Fivefold‐Coordinated Silicon in MgSiO3 Melt Accommodates Viscous Flow up to Transition Zone Pressures

Abstract

Abstract Fivefold‐coordinated silicon is assumed to be the transitional species that accommodates the structural relaxation needed for viscous flow of silicate melts, but so far robust insight at the atomic scale is missing. Furthermore, the role of fivefold‐coordinated silicon in melts at Earth's mantle pressures is unknown. Here we discuss the relative dynamic stabilities of four‐, five‐ and sixfold Si–O coordination polyhedra in MgSiO3 melt at pressures up to 40 GPa, obtained from ab initio molecular dynamics simulations and a novel concept based on a coordination auto‐correlation function. Successfully benchmarked against the relaxation time predicted by Maxwell's relation, our results show that fivefold‐coordinated silicon accommodates structural relaxation not only at ambient pressure, but at least up to the pressures of the mantle transition zone. These results shed new light on the interplay between structure and viscosity of silicate melts.

Keywords

• MgSiO3 melt
• viscosity
• Earth's mantle
• coordination
• lifetime analysis
• relaxation