Iron–silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle

Abstract

The boundary between the Earth's metallic core and its silicate mantle is characterized by strong lateral heterogeneity and sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition. To investigate the composition and properties of the lowermost mantle, an understanding of the chemical reactions that take place between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earth's lower mantle is first required. Here we present a study of the interaction between iron and silica (SiO2) in electrically and laser-heated diamond anvil cells. In a multianvil apparatus at pressures up to 140 GPa and temperatures over 3,800 K we simulate conditions down to the core-mantle boundary. At high temperature and pressures below 40 GPa, iron and silica react to form iron oxide and an iron-silicon alloy, with up to 5 wt% silicon. At pressures of 85-140 GPa, however, iron and SiO2 do not react and iron-silicon alloys dissociate into almost pure iron and a CsCl-structured (B2) FeSi compound. Our experiments suggest that a metallic silicon-rich B2 phase, produced at the core-mantle boundary (owing to reactions between iron and silicate), could accumulate at the boundary between the mantle and core and explain the anomalously high electrical conductivity of this region.

Configurational thermodynamics of Fe–Ni alloys at Earth's core conditions

“…We used the 64 generalised gradient approximation (GGA) to the DFT exchange-correlation 65 functional as parametrised by Perdew et al (1996). This technique has pre-66 viously been successfully used for simulating the effect of pressure on the 67 properties of Fe-based alloys (Dubrovinsky et al, 2003(Dubrovinsky et al, , 2007Asker et al, 68 ment with experiment for properties of interest in this study, e.g., short-range 70 order and mixing enthalpy (Ruban and Abrikosov, 2008) …”

Axial poloidal electromagnetic core-mantle coupling torque: A re-examination for different conductivity and satellite supported geomagnetic field models

“…Buffett (1992) concluded that EM core-mantle coupling is required for the retrograde annual nutation of the Earth to be out of phase and estimated a shell of 0.2 km at the bottom of the mantle with σ M = 5 · 10 5 S m −1 (a shell of 1 km with 10 5 S m −1 would have the same conductance). New laboratory experiments show that such high conductivity values are possible at the bottom of the mantle (e. g. Dubrovinsky et al, 2003). However, other mechanisms could also give such values, including Buffett's et al (2000) sedimentation in the core mechanism, and most recently the possibility that the post-perovsike phase might have an elevated electrical conductivity (e. g. Ono et al, 2006).…”

Ab initio theory of planetary materials

“…Recent experiments [165] have shown that at high pressures FeSi crystallises with the CsCl-structure (i.e. has identical atomic co-ordinates to bcc-Fe), and it has been found that at low concentrations Si favours the formation of bcc Fe over the hcp polymorph [166,167]. Vočadlo et al [160] therefore investigated the energetic effect of the substitution of S and Si in bcc and hcp Fe at representative core densities.…”