High-pressure X-ray diffraction and X-ray emission studies on iron-bearing silicate perovskite under high pressures

Abstract

Iron-bearing silicate perovskite is believed to be the most abundant mineral of the Earth's lower mantle. Recent studies have shown that Fe²⁺ exists predominantly in the intermediate-spin state with a total spin number of 1 in silicate perovskite in the lower part of the lower mantle. Here we have measured the spin states of iron and the pressure–volume relation in silicate perovskite [(Mg_0.6,Fe_0.4)SiO₃] at pressure conditions relevant to the lowermost mantle using in situ X-ray emission and X-ray diffraction in a diamond cell. Our results showed that the intermediate-spin Fe²⁺ is stable in the silicate perovskite up to ∼ 125 GPa but starts to transition to the low-spin state at approximately 135 GPa. Concurrent X-ray diffraction measurements showed a decrease of approximately 1% in the unit cell volume in the silicate perovskite [(Mg_0.6,Fe_0.4)SiO₃], which is attributed to the intermediate-spin to the low-spin transition. The transition pressure coincides with the pressure conditions of the lowermost mantle, raising the possibility of the existence of the silicate perovskite phase with the low-spin Fe²⁺ across the transition from the post-perovskite to the perovskite phases in the bottom of the D″ layer.

Keywords:

• silicate perovskite
• electronic spin transition
• X-ray emission
• equation of state
• high pressure

Acknowledgements

We acknowledge GSECARS and XOR-3, APS, ANL for the use of the synchrotron and laser facilities. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under contract no. DE-AC02-06CH11357. GSECARS is supported by NSF Earth Sciences (EAR-0622171) and DOE Geosciences (DE-FG02-94ER14466). J.F.L. acknowledges the financial support from NSF Earth Sciences (EAR-0838221), Carnegie/DOE Alliance Center (CDAC) and Energy Frontier Research under Extreme Environments, Office of Basic Energy Sciences of the US Department of Energy as part of an Energy Frontier Research Center.