by | May 11, 2022 | mainpost, vol21

Stability of Fe2S and Fe12S7 to 125 GPa; implications for S-rich planetary cores

C.C. Zurkowski1,

1University of Chicago, Department of the Geophysical Sciences, 5734 S Ellis Ave, Chicago, IL 60637, USA

B. Lavina2,3,

2X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA
3Center for Advanced Radiation Sources, 9700 South Cass Avenue, Building 434A, Argonne, IL 60439, USA

S. Chariton3,

3Center for Advanced Radiation Sources, 9700 South Cass Avenue, Building 434A, Argonne, IL 60439, USA

V. Prakapenka3,

3Center for Advanced Radiation Sources, 9700 South Cass Avenue, Building 434A, Argonne, IL 60439, USA

A.J. Campbell1

1University of Chicago, Department of the Geophysical Sciences, 5734 S Ellis Ave, Chicago, IL 60637, USA

Affiliations | Corresponding Author | Cite as | Funding information

Geochemical Perspectives Letters v21 | https://doi.org/10.7185/geochemlet.2217
Received 29 October 2021 | Accepted 28 March 2022 | Published 11 May 2022

Copyright © 2022 The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY 4.0

Keywords: planetary cores, crystallization sequences, crystal chemistry, iron sulfides, high pressure, high temperature, iron alloys, phase relations


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Abstract

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information

The Fe-FeS phase relations were explored in the 22–25 wt. % S compositional range using single crystal X-ray diffraction in a laser heated diamond anvil cell. At pressures up to 125 GPa and at high temperatures, Fe2S and Fe12S7 were determined to co-crystallise. The novel Fe12S7 compound adopts the Co12P7 structure and Fe2S assumes the Fe2P-type structure. Applying these results to an Fe-FeS binary phase diagram exposes a complex series of FeS phase assemblages in the 16–25 wt. % S range, whereby minor changes in S content significantly affect the crystallisation sequence of Fe-S rich planetary cores. For core compositions S-rich of the Fe2S-Fe12S7 eutectic, the small density difference between solid Fe12S7 and Fe2S is likely to result in the formation of a core slush rather than a gravitationally stable inner core. Crystallisation of denser Fe2S at eutectic conditions could then result in gravitational settling of an Fe2S-rich inner core over time. As the Fe2P-type Fe2S has previously been identified forming at high temperatures to pressures as low as 22 GPa, the core crystallisation regimes determined here also elucidate that the Martian core sulfur composition must lie on the S-rich side of the Fe-Fe3S eutectic or even the S-rich side of the Fe3S-Fe2S eutectic to maintain a fully molten core.

Figures and Tables

Figure 1 (a) Diffraction mapping of the hexagonal Fe2S lattice collected at 105 GPa upon quenching from 2400 K. The reflections shown all satisfy the condition hk-2. (b) The crystal structure of Fe2P-type Fe2S (Pearson symbol C22). (c) Diffraction mapping of an Fe12S7 lattice identified at 105 GPa upon quenching from 2400 K. The reflections shown all satisfy the condition hk-3. (d) The crystal structure of Co12P7-type Fe12S7. The FeS5 and FeS4 building blocks observed in both the Fe2S and Fe12S7 structures are shown in the centre-right inset.

Figure 2 (a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013) and Boehler (1992), and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012) and Mori et al. (2017). (b) In the 16–25 wt. % S range, four core crystallisation scenarios are possible depending on the crystallising phase and the density difference between it and the remaining core liquid. In case 4 in (b), the minimal density difference between solid Fe2S and an Fe2S-rich liquid, may result in a neutrally buoyant solid and liquid and the formation of a core slush. (c) A crystallisation model of an Fe + 24 wt. % S core spanning 80–150 GPa indicates that Fe2S may begin to crystallise with Fe12S7 around 115 GPa, causing the denser Fe2S to precipitate out and settle downwards.

Table 1 Unit cell parameters, volumes, and densities of hexagonal Fe2S and Fe12S7 indexed at each pressure step upon temperature quenching. Values in parentheses are propagated uncertainties on the last reported digits.

Table 2 Atomic coordinates refined for the C22 Fe2S and Fe12S7 structure models at 105 GPa.

Figure 1 Figure 2 Table 1 Table 2

View all figures and tables





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Introduction

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


Earth and other terrestrial planets are composed of silicate mantles and Fe-alloy metallic cores (e.g., McDonough and Sun, 1995

McDonough, W.F., Sun, S.S. (1995) The composition of the Earth. Chemical Geology 120, 223–253. https://doi.org/10.1016/0009-2541(94)00140-4

; Righter and O’Brien, 2011

Righter, K., O’Brien, D.P. (2011) Terrestrial planet formation. Proceedings of the National Academy of Sciences 108, 19165–19170. https://doi.org/10.1073/pnas.1013480108

). Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000

Fei, Y., Li, J., Bertka, C.M., Prewitt, C.T. (2000) Structure type and bulk modulus of Fe3S, a new iron-sulfur compound. American Mineralogist 85, 1830–1833. https://doi.org/10.2138/am-2000-11-1229

; Campbell et al., 2007

Campbell, A.J., Seagle, C.T., Heinz, D.L., Shen, G., Prakapenka, V.B. (2007) Partial melting in the iron-sulfur system at high pressure: A synchrotron X-ray diffraction study. Physics of the Earth and Planetary Interiors 162, 119–128. https://doi.org/10.1016/j.pepi.2007.04.001

; Kamada et al., 2012

Kamada, S., Ohtani, E., Terasaki, H., Sakai, T., Miyahara, M., Ohishi, Y., Hirao, N. (2012) Melting relationships in the Fe–Fe3S system up to the outer core conditions. Earth and Planetary Science Letters 359–360, 26–33. https://doi.org/10.1016/j.epsl.2012.09.038

; Mori et al., 2017

Mori, Y., Ozawa, H., Hirose, K., Sinmyo, R., Tateno, S., Morard, G., Ohishi, Y. (2017) Melting experiments on Fe–Fe3S system to 254 GPa. Earth and Planetary Science Letters 464, 135–141. https://doi.org/10.1016/j.epsl.2017.02.021

), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970

Murthy, V.R., Hall, H.T. (1970) The chemical composition of the Earth’s core: Possibility of sulphur in the core. Physics of the Earth and Planetary Interiors 2, 276–282. https://doi.org/10.1016/0031-9201(70)90014-2

; Kruijer et al., 2014

Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150–1154. https://doi.org/10.1126/science.1251766

; Terasaki et al., 2008

Terasaki, H., Frost, D.J., Rubie, D.C., Langenhorst, F. (2008) Percolative core formation in planetesimals. Earth and Planetary Science Letters 273, 132–137. https://doi.org/10.1016/j.epsl.2008.06.019

). Therefore, the Fe-FeS phase equilibria and eutectic melting relations relevant to the P-T-X conditions of a given planetary core play a significant role in the core’s evolving structure as it crystallises over time.

The terrestrial planets each have different interior structures, oxidation states, and proposed core sulfur contents. Cosmochemical models and metal-silicate partitioning experiments estimate that Earth’s core contains ∼2 wt. % sulfur (McDonough, 2003

McDonough, W.F. (2003) 2.15 - Compositional Model for the Earth’s Core. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry, Volume 2: The Mantle and Core. First Edition, Elsevier, Amsterdam, 547–568. https://doi.org/10.1016/B0-08-043751-6/02015-6

; Suer et al., 2017

Suer, T.A., Siebert, J., Remusat, L., Menguy, N., Fiquet, G. (2017) A sulfur-poor terrestrial core inferred from metal–silicate partitioning experiments. Earth and Planetary Science Letters 469, 84–97. https://doi.org/10.1016/j.epsl.2017.04.016

), and the sulfur content of the core is limited to <6 wt. % to explain the presence of an Fe-rich inner core (Mori et al., 2017

Mori, Y., Ozawa, H., Hirose, K., Sinmyo, R., Tateno, S., Morard, G., Ohishi, Y. (2017) Melting experiments on Fe–Fe3S system to 254 GPa. Earth and Planetary Science Letters 464, 135–141. https://doi.org/10.1016/j.epsl.2017.02.021

). Mercury is small and dense, with a large core-mass fraction and a reduced core composition with sulfur contents <1.5 wt. % (Smith, D.E., Zuber, M.T., Phillips, R.J., Solomon, S.C., Hauck, S.A., et al. (2012)

Smith, D.E., Zuber, M.T., Phillips, R.J., Solomon, S.C., Hauck, S.A., et al. (2012) Gravity Field and Internal Structure of Mercury from MESSENGER. Science 336, 214–217. https://doi.org/10.1126/science.1218809

; Namur et al., 2016

Namur, O., Charlier, B., Holtz, F., Cartier, C., McCammon, C. (2016) Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury. Earth and Planetary Science Letters 448, 102–114. https://doi.org/10.1016/j.epsl.2016.05.024

; Genova et al., 2019

Genova, A., Goossens, S., Mazarico, E., Lemoine, F.G., Neumann, G.A., Kuang, W., Sabaka, T.J., Hauck, S.A., Smith, D.E., Solomon, S.C., Zuber, M.T. (2019) Geodetic Evidence That Mercury Has A Solid Inner Core. Geophysical Research Letters 46, 3625–3633. https://doi.org/10.1029/2018GL081135

). Recent analysis of Marsquakes and geodetic data from the InSight mission reveal that Mars’ core accounts for ∼½ the planet’s radius and is fully molten, such that it likely has a high sulfur content (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)

Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021) Seismic detection of the Martian core. Science 373, 443–448. https://doi.org/10.1126/science.abi7730

). In a purely Fe-S Martian core, 25 wt. % sulfur is required to explain the observations, while in a multicomponent core with cosmochemically plausible light element concentrations, a minimum of 10–15 wt. % sulfur is required (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)

Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021) Seismic detection of the Martian core. Science 373, 443–448. https://doi.org/10.1126/science.abi7730

). Less is known about Venus: its core is likely fully molten, due to the lack of heat release through plate tectonics (Nimmo, 2002

Nimmo, F. (2002) Why does Venus lack a magnetic field? Geology 30, 987–990. https://doi.org/10.1130/0091-7613(2002)030<0987:WDVLAM>2.0.CO;2

), which lends little constraint on how the core composition contributes to the core structure.

Reflecting on the 6–360 GPa pressure range and 1.5–25 wt. % potential sulfur concentrations in the terrestrial planetary cores in our solar system alone, it is inevitable that a much wider variety of core structures and compositions exists in planetary bodies outside of our solar system. Continuing to build the Fe sulfide phase diagram with pressure, temperature and composition is critical for interpreting core crystallisation in our solar system and beyond. Here we report new findings on the high P-T polymorphism of Fe2S (22 wt. % S) and the discovery of Fe12S7 (25 wt. % S) to 125 GPa, and use these findings to model the structures of planetary cores with 16–25 wt. % S.

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Experimental Methods

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


High pressure-temperature conditions were achieved on Fe66S34 (by atom) samples using a laser heated diamond anvil cell. Powder and single crystal X-ray diffraction techniques were employed during heating and after temperature quenching, respectively, at beamline 13 ID-D of the Advanced Photon Source, Argonne National Lab. Analysis of the diffraction data is described in detail in the Supplementary Information.

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Synthesis and Structure Determination of Fe2S and Fe12S7

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


Two high P-T experiments were conducted at 105.3 (9) GPa (values in parentheses are propagated uncertainties on the last reported digits) and quenched from 2400 (120) K and 125 (1) GPa and quenched from 2260 (140) K (Table S-2). After each heating, rotational diffraction scans were collected at various locations around the heated region to assess the phase relations. At each pressure step two distinct lattices were identified at the centre of the laser heated spot and are likely the relevant high temperature sulfides at these P-X conditions (Fig. 1).


Figure 1 (a) Diffraction mapping of the hexagonal Fe2S lattice collected at 105 GPa upon quenching from 2400 K. The reflections shown all satisfy the condition hk-2. (b) The crystal structure of Fe2P-type Fe2S (Pearson symbol C22). (c) Diffraction mapping of an Fe12S7 lattice identified at 105 GPa upon quenching from 2400 K. The reflections shown all satisfy the condition hk-3. (d) The crystal structure of Co12P7-type Fe12S7. The FeS5 and FeS4 building blocks observed in both the Fe2S and Fe12S7 structures are shown in the centre-right inset.
Full size image


One set of grains was indexed to a hexagonal cell with parameters a = 5.326 (7) Å and c = 3.125 (3) Å, compatible with 3 formula units of Fe2S (Table 1, Fig. 1a). The systematic absences identified for this hexagonal phase are compatible with a P-62m space group, and a Fe2P-type (Pearson symbol: C22, Z = 3) structure was solved and refined to the data (Table 1, Table S-2, Fig. 1b) (Rundqvist and Jellinek, 1959

Rundqvist, S., Jellinek, F. (1959) The structures of Ni6Si2B, Fe2P and some related phases. Acta Chemica Scandinavica 13, 425–432. https://doi.org/10.3891/acta.chem.scand.13-0425

). This C22 structure, shown in Figure 1b, is composed of 4 fold coordinated and 5 fold coordinated iron sites arranged into columns of edge sharing square pyramids (green) and columns of corner sharing tetrahedra (blue) linked along edges in the c direction (Fig. 1a; Appendix A-1). Additional Co2P-type and Cr2P-type Fe2S polymorphs were identified in the lower temperature regions of the laser heated spot after each run and discussion of their structures are provided in the Supplementary Information.

Table 1 Unit cell parameters, volumes, and densities of hexagonal Fe2S and Fe12S7 indexed at each pressure step upon temperature quenching. Values in parentheses are propagated uncertainties on the last reported digits.
PhaseP (GPa)Synthesis T (K)a (Å)c (Å)V (Å3)Zρ (g/cm3)
Fe2S105.3 (9)2400 (120)5.340 (3)3.133 (2)77.38 (9)39.27 (1)
125 (1)2260 (140)5.282 (5)3.062 (4)74.0 (2)9.69 (1)
Fe12S7105.3 (9)2400 (120)7.794 (2)3.1234 (8)164.33 (8)19.05 (1)
125 (1)2260 (140)7.700 (7)3.104 (4)159.4 (3)9.32 (1)


The second set of grains was indexed to a hexagonal cell with a = 7.794 (2) Å and c = 3.1234 (8) Å, compatible with 1 formula unit of Fe12S7 (Table 2, Fig. 1c). Crystal structure solution and refinement reveal that Fe12S7 adopts the Co12P7 structure (P-6, Z = 1) (Zurkowski et al., 2020

Zurkowski, C., Lavina, B., Chariton, S., Tkachev, S., Prakapenka, V., Campbell, A. (2020). The novel high-pressure/high-temperature compound Co12P7 determined from synchrotron data. Acta Crystallographica Section E: Crystallographic Communications 76, 1665–1668. https://doi.org/10.1107/S2056989020012657

) (Fig. 1d; Appendix A-2). The Co12P7 structure is closely related to the C22 Fe2S structure, in that it is composed of the same building blocks arranged into similar edge-sharing columns, but an increased ratio of square pyramid to tetrahedral building blocks results in the formation of trigonal channels along the c direction in Fe12S7 (Fig. 1d). Previous studies have asserted that this structure is not likely stable in Fe-rich systems as it is composed of a majority 5 fold coordinated sites (Dhahri, 1996

Dhahri, E. (1996) Etude des problèmes d’ordre et de stabilité dans les phases Ln2M12X7 (terre rare-métal de transition-non métal). Journal of Physics: Condensed Matter 8, 4351. https://doi.org/10.1088/0953-8984/8/24/004

), but this work reveals that this structure stabilises in the Fe-S systems with sufficient pressure and temperature.

Table 2 Atomic coordinates refined for the C22 Fe2S and Fe12S7 structure models at 105 GPa.
PhaseSitexyzsofU11U22U33U23U13U12Ueq
C22 Fe2SFe10.59900−1/21/40.0100.0100.009000.00520.0095
error0.00080.0010.0010.0020.00070.0008
Fe20.2594001/40.0090.0080.011000.00410.0092
error0.00060.0010.0020.0010.00080.0007
S12/31/301/60.007
error0.001
S2001/21/120.011
error0.002
Fe12S7Fe10.01510.254901/20.0100.0200.014000.0050.0159
error0.00060.00060.0010.0020.0010.0010.0008
Fe20.13350.623301/20.0090.0100.014000.0040.0112
error0.00060.00050.0010.0010.0020.0010.0009
Fe30.23240.21781/21/20.0320.0240.012000.0240.0185
error0.00060.00070.0020.0020.0010.0020.0008
Fe40.51700.13341/21/20.0090.0090.014000.0050.0106
error0.00050.00050.0010.0010.0020.0010.0009
S50.16380.44761/21/20.011
error0.00080.00100.001
S60.4460.28101/20.013
error0.0010.0010.001
S70001/60.020
error0.002


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Crystallisation Sequences in S-rich Planetary Cores

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


The novel Fe2P-type Fe2S (22 wt. % S) and Co12P7-type Fe12S7 (25 wt. % S) phases were then used to construct an updated binary X-T phase diagram between 100 and 125 GPa (Fig. 2). The Fe-rich portion (<16 wt. % S) of this phase diagram has been characterised at these conditions previously (e.g., Kamada et al., 2012

Kamada, S., Ohtani, E., Terasaki, H., Sakai, T., Miyahara, M., Ohishi, Y., Hirao, N. (2012) Melting relationships in the Fe–Fe3S system up to the outer core conditions. Earth and Planetary Science Letters 359–360, 26–33. https://doi.org/10.1016/j.epsl.2012.09.038

; Mori et al., 2017

Mori, Y., Ozawa, H., Hirose, K., Sinmyo, R., Tateno, S., Morard, G., Ohishi, Y. (2017) Melting experiments on Fe–Fe3S system to 254 GPa. Earth and Planetary Science Letters 464, 135–141. https://doi.org/10.1016/j.epsl.2017.02.021

), but the addition of this study exposes the complexity of Fe-S phase assemblages in the limited 16–25 wt. % S range: Fe3S + Fe2S (16–22 wt. %) and Fe2S + Fe12S7 (22–25 wt. %) (Fig. 2a).


Figure 2 (a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013)

Anzellini, S., Dewaele, A., Mezouar, M., Loubeyre, P., Morard, G. (2013) Melting of iron at Earth’s inner core boundary based on fast X-ray diffraction. Science 340, 464–466. https://doi.org/10.1126/science.1233514

and Boehler (1992)

Boehler, R. (1992) Melting of the Fe-FeO and the Fe-FeS systems at high pressure: Constraints on core temperatures. Earth and Planetary Science Letters 111, 217–227. https://doi.org/10.1016/0012-821X(92)90180-4

, and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012)

Kamada, S., Ohtani, E., Terasaki, H., Sakai, T., Miyahara, M., Ohishi, Y., Hirao, N. (2012) Melting relationships in the Fe–Fe3S system up to the outer core conditions. Earth and Planetary Science Letters 359–360, 26–33. https://doi.org/10.1016/j.epsl.2012.09.038

and Mori et al. (2017)

Mori, Y., Ozawa, H., Hirose, K., Sinmyo, R., Tateno, S., Morard, G., Ohishi, Y. (2017) Melting experiments on Fe–Fe3S system to 254 GPa. Earth and Planetary Science Letters 464, 135–141. https://doi.org/10.1016/j.epsl.2017.02.021

. (b) In the 16–25 wt. % S range, four core crystallisation scenarios are possible depending on the crystallising phase and the density difference between it and the remaining core liquid. In case 4 in (b), the minimal density difference between solid Fe2S and an Fe2S-rich liquid, may result in a neutrally buoyant solid and liquid and the formation of a core slush. (c) A crystallisation model of an Fe + 24 wt. % S core spanning 80–150 GPa indicates that Fe2S may begin to crystallise with Fe12S7 around 115 GPa, causing the denser Fe2S to precipitate out and settle downwards.
Full size image


As the structures of planetary cores are dictated by the density difference between the crystallising phases and remaining liquid, these Fe-S phase relations determine the possible S-rich core crystallisation regimes for an approximately Venus-sized planet (Aitta, 2012

Aitta, A. (2012) Venus’ internal structure, temperature and core composition. Icarus 218, 967–974. https://doi.org/10.1016/j.icarus.2012.01.007

), such as the rocky exoplanet TRAPPIST-1e (RT1e = 0.96 × RVenus) (Grimm, S.L., Demory, B.-O., Gillon, M., Dorn, C., Agol, E., et al. (2018)

Grimm, S.L., Demory, B.-O., Gillon, M., Dorn, C., Agol, E., et al. (2018) The nature of the TRAPPIST-1 exoplanets. Astronomy & Astrophysics 613, A68. https://doi.org/10.1051/0004-6361/201732233

). The visualisations in Figure 2b depict the intricacies of these S-rich crystallising cores. With core compositions ranging from 16–22 wt. % sulfur, two crystallisation models are possible: Fe3S may crystallise into the inner core leaving a lower density, more S-rich liquid outer core or, for more S-rich compositions, Fe2S would be the crystallising phase from a more Fe-rich liquid, producing gravitationally buoyant 'snow’ that would remix and not produce a solid inner core (Fig. 2b; Scenarios 1, 2).

Then for 22–25 wt. % sulfur concentrations, further variations are possible. A gravitationally stable Fe2S-rich inner core could form for core compositions on the Fe-rich side of the Fe2S-Fe12S7 eutectic (Fig. 2b; Scenario 3). Alternatively, for more S-rich compositions shown in Scenario 4 in Figure 2b, the crystallisation of Fe12S7 from a more Fe-rich liquid could result in a neutrally buoyant slush due to the near equivalent densities of an Fe2S-rich liquid and Fe12S7 solid. At 105 GPa, a ∼2.5 % volume increase of Fe2S upon melting results in equivalent solid Fe12S7 and liquid Fe2S densities. As changes in volume from 15 % have been estimated upon melting in previous studies of iron-rich systems (Anderson 2003

Anderson, O.L. (2003) The Three-Dimensional Phase Diagram of Iron. In: Dehant, V., Creager, K.C., Karato, S.-I., Zatman, S. (Eds.) Earth’s Core: Dynamics, Structure, Rotation. American Geophysical Union, Washington, D.C., 83–103. https://doi.org/10.1029/GD031

; Kuwayama et al., 2020

Kuwayama, Y., Morard, G., Nakajima, Y., Hirose, K., Baron, A.Q.R., Kawaguchi, S.I., Tsuchiya, T., Ishikawa, D., Hirao, N. Ohishi, Y. (2020) Equation of State of Liquid Iron under Extreme Conditions. Physical Review Letters 124, 165701. https://doi.org/10.1103/PhysRevLett.124.165701

), these values credit the possibility of neutrally buoyant rather than density settling crystallisation in such an S-rich core. Once eutectic temperature and composition is reached in this system, the crystallisation of both Fe12S7 and Fe2S will result in solidification of an inner core over time.

A core crystallisation model of an S-rich core was then constructed to further assess this scenario (Fig. 2c). The model begins with a fully molten exoplanetary core with Fe + 24 wt. % S composition that spans an arbitrary pressure range of 80–50 GPa with radius of 950 km. These pressure-depth values were assumed using a similar relationship as constrained for Earth’s core. Equations of state of Fe and Fe2S in this pressure range were then used convert from weight percent to volume percent of sulfur in the bulk starting liquid (Dewaele et al., 2006

Dewaele, A., Loubeyre, P., Occelli, F., Mezouar, M., Dorogokupets, P.I., Torrent, M. (2006) Quasihydrostatic Equation of State of Iron above 2 Mbar. Physical Review Letters 97, 215504. https://doi.org/10.1103/PhysRevLett.97.215504

; Zurkowski et al., 2022

Zurkowski, C.C., Lavina, B., Brauser, N.M., Davis, A.H., Chariton, S., Tkachev, S., Greenberg, E., Prakapenka, V.B., Campbell, A.J. (2022) Pressure-induced C23-C37 transition and compression behavior of orthorhombic Fe2S to Earth’s core pressures and high temperatures. American Mineralogist, in press. https://doi.org/10.2138/am-2022-8187

). The model proceeds by crystallising along the S-rich side of the Fe12S7 eutectic in radial shells, assuming temperatures are at the liquidus in each step and following the approximate Fe12S7 liquidus in Figure 2a. The volume fractions of Fe and S were then deducted from the remaining core liquid and the S content of the core liquid was recalculated. For this model core, eutectic conditions would be met around 115 GPa, resulting in the co-crystallisation of a denser Fe2S with Fe12S7 (Fig. 2c). Depending on the dynamics of a specific iron sulfide core, further texturing of this multiphase solid inner core would be possible.

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Applications for the Martian Core

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


Marsquake and geodetic data obtained from the recent InSight mission on Mars indicates a large, fully molten core requiring more sulfur-rich core compositions than previously thought (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)

Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021) Seismic detection of the Martian core. Science 373, 443–448. https://doi.org/10.1126/science.abi7730

). As grains of C22 Fe2S have also been synthesised at 22 GPa and 1300 K (Koch-Müller et al., 2002

Koch-Müller, M., Fei, Y., Wirth, R., Bertka, C.M. (2002) Characterization of high-pressure iron-sulfur compounds. Lunar and Planetary Science Conference XXXIII, Houston, TX, abstract no. 1424.

), it is likely that the C22 Fe2S high temperature stability field extends from at least 19–125 GPa; encompassing pressures relevant to the Martian core (18–40 GPa) (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)

Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021) Seismic detection of the Martian core. Science 373, 443–448. https://doi.org/10.1126/science.abi7730

). Fe3S is also known to be stable at the pressures spanning the Martian core; therefore, models of an iron sulfide Martian core may involve crystallisation of Fe3S and/or Fe2S, like the higher pressure phase diagram in Figure 2a. The regions of the phase diagram in Figure 2a that are Fe-rich of Fe2S may therefore be generally applied to assess possible core S contents to sustain a molten core. Based on these core-structure models (Fig. 2), the Martian core sulfur composition must lie either on the S-rich side of the Fe-Fe3S eutectic, such that Fe3S is the crystallising phase from a more Fe-rich liquid, or even S-rich of the Fe3S-Fe2S eutectic, such that Fe2S is crystallising from a denser Fe-S liquid. Core compositions more sulfur-rich than Fe2S are geochemically unlikely (Steenstra and van Westrenen, 2018

Steenstra, E.S., van Westrenen, W. (2018) A synthesis of geochemical constraints on the inventory of light elements in the core of Mars. Icarus 315, 69–78. https://doi.org/10.1016/j.icarus.2018.06.023

).

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Conclusions

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


Here we present the novel characterisation of Fe2S and Fe12S7 from in situ X-ray diffraction experiments conducted up to 125 GPa and apply these findings to exoplanetary core crystallisation regimes in the 16–25 wt. % sulfur range. The Fe3S-Fe2S-Fe12S7 phase stabilities based on core sulfur contents result in intricate core solidification structures including: gravitationally stable inner core crystallisation, low density sulfide snow and remixing in a sustained molten core, and neutrally buoyant sulfide crystallisation forming a core slush. As the stability fields of Fe3S and Fe2S also extend as low as 21 GPa, the phase relations Fe-rich of Fe2S were generally applied to the molten Martian core to constrain that its sulfur contents must be S-rich of the Fe-Fe3S eutectic or S-rich of the Fe3S-Fe2S eutectic. The presented experimental results and core crystallisation models highlight the sensitivity of core structure to sulfur content and the utility of the Fe-FeS phase relationships for interpreting seismically detected core structures.

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Acknowledgments

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR - 1634415). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR-1606856 and by GSECARS through NSF grant EAR-1634415 and DOE grant DE-FG02-94ER14466. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.This material is based upon work supported by a National Science Foundation Graduate Research Fellowship to CCZ. This work was also supported by the National Science Foundation by grant EAR-1651017 to AJC.

Editor: Anat Shahar

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References

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information

Aitta, A. (2012) Venus’ internal structure, temperature and core composition. Icarus 218, 967–974. https://doi.org/10.1016/j.icarus.2012.01.007
Show in context

As the structures of planetary cores are dictated by the density difference between the crystallising phases and remaining liquid, these Fe-S phase relations determine the possible S-rich core crystallisation regimes for an approximately Venus-sized planet (Aitta, 2012), such as the rocky exoplanet TRAPPIST-1e (RT1e = 0.96 × RVenus) (Grimm, S.L., Demory, B.-O., Gillon, M., Dorn, C., Agol, E., et al. (2018)).
View in article


Anderson, O.L. (2003) The Three-Dimensional Phase Diagram of Iron. In: Dehant, V., Creager, K.C., Karato, S.-I., Zatman, S. (Eds.) Earth’s Core: Dynamics, Structure, Rotation. American Geophysical Union, Washington, D.C., 83–103. https://doi.org/10.1029/GD031
Show in context

As changes in volume from 15 % have been estimated upon melting in previous studies of iron-rich systems (Anderson 2003; Kuwayama et al., 2020), these values credit the possibility of neutrally buoyant rather than density settling crystallisation in such an S-rich core.
View in article


Anzellini, S., Dewaele, A., Mezouar, M., Loubeyre, P., Morard, G. (2013) Melting of iron at Earth’s inner core boundary based on fast X-ray diffraction. Science 340, 464–466. https://doi.org/10.1126/science.1233514
Show in context

(a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013) and Boehler (1992), and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012) and Mori et al. (2017).
View in article


Boehler, R. (1992) Melting of the Fe-FeO and the Fe-FeS systems at high pressure: Constraints on core temperatures. Earth and Planetary Science Letters 111, 217–227. https://doi.org/10.1016/0012-821X(92)90180-4
Show in context

(a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013) and Boehler (1992), and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012) and Mori et al. (2017).
View in article


Campbell, A.J., Seagle, C.T., Heinz, D.L., Shen, G., Prakapenka, V.B. (2007) Partial melting in the iron-sulfur system at high pressure: A synchrotron X-ray diffraction study. Physics of the Earth and Planetary Interiors 162, 119–128. https://doi.org/10.1016/j.pepi.2007.04.001
Show in context

Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article


Dewaele, A., Loubeyre, P., Occelli, F., Mezouar, M., Dorogokupets, P.I., Torrent, M. (2006) Quasihydrostatic Equation of State of Iron above 2 Mbar. Physical Review Letters 97, 215504. https://doi.org/10.1103/PhysRevLett.97.215504
Show in context

Equations of state of Fe and Fe2S in this pressure range were then used convert from weight percent to volume percent of sulfur in the bulk starting liquid (Dewaele et al., 2006; Zurkowski et al., 2022).
View in article


Dhahri, E. (1996) Etude des problèmes d’ordre et de stabilité dans les phases Ln2M12X7 (terre rare-métal de transition-non métal). Journal of Physics: Condensed Matter 8, 4351. https://doi.org/10.1088/0953-8984/8/24/004
Show in context

Previous studies have asserted that this structure is not likely stable in Fe-rich systems as it is composed of a majority 5 fold coordinated sites (Dhahri, 1996), but this work reveals that this structure stabilises in the Fe-S systems with sufficient pressure and temperature.
View in article


Fei, Y., Li, J., Bertka, C.M., Prewitt, C.T. (2000) Structure type and bulk modulus of Fe3S, a new iron-sulfur compound. American Mineralogist 85, 1830–1833. https://doi.org/10.2138/am-2000-11-1229
Show in context

Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article


Genova, A., Goossens, S., Mazarico, E., Lemoine, F.G., Neumann, G.A., Kuang, W., Sabaka, T.J., Hauck, S.A., Smith, D.E., Solomon, S.C., Zuber, M.T. (2019) Geodetic Evidence That Mercury Has A Solid Inner Core. Geophysical Research Letters 46, 3625–3633. https://doi.org/10.1029/2018GL081135
Show in context

Mercury is small and dense, with a large core-mass fraction and a reduced core composition with sulfur contents <1.5 wt. % (Smith, D.E., Zuber, M.T., Phillips, R.J., Solomon, S.C., Hauck, S.A., et al. (2012); Namur et al., 2016; Genova et al., 2019).
View in article


Grimm, S.L., Demory, B.-O., Gillon, M., Dorn, C., Agol, E., et al. (2018) The nature of the TRAPPIST-1 exoplanets. Astronomy & Astrophysics 613, A68. https://doi.org/10.1051/0004-6361/201732233
Show in context

As the structures of planetary cores are dictated by the density difference between the crystallising phases and remaining liquid, these Fe-S phase relations determine the possible S-rich core crystallisation regimes for an approximately Venus-sized planet (Aitta, 2012), such as the rocky exoplanet TRAPPIST-1e (RT1e = 0.96 × RVenus) (Grimm, S.L., Demory, B.-O., Gillon, M., Dorn, C., Agol, E., et al. (2018)).
View in article


Kamada, S., Ohtani, E., Terasaki, H., Sakai, T., Miyahara, M., Ohishi, Y., Hirao, N. (2012) Melting relationships in the Fe–Fe3S system up to the outer core conditions. Earth and Planetary Science Letters 359–360, 26–33. https://doi.org/10.1016/j.epsl.2012.09.038
Show in context

The Fe-rich portion (<16 wt. % S) of this phase diagram has been characterised at these conditions previously (e.g., Kamada et al., 2012; Mori et al., 2017), but the addition of this study exposes the complexity of Fe-S phase assemblages in the limited 16–25 wt. % S range: Fe3S + Fe2S (16–22 wt. %) and Fe2S + Fe12S7 (22–25 wt. %) (Fig. 2a).
View in article
Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article
(a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013) and Boehler (1992), and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012) and Mori et al. (2017).
View in article


Koch-Müller, M., Fei, Y., Wirth, R., Bertka, C.M. (2002) Characterization of high-pressure iron-sulfur compounds. Lunar and Planetary Science Conference XXXIII, Houston, TX, abstract no. 1424.
Show in context

As grains of C22 Fe2S have also been synthesised at 22 GPa and 1300 K (Koch-Müller et al., 2002), it is likely that the C22 Fe2S high temperature stability field extends from at least 19–125 GPa; encompassing pressures relevant to the Martian core (18–40 GPa) (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)).
View in article


Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150–1154. https://doi.org/10.1126/science.1251766
Show in context

Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article


Kuwayama, Y., Morard, G., Nakajima, Y., Hirose, K., Baron, A.Q.R., Kawaguchi, S.I., Tsuchiya, T., Ishikawa, D., Hirao, N. Ohishi, Y. (2020) Equation of State of Liquid Iron under Extreme Conditions. Physical Review Letters 124, 165701. https://doi.org/10.1103/PhysRevLett.124.165701
Show in context

As changes in volume from 15 % have been estimated upon melting in previous studies of iron-rich systems (Anderson 2003; Kuwayama et al., 2020), these values credit the possibility of neutrally buoyant rather than density settling crystallisation in such an S-rich core.
View in article


McDonough, W.F., Sun, S.S. (1995) The composition of the Earth. Chemical Geology 120, 223–253. https://doi.org/10.1016/0009-2541(94)00140-4
Show in context

Earth and other terrestrial planets are composed of silicate mantles and Fe-alloy metallic cores (e.g., McDonough and Sun, 1995; Righter and O’Brien, 2011).
View in article


McDonough, W.F. (2003) 2.15 - Compositional Model for the Earth’s Core. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry, Volume 2: The Mantle and Core. First Edition, Elsevier, Amsterdam, 547–568. https://doi.org/10.1016/B0-08-043751-6/02015-6
Show in context

Cosmochemical models and metal-silicate partitioning experiments estimate that Earth’s core contains ∼2 wt. % sulfur (McDonough, 2003; Suer et al., 2017), and the sulfur content of the core is limited to <6 wt. % to explain the presence of an Fe-rich inner core (Mori et al., 2017).
View in article


Mori, Y., Ozawa, H., Hirose, K., Sinmyo, R., Tateno, S., Morard, G., Ohishi, Y. (2017) Melting experiments on Fe–Fe3S system to 254 GPa. Earth and Planetary Science Letters 464, 135–141. https://doi.org/10.1016/j.epsl.2017.02.021
Show in context

Cosmochemical models and metal-silicate partitioning experiments estimate that Earth’s core contains ∼2 wt. % sulfur (McDonough, 2003; Suer et al., 2017), and the sulfur content of the core is limited to <6 wt. % to explain the presence of an Fe-rich inner core (Mori et al., 2017).
View in article
Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article
(a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013) and Boehler (1992), and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012) and Mori et al. (2017).
View in article


Murthy, V.R., Hall, H.T. (1970) The chemical composition of the Earth’s core: Possibility of sulphur in the core. Physics of the Earth and Planetary Interiors 2, 276–282. https://doi.org/10.1016/0031-9201(70)90014-2
Show in context

Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article


Namur, O., Charlier, B., Holtz, F., Cartier, C., McCammon, C. (2016) Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury. Earth and Planetary Science Letters 448, 102–114. https://doi.org/10.1016/j.epsl.2016.05.024
Show in context

Mercury is small and dense, with a large core-mass fraction and a reduced core composition with sulfur contents <1.5 wt. % (Smith, D.E., Zuber, M.T., Phillips, R.J., Solomon, S.C., Hauck, S.A., et al. (2012); Namur et al., 2016; Genova et al., 2019).
View in article


Nimmo, F. (2002) Why does Venus lack a magnetic field? Geology 30, 987–990. https://doi.org/10.1130/0091-7613(2002)030<0987:WDVLAM>2.0.CO;2
Show in context

Less is known about Venus: its core is likely fully molten, due to the lack of heat release through plate tectonics (Nimmo, 2002), which lends little constraint on how the core composition contributes to the core structure.
View in article


Righter, K., O’Brien, D.P. (2011) Terrestrial planet formation. Proceedings of the National Academy of Sciences 108, 19165–19170. https://doi.org/10.1073/pnas.1013480108
Show in context

Earth and other terrestrial planets are composed of silicate mantles and Fe-alloy metallic cores (e.g., McDonough and Sun, 1995; Righter and O’Brien, 2011).
View in article


Rundqvist, S., Jellinek, F. (1959) The structures of Ni6Si2B, Fe2P and some related phases. Acta Chemica Scandinavica 13, 425–432. https://doi.org/10.3891/acta.chem.scand.13-0425
Show in context

The systematic absences identified for this hexagonal phase are compatible with a P-62m space group, and a Fe2P-type (Pearson symbol: C22, Z = 3) structure was solved and refined to the data (Table 1, Table S-2, Fig. 1b) (Rundqvist and Jellinek, 1959).
View in article


Smith, D.E., Zuber, M.T., Phillips, R.J., Solomon, S.C., Hauck, S.A., et al. (2012) Gravity Field and Internal Structure of Mercury from MESSENGER. Science 336, 214–217. https://doi.org/10.1126/science.1218809
Show in context

Mercury is small and dense, with a large core-mass fraction and a reduced core composition with sulfur contents <1.5 wt. % (Smith, D.E., Zuber, M.T., Phillips, R.J., Solomon, S.C., Hauck, S.A., et al. (2012); Namur et al., 2016; Genova et al., 2019).
View in article


Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021) Seismic detection of the Martian core. Science 373, 443–448. https://doi.org/10.1126/science.abi7730
Show in context

Recent analysis of Marsquakes and geodetic data from the InSight mission reveal that Mars’ core accounts for ∼½ the planet’s radius and is fully molten, such that it likely has a high sulfur content (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)).
View in article
In a purely Fe-S Martian core, 25 wt. % sulfur is required to explain the observations, while in a multicomponent core with cosmochemically plausible light element concentrations, a minimum of 10–15 wt. % sulfur is required (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)).
View in article
Marsquake and geodetic data obtained from the recent InSight mission on Mars indicates a large, fully molten core requiring more sulfur-rich core compositions than previously thought (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)).
View in article
As grains of C22 Fe2S have also been synthesised at 22 GPa and 1300 K (Koch-Müller et al., 2002), it is likely that the C22 Fe2S high temperature stability field extends from at least 19–125 GPa; encompassing pressures relevant to the Martian core (18–40 GPa) (Stähler, S.C., Khan, A., Banerdt, W.B., Lognonné, P., Giardini, D., et al. (2021)).
View in article


Steenstra, E.S., van Westrenen, W. (2018) A synthesis of geochemical constraints on the inventory of light elements in the core of Mars. Icarus 315, 69–78. https://doi.org/10.1016/j.icarus.2018.06.023
Show in context

Core compositions more sulfur-rich than Fe2S are geochemically unlikely (Steenstra and van Westrenen, 2018).
View in article


Suer, T.A., Siebert, J., Remusat, L., Menguy, N., Fiquet, G. (2017) A sulfur-poor terrestrial core inferred from metal–silicate partitioning experiments. Earth and Planetary Science Letters 469, 84–97. https://doi.org/10.1016/j.epsl.2017.04.016
Show in context

Cosmochemical models and metal-silicate partitioning experiments estimate that Earth’s core contains ∼2 wt. % sulfur (McDonough, 2003; Suer et al., 2017), and the sulfur content of the core is limited to <6 wt. % to explain the presence of an Fe-rich inner core (Mori et al., 2017).
View in article


Terasaki, H., Frost, D.J., Rubie, D.C., Langenhorst, F. (2008) Percolative core formation in planetesimals. Earth and Planetary Science Letters 273, 132–137. https://doi.org/10.1016/j.epsl.2008.06.019
Show in context

The Fe-rich portion (<16 wt. % S) of this phase diagram has been characterised at these conditions previously (e.g., Kamada et al., 2012; Mori et al., 2017), but the addition of this study exposes the complexity of Fe-S phase assemblages in the limited 16–25 wt. % S range: Fe3S + Fe2S (16–22 wt. %) and Fe2S + Fe12S7 (22–25 wt. %) (Fig. 2a).
View in article
Sulfur easily alloys with and lowers the melting temperature of iron (Fei et al., 2000; Campbell et al., 2007; Kamada et al., 2012; Mori et al., 2017), and plays a key role in early metal-melt formation and differentiation (Murthy and Hall, 1970; Kruijer et al., 2014; Terasaki et al., 2008).
View in article


Zurkowski, C., Lavina, B., Chariton, S., Tkachev, S., Prakapenka, V., Campbell, A. (2020). The novel high-pressure/high-temperature compound Co12P7 determined from synchrotron data. Acta Crystallographica Section E: Crystallographic Communications 76, 1665–1668. https://doi.org/10.1107/S2056989020012657
Show in context

Crystal structure solution and refinement reveal that Fe12S7 adopts the Co12P7 structure (P-6, Z = 1) (Zurkowski et al., 2020) (Fig. 1d; Appendix A-2).
View in article


Zurkowski, C.C., Lavina, B., Brauser, N.M., Davis, A.H., Chariton, S., Tkachev, S., Greenberg, E., Prakapenka, V.B., Campbell, A.J. (2022) Pressure-induced C23-C37 transition and compression behavior of orthorhombic Fe2S to Earth’s core pressures and high temperatures. American Mineralogist, in press. https://doi.org/10.2138/am-2022-8187
Show in context

Equations of state of Fe and Fe2S in this pressure range were then used convert from weight percent to volume percent of sulfur in the bulk starting liquid (Dewaele et al., 2006; Zurkowski et al., 2022).
View in article



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Supplementary Information

Abstract | Introduction | Experimental Methods | Synthesis and Structure Determination of Fe2S and Fe12S7 | Crystallisation Sequences in S-rich Planetary Cores | Applications for the Martian Core | Conclusions | Acknowledgments | References | Supplementary Information


The Supplementary Information includes:
  • Experimental Methods
  • Structural Relationship of the Fe2S Polymorphs and Fe12S7
  • Tables S-1 and S-2
  • Figures S-1 to S-3
  • Appendices A-1 to A-4
  • Supplementary Information References
    •

    Download Appendix A-1 (.cif).
    Download Appendix A-2 (.cif).
    Download Appendix A-3 (.cif).
    Download Appendix A-4 (.cif).

    Download the Supplementary Information (PDF).
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    Figures



    Figure 1 (a) Diffraction mapping of the hexagonal Fe2S lattice collected at 105 GPa upon quenching from 2400 K. The reflections shown all satisfy the condition hk-2. (b) The crystal structure of Fe2P-type Fe2S (Pearson symbol C22). (c) Diffraction mapping of an Fe12S7 lattice identified at 105 GPa upon quenching from 2400 K. The reflections shown all satisfy the condition hk-3. (d) The crystal structure of Co12P7-type Fe12S7. The FeS5 and FeS4 building blocks observed in both the Fe2S and Fe12S7 structures are shown in the centre-right inset.
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    Figure 2 (a) Phase diagram for the Fe-FeS system between 100–125 GPa inferred from this study in the 16–26 wt. % S (25–36 at. %) range along with previous studies: Fe and FeS melting temperatures were taken from Anzellini et al. (2013)

    Anzellini, S., Dewaele, A., Mezouar, M., Loubeyre, P., Morard, G. (2013) Melting of iron at Earth’s inner core boundary based on fast X-ray diffraction. Science 340, 464–466. https://doi.org/10.1126/science.1233514

    and Boehler (1992)

    Boehler, R. (1992) Melting of the Fe-FeO and the Fe-FeS systems at high pressure: Constraints on core temperatures. Earth and Planetary Science Letters 111, 217–227. https://doi.org/10.1016/0012-821X(92)90180-4

    , and eutectic melting temperature and composition in the Fe-Fe3S system are based on Kamada et al. (2012)

    Kamada, S., Ohtani, E., Terasaki, H., Sakai, T., Miyahara, M., Ohishi, Y., Hirao, N. (2012) Melting relationships in the Fe–Fe3S system up to the outer core conditions. Earth and Planetary Science Letters 359–360, 26–33. https://doi.org/10.1016/j.epsl.2012.09.038

    and Mori et al. (2017)

    Mori, Y., Ozawa, H., Hirose, K., Sinmyo, R., Tateno, S., Morard, G., Ohishi, Y. (2017) Melting experiments on Fe–Fe3S system to 254 GPa. Earth and Planetary Science Letters 464, 135–141. https://doi.org/10.1016/j.epsl.2017.02.021

    . (b) In the 16–25 wt. % S range, four core crystallisation scenarios are possible depending on the crystallising phase and the density difference between it and the remaining core liquid. In case 4 in (b), the minimal density difference between solid Fe2S and an Fe2S-rich liquid, may result in a neutrally buoyant solid and liquid and the formation of a core slush. (c) A crystallisation model of an Fe + 24 wt. % S core spanning 80–150 GPa indicates that Fe2S may begin to crystallise with Fe12S7 around 115 GPa, causing the denser Fe2S to precipitate out and settle downwards.
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