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

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.Plain Language Summary Silicate melts are present not only near the Earth's surface, but also at great depths in the Earth's mantle. It is important not only to know their viscosity, but also to understand the deep connection between the viscosity and the structure of silicate melts. To shed light on this connection, it is needed to not only investigate the structure at the atomic scale, but also the stability of the fundamental structural building blocks of silicate melts as a function of time, which we do for the geophysically important melt composition of MgSiO3. The fundamental building blocks are Si-O coordination polyhedra, of which there are three important in MgSiO3 melt between ambient pressure and the conditions of the Earth's mantle: Four-, five-and six-fold coordinated silicon. Using molecular dynamics simulations of MgSiO3 melt and a novel approach to compute coordination lifetimes from them, we show that fivefold coordinated silicon is not only highly abundant, but also short-lived at all simulated pressures. This means that the macroscopically observed viscosity results primarily from the formation and destruction of fivefold coordinated silicon on the space and time scale of atomic movements. For such fundamental insight, we developed the concept presented here.