Viscosity and viscosity anomalies of model silicates and magmas: a numerical investigation

TL;DR

This study uses computer simulations to explore how viscosity and diffusion behave in model silicates and magmas under pressure, revealing anomalies and testing the validity of theoretical relations.

Contribution

It provides new insights into the pressure-dependent transport properties of silicate melts and evaluates the applicability of the Eyring equation in complex magmatic liquids.

Findings

Viscosity anomalies correlate with diffusion behavior under pressure.

Sodium ions exhibit three distinct diffusion regimes in densified NS2.

Eyring equation is valid for high-viscosity melts.

Abstract

We present results for transport properties (diffusion and viscosity) using computer simulations. Focus is made on a densified binary sodium disilicate 2SiO$_2$-Na$_2$O (NS2) liquid and on multicomponent magmatic liquids (MORB, basalt). In the NS2 liquid, results show that a certain number of anomalies appear when the system is densified: the usual diffusivity maxima/minima is found for the network-forming ions (Si,O) whereas the sodium atom displays three distinct r\'egimes for diffusion. Some of these features can be correlated with the obtained viscosity anomaly under pressure, the latter being be fairly well reproduced from the simulated diffusion constant. In model magmas (MORB liquid), we find a plateau followed by a continuous increase of the viscosity with pressure. Finally, having computed both diffusion and viscosity independently, we can discuss the validity of the Eyring equation for viscosity which relates diffusion and viscosity. It is shown that it can be considered as valid in melts with a high viscosity. On the overall, these results highlight the difficulty of establishing a firm relationship between dynamics, structure and thermodynamics in complex liquids.