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Greigite (Fe3s4) Is Thermodynamically Stable: Implications for Its Terrestrial and Planetary Occurrence

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Greigite (Fe3s4) Is Thermodynamically Stable: Implications for Its Terrestrial and Planetary Occurrence Greigite (Fe3S4) is thermodynamically stable: Implications for its terrestrial and planetary occurrence Tamilarasan Subramania,1, Kristina Lilovaa,1, Mykola Abramchuka, Kurt D. Leinenwebera, and Alexandra Navrotskya,2 aSchool of Molecular Sciences and Center for Materials of the Universe, Arizona State University, Tempe, AZ 85281 Contributed by Alexandra Navrotsky, September 29, 2020 (sent for review August 18, 2020; reviewed by Robert M. Hazen and Juraj Majzlan) Iron sulfide minerals are widespread on Earth and likely in planetary (16–19), we determined the energetic stability of bulk, nano- bodies in and beyond our solar system. Using measured enthalpies of sized, and high-pressure Fe3S4 phases. We conclude that grei- formation for three magnetic iron sulfide phases: bulk and nanophase gite is a thermodynamically stable phase in the Fe–Ssystem Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show near ambient temperature, that it has a low surface energy that greigite is a stable phase in the Fe–S phase diagram at ambient comparable to that of magnetite and other oxide spinels, and temperature. The thermodynamic stability and low surface energy of that the monoclinic phase has a field of thermodynamic sta- greigite supports the common occurrence of fine-grained Fe3S4 in many bility above about 3 GPa. Implications for the occurrence of anoxic terrestrial settings. The high-pressure monoclinic phase, thermo- Fe3S4 in terrestrial and planetary settings are discussed. dynamically metastable below about 3 GPa, shows a calculated nega- tive P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their Results and Discussion magnetism may contribute to its present magnetic field. All thermodynamic data in this paper are calculated on the basis of 1 mole of iron to facilitate comparison between different greigite | thermodynamic stability | terrestrial planets | magnetism | iron sulfides. Using the obtained calorimetric data (Table 1), we high pressure calculated the enthalpy of decomposition of greigite to the neighboring phases (pyrite and pyrrhotite) without the involve- ron sulfides are widespread in the Earth and may be important ment of either sulfur or iron as separate phases. The free energy EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Ion other planets. The spinel mineral greigite Fe3S4, the sulfide of this potential disproportion reaction holds the key to whether analogs of magnetite Fe3O4, has long been considered a meta- greigite is stable or metastable on the iron–sulfur phase diagram. stable phase and a rare mineral (1). Nevertheless, fine-grained Fe3S4 has been reported in a variety of geologic and environ- FeS1.33 → 0.726FeS1.092 + 0.274FeS2 , mental settings. Greigite, mackinawite FeS , and pyrite FeS 0.9 2 ΔH = 18.9 ± 8.6 kJ=mol. can be found in sediments formed as a product of sulfate re- r duction (2). Fine‐grained magnetic iron sulfides such as greigite The reaction is energetically substantially unfavorable and would and pyrrhotite have been reported in marine sediments associ- > ated with anaerobic oxidation of methane and formation of require a positive entropy change of 63.4 J/mol.K at 300 K to methane hydrates (3). Small greigite particles occur in magne- Significance totactic bacteria, which produce both Fe3S4 and Fe3O4 in mag- netosome organelles (4). Greigite is a possible catalyst for CO2 reduction of small organic molecules and for H2 and CO2 fixa- The new experimental thermodynamic studies of bulk, nano- tion, relevant to the origin of life (5, 6). Fe3S4 can possibly host phase, and monoclinic high-pressure forms of greigite (Fe3S4) remanent magnetism, thus potentially contributing to the weak confirm its stability in the Fe–S system and explain its common magnetic field of the planet Mercury (7). Greigite has been occurrence in sediments, magnetotactic bacteria, and other am- reported to form as one of the products in the corrosion of crude bient temperature environments under anoxic conditions. The oil tanks when the rust reacts with H2S gas above the crude oil data disprove the commonly held hypothesis that greigite is a surface (8). Given this widespread occurrence, one must ques- metastable phase. The low surface energy of greigite provides tion the assumptions of rarity and metastability. insight not only into the stability of greigite nanoparticles but The change of mineral properties with particle size has been also into their role in prebiotic, biological, and planetary pro- recognized as a critical aspect of geochemical and biochemical re- cesses. Our experimental data for the stability of the three forms actions in natural systems (9). An important question is whether of Fe3S4 confirm that it could be stable on Mercury, thus con- greigite, a sulfide spinel, has a relatively low surface energy analogous tributing to Mercury’s weak magnetic field. to that of magnetite and other oxide spinels (10, 11). If the surface energy of greigite is lower than that of other sulfides in the Fe–S Author contributions: K.L. and A.N. designed research; T.S., K.L., M.A., K.D.L., and A.N. system, its stability at the nanoscale may be enhanced. Particle-size performed research; T.S., K.L., M.A., K.D.L., and A.N. analyzed data; T.S., K.L., M.A., K.D.L., – – and A.N. wrote the paper; T.S, M.A., and K.D.L. carried out the synthesis, characterization, decrease also results in shift to lower oxygen fugacity of the Fe Si O and surface area measurements of various Fe3S4 phases, did calorimetric measurements, oxygen buffers (11). The sulfur and oxygen fugacities are directly and carried out the synthesis, characterization, and surface area measurements of various linked and control the partitioning of metals between sulfide and Fe3S4 phases. Reviewers: R.M.H., Carnegie Institution for Science; and J.M., Mineralogy Department. oxide phases. Hence, the fS2 and fS2–fO2 buffers could be affected by particle-size effects in the iron sulfides, oxides, and silicates (12). University of Jena. Greigite transforms from a cubic spinel structure to a monoclinic The authors declare no competing interest. Published under the PNAS license. Cr3S4-type structure at 3–4 GPa (13, 14). A natural nanosized 1 monoclinic Fe3S4 phase, probably the same as the reported high- T.S. and K.L. contributed equally to this work. pressure phase, has been found in the Sudbury ore deposit (15). 2To whom correspondence may be addressed. Email: Alexandra.Navrotsky@asu.edu. Up to now the thermodynamic properties of greigite have not This article contains supporting information online at https://www.pnas.org/lookup/suppl/ been determined experimentally. Using high-temperature oxide doi:10.1073/pnas.2017312117/-/DCSupplemental. melt solution calorimetric techniques developed for sulfides www.pnas.org/cgi/doi/10.1073/pnas.2017312117 PNAS Latest Articles | 1of4 Downloaded by guest on September 26, 2021 Table 1. Enthalpies of drop solution and of formation from elements of iron sulfide phases per mole of iron Compound ΔHds (kJ/mol) ΔHf,el (kJ/mol) Reference FeS2 (pyrite) −1,471.1 ± 4.2 −175.5 ± 6.0 Xu and Navrotsky, 2010 (16) FeS (troilite) −913.5 ± 3.3 −101.4 ± 4.0 Xu and Navrotsky, 2010 (16) FeS1.092 (pyrrhotite) −966.3 ± 3.3 −106.2 ± 3.5 Xu and Navrotsky, 2010 (16) FeS1.33 (1/3 Fe3S4)* † ‡ Bulk (greigite) −1,071.6 ± 6.2 (6) −144.1 ± 7.3 This work Nanophase (greigite) −1,074.5 ± 4.3 (7) −141.2 ± 5.9 This work High pressure (monoclinic) −1,085.7 ± 5.6 (8) −130.0 ± 6.9 This work *The data reported refer to the molecular formula FeS1.33. † Number of drops given in parentheses; error is 2 SDs of the mean. ‡ The thermochemical cycle used to calculate the enthalpies of formation is given in SI Appendix, Table S2. change the sign of ΔG. Although the standard entropy of greigite The surface energetics of greigite might be important for the is not known, such a large ΔS is very unlikely for a reaction catalytic formation of prebiotic organic molecules on its surface involving only solid phases. We conclude that greigite is a stable and thus for the origin of life according to the iron–sulfur world – phase in the iron sulfur system, at least near ambient tempera- hypothesis (6). The cubic Fe4S4 units resemble 4Fe-4S clusters, ture, and it warrants a place on the equilibrium iron–sulfur phase which are found in the proteins of all living organisms and act as diagram. Greigite is known as an intermediate in pyrite forma- catalysts for synthesis of biomolecules (28). tion, (pyritization) under anoxic sulfate-reducing sedimentary The low oxygen fugacity and the high sulfur concentration conditions (1, 20). Under a diluted carbon supply, the iron spe- recorded during the MESSENGER mission support possible cies will react with sulfide produced by anaerobic decomposition widespread presence of iron sulfide phases on Mercury (29, 30) of organic matter until the reactive iron exhausts the available due to the lack of atmosphere, slow rotation, and high eccen- sulfide and stops the reaction, thus resulting in preservation of tricity, the surface temperature of Mercury varies considerably. greigite (21). The formation of pyrite from greigite will occur, not While the highest temperature on the surface at the equator could because greigite is intrinsically metastable at its own composition, reach 700 K, the temperature at depths below 1 m is modeled to but because excess sulfur is present. Our thermodynamic data sup- be 450 K or less (31), while the temperature in polar craters could port the idea (21) that greigite is a stable compound once formed drop to 100 K. Thus, the surface and near-surface regions of unless there is a change in sulfidizing environment, which explains Mercury are likely to be within the stability field of greigite.
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