy (SEM)

The SEM images were recorded with a TESCAN Vega 5130 SB instrument (20 kV accelerating voltage). The sample was coated with gold.

2.6. Vibrational spectroscopy

For the FT–IR spectrum, a Thermo Scientific Nicolet 380 FTIR spectrometer (spectral resolution: 6 cm⁻¹, 256 scans per measurement) with KBr blanks was used.

The Raman spectrum was recorded shortly after synthesis with a Bruker RFS100/S FT spectrometer at room temperature (Nd/YAG-laser, wavelength of the laser: 1064 nm).

2.7. Refinement

Crystal data, data collection and structure refinement details are given in Table 1. Due to the small crystals and their low scattering power, the crystal structure solution was carried out by single-crystal diffraction at a synchrotron radiation source. The quality of the crystals affected the measured data set with the effect that only reflections to sin θ_max/λ = 0.56 Å⁻¹ could be considered for the structure refinement. The crystal structure was solved by direct methods. The resulting structure solution exhibits a chemically reasonable atomic arrangement, distances, angles and displacement parameters.

Table 1
Experimental details

Crystal data
Chemical formula	MgCO₃·MgCl₂·7H₂O
M_r	305.64
Crystal system, space group	Monoclinic, Cc
Temperature (K)	150
a, b, c (Å)	13.368 (5), 11.262 (5), 9.266 (4)
β (°)	118.83 (3)
V (Å³)	1222.0 (9)
Z	4
Radiation type	Synchrotron, λ = 0.8000 Å
μ (mm⁻¹)	0.93
Crystal size (mm)	0.13 × 0.07 × 0.01 × 0.02 (radius)

Data collection
Diffractometer	Stoe IPDS II
Absorption correction	For a sphere (Coppens, 1970 )
No. of measured, independent and observed [I > 2σ(I)] reflections	8746, 6975, 5476
R_int	0.0613
θ_max (°)	26.7
(sin θ/λ)_max (Å⁻¹)	0.561

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.053, 0.161, 1.12
No. of reflections	4791
No. of parameters	179
No. of restraints	22
H-atom treatment	Only H-atom coordinates refined
Δρ_max, Δρ_min (e Å⁻³)	0.36, −0.43
Absolute structure	Flack x determined using 647 quotients [(I⁺) − (I⁻)]/[(I⁺) + (I⁻)] (Parsons et al., 2013 )
Absolute structure parameter	0.43 (13)
Computer programs: X-AREA (Stoe & Cie, 2015 ), X-RED (Stoe & Cie, 2015), SHELXS97 (Sheldrick, 2008 ), SHELXL2016 (Sheldrick, 2015 ), DIAMOND (Brandenburg, 2017 ) and publCIF (Westrip, 2010 ).

H atoms were placed in the positions indexed by difference Fourier maps and their U_iso values were set at 1.2U_eq(O) using a riding-model approximation.

The crystal exhibits nonmerohedral twinning. The matrix that relates the individual diffraction pattern was determined as (1 0 1.38, 0 −1 0, 0 0 −1). The reflections of both domains were integrated (number of reflections in domain 1: 2829; domain 2: 3505; overlaid: 641; major twin component fraction: 56.45%).

3. Results and discussion

3.1. Characterization of magnesium carbonate chloride heptahydrate

The characterization of the unwashed product with PXRD is in accordance with the reference pattern PDF 21-1254 for MgCO₃·MgCl₂·7H₂O (Schmittler, 1964). The filtered product was stored in a sealed vessel. After 19 months, the powder pattern remained constant, i.e. the product did not alter. If the product was washed with ethanol and stored in the air, decomposition to chlorartinite [Mg₂(OH)Cl(CO₃)·3H₂O] begins within a few days (Fig. 1). This observation confirms the information of Gloss (1937).

Figure 1
Powder XRD patterns of MgCO₃·MgCl₂·7H₂O under ambient conditions (Cu Kα₁ radiation) for (a) the unwashed product immediately after the synthesis, (b) the unwashed product stored in the air after 19 months, (c) the product washed with ethanol after storage in the air for 10 d and (d) the product washed with ethanol after storage in the air for 19 months. Reference data: MgCO₃·MgCl₂·7H₂O (PDF 21-1254) and Mg₂(OH)Cl(CO₃)·3H₂O (PDF 07-0278).

The thermal decomposition of MgCO₃·MgCl₂·7H₂O starts as early as the heating begins and shows two main steps (Fig. 2). H₂O, CO₂ and HCl are evaporated off. This is in accordance with the observation of Serowy & Liebmann (1964). A precise assignment of the stepwise mass loss is not possible. The characterization of the residue with PXRD at 573 K exhibits the presence of a mixture of basic magnesium carbonates, i.e. hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O] and amorphous phases. At 803 K the decomposition is complete and only MgO remains in the residue. The observed mass loss of 74.3 (1)% confirms the theoretical mass loss of 73.6%.

Figure 2
Thermal analysis of MgCO₃·MgCl₂·7H₂O.

The SEM images of MgCO₃·MgCl₂·7H₂O show thin needles (50 × 5 µm), which are twinned or even more intergrown (Fig. 3). Numerous crystallization experiments with the aim of obtaining larger crystals were not successful.

Figure 3
SEM images of MgCO₃·MgCl₂·7H₂O, with the crystals exhibiting twinning or even further intergrowth.

The FT–IR (Fig. 4) and Raman spectra (Fig. 5) of MgCO₃·MgCl₂·7H₂O confirm the absence of hydroxide ions in the crystal structure, because there are no bands above 3500 cm⁻¹ as in chlorartinite, Mg₂(OH)Cl(CO₃)·3H₂O (Vergasova et al., 1998 ). The assignment of the bands was concluded from a comparison with the vibrational spectra of other neutral magnesium carbonates and chlorartinite (Coleyshaw et al., 2003 ; Vergasova et al., 1998) (Table 2).

Table 2
Assignment of the IR and Raman bands of MgCO₃·MgCl₂·7H₂O

IR	Raman	Assignment (Coleyshaw et al., 2003)
3407, 3240	3386, 3250	ν(OH)_W
1635	1660	δ(OH)_W
1550, 1449, 1401	1544	ν_as(CO)
1114	1111	ν_s(CO)
845	794	γ(CO)
620	599	δ_as(CO)
457	403, 227, 181, 154, 124	lattice vibrations
Notes: ν = valence vibration, δ = deformation vibration (in the plane), γ = deformation vibration out of the plane, W = water, s = symmetric and as = asymmetric.

Figure 4
IR spectrum of MgCO₃·MgCl₂·7H₂O under ambient conditions.

Figure 5
Raman spectrum of MgCO₃·MgCl₂·7H₂O under ambient conditions.

3.2. Crystal structure of magnesium carbonate chloride heptahydrate

The monoclinic crystal structure of MgCO₃·MgCl₂·7H₂O with the space group Cc and the lattice parameters published by Schmittler (1964) were confirmed. There are two distinguishable magnesium ions. Mg1 is coordinated by three water molecules and two carbonate anions. One carbonate acts as a monodentate ligand via atom O9 and the other as a bidentate ligand via atoms O2 and O6. The octahedra of Mg2 are formed by four water molecules and two carbonate units which are connected to the magnesium ion in a monodentate manner via atoms O2 and O6 (Fig. 6). The corner-linked Mg–O octahedra are arranged in a zigzag manner and together with the carbonate units form double chains parallel to the (100) plane (Fig. 7).

Figure 6
The asymmetric unit and coordination units of MgCO₃·MgCl₂·7H₂O [symmetry codes: (i) x, −y, z − $[{1\over 2}]$ ; (ii) x, y, z − 1; (iii) x, −y, z + $[{1\over 2}]$ ; (iv) x, y, z + 1].

Figure 7
The characteristic structural motif in MgCO₃·MgCl₂·7H₂O, showing the double chain of MgO octahedra linked by corners and carbonate units parallel to the (100) plane.

All the carbonate units are crystallographically equivalent and exhibit a C_s geometry, because they are planar, but the C—O bonds have different lengths. Each carbonate unit is coordinated by three magnesium ions: monodentate to Mg1, bidentate to Mg1ⁱ and monodentate to Mg2ⁱⁱ (see Fig. 6 for symmetry codes). In addition, the carbonate units stabilize the double chains (Fig. 7).

Between the double chains, which are arranged in a zigzag-like stacking order parallel to the (001) plane, are located the chloride ions Cl1 and Cl2 (Fig. 8). The positions of atoms H1A and H3B are fixed by short hydrogen bonds to atoms O9^iv and O4^vi, and the other H atoms by interactions with the chloride ions (Table 3 and Fig. 9). As a consequence, a three-dimensional network is formed.

Table 3
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1A⋯O9^iv	0.82 (3)	1.94 (6)	2.688 (14)	153 (13)
O1—H1B⋯Cl2^iv	0.82 (3)	2.38 (4)	3.186 (11)	167 (12)
O3—H3A⋯Cl2^v	0.82 (3)	2.32 (3)	3.135 (11)	174 (17)
O3—H3B⋯O4^vi	0.82 (3)	2.10 (11)	2.796 (13)	143 (17)
O4—H4A⋯Cl1^vii	0.82 (3)	2.36 (3)	3.176 (10)	171 (14)
O4—H4B⋯Cl1^viii	0.82 (3)	2.49 (7)	3.251 (10)	155 (13)
O5—H5A⋯Cl2^viii	0.82 (3)	2.45 (7)	3.222 (11)	157 (16)
O5—H5B⋯Cl1^ix	0.81 (3)	2.54 (6)	3.327 (11)	164 (17)
O7—H7A⋯Cl2^viii	0.82 (3)	2.42 (6)	3.212 (11)	163 (16)
O7—H7B⋯Cl1^x	0.82 (3)	2.30 (4)	3.111 (11)	169 (16)
O8—H8A⋯Cl2^vi	0.82 (3)	2.46 (5)	3.254 (10)	163 (11)
O8—H8B⋯Cl2^v	0.82 (3)	2.41 (3)	3.233 (10)	178 (11)
O10—H10A⋯Cl1^vii	0.82 (3)	2.34 (5)	3.146 (10)	166 (15)
O10—H10B⋯Cl1^xi	0.82 (3)	2.67 (7)	3.424 (10)	154 (12)
Symmetry codes: (iv) x, y, z + 1; (v) x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , z + 1; (vi) x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (vii) x, y − 1, z; (viii) x, −y + 1, z + $[{1\over 2}]$ ; (ix) x + $[{1\over 2}]$ , −y + $[{3\over 2}]$ , z + $[{1\over 2}]$ ; (x) x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , z; (xi) x, −y + 1, z − $[{1\over 2}]$ .

Figure 8
Excerpt of the crystal structure of MgCO₃·MgCl₂·7H₂O, showing the zigzag-like stacking order of the double chains and the chloride ions between them.

Figure 9
Excerpt of the crystal structure of MgCO₃·MgCl₂·7H₂O, showing the hydrogen-bond interactions of the H atoms with chloride ions (dashed lines) [symmetry codes: (iv) x, y, z + 1; (v) x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , z + 1; (vi) x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (vii) x, y − 1, z; (viii) x, −y + 1, z + $[{1\over 2}]$ ; (ix) x + $[{1\over 2}]$ , −y + $[{3\over 2}]$ , z + $[{1\over 2}]$ ; (x) x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , z; (xi) x, −y + 1, z − $[{1\over 2}]$ ].

The structural motifs of such double chains are similar in MgCO₃·MgCl₂·7H₂O and MgCO₃·3H₂O (Giester et al., 2000 ), but in contrast to MgCO₃·3H₂O in MgCO₃·MgCl₂·7H₂O, only two of three carbonate units and three and four water molecules instead of two water molecules are linked to each Mg atom. Furthermore, no free water molecules are positioned between these double chains in MgCO₃·MgCl₂·7H₂O. The crystal structures of other neutral magnesium carbonates, e.g. MgCO₃·5H₂O, MgCO₃·6H₂O and the chloride-containing magnesium carbonates Mg₂(OH)Cl(CO₃)·2H₂O (chlorartinite) and Mg₂(OH)Cl(CO₃)·H₂O (dehydrated clorartinite), do not exhibit such double chains (Liu et al., 1990 ; Rincke et al., 2020 ; Sugimoto et al., 2006 , 2007 ). Therefore, the crystal structure of MgCO₃·MgCl₂·7H₂O is unique.

Supporting information

CCDC reference: 2010753

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2053229620008153/uk3196sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2053229620008153/uk3196Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2053229620008153/uk3196Isup3.cml

Computing details top

Data collection: X-AREA (Stoe & Cie, 2015); cell refinement: X-AREA (Stoe & Cie, 2015); data reduction: X-RED (Stoe & Cie, 2015); 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).

Magnesium carbonate chloride heptahydrate top

Crystal data top

MgCO₃·MgCl₂·7H₂O	F(000) = 632
M_r = 305.64	D_x = 1.661 Mg m⁻³
Monoclinic, Cc	Synchrotron radiation, λ = 0.8000 Å
a = 13.368 (5) Å	Cell parameters from 4192 reflections
b = 11.262 (5) Å	θ = 2.7–29.5°
c = 9.266 (4) Å	µ = 0.93 mm⁻¹
β = 118.83 (3)°	T = 150 K
V = 1222.0 (9) Å³	Needle, colourless
Z = 4	0.13 × 0.07 × 0.01 × 0.02 (radius) mm

Data collection top

Stoe IPDS II diffractometer	5476 reflections with I > 2σ(I)
Radiation source: synchrotron	R_int = 0.061
rotation method scans	θ_max = 26.7°, θ_min = 3.3°
Absorption correction: for a sphere (Coppens, 1970)	h = −14→14
	k = −12→12
8746 measured reflections	l = −10→10
6975 independent reflections

Refinement top

Refinement on F²	Hydrogen site location: difference Fourier map
Least-squares matrix: full	Only H-atom coordinates refined
R[F² > 2σ(F²)] = 0.053	w = 1/[σ²(F_o²) + (0.094P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.161	(Δ/σ)_max < 0.001
S = 1.12	Δρ_max = 0.36 e Å⁻³
4791 reflections	Δρ_min = −0.43 e Å⁻³
179 parameters	Absolute structure: Flack x determined using 647 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013)
22 restraints	Absolute structure parameter: 0.43 (13)
Primary atom site location: structure-invariant direct methods

Special details top

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.

Refinement. Refined as a 2-component twin

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Mg1	0.4801 (4)	0.1376 (3)	0.5771 (6)	0.0178 (9)
Mg2	0.4708 (3)	0.1837 (4)	0.9931 (5)	0.0182 (10)
Cl1	0.1825 (3)	0.9715 (3)	0.5763 (4)	0.0262 (9)
Cl2	0.2944 (3)	0.5078 (3)	0.0497 (4)	0.0249 (8)
C1	0.4795 (10)	0.0190 (12)	0.2714 (13)	0.017 (3)
O1	0.4661 (9)	0.2884 (9)	1.1718 (12)	0.028 (2)
H1B	0.419 (11)	0.341 (10)	1.152 (18)	0.034*
H1A	0.468 (13)	0.250 (11)	1.247 (14)	0.034*
O2	0.4771 (7)	0.0395 (8)	1.1333 (10)	0.0196 (19)
O3	0.6440 (9)	0.1968 (9)	1.1050 (12)	0.028 (2)
H3B	0.680 (13)	0.211 (16)	1.203 (6)	0.034*
H3A	0.688 (12)	0.153 (13)	1.09 (2)	0.034*
O4	0.2896 (8)	0.1586 (9)	0.8686 (11)	0.025 (2)
H4B	0.264 (12)	0.148 (13)	0.932 (14)	0.029*
H4A	0.255 (12)	0.112 (11)	0.792 (12)	0.029*
O5	0.4634 (9)	0.3447 (9)	0.8750 (12)	0.029 (2)
H5A	0.433 (12)	0.371 (12)	0.781 (7)	0.034*
H5B	0.509 (12)	0.395 (10)	0.933 (13)	0.034*
O6	0.4774 (8)	0.0931 (8)	0.8055 (12)	0.0184 (19)
O7	0.4802 (9)	0.3153 (9)	0.5418 (13)	0.028 (2)
H7A	0.427 (8)	0.361 (9)	0.52 (2)	0.034*
H7B	0.536 (7)	0.358 (10)	0.57 (2)	0.034*
O8	0.6599 (8)	0.1449 (9)	0.7010 (12)	0.025 (2)
H8B	0.694 (12)	0.111 (12)	0.790 (8)	0.029*
H8A	0.686 (11)	0.116 (12)	0.645 (12)	0.029*
O9	0.4820 (8)	0.1034 (8)	0.3651 (11)	0.022 (2)
O10	0.3036 (8)	0.1571 (9)	0.4583 (11)	0.024 (2)
H10B	0.278 (11)	0.150 (13)	0.359 (5)	0.029*
H10A	0.262 (10)	0.117 (12)	0.481 (14)	0.029*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Mg1	0.023 (2)	0.017 (2)	0.0156 (18)	−0.0018 (19)	0.0114 (15)	−0.004 (2)
Mg2	0.022 (2)	0.019 (2)	0.0152 (18)	0.0026 (17)	0.0102 (16)	0.0000 (17)
Cl1	0.0248 (16)	0.0273 (19)	0.0302 (17)	−0.0052 (14)	0.0162 (14)	−0.0031 (15)
Cl2	0.0242 (15)	0.0255 (16)	0.0257 (15)	0.0023 (14)	0.0126 (12)	0.0018 (14)
C1	0.021 (6)	0.023 (8)	0.010 (7)	0.001 (5)	0.008 (6)	−0.009 (6)
O1	0.048 (6)	0.021 (5)	0.024 (5)	0.008 (4)	0.023 (5)	0.009 (4)
O2	0.027 (5)	0.017 (5)	0.017 (4)	0.005 (4)	0.013 (4)	−0.003 (4)
O3	0.030 (6)	0.032 (6)	0.023 (5)	0.005 (4)	0.013 (5)	−0.001 (4)
O4	0.028 (5)	0.030 (5)	0.017 (5)	−0.002 (4)	0.012 (4)	0.001 (4)
O5	0.040 (6)	0.023 (5)	0.020 (5)	−0.006 (4)	0.013 (4)	−0.001 (4)
O6	0.025 (5)	0.016 (5)	0.017 (4)	0.000 (4)	0.012 (4)	0.002 (4)
O7	0.030 (5)	0.022 (5)	0.038 (7)	0.006 (4)	0.021 (5)	0.004 (4)
O8	0.024 (5)	0.029 (5)	0.020 (5)	0.003 (4)	0.010 (4)	−0.003 (4)
O9	0.034 (6)	0.018 (5)	0.020 (5)	0.002 (4)	0.018 (4)	0.005 (4)
O10	0.025 (5)	0.029 (6)	0.019 (5)	0.001 (4)	0.012 (4)	0.002 (4)

Geometric parameters (Å, º) top

Mg1—O9	2.014 (10)	Mg2—O2	2.055 (10)
Mg1—O7	2.028 (11)	Mg2—O1	2.058 (11)
Mg1—O2ⁱ	2.066 (9)	Mg2—O5	2.095 (11)
Mg1—O10	2.079 (11)	Mg2—O4	2.140 (11)
Mg1—O8	2.107 (11)	C1—O9	1.277 (15)
Mg1—O6	2.192 (10)	C1—O2ⁱⁱ	1.285 (15)
Mg2—O3	2.036 (11)	C1—O6ⁱ	1.305 (17)
Mg2—O6	2.054 (11)

O9—Mg1—O7	91.7 (4)	O3—Mg2—O1	91.0 (5)
O9—Mg1—O2ⁱ	94.2 (4)	O6—Mg2—O1	174.8 (5)
O7—Mg1—O2ⁱ	174.1 (5)	O2—Mg2—O1	87.3 (4)
O9—Mg1—O10	92.8 (4)	O3—Mg2—O5	87.6 (4)
O7—Mg1—O10	84.2 (4)	O6—Mg2—O5	89.9 (4)
O2ⁱ—Mg1—O10	94.6 (4)	O2—Mg2—O5	172.2 (4)
O9—Mg1—O8	89.5 (4)	O1—Mg2—O5	85.0 (4)
O7—Mg1—O8	87.7 (4)	O3—Mg2—O4	176.1 (5)
O2ⁱ—Mg1—O8	93.2 (4)	O6—Mg2—O4	88.7 (4)
O10—Mg1—O8	171.6 (4)	O2—Mg2—O4	85.8 (4)
O9—Mg1—O6	155.8 (4)	O1—Mg2—O4	92.5 (5)
O7—Mg1—O6	112.5 (4)	O5—Mg2—O4	94.4 (4)
O2ⁱ—Mg1—O6	61.6 (4)	O3—Mg2—Mg1ⁱⁱⁱ	92.8 (3)
O10—Mg1—O6	89.6 (4)	O6—Mg2—Mg1ⁱⁱⁱ	71.4 (3)
O8—Mg1—O6	91.6 (4)	O2—Mg2—Mg1ⁱⁱⁱ	26.5 (3)
O9—Mg1—C1ⁱⁱⁱ	124.7 (4)	O1—Mg2—Mg1ⁱⁱⁱ	113.8 (3)
O7—Mg1—C1ⁱⁱⁱ	143.6 (5)	O5—Mg2—Mg1ⁱⁱⁱ	161.2 (3)
O2ⁱ—Mg1—C1ⁱⁱⁱ	30.5 (4)	O4—Mg2—Mg1ⁱⁱⁱ	84.2 (3)
O10—Mg1—C1ⁱⁱⁱ	93.4 (4)	O9—C1—O2ⁱⁱ	121.5 (12)
O8—Mg1—C1ⁱⁱⁱ	92.0 (4)	O9—C1—O6ⁱ	123.5 (10)
O6—Mg1—C1ⁱⁱⁱ	31.1 (4)	O2ⁱⁱ—C1—O6ⁱ	115.0 (10)
O9—Mg1—Mg2ⁱ	67.8 (3)	O9—C1—Mg1ⁱ	175.9 (10)
O7—Mg1—Mg2ⁱ	159.4 (3)	O2ⁱⁱ—C1—Mg1ⁱ	54.7 (6)
O2ⁱ—Mg1—Mg2ⁱ	26.4 (2)	O6ⁱ—C1—Mg1ⁱ	60.3 (6)
O10—Mg1—Mg2ⁱ	94.5 (3)	C1^iv—O2—Mg2	138.1 (8)
O8—Mg1—Mg2ⁱ	93.9 (3)	C1^iv—O2—Mg1ⁱⁱⁱ	94.7 (8)
O6—Mg1—Mg2ⁱ	88.0 (3)	Mg2—O2—Mg1ⁱⁱⁱ	127.1 (4)
C1ⁱⁱⁱ—Mg1—Mg2ⁱ	56.9 (3)	C1ⁱⁱⁱ—O6—Mg2	134.5 (8)
O3—Mg2—O6	87.9 (4)	C1ⁱⁱⁱ—O6—Mg1	88.5 (7)
O3—Mg2—O2	92.6 (4)	Mg2—O6—Mg1	137.0 (5)
O6—Mg2—O2	97.9 (4)	C1—O9—Mg1	142.8 (8)

Symmetry codes: (i) x, −y, z−1/2; (ii) x, y, z−1; (iii) x, −y, z+1/2; (iv) x, y, z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O9^iv	0.82 (3)	1.94 (6)	2.688 (14)	153 (13)
O1—H1B···Cl2^iv	0.82 (3)	2.38 (4)	3.186 (11)	167 (12)
O3—H3A···Cl2^v	0.82 (3)	2.32 (3)	3.135 (11)	174 (17)
O3—H3B···O4^vi	0.82 (3)	2.10 (11)	2.796 (13)	143 (17)
O4—H4A···Cl1^vii	0.82 (3)	2.36 (3)	3.176 (10)	171 (14)
O4—H4B···Cl1^viii	0.82 (3)	2.49 (7)	3.251 (10)	155 (13)
O5—H5A···Cl2^viii	0.82 (3)	2.45 (7)	3.222 (11)	157 (16)
O5—H5B···Cl1^ix	0.81 (3)	2.54 (6)	3.327 (11)	164 (17)
O7—H7A···Cl2^viii	0.82 (3)	2.42 (6)	3.212 (11)	163 (16)
O7—H7B···Cl1^x	0.82 (3)	2.30 (4)	3.111 (11)	169 (16)
O8—H8A···Cl2^vi	0.82 (3)	2.46 (5)	3.254 (10)	163 (11)
O8—H8B···Cl2^v	0.82 (3)	2.41 (3)	3.233 (10)	178 (11)
O10—H10A···Cl1^vii	0.82 (3)	2.34 (5)	3.146 (10)	166 (15)
O10—H10B···Cl1^xi	0.82 (3)	2.67 (7)	3.424 (10)	154 (12)

Symmetry codes: (iv) x, y, z+1; (v) x+1/2, y−1/2, z+1; (vi) x+1/2, −y+1/2, z+1/2; (vii) x, y−1, z; (viii) x, −y+1, z+1/2; (ix) x+1/2, −y+3/2, z+1/2; (x) x+1/2, y−1/2, z; (xi) x, −y+1, z−1/2.

Assignment of the IR- and Raman-bands of MgCO₃·MgCl₂·7H₂O top

IR	Raman	Assignment (Coleyshaw et al., 2003)
3407, 3240	3386, 3250	ν(OH)_W
1635	1660	δ(OH)_W
1550, 1449, 1401	1544	ν^as(CO)
1114	1111	ν^s(CO)
845	794	γ(CO)
620	599	δ^as(CO)
457	403, 227, 181, 154, 124	lattice vibrations

Notes: ν valence vibration, δ deformation vibration (in the plane), γ deformation vibration out of the plane, W = water, s = symmetric and as = asymmetric.

Hydrogen-bond geometry (Å, °) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O9^iv	0.82 (3)	1.94 (6)	2.688 (14)	153 (13)
O1—H1B···Cl2^iv	0.82 (3)	2.38 (4)	3.186 (11)	167 (12)
O3—H3A···Cl2^v	0.82 (3)	2.32 (3)	3.135 (11)	174 (17)
O3—H3B···O4^vi	0.82 (3)	2.10 (11)	2.796 (13)	143 (17)
O4—H4A···Cl1^vii	0.82 (3)	2.36 (3)	3.176 (10)	171 (14)
O4—H4B···Cl1^viii	0.82 (3)	2.49 (7)	3.251 (10)	155 (13)
O5—H5A···Cl2^viii	0.82 (3)	2.45 (7)	3.222 (11)	157 (16)
O5—H5B···Cl1^ix	0.81 (3)	2.54 (6)	3.327 (11)	164 (17)
O7—H7A···Cl2^viii	0.82 (3)	2.42 (6)	3.212 (11)	163 (16)
O7—H7B···Cl1^x	0.82 (3)	2.30 (4)	3.111 (11)	169 (16)
O8—H8A···Cl2^vi	0.82 (3)	2.46 (5)	3.254 (10)	163 (11)
O8—H8B···Cl2^v	0.82 (3)	2.41 (3)	3.233 (10)	178 (11)
O10—H10A···Cl1^vii	0.82 (3)	2.34 (5)	3.146 (10)	166 (15)
O10—H10B···Cl1^xi	0.82 (3)	2.67 (7)	3.424 (10)	154 (12)

Symmetry codes: (iv) x, y, z+1; (v) x+1/2, y-1/2, z+1; (vii) x+1/2, -y+1/2, z+1/2; (vii) x, y-1, z; (viii) x, -y+1, z+1/2; (ix) x+1/2, -y+3/2, z+1/2; (x) x+1/2, y-1/2, z; (xi) x, -y+1, z-1/2.

Acknowledgements

The award of synchrotron beamtime at KIT Synchrotron Radiation Source, Karlsruhe, Germany, is gratefully acknowledged.

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