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Greigite (Fe3S4) is thermodynamically stable: Implications for its terrestrial and planetary occurrence
Tamilarasan Subramani1, Kristina Lilova1, Mykola Abramchuk1
1School of Molecular Sciences and Center for Materials of the Universe, Arizona State University, Tempe, AZ 85281.
Magnetic iron sulfide minerals, like greigite (Fe3S4), are stable on Earth and may exist on other planets. Their properties explain their common occurrence and potential role in planetary magnetic fields.
Area of Science:
- Geochemistry
- Mineral Physics
- Planetary Science
Background:
- Iron sulfide minerals are abundant in terrestrial and extraterrestrial environments.
- Greigite (Fe3S4) is a magnetic iron sulfide with implications for geochemistry and planetary magnetism.
Purpose of the Study:
- To determine the thermodynamic stability of magnetic iron sulfide phases, including greigite and its high-pressure form.
- To investigate the occurrence and magnetic properties of iron sulfides in various settings.
Main Methods:
- Measured enthalpies of formation for bulk and nanophase Fe3S4 spinel (greigite) and its high-pressure monoclinic phase.
- Phase diagram analysis at ambient and high-pressure conditions.
Main Results:
- Greigite (Fe3S4) is a stable phase in the Fe-S phase diagram at ambient temperature.
- A high-pressure monoclinic phase of Fe3S4 is thermodynamically metastable below 3 GPa.
- Calculated negative P-T slope for the formation of the monoclinic phase from the spinel.
Conclusions:
- The thermodynamic stability and low surface energy of greigite support its widespread occurrence in anoxic terrestrial environments.
- The stability of these iron sulfide phases suggests their potential existence on Mercury.
- The magnetic properties of these minerals may contribute to Mercury's magnetic field.
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