Iron-silicon alloy in Earth's core?

Area of Science:

• Geophysics
• Materials Science
• High-Pressure Physics

Background:

• The composition of Earth's inner core is crucial for understanding planetary formation and dynamics.
• Iron alloys are primary candidates for inner core constituents, but their high-pressure phase behavior is complex.
• Previous studies on iron-silicon (Fe-Si) phase relations have been limited in pressure and temperature ranges.

Purpose of the Study:

• To investigate the phase diagram of iron-rich Fe-Si alloys under extreme pressures and temperatures.
• To determine the stability fields of different crystallographic phases (e.g., bcc, hcp) in Fe-Si alloys.
• To assess the potential role of silicon in the Earth's inner core.

Main Methods:

• High-pressure and high-temperature experiments utilizing techniques such as laser-heated diamond anvil cells.
• In-situ X-ray diffraction to identify crystal structures and phase transitions.
• Analysis of recovered samples to confirm phase compositions.

Main Results:

• Silicon (Si) addition significantly stabilizes the body-centered cubic (bcc) phase in iron (Fe) alloys.
• The bcc phase in Fe-Si alloys remains stable up to at least 84 gigapascals and 2400 kelvin.
• Pure Fe exhibits a hexagonal close-packed (hcp) phase at lower pressures (around 10 gigapascals).

Conclusions:

• Earth's inner core could be composed of hexagonal close-packed (hcp) iron with up to 4 weight percent silicon.
• Alternatively, the inner core might consist of a mixture of a silicon-rich bcc phase and a silicon-poor hcp phase.
• Silicon is a plausible light element impurity in Earth's inner core, influencing its structure and properties.