The crystal structures of Fe-bearing MgCO3 sp 2- and sp 3-carbonates at 98 GPa from single-crystal X-ray diffraction using synchrotron radiation.

The crystal structure of MgCO3-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp 2 to the sp 3 type of bonding. We have now determined the crystal structure of iron-bearing MgCO3-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg0.85Fe0.15)CO3 single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg2.53Fe0.47)C3O9 in the space group C2/m that contains tetra-hedrally coordinated carbon, where CO4 4- tetra-hedra are linked by corner-sharing oxygen atoms to form three-membered C3O9 6- ring anions. The crystal structure of (Mg0.85Fe0.15)CO3 (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles. © Chariton et al. 2020.

Chemical context

Carbonates and their high-pressure behaviour have attracted significant inter­est because of their potential role as carbon-bearing phases in the deep Earth. Recent discoveries of novel compounds that contain tetra­hedral CO4 4− units (e.g., Merlini et al., 2015 ▸; Cerantola et al., 2017 ▸) increase the relevance of such studies, as the new high-pressure phases may be stable at conditions prevalent in the deep part of Earth’s lower mantle. In addition, theoretical modelling predictions imply potential structural analogues of CO4 4−-bearing carbonates and silicates, and thus carbonates with tetra­hedrally coordinated carbon may be important to understanding the complex geochemistry of Earth’s mantle.

Carbonates with tetra­hedrally coordinated carbon are not well characterized, despite their potential significance, as structural studies have to be carried out under high-pressure conditions and are therefore challenging. A reliable structural characterization is, however, a prerequisite for determining phase stabilities and to understand, for example, why the p,T-phase diagram of MgCO3 is relatively simple compared to the dense phase diagram of CaCO3 (see summary in Bayarjargal et al., 2018 ▸).

It is generally accepted that magnesite (MgCO3) transforms to MgCO3-II at 80–115 GPa (Isshiki et al., 2004 ▸; Boulard et al., 2011 ▸,2015 ▸; Maeda et al., 2017 ▸). Models based on density functional theory (DFT) (Oganov et al., 2008 ▸) and inter­pretation of X-ray diffraction data and IR spectra imply that MgCO3-II contains carbon in a tetra­hedral coordination (Boulard et al. 2011 ▸, 2015 ▸). While structure-prediction techniques are undoubtedly useful for preliminary surveys of phase stabilities, they provide a range of possible new phases, derived under constraints such as unit-cell contents. Powder diffraction data obtained at pressures around 100 GPa generally do not yield accurate structure determinations, and typically do not allow unambiguous assignment of the space group or site occupancies. In contrast, single-crystal X-ray diffraction is a powerful and unique tool that can provide accurate structure refinements under these conditions (Boffa Ballaran et al., 2013 ▸). Well-established statistical parameters allow an assessment of the reliability of the structural model. Other carbonate structures with tetra­hedral CO4 4− units at extreme conditions have previously been reported using this method, such as the novel phases Fe4C3O12 in space group R3c, (Mg,Fe)4C4O13 in C2/c (Merlini et al., 2015 ▸; Cerantola et al., 2017 ▸) and Ca(Fe,Mg)2C3O9 in Pnma (Merlini et al., 2017 ▸). These results lead to two conclusions. Firstly, the stability fields of carbonates strongly depend on their composition. Secondly, CO4 4− units have the ability to form polymeric networks, and thus are potential analogues to silicates.

Structural commentary

Under ambient conditions (Mg0.85Fe0.15)CO3 crystallizes in the calcite-type structure in space group R c. Iron and magnesium share the same crystallographic site (Wyckoff position 6b; site symmetry .) and are coordinated by six oxygen atoms, while the CO3 2− units form planar equilateral triangles with point-group symmetry 32 (e.g. Lavina et al., 2010 ▸). After compression to 98 (2) GPa at ambient temperature, X-ray diffraction data of (Mg0.85Fe0.15)CO3 can still be indexed in the R c space group (Fig. 1 ▸, Table 1 ▸). However, the unit-cell volume is decreased by nearly 32% compared to ambient conditions. This result challenges a recent suggestion based on DFT-based calculations that predicted a structural transformation of MgCO3 to a triclinic phase at 85–101 GPa and 300 K (Pickard & Needs, 2015 ▸). At 98 GPa, the C—O bond length [1.195 (8) Å] has decreased only by ∼7% compared to the structure at ambient conditions, thus reflecting the highly incompressible nature of the CO3 2− units. On the other hand, the (Mg/Fe)—O bonds [1.855 (5) Å at 98 GPa] display a much more compressible behavior (∼12% bond-length and ∼32% octa­hedra-volume shrinkage compared to ambient conditions). On a last note, it is well known that rhombohedral carbonates can be described as a distortion of the NaCl (B1) structure. Previously, the t parameter, , where a and c are the lattice parameters) has been used to evaluate the degree of distortion (Gao et al., 2014 ▸). We observed that at 98 GPa and 300 K, t ≃1 for (Mg0.85Fe0.15)CO3, which means that at the conditions of our experiment the (Mg/Fe) cations and the CO3 2− anions are arranged in the manner of a nearly ideal NaCl (B1) structure.

Figure 1

Crystal structure of (Mg0.85Fe0.15)CO3 at 98 GPa and prior to laser-heating shown in a projection along the c axis. The building blocks of the unit cell appear on the right. Here, iron occupies the same sites as the magnesium atoms.

Table 1

Experimental details

	MgCO3-II at 98 GPa	MgCO3 at 98 GPa
Crystal data
Chemical formula	3[(Mg0.85Fe0.15)CO3]	(Mg0.85Fe0.15)CO3
M r	265.6	89
Crystal system, space group	Monoclinic, C2/m	Trigonal, R c
Temperature (K)	293	293
a, b, c (Å)	8.238 (3), 6.5774 (12), 6.974 (5)	4.281 (7), 4.281 (7), 12.12 (2)
α, β, γ (°)	90, 104.40 (6), 90	90, 90, 120
V (Å3)	366.0 (3)	192.3 (5)
Z	4	6
Radiation type	Synchrotron, λ = 0.41107 Å	Synchrotron, λ = 0.2952 Å
μ (mm−1)	0.58	0.25
Crystal size (mm)	0.01 × 0.01 × 0.01	0.01 × 0.01 × 0.01
Data collection
Diffractometer	ID15b @ ESRF	13IDD @ APS (GSECARS)
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2019 ▸)	Multi-scan (CrysAlis PRO; Rigaku OD, 2019 ▸)
T min, T max	0.104, 1	0.95, 1
No. of measured, independent and observed [I > 3σ(I)] reflections	522, 298, 211	176, 60, 33
R int	0.020	0.053
(sin θ/λ)max (Å−1)	0.860	0.900
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.084, 0.095, 3.21	0.100, 0.084, 2.89
No. of reflections	298	60
No. of parameters	39	5
Δρmax, Δρmin (e Å−3)	1.76, −1.21	0.66, −0.50

Computer programs: CrysAlis PRO (Rigaku OD, 2019 ▸), SUPERFLIP (Palatinus & Chapuis, 2007 ▸), JANA2006 (Petříček et al., 2014 ▸), VESTA (Momma & Izumi, 2011 ▸) and publCIF (Westrip, 2010 ▸).

After annealing at 2500 K and 98 GPa, we observed a phase transition to a polymorph in which carbon is tetra­hedrally coordinated by oxygen. The newly formed phase with chemical formula (Mg2.53Fe0.47)C3O9 (as determined from structural refinements, see below) has monoclinic symmetry, and the diffraction pattern indicates space group C2/m (Fig. 2 ▸, Table 1 ▸). We identify this phase as the MgCO3-II structure that was previously predicted (Oganov et al., 2008 ▸; Boulard et al., 2015 ▸). In contrast to previous studies, we provide an accurate structure solution and refinement based on single crystal X-ray diffraction data. The structure consists of three-membered C3O9 6− rings formed by corner-sharing CO4 tetra­hedra (Fig. 2 ▸ c) that alternate with [Fe,Mg]O polyhedra (x = 6–8) perpendicular to the b axis. We can distinguish three crystallographic cation positions (Fig. 2 ▸ b):

Figure 2

(a) The crystal structure of (Mg2.53Fe0.47)C3O9 according to this study, in a projection along the c axis; CO4 tetra­hedra are given in the polyhedral representation. (b) The three cation sites that host Mg/Fe atoms and their respective polyhedra. (c) C3O9 6− ring anions are formed from three edge-sharing CO4 tetra­hedra. Atomic positions are shaded according to colours in (b) and oxygen atoms appear as small white spheres. [Symmetry codes: (i) x, −y, z; (v) −x + , −y + , −z + 1; (x) −x, y, −z + 1; (xi) x, y, z + 1.]

(1) The M1 site is located on a twofold rotation axis (Wyckoff position 4g) and is occupied by Mg and Fe in a 0.917 (17):0.083 (17) ratio. This site is surrounded by eight oxygen atoms forming a distorted square anti­prism (dark blue); (2) The M3 site is situated on a mirror plane (4 i) in a 0.61 (2):0.39 (2) Mg:Fe ratio and a coordination number of 10 (blue; can be described as half cubocta­hedra merged through hexa­gonal-based faces with hexa­gonal pyramids); (3) M2 is likewise situated on a mirror plane (4 i) and is fully occupied by Mg in [MgO6] octa­hedra (magenta). The maximum and minimum bond lengths of each cation site from its neighbouring oxygen atoms are shown in Table 2 ▸. At 98 GPa the C—O bond lengths of the two different CO4 4− carbonate groups [C1 is located on a general site (8 j) and C2 on a mirror plane (4 i) vary from 1.287 (18)–1.409 (13) Å and the C—O—C inter-tetra­hedral angle is ∼112°.

Table 2

Geometric parameters of (Mg2.53Fe0.47)C3O9 at 98 GPa

Group	Maximal bond length (Å)	Minimal bond length (Å)	Polyhedron volume (Å3)	Distortion indexa
CO4 (C1—O)	1.409 (19)	1.287 (18)	1.25	0.045
CO4 (C2—O)	1.38 (3)	1.29 (4)	1.25	0.022
M2O6 b	1.87 (3)	1.813 (10)	7.78	0.010
M1O8 c	2.039 (13)	1.908 (14)	13.24	0.020
M3O8 d	2.358 (14)e	1.828 (19)	14.59	0.068

Notes: (a) as defined in Baur (1974 ▸); (b) Mg:Fe ratio for M = 1:0; (c) Mg:Fe ratio for M = 0.917 (17):0.083 (17); (d) Mg:Fe ratio for M = 0.61 (2):0.39 (2); (e) alternatively, for CN = 10 the maximal distance is 2.451 (14) Å, the polyhedral volume is 20.58 Å3 and the distortion index is 0.080.

From all proposed structural models for MgCO3-II over the last two decades, only one appears to successfully match the structure model that we report here. On the basis of powder X-ray diffraction (PXRD) experiments and variable-cell simulations, Oganov et al. (2008 ▸) suggested several energet­ically favourable structural models for MgCO3-II, one of which is in space group C2/m. While our structural solution and refinement from the experimental data is clearly similar to the theoretical predictions by Oganov et al. (2008 ▸), the different composition of the materials and the small differences in the structural parameters required us to check additionally whether theoretical calculations with our model as the starting one would lead to the same result as that reported by Oganov et al. (2008 ▸). We performed such a test and confirm that our results and those of Oganov et al. (2008 ▸) are the same within the accuracy of the methods. More concretely, we performed DFT-based model calculations using the plane wave/pseudopotential CASTEP package (Clark et al., 2005 ▸). Pseudopotentials were generated ‘on the fly’ using the parameters provided with the CASTEP distribution. These pseudopotentials have been tested extensively for accuracy and transferability (Lejaeghere et al., 2016 ▸). The pseudopotentials were employed in conjunction with plane waves up to a kinetic energy cutoff of 1020 eV. The calculations were carried out with the PBE exchange–correlation function (Perdew et al., 1996 ▸). For simplicity, we assumed that all three M1, M2 and M3 positions are fully occupied by Mg2+. The calculations revealed that the energies of our structural model and that of Oganov et al. (2008 ▸) are indeed, identical. The DFT calculations gave C—O distances in good agreement with experimental data. Each carbon atom is coordinated by two oxygen atoms that are each shared with another tetra­hedrally coord­inated carbon, and two that are not shared. The C—O distances for the latter are significantly shorter [1.29 Å < d(C—O) < 1.32 Å] than the former [1.33 Å < (C—O) < 1.41 Å]. A Mulliken bond-population analysis shows that for the long C—O bonds there is a significant bond population of ∼0.5 e− Å−3. This is less than the value for the short bonds, where the bond population is ∼0.9 e− Å−3, but this still is a predominantly covalent bond, and justifies the description as a tetra­hedrally coordinated carbon atom. The formation of (C3O9)6− carbonate rings was previously observed in Ca(Fe,Mg)C3O9 (dolomite-IV) after laser heating of Ca(Fe,Mg)CO3 at 115 GPa (Merlini et al., 2017 ▸). However, dolomite-IV is topologically different from the MgCO3-II structure that we report here. Unlike (Mg2.53Fe0.47)C3O9, Ca(Fe,Mg)C3O9 crystallizes in the ortho­rhom­bic system (space group Pnma), thus highlighting the significance of the metal cations that are present in the carbonate.

Upon decompression at ambient temperature, (Mg2.53Fe0.47)C3O9 reflections become broad and weak, and almost disappear at ∼74 GPa (Fig. 3 ▸ a–c). This may be an indication of either amorphization or sluggish back-transformation to a carbonate with trigonal symmetry. Anti­cipating that further heating would aid recrystallization, we laser-heated the sample at 74 GPa and 2000 (150) K for a few seconds. Wide images collected on the temperature-quenched sample indicated the formation of the calcite structure-type carbonate (Fig. 3 ▸ d).

Figure 3

Unrolled X-ray diffraction images collected at room temperature (λ = 0.411 Å). (a) Sharp and intense reflections of (Mg2.53Fe0.47)C3O9 appear after laser-heating of the starting material at 98 GPa and 2500 K. (b) The crystal phase gradually deteriorates during decompression and (c) nearly disappears at ∼74 GPa. (d) Consequent laser-heating treatment results in the formation of the initial carbonate structure. Green circles mark a few of the characteristic reflections of (Mg2.53Fe0.47)C3O9, the position of Ne reflections and in some cases Re reflections are marked with blue and orange arrows, respectively. The 2θ positions of three characteristic carbonate (R c) reflections are indicated with white arrows. Diamond reflections are marked in red.

Synthesis and crystallization

Magnesium carbonate crystals with 15(±4) mol% Fe were grown following the procedure reported by Chariton et al. (2020 ▸). The composition of the starting material was determined by single-crystal X-ray diffraction under ambient conditions as (Mg0.85Fe0.15)CO3. A single crystal of ∼7 µm size in all dimensions was loaded inside the sample chamber of a BX90-type diamond anvil cell equipped with bevelled Boehler–Almax type diamonds (culet diameter 80 mm). Rhenium and neon were used as the gasket material and pressure-transmitting medium, respectively. The pressure was determined using the equation of state (EoS) of solid Ne (Fei et al., 2007 ▸). First, the sample was compressed up to 98 GPa and a single-crystal collection took place at 300 K. Consequently, the same crystal was laser-heated from both sides up to 2500 (150) K for a few seconds and then quenched to room temperature. Finally, we performed a 5×5 grid of still-image collection with a 2 µm step and 1 s exposure time around the center of the sample. This strategy was used to locate the most heated area of the crystal and the best spot to collect single-crystal X-ray diffraction patterns during rotation of the cell. Single-crystal data collection was performed as a series of ω scans over the range ±35° with a step of 0.5°.

Refinement

Details of the data collection, structure solution and refinement are summarized in Table 1 ▸. In the case of the (Mg0.85Fe0.15)CO3 dataset collected at 98 GPa, the limited number of available reflections required us to fix the Fe content according to our ambient condition estimates (see also "Synthesis and Crystallization" section). On the other hand, during the structure refinements of (Mg2.53Fe0.47)C3O9 all three cation sites (i.e. M1, M2 and M3) were tested for their ability to host Fe by refining the site occupancies. As described above, only the M1 and M3 sites were eventually found to accommodate ∼16(±3) mol % Fe in total. Note that the resulting 5.38 Mg:Fe ratio of (Mg2.53Fe0.47)C3O9 is almost identical to the starting 5.67 Mg:Fe ratio of (Mg0.85Fe0.15)CO3 within the accuracy of our method. Therefore, it is safe to conclude that nearly none or only a negligible amount of Fe was lost during the observed phase transition. The crystal structure of (Mg2.53Fe0.47)C3O9 solved at 98 GPa was used for the structure refinements of the data of the same phase collected during decompression. Due to the limited angular range caused by the laser-heated DAC, the resolution of the data set was not sufficient to refine the anisotropic displacement parameters. Therefore, all atoms were refined with the isotropic approximation.

Crystal structure: contains datablock(s) MgCO3-II_98GPa, MgCO3_98GPa. DOI: 10.1107/S2056989020005411/wm5543sup1.cif

CCDC references: 1998018, 1998019

Additional supporting information: crystallographic information; 3D view; checkCIF report

3[(Mg0.85Fe0.15)CO3]	F(000) = 530
Mr = 265.6	Dx = 4.861 Mg m−3
Monoclinic, C2/m	Synchrotron radiation, λ = 0.41107 Å
Hall symbol: -C 2y	Cell parameters from 146 reflections
a = 8.238 (3) Å	θ = 2.3–19.0°
b = 6.5774 (12) Å	µ = 0.58 mm−1
c = 6.974 (5) Å	T = 293 K
β = 104.40 (6)°	Irregular, colourless
V = 366.0 (3) Å3	0.01 × 0.01 × 0.01 mm
Z = 4

ID15b @ ESRF diffractometer	298 independent reflections
Radiation source: synchrotron	211 reflections with I > 3σ(I)
Synchrotron monochromator	Rint = 0.020
ω scans	θmax = 20.7°, θmin = 2.3°
Absorption correction: multi-scan (CrysAlisPro; Rigaku OD, 2019)	h = −11→12
Tmin = 0.104, Tmax = 1	k = −8→8
522 measured reflections	l = −7→9

Refinement on F	0 restraints
Least-squares matrix: full	5 constraints
R[F2 > 2σ(F2)] = 0.084	Weighting scheme based on measured s.u.'s w = 1/(σ2(F) + 0.000144F2)
wR(F2) = 0.095	(Δ/σ)max = 0.001
S = 3.21	Δρmax = 1.76 e Å−3
298 reflections	Δρmin = −1.21 e Å−3
39 parameters

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mg3	0.4441 (6)	0	0.6503 (9)	0.0177 (11)*	0.61 (2)
Fe3	0.4441 (6)	0	0.6503 (9)	0.0177 (11)*	0.39 (2)
Mg2	0.1712 (7)	0	0.3146 (12)	0.0086 (11)*
Mg1	0	0.2457 (6)	0	0.0117 (13)*	0.917 (17)
Fe1	0	0.2457 (6)	0	0.0117 (13)*	0.083 (17)
O4	0.1395 (17)	0	0.044 (3)	0.020 (2)*
O6	0.2736 (13)	0.1662 (9)	0.847 (2)	0.0179 (17)*
O2	0.3442 (12)	0.1683 (9)	0.4218 (18)	0.0157 (15)*
O1	0.4097 (18)	0	0.105 (3)	0.021 (2)*
O5	0.1487 (16)	0	0.575 (3)	0.016 (2)*
O3	0.0062 (12)	0.1898 (9)	0.2702 (19)	0.0159 (17)*
C1	0.1347 (19)	0.1774 (13)	0.683 (3)	0.017 (2)*
C2	0.265 (3)	0	0.964 (4)	0.024 (3)*

Mg3—O6	2.451 (14)	Mg2—O3i	1.814 (9)
Mg3—O6i	2.451 (14)	Mg1—O4	1.962 (8)
Mg3—O2	1.947 (11)	Mg1—O4vi	1.962 (8)
Mg3—O2ii	2.226 (11)	Mg1—O6v	1.991 (10)
Mg3—O2iii	2.226 (11)	Mg1—O6vii	1.991 (10)
Mg3—O2i	1.947 (11)	Mg1—O1viii	2.039 (12)
Mg3—O1ii	1.829 (18)	Mg1—O1ix	2.039 (12)
Mg3—O5	2.359 (14)	Mg1—O3	1.908 (13)
Mg3—O3iv	2.127 (7)	Mg1—O3vi	1.908 (13)
Mg3—O3v	2.127 (7)	Fe1—O4	1.962 (8)
Fe3—O6	2.451 (14)	Fe1—O4vi	1.962 (8)
Fe3—O6i	2.451 (14)	Fe1—O6v	1.991 (10)
Fe3—O2	1.947 (11)	Fe1—O6vii	1.991 (10)
Fe3—O2ii	2.226 (11)	Fe1—O1viii	2.039 (12)
Fe3—O2iii	2.226 (11)	Fe1—O1ix	2.039 (12)
Fe3—O2i	1.947 (11)	Fe1—O3	1.908 (13)
Fe3—O1ii	1.829 (18)	Fe1—O3vi	1.908 (13)
Fe3—O5	2.359 (14)	C1—O6	1.403 (19)
Fe3—O3iv	2.127 (7)	C1—O2v	1.288 (18)
Fe3—O3v	2.127 (7)	C1—O5	1.411 (17)
Mg2—O4	1.84 (2)	C1—O3x	1.28 (2)
Mg2—O2	1.813 (9)	C2—O4xi	1.29 (3)
Mg2—O2i	1.813 (9)	C2—O6	1.38 (2)
Mg2—O5	1.87 (2)	C2—O6i	1.38 (2)
Mg2—O3	1.814 (9)	C2—O1xi	1.34 (3)
O6—Mg3—O6i	53.0 (3)	O5—Fe3—O3iv	100.3 (3)
O6—Mg3—O2	91.1 (4)	O5—Fe3—O3v	100.3 (3)
O6—Mg3—O2ii	119.7 (3)	O3iv—Fe3—O3v	147.1 (5)
O6—Mg3—O2iii	159.8 (5)	O4—Mg2—O2	108.6 (6)
O6—Mg3—O2i	121.8 (4)	O4—Mg2—O2i	108.6 (6)
O6—Mg3—O1ii	79.3 (6)	O4—Mg2—O5	166.6 (7)
O6—Mg3—O5	54.7 (5)	O4—Mg2—O3	85.1 (6)
O6—Mg3—O3iv	112.4 (5)	O4—Mg2—O3i	85.1 (6)
O6—Mg3—O3v	61.7 (4)	O2—Mg2—O2i	75.3 (4)
O6i—Mg3—O2	121.8 (4)	O2—Mg2—O5	81.8 (6)
O6i—Mg3—O2ii	159.8 (5)	O2—Mg2—O3	97.4 (4)
O6i—Mg3—O2iii	119.7 (3)	O2—Mg2—O3i	165.8 (7)
O6i—Mg3—O2i	91.1 (4)	O2i—Mg2—O5	81.8 (6)
O6i—Mg3—O1ii	79.3 (6)	O2i—Mg2—O3	165.8 (7)
O6i—Mg3—O5	54.7 (5)	O2i—Mg2—O3i	97.4 (4)
O6i—Mg3—O3iv	61.7 (4)	O5—Mg2—O3	85.1 (6)
O6i—Mg3—O3v	112.4 (5)	O5—Mg2—O3i	85.1 (6)
O2—Mg3—O2ii	74.2 (4)	O3—Mg2—O3i	87.0 (4)
O2—Mg3—O2iii	107.0 (5)	O4—Mg1—O4vi	69.1 (5)
O2—Mg3—O2i	69.3 (4)	O4—Mg1—O6v	73.9 (4)
O2—Mg3—O1ii	144.2 (3)	O4—Mg1—O6vii	139.3 (4)
O2—Mg3—O5	67.3 (5)	O4—Mg1—O1viii	150.6 (7)
O2—Mg3—O3iv	140.7 (5)	O4—Mg1—O1ix	118.7 (6)
O2—Mg3—O3v	71.5 (4)	O4—Mg1—O3	79.4 (6)
O2ii—Mg3—O2iii	59.6 (3)	O4—Mg1—O3vi	82.3 (6)
O2ii—Mg3—O2i	107.0 (5)	O4vi—Mg1—O6v	139.3 (4)
O2ii—Mg3—O1ii	80.8 (6)	O4vi—Mg1—O6vii	73.9 (4)
O2ii—Mg3—O5	140.5 (4)	O4vi—Mg1—O1viii	118.7 (6)
O2ii—Mg3—O3iv	115.2 (4)	O4vi—Mg1—O1ix	150.6 (7)
O2ii—Mg3—O3v	58.1 (4)	O4vi—Mg1—O3	82.3 (6)
O2iii—Mg3—O2i	74.2 (4)	O4vi—Mg1—O3vi	79.4 (6)
O2iii—Mg3—O1ii	80.8 (6)	O6v—Mg1—O6vii	146.1 (3)
O2iii—Mg3—O5	140.5 (4)	O6v—Mg1—O1viii	86.9 (5)
O2iii—Mg3—O3iv	58.1 (4)	O6v—Mg1—O1ix	64.9 (5)
O2iii—Mg3—O3v	115.2 (4)	O6v—Mg1—O3	74.6 (5)
O2i—Mg3—O1ii	144.2 (3)	O6v—Mg1—O3vi	112.2 (5)
O2i—Mg3—O5	67.3 (5)	O6vii—Mg1—O1viii	64.9 (5)
O2i—Mg3—O3iv	71.5 (4)	O6vii—Mg1—O1ix	86.9 (5)
O2i—Mg3—O3v	140.7 (5)	O6vii—Mg1—O3	112.2 (5)
O1ii—Mg3—O5	127.7 (8)	O6vii—Mg1—O3vi	74.6 (5)
O1ii—Mg3—O3iv	73.6 (3)	O1viii—Mg1—O1ix	69.7 (7)
O1ii—Mg3—O3v	73.6 (3)	O1viii—Mg1—O3	74.1 (6)
O5—Mg3—O3iv	100.3 (3)	O1viii—Mg1—O3vi	126.1 (6)
O5—Mg3—O3v	100.3 (3)	O1ix—Mg1—O3	126.1 (6)
O3iv—Mg3—O3v	147.1 (5)	O1ix—Mg1—O3vi	74.1 (6)
O6—Fe3—O6i	53.0 (3)	O3—Mg1—O3vi	157.8 (4)
O6—Fe3—O2	91.1 (4)	O4—Fe1—O4vi	69.1 (5)
O6—Fe3—O2ii	119.7 (3)	O4—Fe1—O6v	73.9 (4)
O6—Fe3—O2iii	159.8 (5)	O4—Fe1—O6vii	139.3 (4)
O6—Fe3—O2i	121.8 (4)	O4—Fe1—O1viii	150.6 (7)
O6—Fe3—O1ii	79.3 (6)	O4—Fe1—O1ix	118.7 (6)
O6—Fe3—O5	54.7 (5)	O4—Fe1—O3	79.4 (6)
O6—Fe3—O3iv	112.4 (5)	O4—Fe1—O3vi	82.3 (6)
O6—Fe3—O3v	61.7 (4)	O4vi—Fe1—O6v	139.3 (4)
O6i—Fe3—O2	121.8 (4)	O4vi—Fe1—O6vii	73.9 (4)
O6i—Fe3—O2ii	159.8 (5)	O4vi—Fe1—O1viii	118.7 (6)
O6i—Fe3—O2iii	119.7 (3)	O4vi—Fe1—O1ix	150.6 (7)
O6i—Fe3—O2i	91.1 (4)	O4vi—Fe1—O3	82.3 (6)
O6i—Fe3—O1ii	79.3 (6)	O4vi—Fe1—O3vi	79.4 (6)
O6i—Fe3—O5	54.7 (5)	O6v—Fe1—O6vii	146.1 (3)
O6i—Fe3—O3iv	61.7 (4)	O6v—Fe1—O1viii	86.9 (5)
O6i—Fe3—O3v	112.4 (5)	O6v—Fe1—O1ix	64.9 (5)
O2—Fe3—O2ii	74.2 (4)	O6v—Fe1—O3	74.6 (5)
O2—Fe3—O2iii	107.0 (5)	O6v—Fe1—O3vi	112.2 (5)
O2—Fe3—O2i	69.3 (4)	O6vii—Fe1—O1viii	64.9 (5)
O2—Fe3—O1ii	144.2 (3)	O6vii—Fe1—O1ix	86.9 (5)
O2—Fe3—O5	67.3 (5)	O6vii—Fe1—O3	112.2 (5)
O2—Fe3—O3iv	140.7 (5)	O6vii—Fe1—O3vi	74.6 (5)
O2—Fe3—O3v	71.5 (4)	O1viii—Fe1—O1ix	69.7 (7)
O2ii—Fe3—O2iii	59.6 (3)	O1viii—Fe1—O3	74.1 (6)
O2ii—Fe3—O2i	107.0 (5)	O1viii—Fe1—O3vi	126.1 (6)
O2ii—Fe3—O1ii	80.8 (6)	O1ix—Fe1—O3	126.1 (6)
O2ii—Fe3—O5	140.5 (4)	O1ix—Fe1—O3vi	74.1 (6)
O2ii—Fe3—O3iv	115.2 (4)	O3—Fe1—O3vi	157.8 (4)
O2ii—Fe3—O3v	58.1 (4)	O6—C1—O2v	107.8 (11)
O2iii—Fe3—O2i	74.2 (4)	O6—C1—O5	103.5 (10)
O2iii—Fe3—O1ii	80.8 (6)	O6—C1—O3x	113.7 (17)
O2iii—Fe3—O5	140.5 (4)	O2v—C1—O5	107.9 (17)
O2iii—Fe3—O3iv	58.1 (4)	O2v—C1—O3x	110.6 (10)
O2iii—Fe3—O3v	115.2 (4)	O5—C1—O3x	112.8 (11)
O2i—Fe3—O1ii	144.2 (3)	O4xi—C2—O6	114.7 (12)
O2i—Fe3—O5	67.3 (5)	O4xi—C2—O6i	114.7 (12)
O2i—Fe3—O3iv	71.5 (4)	O4xi—C2—O1xi	110 (3)
O2i—Fe3—O3v	140.7 (5)	O6—C2—O6i	105 (2)
O1ii—Fe3—O5	127.7 (8)	O6—C2—O1xi	105.7 (13)
O1ii—Fe3—O3iv	73.6 (3)	O6i—C2—O1xi	105.7 (13)
O1ii—Fe3—O3v	73.6 (3)

Mg0.85Fe0.15CO3	Dx = 4.614 Mg m−3
Mr = 89	Synchrotron radiation, λ = 0.2952 Å
Trigonal, R3c	Cell parameters from 65 reflections
Hall symbol: -R 3 2"c	θ = 2.7–13.9°
a = 4.281 (7) Å	µ = 0.25 mm−1
c = 12.12 (2) Å	T = 293 K
V = 192.3 (5) Å3	Irregular, colourless
Z = 6	0.01 × 0.01 × 0.01 mm
F(000) = 265

13IDD @ APS (GSECARS) diffractometer	60 independent reflections
Radiation source: synchrotron	33 reflections with I > 3σ(I)
Synchrotron monochromator	Rint = 0.053
ω scans	θmax = 15.4°, θmin = 2.7°
Absorption correction: multi-scan (CrysAlisPro; Rigaku OD, 2019)	h = −6→6
Tmin = 0.95, Tmax = 1	k = −7→5
176 measured reflections	l = −18→18

Refinement on F	0 restraints
Least-squares matrix: full	1 constraint
R[F2 > 2σ(F2)] = 0.100	Weighting scheme based on measured s.u.'s w = 1/(σ2(F) + 0.000144F2)
wR(F2) = 0.084	(Δ/σ)max < 0.001
S = 2.89	Δρmax = 0.66 e Å−3
60 reflections	Δρmin = −0.50 e Å−3
5 parameters

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mg1	0	0	0	0.0373 (13)*	0.85
Fe1	0	0	0	0.0373 (13)*	0.15
O1	0.2791 (17)	0	0.25	0.0382 (16)*
C1	0	0	0.25	0.040 (3)*

Mg1—O1i	1.855 (7)	Fe1—O1iii	1.855 (8)
Mg1—O1ii	1.855 (5)	Fe1—O1iv	1.855 (7)
Mg1—O1iii	1.855 (8)	Fe1—O1v	1.855 (5)
Mg1—O1iv	1.855 (7)	Fe1—O1vi	1.855 (8)
Mg1—O1v	1.855 (5)	C1—O1	1.195 (8)
Mg1—O1vi	1.855 (8)	C1—O1vii	1.195 (8)
Fe1—O1i	1.855 (7)	C1—O1viii	1.195 (8)
Fe1—O1ii	1.855 (5)
O1i—Mg1—O1ii	93.2 (2)	O1i—Fe1—O1iv	180
O1i—Mg1—O1iii	93.2 (2)	O1i—Fe1—O1v	86.8 (2)
O1i—Mg1—O1iv	180	O1i—Fe1—O1vi	86.8 (2)
O1i—Mg1—O1v	86.8 (2)	O1ii—Fe1—O1iii	93.2 (3)
O1i—Mg1—O1vi	86.8 (2)	O1ii—Fe1—O1iv	86.8 (2)
O1ii—Mg1—O1iii	93.2 (3)	O1ii—Fe1—O1v	180
O1ii—Mg1—O1iv	86.8 (2)	O1ii—Fe1—O1vi	86.8 (3)
O1ii—Mg1—O1v	180	O1iii—Fe1—O1iv	86.8 (2)
O1ii—Mg1—O1vi	86.8 (3)	O1iii—Fe1—O1v	86.8 (3)
O1iii—Mg1—O1iv	86.8 (2)	O1iii—Fe1—O1vi	180
O1iii—Mg1—O1v	86.8 (3)	O1iv—Fe1—O1v	93.2 (2)
O1iii—Mg1—O1vi	180	O1iv—Fe1—O1vi	93.2 (2)
O1iv—Mg1—O1v	93.2 (2)	O1v—Fe1—O1vi	93.2 (3)
O1iv—Mg1—O1vi	93.2 (2)	O1—C1—O1vii	120.00 (10)
O1v—Mg1—O1vi	93.2 (3)	O1—C1—O1viii	120.0 (5)
O1i—Fe1—O1ii	93.2 (2)	O1vii—C1—O1viii	120.0 (5)
O1i—Fe1—O1iii	93.2 (2)

Atom label	x	y	z	Site symmetry	Uisco[a]	Occupancy
Mg1	0	0.2457 (6)	0	4g	0.0117 (13)	0.917 (17)
Fe1	0	0.2457 (6)	0	4g	0.0117 (13)	0.083 (17)
Mg2	0.1712 (7)	0	0.3146 (12)	4i	0.0086 (11)	1
Mg3	0.4441 (6)	0	0.6503 (9)	4i	0.0177 (11)	0.61 (2)
Fe3	0.4441 (6)	0	0.6503 (9)	4i	0.0177 (11)	0.39 (2)
O1	0.4097 (18)	0	0.105 (3)	4i	0.021 (2)	1
O2	0.3442 (12)	0.1683 (9)	0.4218 (18)	8j	0.0157 (15)	1
O3	0.0062 (12)	0.1898 (9)	0.2702 (19)	8j	0.0159 (17)	1
O4	0.1395 (17)	0	0.044 (3)	4i	0.020 (2)	1
O5	0.1487 (16)	0	0.575 (3)	4i	0.016 (2)	1
O6	0.2736 (13)	0.1662 (9)	0.847 (2)	8j	0.0179 (17)	1
C1	0.1347 (19)	0.1774 (13)	0.683 (3)	8j	0.017 (2)	1
C2	0.265 (3)	0	0.964 (4)	4i	0.024 (3)	1

Atom label	x	y	z	Site symmetry	Uisco[a]	Occupancy
Mg1	0	0	0	6b	0.0373 (13)	0.85
Fe1	0	0	0	6b	0.0373 (13)	0.15
O1	0.2791 (17)	0	0.25	18e	0.0382 (16)	1
C1	0	0	0.25	6a	0.04 (3)	1