The crystal structures of Mg 2 Fe 2 C 4 O 13 , with tetrahedrally coordinated carbon, and Fe 13 O 19 , synthesized at deep mantle conditions
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We simulated the redox decomposition of magnesium-siderite at pressures and temperatures corresponding to the top of the Earth’s D″ layer (135 GPa and 2650 K). It transforms into new phases, with unexpected stoichiometry. We report their crystal structure, based on single-crystal synchrotron radiation diffraction on a multi-grain sample, using a charge-flipping algorithm. Mg 2 Fe 2 (C 4 O 13 ) is monoclinic, a = 9.822(3), b = 3.9023(13), c = 13.154(5) Å, β = 108.02(3)°, V = 479.4(3) Å 3 (at 135 GPa). It contains tetrahedrally coordinated carbon units, corner-shared in truncated C 4 O 13 chains. Half of the cations are divalent, and half trivalent. The carbonate coexists with a new iron oxide, Fe 13 O 19 , monoclinic, a = 19.233(2), b = 2.5820(13), c = 9.550(11) Å, β = 118.39(3)°, V = 417.2(5) Å 3 (at 135 GPa). It has a stoichiometry between hematite, Fe2O3, and magnetite, Fe 3 O 4 . The formation of these unquenchable phases indicates, indirectly, the formation of reduced-carbon species, possibly diamond. These structures suggest the ideas that the mineralogy of the lower mantle and D″ region may be more complex than previously estimated. This is especially significant concerning accessory phases of fundamental geochemical significance and their role in ultra-deep iron-carbon redox coupling processes, as well as the iron-oxygen system, which certainly play an important role in the lower mantle mineral phase equilibria.
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Iron carbonates change their structure to CO4-based carbonates at ~70 GPa.
“experiments performed on pure synthetic single crystals of FeCO 3 to ~ 100 GPa and 2500 K brought evidence that even iron carbonates change their structure to CO 4 -based carbonates at ~ 70 GPa”
From Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions (2020) · cites this paper as Merlini et al., 2015
CO4-based carbonates are the most favourable C-carrier in the lower mantle.
“proving that the CO 4based carbonates are the most favourable C-carrier in the lower mantle (Cerantola et al., 2017;Merlini et al., 2015).”
From Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions (2020) · cites this paper as Merlini et al., 2015
Tetrahedral carbonate units can polymerize into chain carbonate structures.
“or chain carbonate structures (Merlini et al., 2015;Cerantola et al., 2017), not yet found in nature.”
From Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions (2020) · cites this paper as Merlini et al., 2015
At HP-HT conditions, carbon in carbonates changes from a trigonal arrangement to tetrahedral carbonate units.
“C transforms from a trigonal arrangement to tetrahedral carbonate units”
From Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions (2020) · cites this paper as Merlini et al., 2015
Laser heating of single crystals with sensible iron content allowed the stabilization of single crystals suitable for ab-initio experimental phase determination of Mg2Fe2(C4O13) carbonate.
“The possibility to laser heat single crystals with sensible iron content for IR laser absorption has successively allowed the stabilization of single crystals suitable for ab-initio experimental phase determination of Mg 2 Fe 2 (C 4 O 13 ) carbonate (Merlini et al., 2015).”
From Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions (2020) · cites this paper as Merlini et al., 2015
Mg2Fe2(C4O13) belongs to a new structural class of carbonates, based on truncated chains constituted by four tetrahedral groups, described by the crystal-chemical formula [C4O13]10-.
“It belongs to a new structural class of carbonates, and it is based on truncated chains constituted by four tetrahedral groups, described by the crystal-chemical formula [C 4 O 13 ] 10-.”
From Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions (2020) · cites this paper as Merlini et al., 2015
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Stability of magnesite and its high-pressure form in the lowermost mantle
Isshiki, M., Irifune, T., Hirose, K., Ono, S., Ohishi, Y., Watanuki, T., Nishibori, E., Takata, M., & Sakata, M. (2004) · Nature · doi:10.1038/nature02181 · Cited by 279 · see citing works
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Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions
Merlini, M., Milani, S., & Maurice, J. (2020) · Geophysical Monograph Series · doi:10.1002/9781119508229.ch9 · Cited by 7 · see citing works
“The possibility to laser heat single crystals with sensible iron content for IR laser absorption has successively allowed the stabilization of single crystals suitable for ab‐initio experimental phase determination of Mg 2 Fe 2 (C 4 O 13 ) carbonate (Merlini et al., 2015).”
BackgroundInfluentialSelf-citation
- Authors
- Merlini, M., Hanfland, M., Salamat, A., Petitgirard, S., & Müller, H.
- Affiliations
- University of Milan (IT)
- European Synchrotron Radiation Facility (FR)
- Harvard University (US)
- University of Bayreuth (DE)
- Corresponding author
- Merlini, Marco
- Venue
- American Mineralogist
- Publisher
- Mineralogical Society of America
- Published
- 2015-08
- Volume
- 100
- Issue
- 8-9
- Pages
- 2001-2004
- ISSN
- 0003-004X (ISSN-L)
- OpenAlex
- W2188244609
- Cited by
- 59
- References
- 1
- Citations by year
- 2026: 3 · 2025: 3 · 2024: 2 · 2023: 2 · 2022: 7 · 2021: 11 · 2020: 8 · 2019: 7 · 2018: 6 · 2017: 8 · 2016: 2
- FWCI
- 5.15
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Licence
- cc-by-nc-nd
- Locations
- American Mineralogist (publishedVersion, cc-by-nc-nd)
- Funders
- European Synchrotron Radiation Facility
- Topics
- Physical Sciences › Earth and Planetary Sciences › Geophysics › High-pressure geophysics and materials
- Physical Sciences › Earth and Planetary Sciences › Geochemistry and Petrology › Mineralogy and Gemology Studies
- Physical Sciences › Chemistry › Inorganic Chemistry › Inorganic Chemistry and Materials
- Keywords
- redox decomposition, deep mantle, D'' layer, high-pressure crystal structure, iron oxide polymorphs, diamond formation, phase equilibria
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