Acta Crystallographica Section E: Crystallographic Communications

Acta Crystallogr E Crystallogr Commun. 76(Pt 5): 715-719

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

https://orcid.org/0000-0001-5522-0498Stella Charitonaf*, Maxim Bykova, Elena Bykovab, Egor Koemetsa, Timofey Fedotenkoc, Björn Winklerd, Michael Hanflande, Vitali B. Prakapenkaf, Eran Greenbergf, Catherine McCammona, Leonid Dubrovinskya

a. Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany

b. Deutsches Elektronen-Synchrotron (DESY), 22607 Hamburg, Germany

c. Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany

d. Institute of Geosciences, Goethe University, 60438 Frankfurt am Main, Germany

e. European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France

f. GeoSoilEnviroCARS, University of Chicago, 60637 Chicago, Illinois, USA

Correspondence e-mail: stellachariton@hotmail.com

© Chariton et al. 2020

DOI: 10.1107/S2056989020005411

Published online: 21 April 2020

Abstract

We report the phase transition of Mg0.85Fe0.15CO₃ sp ²- to Mg2.53Fe0.47C₃O₉ sp ³-carbonate (MgCO₃–II phase) at 98 GPa and describe their crystal structures by means of single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells.

Abstract

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 tetrahedrally coordinated carbon, where CO₄ ⁴⁻ tetrahedra 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.

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.

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]Ox polyhedra (x = 6–8) perpendicular to the b axis. We can distinguish three crystallographic cation positions (Fig. 2 ▸ b):

(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°.

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).

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.

Figure 1View larger versionFigure 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.

Figure 2View larger versionFigure 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.]

Figure 3View larger versionFigure 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.

See full table Table 1. Experimental details

See full table Table 2. Geometric parameters of (Mg2.53Fe0.47)C₃O₉ at 98 GPa

Supplementary Material

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Acknowledgments

The large majority of diffraction experiments were performed on the X-ray diffraction beamline ID15b at the European Synchrotron Radiation Facility, Grenoble, France. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source, Argonne National Laboratory.

supplementary crystallographic information

Magnesium(II) iron(II) carbonate (MgCO3-II_98GPa) . Crystal data

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Magnesium(II) iron(II) carbonate (MgCO3-II_98GPa) . Data collection

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Magnesium(II) iron(II) carbonate (MgCO3-II_98GPa) . Refinement

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Magnesium(II) iron(II) carbonate (MgCO3-II_98GPa) . Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²)

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Magnesium(II) iron(II) carbonate (MgCO3-II_98GPa) . Geometric parameters (Å, º)

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

Iron (II) Magnesium (II) carbonate (MgCO3_98GPa) . Crystal data

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Iron (II) Magnesium (II) carbonate (MgCO3_98GPa) . Data collection

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Iron (II) Magnesium (II) carbonate (MgCO3_98GPa) . Refinement

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Iron (II) Magnesium (II) carbonate (MgCO3_98GPa) . Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²)

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Iron (II) Magnesium (II) carbonate (MgCO3_98GPa) . Geometric parameters (Å, º)

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

Fractional atomic coordinates and isotropic displacement parameters of (Mg2.53Fe0.47)C₃O₉ at 98 GPa.

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[a] All atomic displacement parameters were refined in the isotropic approximation

Fractional atomic coordinates and isotropic displacement parameters of (Mg0.85Fe0.15)CO₃ at 98 GPa.

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[a] All atomic displacement parameters were refined in the isotropic approximation

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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 |

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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

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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 |

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| 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)* |

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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) |  |

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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 |

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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

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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 |

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| 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)* |

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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) |  |

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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

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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

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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.

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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.]

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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.

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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 ▸).

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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.

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