Stability of magnetite at lower mantle conditions
Abstract
The structural and electronic/magnetic properties of the spinel magnetite Fe3O4 and its high-pressure polymorphs have raised scientific interest both due to the abundance of spinel and post-spinel phases in Earth's interior, and also due to the coexistence of Fe2+ and Fe3+. Earlier studies claimed that magnetite undergoes a pressure-induced structural phase transition to a monoclinic structure above 25 GPa [1]. However, more recently multiple studies using synchrotron x-ray diffraction have shown that the high-pressure phase is actually orthorhombic [2]. It was also claimed that Fe3O4 would disproportionate to a combination of FeO and Fe2O3 at higher pressures near 50 GPa [3]. However, a recent study revealed that at 70 GPa h-Fe3O4 undergoes another structural transition to a different orthorhombic space group (Pnma) which is stable up to at least 103 GPa [4]. We have studied Fe3O4 up to pressures above 164 GPa, using diamond anvil cells in combination with synchrotron x-ray diffraction and online laser-heating system to study the stability of Fe3O4 at high pressure-temperature conditions. Our results show that Fe3O4 is stable up to at least 164 GPa and 2000 K. At these conditions we observe a structure similar to the one reported by Ricolleau and Fei [4], although due to the stress released by laser heating we have very sharp diffraction peaks, which allows us to refine the structure. We were able to index all observed diffraction peaks and perform a full profile refinement of XRD patterns at high temperatures and quenched to room-temperature at various pressures. We have observed a significant effect of temperature on the unit-cell, resulting in contraction and expansion in different crystallographic directions. The high-pressure high-temperature stability of Fe3O4 and it properties across mantle conditions will be discussed in terms of impact on structure and evolution of Earth's deep interior. [1] Mao et al. J Geophys Res 79 (1974) [2] Haavik et al. Am Mineral 85 (2000) [3] Lazor et al. J Geophys Res 109 (2004) [4] Ricolleau and Fei. Am Mineral 101 (2016)
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMMR21B2638G
- Keywords:
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- 3909 Elasticity and anelasticity;
- MINERAL PHYSICSDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 3939 Physical thermodynamics;
- MINERAL PHYSICSDE: 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY