High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V

Journal Pre-proof High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V

Jannes Binck, Stella Chariton, Michal Stekiel, Lkhamsuren Bayarjargal, Wolfgang Morgenroth, Victor Milman, Leonid Dubrovinsky, Björn Winkler PII:

S0031-9201(19)30087-1

DOI:

https://doi.org/10.1016/j.pepi.2019.106403

Reference:

PEPI 106403

To appear in: Received date:

9 April 2019

Revised date:

1 November 2019

Accepted date:

18 November 2019

Please cite this article as: J. Binck, S. Chariton, M. Stekiel, et al., High-pressure, hightemperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V, (2019), https://doi.org/10.1016/j.pepi.2019.106403

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© 2019 Published by Elsevier.

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High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V Jannes Bincka,∗, Stella Charitonb , Michal Stekiela , Lkhamsuren Bayarjargala , Wolfgang Morgenrotha , Victor Milmanc , Leonid Dubrovinskyb , Bj¨ orn Winklera a Institut

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f¨ ur Geowissenschaften, Goethe-Universit¨ at Frankfurt, Altenh¨ oferallee 1, 60438 Frankfurt am Main, Germany b Bayerisches Geoinstitut, Universit¨ at Bayreuth, Bayreuth 95447, Germany c BIOVIA Dassault Syst` emes, 334 Science Park, Cambridge CB4 0WN, UK

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Abstract

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The stability of Fe-poor dolomite, CaMg0.98 Fe0.02 (CO3 )2 , was studied by Raman spectroscopy and single-crystal X-ray diffraction at high pressures (P < 60 GPa)

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and high temperatures (T < 2300 K). Density functional theory calculations were employed to complement the experimental study. Between 40–60 GPa and 1800–2300 K, we observed the formation of a high P, T phase, “dolomite-V”,

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which, after quenching to ambient temperature, remained stable down to 12 GPa. The dolomite-V phase crystallizes in the space group C2/c with Z = 4 formula

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units. The structure of the high pressure polymorph “dolomite-IIIc” was solved, which crystallizes in the space group P ¯1 with Z = 8 formula units. The combined

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experimental and theoretical findings show that at high pressures and low to moderate temperatures Dol-IIIc is formed, while at high pressures and high temperatures Dol-V becomes stable. Assuming that thermodynamic equilibrium is obtained at high-pressure, high-temperature conditions, the current study extends our understanding of the phase stability of Fe-poor dolomite polymorphs at upper and lower mantle conditions. Keywords: dolomite, structure, polymorph, carbonate

∗ Corresponding

author. Email address: [email protected] (Jannes Binck)

Preprint submitted to Journal of LATEX Templates

November 1, 2019

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1. Introduction Carbonates are well-known host minerals for carbon and thus are important components of the global carbon cycle [1]. The Earth’s crust contains carbon mainly stored in carbonate sediments [1]. Large amounts of oceanic crust, 5

including carbonate deposits, are continuously subducted along convergent plate margins [2]. Estimates suggest that teragram of carbon are subducted into the

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deep Earth every year [3]. The solubility of carbon in major mantle minerals was shown to be very low and led to the conclusion that carbon may be stored

environments, which are consistent with conditions in subducting slabs, are

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in carbonates that survive decomposition during subduction [4]. Cold oxidized

believed to prevent carbonate break down [5]. However, carbonates may also

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react with silicates or metals and decompose into diamond, when encountering reducing conditions in the Earth’s mantle [6, 7].

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Dolomite CaMg(CO3 )2 is thought to constitute up to 50 % of the Earth’s accessible carbonate reservoirs [8]. Since there is evidence for dolomite inclusions in ultra-deep diamonds [9], high-pressure, high-temperature polymorphs of

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dolomite could also be important carbon-containing phases in the Earth’s mantle. Some experimental constraints concerning the stability of dolomite and Fe-

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bearing dolomite at mantle conditions are available [10, 11, 12, 13]. Iron is a major chemical component of the mantle and, depending on its redox state,

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affects the thermodynamic stability of carbonates [14]. Recently it has been shown that at high pressures (>35 GPa) distinct polymorphs of dolomite are formed as a function of Fe-content [12, 15]. In order to understand the fate of subducted carbon it is essential to study both Fe-poor and Fe-rich solid solutions 25

of the largest known carbonate reservoir at conditions like they are assumed to prevail in the mantle. In the following paragraphs, results of several earlier studies are discussed which have investigated solid solutions of Fe-poor and Fe-rich dolomite at P, T conditions of the Earth’s mantle.

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Dolomite (Dol-I): Dolomite has been used as a term for solid solutions 30

with an Fe content of up to 40 at % [10, 11, 12, 13]. Hence, Ca(Mg,Fe)(CO3 )2 polymorphs were labeled as “dolomite” and a consecutive index. At ambient conditions, dolomite (Dol-I) crystallizes in the space group R¯3 with Z = 3 formula units [16]. The ordered structure is build up by alternating layers of octahedral CaO6 , triangular planar CO3 and octahedral MgO6 groups along the c-axis. A partial substitution of Mg2+ by Fe2+ is observed in natural dolomites

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[17], which form solid solutions with the isostructural ankerite. Nearly Fe-free

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dolomite (Fe < 2 at %) was shown to decompose into aragonite and magnesite at higher pressures and higher temperatures[18, 19, 20]. Only recently, however,

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pure dolomite was shown to undergo two high pressure phase transitions rather than decomposing [12]. An increasing Fe content may stabilize the dolomite

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structure at higher pressures [21, 11]. For a nearly Fe-free (Fe < 2 at %) dolomite, Raman spectra have indicated the formation of a dolomite-Ib (Dol-Ib) phase at

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around 11 GPa [13]. However, the Dol-Ib phase was not found by a more recent study that has used a sample with a slightly higher Fe content (Fe = 8 at %) [22]. At temperatures > 1100 K, dolomite shows an order-disorder transition

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into a phase isostructural with calcite (R¯3c, Z = 6) between ambient pressure

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and ∼3 GPa [23, 24].

Dolomite-II (Dol-II):

Pure and Fe-rich (Fe = 40 at %) Dol-II crystallizes

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in space group P ¯ 1 with Z = 2 formula units. Its structure was solved by Merlini et al. [11, 12]. The structure has topological similarities with that of Dol-I, but has a lower symmetry due to the tilting of the triangular CO3 groups. It was further shown, that the structure of Dol-II is equivalent to that of calcite-II [11]. The phase transition of Dol-I into Dol-II is of second order [11], soft phonon driven [25] and was reported for Fe-poor dolomite (< 8 at %) at pressures between 55

14–18 GPa [26, 12, 13, 22] and Fe-rich dolomite (40 at %) at ∼17 GPa [12], respectively. At around 30 GPa, laser annealed Dol-II decomposes into aragonite + magnesite in a temperature range between 1600–1800 K [10].

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Dolomite-III, -IIIb and -IIIc (Dol-III, IIIb, IIIc): Dolomite-III was first introduced by Mao et al. [10]. A structure model was elucidated one year 60

later by Merlini et al. [11]. Two different high-pressure stuctures have been proposed for a Fe-rich (Fe = 40 at %) dolomite at ambient temperature: Dol-III (P ¯ 1, Z = 8) at 56 GPa [11], and Dol-IIIb (R3, Z = 3) at 37 GPa [12]. Another high-pressure polymorph (Dol-IIIc) was reported for pure CaMg(CO3 )2

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at about 41 GPa and 300 K [12], but due to a limited number of reflections the authors could not solve its structure. The pressure at which Dol-II transforms

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into Dol-III, Dol-IIIb, or Dol-IIIc depends on the Fe content, and ranges from 35 to 41 GPa [11, 12]. Raman spectra on Fe-poor (Fe < 8 at %) dolomite at

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high pressures have been reported by Efthimiopoulos et al. [13] and Vennari and

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or Dol-IIIc.

Dol-IV (P nma, Z = 6) is a high-pressure, high-

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Dolomite IV (Dol-IV):

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Williams [22]. It is unclear whether those spectra correspond to Dol-III, Dol-IIIb

temperature polymorph obtained at 115 GPa and 2500 K of an iron-rich (Fe =

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40 at %) dolomite. Its structure was determined at a pressure of 115 GPa and ambient temperature by Merlini et al. [12]. The coordination of the carbon atoms 75

was found to be no longer triangular, but tetrahedral. Dol-IV was preserved

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upon decompression to at least 70 GPa, before the diamond anvils failed.

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Further potential polymorphs of dolomite:

The report of the Dol-III

phase described by Mao et al. [10] was based on diffraction data of a dolomite powder sample (Fe = 8 at %) in a pressure range between 26–38 GPa and upon 80

heating to 1500 K. Since the diffraction patterns of Dol-III reported by Mao et al. [10] differ from those reported by Merlini et al. [11, 12], speculations arose that another high-pressure, high-temperature phase had been found [13]. Based on density functional theory calculations, equations of state have been reported for CaMg(CO3 )2 up to 80 GPa for hypothetical low-energy structures

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with space groups P ¯ 1, P 2/c, C2/c (trigonally coordinated carbon) and C2/c (4-fold C) [27]. The structure with C2/c space group and Z = 4 formula units

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(trigonally coordinated carbon) turned out to be the most stable phase [27]. Furthermore, the thermodynamic stability of the C2/c phase was shown to increase when significant proportions of iron are incorporated into the structure 90

[15]. Summing up the current state of knowledge, polymorphs of both Fe-free

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and Fe-bearing dolomite can exist at pressures up to at least 112 GPa [12]. At pressures > 35 GPa, heating experiments on Fe-rich dolomite (40 at %) showed

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that polymorphs can exist at conditions of the Earth’s deep lower mantle [11, 12]. In contrast much less in known regarding the phase stability of Fe-poor dolomite

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at the P, T conditions of the mantle. Hence, the aim of our study was to extend the phase diagram of Fe-poor (≤ 2 at %) dolomite up to pressures of 60 GPa

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and temperatures of 2300 K. Also, we report the crystal structures of Dol-IIIc

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here.

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2. Experimental

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[12] and the C2/c phase [27], which will be referred to as “dolomite-V (Dol-V)”

2.1. Characterization of samples

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A natural iron-bearing dolomite (CaMg0.98 Fe0.02 (CO3 )2 ) from Algeria has been used for high P, T experiments in the laser-heated diamond anvil cell (LHDAC). The stoichiometry and homogeneity of this sample has been previously

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characterized by Heinrich et al. [28]. In addition, the sample was further characterized by single-crystal X-ray diffraction with a Bruker IµS Inside diffractometer at the Bayerisches Geoinstitut (Germany). The diffractometer was operated using AgKα radiation (λ = 0.55941 ˚ A) at 50 kV and 880 µA. Lattice param110

eters of the ordered dolomite at ambient conditions were a = b = 4.796(1) ˚ A, c = 15.974(4) ˚ A and V = 318.23(12) ˚ A3 , which are consistent with the parameters of the established ordered structure of iron-free dolomite [16].

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2.2. Preparation of high-pressure, high-temperature experiments High-pressure, high-temperature experiments were carried out using Boehler115

Almax diamond anvil cells (DAC) [29]. Diamonds with culets of either 300 or 350 µm diameter were used for high P, T Raman experiments. For high pressure single-crystal X-ray diffraction studies, diamonds with 300 µm culets and Boehler-Almax cells with opening angles of 70◦ were used. Sample chambers

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of 150–175 µm in diameter were laser drilled in Re gaskets pre-indented to 40–48 µm. Samples investigated by Raman spectroscopy were loaded together

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with KCl as thermal insulation. No additional thermal insulator was used for samples investigated by X-ray diffraction. All cells were loaded with a ruby

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crystal pressure reference and with Ne as pressure transmitting medium. Before and after each Raman or X-ray diffraction measurement the pressure was determined using ruby reference scales for non- and quasi-hydrostatic con-

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ditions [30, 31]. The accuracy of the pressure determination by Mao et al. [31]

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is estimated to fall between 0.05 and 1 GPa for a pressure range up to ∼60 GPa [32]. Pressures were determined before and after the measurement with an

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accuracy of better than ±1 GPa. In some cases, laser heating led to changes in the pressure by up to 8 %.

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2.3. Raman spectroscopy in the LH-DAC Raman spectroscopy was carried out at the Institute of Geosciences at the

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Goethe Universit¨at Frankfurt (Germany). Single crystals of 40–90 µm diameter and 20 µm thickness were measured in three different runs. Raman spectra were 135

measured in 2–4 GPa steps upon compression or decompression covering a range between ambient pressure and ∼55 GPa. The measured frequency range was 100– 1300 cm−1 . A frequency doubled 532.14 nm Nd:YAG Oxxius laser (LCX-532S) was focused on the sample and spectra were collected in backscattering geometry, using a grating spectrometer (Acton, SP-2356) equipped with a CCD detector

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(Pixis 256E) and a microscope objective (Mitutoyo) [33]. The laser power was set to 430 mW and spectra were collected for 100 s. The sample was heated from both sides with a pulsed CO2 laser (Diamond K-250 from Coherent, λ = 6

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10.6 µm) [33]. For the spectroradiometric temperature determination we used a grating spectrometer in combination with a CCD camera (Acton with CCD 145

camera (Pixis 256E)). The temperatures during laser heating were determined by the two-colour pyrometer method, employing Planck and Wien fits [34]. The accuracy of temperature measurement is around ±150 K.

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2.4. High-pressure X-ray diffraction The synthesis of Dol-V was carried out using the laser set-up [33] described above. A single crystal of dolomite (Dol-I) with dimensions of 25 × 20 × 15 µm3

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was compressed to 46.2(8) GPa in a DAC and laser-heated to 1800(150) K

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for 5 min. The formation of the Dol-V phase was initially identified using Raman spectroscopy. X-ray diffraction experiments were then performed at the

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high-pressure X-ray diffraction beamline ID15b at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), with a wavelength of 0.4113 ˚ A

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and a MAR555 flat-panel detector (experimental details are listed in Tab. 1). The spot-size of the X-ray beam was adjusted to 10 × 10 µm2 .

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After the measurement at the ESRF the pressure of the laser-heated sample was increased to 59.3(8) GPa and again heated to 2300(150) K for 5 min in 160

Frankfurt. Powder diffraction measurements were eventually carried out upon

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decompression down to ambient conditions at the extreme conditions beamline P02.2 at PETRA III (Desy, Hamburg, Germany) using a wavelength of 0.2893 ˚ A

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and a Perkin Elmer XRD 1621 flat-panel detector. The spot-size of the X-ray beam was set to 9 × 3 µm2 . 165

Further single crystal diffraction measurements at P02.2 were aimed for determining the structure of Dol-IIIc. Single crystals of dolomite (Dol-I) with dimensions of 10 × 15 × 10 µm3 were compressed and measured in intervals of 5-10 GPa up to a maximal pressure of ∼43 GPa. For Dol-IIIc, diffraction data with two different orientations of the DAC have been obtained in order to increase

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the completeness of the data set. Integration of the reflection intensities and absorption correction were performed with the CrysAlisPro software (Agilent technology). The data collected 7

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for Dol-IIIc in two different orientations were rescaled and combined using XPREP [35]. Structure solution and refinements of Dol-IIIc and Dol-V were 175

carried out using the JANA software package [36]. The solution of the Dol-IIIc and Dol-V structures was achieved with a charge-flipping algorithm [37], using superflip [38] and Fourier-difference analysis.

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2.5. Density functional theory In order to obtain theoretical Raman spectra of the phases investigated density functional perturbation theory (DFPT) calculations were performed employing

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the CASTEP code [39]. The amount of iron in our samples is ≈ 2 at %. It is to

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be expected that this will not lead to a measureable change in the Raman spectra at extreme conditions. Hence we chose to compute structure-property relations

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of the iron-free end-member. The code is an implementation of Kohn–Sham DFT based on a plane wave basis set in conjunction with pseudopotentials. The plane

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wave basis set makes converged results straightforward to obtain in practice, as the convergence is controlled by a single adjustable parameter, the plane wave

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cut-off, which we set to 1020 eV. The norm-conserving pseudopotentials were generated “on the fly” from the CASTEP information provided in the CASTEP 190

data base. These pseudopotentials have extensively been tested for accuracy

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and transferability [40]. The number of valence electrons were 4, 6, 2, and 10 for C, O, Mg, and Ca, respectively. The corresponding core radii (in a.u.) were 1.2,

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1.2, 1.8, and 2.0 for C, O, Mg, and Ca, respectively. All calculations employed the GGA-PBE exchange-correlation functional [41]. The Brillouin zone integrals 195

were performed using Monkhorst–Pack grids [42] with spacings between grid ˚−1 . Geometry optimizations were defined as being points of less than 0.037 A converged when the energy charge between iterations was <0.5×10−6 eV/atom, the maximal residual force was < 0.01 eV/˚ A, and the maximal residual stress was < 0.02 GPa.

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3. Results and discussion 3.1. Crystal structure of Dol-IIIc Single crystal diffraction data could be indexed with the Dol-I phase up to a pressure of 11.8(2) GPa, while diffraction data collected between 20.5(4) and 37.0(8) GPa were indexed with the Dol-II phase [11]. Diffraction pat-

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terns changed again at ∼43 GPa, which is indicative of the Dol-II to Dol-IIIc

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transition reported by Merlini et al. [12]. The phase transition is of first or-

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der as a significant volume decrease of 3.1 % was observed (see appendix Fig. S1). A similar volume decrease was also shown for the transitions of iron-rich

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Dol-II into the iron-rich Dol-III and Dol-IIIb phases [11, 12]. We could index a Dol-IIIc data set measured at 43.4(8) GPa with the triclinic unit cell

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reported by Merlini et al. [12] and solved its structure (Tab. 2 and appendix Tab. S1). Dol-IIIc crystallizes in space group P ¯1 with Z = 8 formula units,

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corresponding to 80 atoms per unit cell. At 43.4(8) GPa the lattice parameters are a = 4.4518(14) ˚ A, b = 11.1683(17) ˚ A, c = 13.6960(17) ˚ A, α = 69.044(13)◦ , β = 88.343(9)◦ , γ = 89.344(10)◦ and V = 635.64(15) ˚ A3 . There is an excellent

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agreement between the experimental and theoretical structural data for the same pressure (Tab. 2). Dol-IIIc is a distorted polymorph of Dol-II, which is

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characterized by planar triangular CO3 groups that are no longer co-planar, alternating distorted MgO6 octahedra, and distorted polyhedra of CaOn with an oxygen coordination of Ca ranging from 7 to 9 (Fig. 1). Some of the Ca and

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Mg cation polyhedra share faces and edges. A significant difference between the Fe-poor Dol-IIIc and its Fe-rich analogues (Dol-III and Dol-IIIb) is the lower degree of tilting of the CO3 groups. It was previously shown, that upon further compression (up to 102 GPa) Dol-IIIc undergoes no phase transition at ambient 225

temperature [12]. 3.2. Crystal structure of Dol-V The Dol-V phase was observed by Raman spectroscopy at 46.2(8) GPa and after heating the crystal to 1800(150) K. The structure of Dol-V was studied

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using synchrotron X-ray diffraction. The reflections could be indexed with a 230

monoclinic unit cell and the structure was readily solved and refined (Tab. 2 and appendix Tab. S2). Dol-V crystallizes in space group C2/c with Z = 4 formula units, corresponding to 40 atoms per unit cell. Unit cell parameters are a = 8.051(14) ˚ A, b = 7.848(3) ˚ A, c = 5.105(15) ˚ A, β = 106.4(3)◦ and V = 309.4(11) ˚ A3 at 46.2(8) GPa. There is very good agreement between the experimental and theoretical structural data (Tab. 2). The structure is ordered

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and consists of CO3 triangular planar units that are tilted against each other,

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distorted MgO6 octahedra and distorted CaO8 square antiprisms. The building blocks of the structure are arranged in alternating layers along the a-axis (Fig.

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2). Cation polyhedra and some of the planar CO3 groups share edges. At around 46 GPa, a volume decrease of about 2.6 % is observed after

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transforming Dol-IIIc upon heating to Dol-V (see appendix Fig. S1). The experimentally determined structure of Dol-V is identical to a DFT-based struc-

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ture model of a pure dolomite reported by Solomatova and Asimow [27]. A comparison between diffraction data of Dol-V (at 46 GPa) and a monoclinic Dol-III phase reported by Mao et al. [10] at 49 GPa showed that these two

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phases are distinct.

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3.3. Decomposition of Dol-V determined by X-ray diffraction Our data displayed some additional reflections at 46.2(8) GPa, which could

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not be assigned to Dol-V, diamond, neon or the rhenium gasket. We conclude that new phases were formed due to the decomposition of Dol-V. When Dol-V was pressurized to ∼46 GPa and heated to ∼1800 K, powder rings started to appear in addition to the single crystal reflections. At ∼59 GPa and after heating to 2300 K the powder rings became dominant. Powder diffraction data were collected from 59 GPa down to ambient conditions in 10 GPa steps. Powder 255

patterns at 59 and 46.2 GPa could be indexed with Dol-V and its decomposition products post-aragonite (CaCO3 ) and ferromagnesite ((Mg,Fe)CO3 ), as reported by Ono et al. [43] and Lavina et al. [44], respectively (Fig. 3a & b). In order to calculate reflection positions of post-aragonite and ferromagnesite at the 10

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pressures at which our experiments have been carried out, we used the equation 260

of state data from Ono et al. [43], Lavina et al. [44]. At ambient conditions, the powder patterns could be indexed with the Dol-I phase using the structure reported by Althoff [45]. Additional weak reflections were observed, which could be indexed with calcite (CaCO3 ) and ferromagnesite reported by Sass et al. [46] and Lavina et al. [44] (Fig. 3c). 3.4. Dolomite high-pressure polymorphs determined by Raman spectroscopy

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Raman spectra were measured up to pressures of ∼42 GPa (Fig. 4a) and at ambient temperatures. According to group theory, the following Raman and

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infrared modes are expected for Dol-I at ambient conditions: Γ = 4Ag (R) + 4Eg (R) + 5 Au (IR) + 5Eu (IR). Six Raman-modes of Dol-I could be traced when increasing the pressure up to ∼6 GPa (Fig. 5a & b). At pressures between 9.1(1)

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and 14.0(1) GPa, the splitting of the characteristic 176 cm −1 (Eg ) mode of Dol-I

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was observed, which indicated that the Dol-Ib phase has formed. Although Dol-Ib was found at a pressure 1.9 GPa lower than reported earlier by Efthimiopoulos

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et al. [13], our observation still shows an excellent reproducibility of the formation of Dol-Ib for the same sample. The second order phase transition of Dol-Ib into Dol-II was found at around 14.5 GPa (Fig. 5a & b). Within an uncertainty of

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2 GPa, our result agrees with that of Efthimiopoulos et al. [13]. The transition from Dol-Ib to Dol-II is characterized by the splitting of Raman modes below

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600 cm−1 , while Raman bands of 720 and 880 cm−1 start to split into doublets at pressures between 18 and 20 GPa. A splitting of the 1140 cm−1 CO3 stretching mode remains up to 28 GPa, but disappears at higher pressures. For the Dol-II structure reported by Merlini et al. [12], the total number of IR and R phonon frequencies is expected to be Γ = 27Au (IR) + 30Ag (R). We observed at least 18 Raman active modes. Our theoretical Raman spectra are in good agreement 285

with experimental data of Dol-II (Fig. 7). The Dol-II to Dol-IIIc first order phase transition was found at around ∼36.2 GPa (Fig. 5a & b). The Dol-IIIc phase is characterized by a further splitting of the Raman-modes of Dol-II and the occurrence of at least 7 new modes that occur in a frequency interval between 11

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700 and 900 cm−1 and between 1150 and 1250 cm−1 . The Dol-IIIc phase was 290

measured up to a pressure of ∼42 GPa. The factor group analysis of Dol-IIIc gives: Γ = 117Au (IR) + 120Ag (R). We observed at least 29 of the Raman-active modes. The spectra recorded here are similar to previously reported Raman spectra of CaMg0.92 Fe0.08 (CO3 )2 that have been measured in a pressure range between 42–86 GPa [22]. The spectra were discussed to correspond to the DolIII phase, which has been found by Merlini et al. [11]. However, the Raman

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spectra have only been measured for iron-poor dolomite [13, 22], making it more

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likely that the formation of Dol-IIIc was observed rather than the presence of Dol-III, or Dol-IIIb. Upon decompression, the back-transformations of Dol-IIIc

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into Dol-II and Dol-II into Dol-I were observed at around 30 GPa and 12 GPa,

3.5. High-pressure high-temperature polymorphs of dolomite determined by Ra-

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man spectroscopy

Raman spectra of quenched samples that had been heated to temperatures of

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1200-2300 K were measured up to pressures of ∼55 GPa (Fig. 4b and appendix Fig. S2). Close to the Dol-I–Dol-II transition, Dol-I remained stable at 13(1) GPa and upon heating to 1600(150) K (appendix Fig. S2a). No indication for Dol-Ib

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was observed before and after heating in this run. At 17 GPa and 1700–1800 K, characteristic Raman-spectra of Dol-II started to change, most notably by the

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appearance of a CO3 stretching mode at 1136 cm−1 (appendix Fig. S2b). The new peak at 1136 cm−1 is very likely the CO3 -stretching mode of aragonite as its Raman shift corresponds to that at ∼18 GPa reported by Bayarjargal et al. [33]. A decomposition of Dol-II into aragonite + magnesite has previously been found for pressures between 26–32 GPa and temperatures between 1600–2350 K [10, 11]. Hence, we infer that decomposition of Dol-II started at these conditions. 315

At ∼39.5(8) GPa, heating Dol-IIIc to 1880(150) K led to significant changes in the Raman-spectra (Fig. 4b). The new modes could not be assigned to Dol-IIIc and indicated the synthesis of a new phase assemblage, that we have identified to be Dol-V + ferromagnesite ((Mg,Fe)CO3 ) + post-aragonite (CaCO3 ) by X-ray 12

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diffraction (Fig. 3). The comparison of experimental and theoretical Raman 320

spectra made it possible to distinguish between characteristic modes of Dol-V and its decomposition products (Fig. 7). In comparison to the theoretical data, the experimental data show additional peaks between 800 and 900 cm−1 and a splitting of the CO3 stretching mode at around 1200 cm−1 . Characteristic features that facilitate the detection of Dol-V are new modes in the low frequency range between 146 and 600 cm−1 , as well as two additional intense bands around

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900 cm−1 . Factor group analyses yields the following Raman- and infrared-active

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modes for Dol-V: Γ = 14Ag (R) + 13Au (IR) + 16 Bg (R) + 14Bu (IR). We observe at least 14 of the Raman-active modes (Fig. 5). The Dol-V phase was further

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pressurized to 55.2(8) GPa and again heated to 2300(150) K. A strong splitting of the CO3 stretching mode is observed, which is indicative of the progressive

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decomposition of Dol-V.

On pressure release, Dol-V was detectable down to 12.6(3) GPa, while char-

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acteristic modes shifted to lower frequencies (Fig. 5a & b). The appearance of new modes that might indicate phase transitions of high pressure CaCO3 polymorphs were also observed during pressure release. The splitting of the CO3

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stretching mode between 1150 and 1250 cm−1 decreases continuously during pressure release. Below 25 GPa, only one broad peak can be observed. Below 44

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GPa, characteristic modes at around 850 cm−1 disappear completely. Although

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it was not possible to properly assign all unknown Raman bands from the high pressure Raman measurements to a particular high pressure carbonate phases, measurements at ambient conditions indicated the reformation of Dol-I, with additional Raman modes that can be explained by a calcite-magnesite-siderite phase assemblage (Fig. 6). Our results from Raman spectroscopy support those from X-ray diffraction and confirm that decomposition occurred at pressures 345

between 39.5-60 GPa and temperatures between 1800-2300 K.

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3.6. Implications of the observation of high-pressure, high-temperature polymorphs of dolomite Summing up the combined observations from Raman spectroscopy and Xray diffraction, we were able to improve the current knowledge regarding the 350

phase stability of Fe-poor dolomite (CaMg0.98 Fe0.02 (CO3 )2 ) at conditions of the Earth’s upper and lower mantle. Based on previously reported data, which have

of

investigated dolomite samples of similar compositions [19, 24, 10, 13, 12], and assuming that thermodynamic equilibrium was reached in our experiments, we

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suggest a phase diagram for Fe-poor dolomite (Fig. 8).

No decomposition of Dol-I or Dol-Ib was found in our experiments, since the

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temperatures were too low (Fig. 8). Our data on heated Dol-II complemented earlier findings indicating that Dol-II decomposes into aragonite and magnesite

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between 17 and 30 GPa at temperatures > 1600 K. Dol-IIIc forms when Dol-II is pressurized to ∼39 GPa at ambient temperature. Upon heating, Dol-IIIc transforms into Dol-V, which after temperature quenching remains present on

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360

pressure release down to ∼13 GPa. It is likely that Dol-IIIc is meta-stable at

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all P, T conditions, since Dol-V formed instead of Dol-IIIc after heating and quenching. Theoretical considerations on Fe-free dolomite [27] indicated that Dol-V is destabilized at higher pressures and 0 K with respect to post-aragonite + magnesite. Our observations indicate that post-aragonite and Fe-bearing

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magnesite (ferromagnesite) would form at P, T conditions of the Earth’s lower mantle, whereas Dol-V starts to decompose. However, Dol-V was present together with post-aragonite and ferromagnesite at 40-52 GPa and 1800 K and at 55-60 GPa and 2300 K. This observation either implies that these conditions 370

are close to the univariant reaction boundary of Dol-V * ) post-aragonite + ferromagnesite, or that the reaction kinetics are very slow, which are considered unlikely. Support for the stability of Fe-rich Dol-V at high pressures and 0 K is given by the thermodynamic calculations of Solomatova and Asimow [15], which show that small amounts of Fe can stabilize the C2/c structure of Dol-V as well

375

as other double carbonates at lower mantle conditions. Then, the stability field of Dol-V could intersect the Earth’s lower mantle geotherm. Carbonates are 14

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likely decomposed in high temperature slabs at deep upper mantle depths [47]. However, cold oceanic crust that incorporates carbonate-silicate assemblages is believed to descent down to possibly the core-mantle boundary [48]. Hence, 380

it is not unlikely that substantial quantities of Fe-poor Dol-V assembled with silicates are hosted by deep subducted oceanic slabs, as temperatures in very

of

cold slabs are assumed to be lower than 1500 K at depths < 1500 km [49].

4. Conclusions

knowledge on the phase stabilities of Fe-poor high-pressure, high-temperature

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Our study on a natural CaMg0.98 Fe0.02 (CO3 )2 dolomite sample extended our

dolomite polymorphs. Structures have been determined for Dol-IIIc at 43 GPa,

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and for Dol-V after heating to 1800 K at 46 GPa. Experimental structural data were shown to be in excellent agreement with those obtained by DFT-based

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calculations. Dol-IIIc transforms into Dol-V at pressures between 40 and 60 GPa and temperatures exceeding 1800 K. Dol-V decomposes into post-aragonite (CaCO3 ) and ferromagnesite ((Mg,Fe)CO3 ) at P, T conditions of the Earth’s

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lower mantle. The results of the present study emphasize the dependence of phase stabilities on composition. Hence, further combined Raman spectroscopy and

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X-ray diffraction studies along the dolomite-ankerite join at extreme conditions would be of great interest with respect to the phase stability of chemically

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complex carbonates in the Earth’s mantle.

5. Acknowledgements The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG)-Germany (FOR2125/CarboPaT, BA4020, 400

WI1232) and BMBF

(05K16RFA, 05K16RFB). DESY (Hamburg, Germany), a member of the Helmholtz Association (HGF), and the ESRF (Grenoble, France) are acknowledged for the provision of experimental facilities. We would like to thank Hanns-Peter Liermann and his team for assistance in using beamline P02.2. Dr. Ilias

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Efthimiopoulos is thanked for supplying sample materials within the CarboPaT 405

collaboration/DFG Research Group.

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c* b*

c*

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b*

Figure 1: Crystal structure of Dol-IIIc in projection along [100] at 43.4(8) GPa (left) and

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reconstructed reciprocal lattice plane in the b∗ c∗ plane (right). (color online)

Figure 2: Crystal structure of Dol-V in projections along [001] and [100] at 46.2(8) GPa and after annealing at 1800(150) K. (color online)

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10

Ne

12

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2

10

12

10

12

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Dol-V Ferromagnesite Post-aragonite c)4 1bar 6

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2

Ne

Ne

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Dol-V Ferromagnesite Post-aragonite 8 K b)4 46.2 GPa, 6annealed at 1800

Intensity (a.u.)

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59 GPa, annealed at 2300 K

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a)

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Dol-I Ferromagnesite Calcite

4

6

8 2 ( )

Figure 3: Phase assemblages measured by X-ray diffraction (wavelength = 0.2893 ˚ A). a) After heating to 2300(150) K and quenching to ambient temperature at 59.3(8) GPa, powder patterns indicated Dol-V + ferromagnesite [44] + post-aragonite [43]. b) After heating to 1800(150) K and quenching to ambient temperature, powder patterns indicated Dol-V + Ferromagnesite [44] + post-aragonite [43]. c) After heating to 2300 K at 59 GPa and quenching to ambient conditions, powder patterns indicated a phase assemblage of Dol-I [45], Calcite [46], and Ferromagnesite [44]. (color online)

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55.2(8) GPa 52.4(8) GPa 47.7(8) GPa

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Dol-V + Decomp.

43.8(8) GPa 36.4(7) GPa 33.9(7) GPa

25.1(5) GPa 21.3(4) GPa 17.9(4) GPa 12.6(3) GPa 1 bar

Dol-I + Decomp.

300 500 700 900 1100 Raman shift (cm 1)

Raman spectra of CaMg0.98 Fe0.02 (CO3 )2 polymorphs. a) Raman spectra of

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Figure 4:

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b)

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Intensity (a.u.)

26 24 22 20 18 16 14 12 10 8 6 4 2 0

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14 a) 12 41.8(8) GPa 39.5(4) GPa Dol-IIIc 10 34.7(8) GPa 27.8(4) GPa 8 21.3(4) GPa Dol-II 17.3(4) GPa 6 14.0(4) GPa 11.5(2) GPa Dol-Ib 4 9.1(1) GPa 6.0(2) GPa 2 1.8(1) GPa Dol-I 0 300 500 700 900 1100 Raman shift (cm 1)

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Intensity (a.u.)

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Dol-I (black), Dol-II (red) and Dol-IIIc (blue) were measured upon compression at ambient

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temperature. b) Raman spectra of quenched Dol-V + decomposition products (green) and Dol-I + decomposition products (black). Spectra between 43 and 52 GPa were measured upon compression and after annealing at 1800 K. The spectrum at 55 GPa was measured after annealing at 2300 K. Spectra between 36 GPa and ambient conditions were measured upon decompression and after heating to 2300 K. (color online)

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Eg

100 0 Figure 5:

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Eg

20 40 Pressure (GPa)

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1000

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Ag

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300

1100

900

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Ag

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Raman shift (cm 1)

500 400

1200

Dol-IIIc

Dol-I Dol-Ib

Dol-II

b)

800 700

Ag

Dol-II

Dol-IIIc Dol-V + decomp. product

Eg

600 0

20 40 Pressure (GPa)

Pressure dependence of the Raman shift for high-pressure, high-temperature

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Raman shift (cm 1)

600

1300

a) Dol-I Dol-Ib

700

dolomite polymorphs as well as its decomposition products in the low (a) and high (b)

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frequency spectrum: Dol-I (black), Dol-II (red), Dol-IIIc (blue) and Dol-V + decomposition products (green). Data are presented for measurements during compression (full circles) and decompression (open circles). Diamond marker correspond to modes of Dol-II (dark-red) and Dol-V (light-green), which have been calculated by DFT. Irreducible representations of Raman modes are labeled for ambient conditions [50]. (color online)

26

Figure 6:

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Quenched sample Dolomite Calcite Magnesite Siderite

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14 12 10 8 6 4 2 0

200

400

600 800 Raman shift (cm 1)

1000

1200

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Intensity (a.u.)

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Raman spectrum of quenched dolomite (CaMg0.98 Fe0.02 (CO3 )2 ) at ambient

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conditions (quenched after laser-heating at 2300 K and 55 GPa), which decomposed into a phase assemblage of dolomite, calcite, and magnesite with slight proportions of a siderite

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component. Reference spectra of calcite, magnesite and siderite are plotted for comparison. All presented Raman spectra were measured at ambient conditions. The dolomite reference is the sample used in the current study before any pressure or temperature treatment was applied. Reference spectra of calcite and siderite were obtained from characterized reference materials [33, 51]. The reference spectrum of magnesite was obtained from the RRUFF open database (R050676). (color online)

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DFT

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43.0(8) GPa, 1800(150) K

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34.7(8) GPa DFT

2 1 400

600

Dol-V + decomposition product Dol-V Dol-II Dol-II

800 1000 1200 1400 1600 Raman shift (cm 1)

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200

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Intensity (a.u.)

6

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Figure 7: Experimental Raman spectra of Dol-II (red) and Dol-V + decomposition products (green) in comparison to DFT-calculated spectra of Dol-II (dark-red) and Dol-V (light-green).

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For Dol-V, the frequency part of 800-1000 cm−1 shows a certain mismatch between theoretical and experimental data. The high-frequency part doesn’t show a splitting of the 1200 cm−1 mode in the calculations, however it is seen in the experiment. The mismatch is explained by additional modes coming from a carbonate decomposition assemblage. (color online)

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Figure 8: Synthesis conditions for polymorphs of Fe-poor dolomite (Fe ≤ 8 at %). Closed and open symbols correspond to data obtained upon compression and decompression, respectively.

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Dol-I (black), disordered Dol-I (orange), Dol-II (red), Dol-IIIc (blue), Dol-V + decomposition products (green), aragonite + magnesite (purple), decomposing Dol-II into aragonite + magnesite (red-purple), Fe-dolomite-III (grey). Data presented as circles were obtained in this

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study on temperature quenched samples, squares by Mao et al. [10], triangles by Merlini et al. [12], left pointing triangles by Martinez et al. [19] and right pointing triangles by Antao et al. [24]. Dashdotted line (purple) corresponds to the dolomite-aragonite + magnesite transition as determined by Martinez et al. [19]. Dashed lines (black, red, blue) correspond to phase boundaries including the Dol-Ib phase described by Efthimiopoulos et al. [13]. Stability fields of the current study are delineated by solid red and green lines corresponding the Dol-I–Dol-II and Dol-II–Dol-V transitions, respectively. Solid grey line corresponds to an adiabatic geotherm, as described in [52]. (color online)

29

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Table 1: Experimental set-up for single-crystal X-ray diffraction measurements of Dol-IIIc and Dol-V Dol-V

CaMg0.98 Fe0.02 (CO3 )2

CaMg0.98 Fe0.02 (CO3 )2

Size of crystal (µm3 )

10×15×10

25×20×15

Synchrotron source

PETRA III

ESRF

Beamline

P02.2

ID15b

Energy (keV)

41.7

30.1

Wavelength (˚ A)

0.2893

0.4113

9×3

10 × 10

ro

Spot-size

(µm2 )

of

Dol-IIIc Composition

Perkin Elmer XRD1621

MAR555

Omega range/step/exposure

±21◦ /0.5◦ /2s

±33◦ /0.5◦ /1s

Jo

ur

na

lP

re

-p

Detector

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Table 2: Crystallographic information about experimental and theoretical data and details of crystal structure refinements of Dol-IIIc and Dol-V. Phase

Dol-IIIc DFT

Exp

DFT

Pressure (GPa) annealing Temp. (K)/duration (min)

43.4(8)

43.4

46.2(8)

43

-

-

1800(150)[a] /5

-

Temperature (K)

298

-

298

-

Crystal system

Triclinic

Triclinic

Monoclinic

Monoclinic

Space group

P¯ 1

P¯ 1

C2/c

C2/c

a (˚ A)

4.4518(4)

b (˚ A)

11.1683(17)

of

Exp

ro

analytical Method

Dol-V

4.4879

8.051(14)

8.1930

11.2572

7.848(3)

7.9595

13.6960(17)

13.7041

5.105(15)

5.1235

α (◦ )

69.044(13)

69.20

90.0

90.0

88.343(9)

88.73

106.4(3)

106.11

89.344(10)

89.77

90.0

90.0

(◦ )

re

β

-p

c (˚ A)

γ (◦ ) V (˚ A3 )

647.08

309.4(11)

320.00

3.85

3.78

3.95

3.81

8

8

4

4

lP

635.64(15)

ρ (g/cm3 ) Z

736

368

Theta range for data collection (◦ )

2.03–15.80

2.88–20.27

Completeness to d = 0.8 ˚ A(%)

30.85

40.52

Index ranges

-7 < h < 7

-13 < h < 12

-13 < k < 9

-13 < k < 13

0 < l < 21

-6 < l < 5

ur

Jo

Redundancy

na

F(000)

Reflections collected Rint

1.00

2.00

2063

410

0.05

0.026

Unique reflections (I>3σ(I))

1289

153

Unique reflections (all)

2062

205

161

24

R1 /wR2 (I>3σ(I))

0.060/0.064

0.057/0.060

R1 /wR2 (all)

0.086/0.065

0.081/0.064

No. of parameters

[a] [b]

[b]

Data collection was performed on temperature quenched samples. Due to the limited amount of available reflections, nearly all atoms were refined in the

isotropic approximation. For Dol-V it was possible to refine Ca anisotropically.

31

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120

Dol-I Dol-II Dol-IIIc Dol-V

100 90 80

of

V/Z (Å 3)

110

600

20

P (GPa)

30

-p

10

ro

70

40

50

re

Figure S1: Pressure dependence of volume per formula unit of Dol-I (black), Dol-II (red),

Jo

ur

na

lP

Dol-IIIc (blue) and Dol-V (green). (color online)

S0

Dol-I

13(1) GPa, 1530(150) K 13.4(1) GPa

700

900

1100

900

1100

500 700 900 1 Raman shift (cm )

1100

ro

500

of

13(1) GPa, 1600(150) K

-p

Dol-II + decomposition Aragonite Dol-II

re

17(1) GPa, 1800(150) K 17(1) GPa, 1700(150) K 17.9(1) GPa 17.2(1) GPa

lP

500

700

Dol-II

ur

na

14 a) 12 10 8 6 4 2 0 300 14 b) 12 10 8 6 4 2 0 300 14 c) 12 10 8 6 4 2 0 300

Jo

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

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27(2) GPa, 1200(150) K 27(2) GPa

Figure S2: Raman spectra of temperature quenched dolomite samples at high pressures. a) Laser-annealed (up to 1600 K) Dol-I. No indication for Dol-Ib is present. b) Laser-annealed Dol-II (up to 1800 K). Splitting of the CO3 stretching mode is indicative for the decomposition of Dol-II. A reference spectrum of aragonite at ∼18 GPa is plotted for a phase identification [33]. c) No decomposition was observed for laser-annealed Dol-II at 1200 K (color online)

S1

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Table S1: Refined atomic coordinates and isotropic displacement factors for Dol-IIIc. Dol-IIIc y

z

Uiso

0.2317(3)

0.4250(2)

0.67558(14)

0.0094(2)

Ca2

0.2485(3)

0.2098(2)

0.44820(14)

0.0090(2)

Ca3

0.7704(3)

0.2974(2)

1.03875(14)

0.0107(2)

Ca4

0.6891(2)

0.0512(2)

0.83600(14)

0.0088(2)

Mg1

0.7370(5)

0.2913(5)

0.5642(3)

0.0092(4)

Mg2

0.7477(4)

0.5835(4)

0.8124(2)

0.0065(4)

Mg3

0.2613(4)

0.2064(4)

0.9133(2)

Mg4

0.2851(4)

0.9212(4)

0.6920(2)

O1

0.9156(9)

0.9255(9)

0.7658(5)

0.0112(8)

O2

0.5139(9)

0.7027(9)

0.7064(5)

0.0110(8)

O3

0.5072(9)

0.2535(8)

0.7897(5)

0.0090(7)

O4

0.9603(9)

0.4691(9)

O5

0.5053(9)

0.4040(9)

O6

0.0151(9)

0.3961(9)

O7

0.9364(9)

O8 O9

of

x

Ca1

0.0081(4) 0.0076(4)

ro

-p

Atoms

0.0099(7)

0.4343(5)

0.0090(7)

0.4511(5)

0.0117(8)

0.1785(8)

0.6946(5)

0.0089(7)

0.6215(9)

0.9544(9)

0.5979(5)

0.0112(8)

0.9396(9)

0.8768(9)

0.9569(5)

0.0111(8)

0.7194(9)

0.8527(5)

0.0127(8)

0.0539(9)

0.8890(5)

0.0104(8)

0.3006(9)

0.8295(5)

0.0136(8)

0.4490(9)

0.7778(5)

0.0129(8)

0.8457(8)

0.8282(5)

0.0087(7)

lP

re

0.9252(5)

0.8697(10)

O11

0.1840(9)

O12

0.9543(10)

O13

0.6037(10)

O14

0.4806(9)

O15

0.3120(10)

0.2731(9)

0.6004(5)

0.0114(8)

O16

0.3799(9)

0.0811(9)

0.7089(5)

0.0112(8) 0.0115(8)

ur

O17

na

O10

0.3511(9)

0.3456(9)

0.9579(5)

0.1112(10)

0.5785(9)

0.7354(5)

0.0134(8)

0.2391(10)

0.5772(10)

0.3828(6)

0.0148(9)

O20

0.0784(9)

0.7847(9)

0.6693(5)

0.0105(8)

O21

0.3866(9)

0.8884(9)

1.0115(5)

0.0115(8)

O22

0.0761(9)

0.0370(8)

0.5750(5)

0.0086(7)

O23

0.4011(9)

0.5534(9)

0.9045(5)

0.0105(8)

O24

0.7027(9)

0.1587(9)

0.5116(5)

0.0108(8)

C1

0.2095(11)

0.1737(11)

0.6724(6)

0.0071(9)

C2

0.2536(12)

0.4575(12)

0.4273(7)

0.0091(9)

C3

0.1735(12)

0.9399(12)

0.9530(7)

0.0097(10)

C4

0.6914(12)

0.3351(11)

0.7997(6)

0.0080(9)

C5

0.2360(12)

0.6828(12)

0.6998(7)

0.0096(9)

C6

0.7937(12)

0.0451(11)

0.5627(7)

0.0092(9)

C7

0.2405(13)

0.4574(13)

0.9282(7)

0.0121(10)

C8

0.7574(13)

0.8313(12)

0.8154(7)

0.0107(10)

O18

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O19

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ro

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-p

Table S2: Refined atomic coordinates and isotropic/anisotropic displacement factors for

re

Dol-V.

Dol-V

Ca1

1

Mg1

0.5

y

z

Uani

0.6680(3)

0.75

0.018(3)

0.5888(4)

0.75

0.0128(8)

0.4406(6)

0.548(3)

0.0176(12)

lP

x

Uiso

0.8662(12)

O2

0.8389(12)

0.2423(6)

0.837(2)

0.0194(12)

O3

0.6224(12)

0.4034(5)

0.641(3)

0.0163(12)

C1

0.3665(8)

0.681(4)

0.0173(15)

na

O1

ur

Atoms

Jo

0.7827(17)

S3

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8