Atomistic insight into viscosity and density of silicate melts under pressure

Abstract

A defining characteristic of silicate melts is the degree of polymerization (tetrahedral connectivity), which dictates viscosity and affects compressibility. While viscosity of depolymerized silicate melts increases with pressure consistent with the free-volume theory, isothermal viscosity of polymerized melts decreases with pressure up to B3-5 GPa, above which it turns over to normal (positive) pressure dependence. Here we show that the viscosity turnover in polymerized liquids corresponds to the tetrahedral packing limit, below which the structure is compressed through tightening of the inter-tetrahedral bond angle, resulting in high compressibility, continual breakup of tetrahedral connectivity and viscosity decrease with increasing pressure. Above the turnover pressure, silicon and aluminium coordination increases to allow further packing, with increasing viscosity and density. These structural responses prescribe the distribution of melt viscosity and density with depth and play an important role in magma transport in terrestrial planetary interiors.

Nanoarchitectured materials composed of fullerene-like spheroids and disordered graphene layers with tunable mechanical properties

“…The anomalous elastic behaviour is generally related to the gradual structural rearrangement, which leads to a more flexible material below the turn-over pressure point. The turnover points have been found to have correlations with bond angles for SiO 2 glass 36 and polymerized silicate glasses 35,37 . Above the turn-over point, the changes in bond angles lead to a more homogeneous material.…”

Fate of MgSiO 3 melts at core–mantle boundary conditions

“…Although many experiments on both melts and glasses densities have been carried out in large volume apparatus to pressures of 20 GPa (12, 13), the few studies conducted in the diamond anvil cell (DAC) were limited to [50][51][52][53][54][55][56][57][58][59][60]15). Studying glasses is a potential alternative for understanding melts because many of their physical properties are similar to those of melt: from the atomistic scale, with the evolution of the silicon coordination number from fourfold to sixfold from 0 to 40 GPa (15), to the macroscopic scale with similarities in compressibility (16,17). Using the X-ray absorption technique adapted to the DAC, we more than double the pressure range for density measurements on amorphous materials.…”

Effect of carbon dioxide on the viscosity of a melt of jadeite composition at high pressure

“…These changes suggest a structural modification by CO 2 in these melts. The viscosity of molten silicate is strongly related to the structure (e.g., Wang et al, 2014). The viscosity of polymerized silicate melts decreases with increasing pressure, whereas that of depolymerized melts increases.…”