Structure and properties of hydrous sodium silicate melts at high pressures and high temperatures by in situ diffraction experiments and ab initio molecular dynamics

概要

Silicate melts are generated and stabilized locally in Earth’s mantle through the presence
of volatiles. Determining the structure of hydrous silicate melts, which is largely different
from anhydrous melts, is therefore important to acquire the microscopic insights into their
properties such as density and viscosity. In addition, our understanding of the liquid state
lags far behind that of solids and gases because of the difficulties in modeling and
simplifying liquid structure with strong many-body correlations without long-range order,
and experimental challenges for in situ experimental observation for hydrous silicate
melts under pressure. Here, the structures of dry and hydrated (with H2O and D2O)
Na6Si8O19 melts and glasses, which is a typical hydrous ternary composition, were
investigated by means of in situ X-ray and neutron diffraction experiments at 0–7 GPa,
combined with ab initio molecular dynamics simulations. This study aims at obtaining a
systematic view of the structural changes with pressure and water content, at
understanding the chemical-bonding states, at comparing to alkaline-earth silicate melts,
and at relating these insights to macroscopic properties.
On compression, the sodium domain structure and the Si-O-Si bond angle rapidly
decrease up to ~4 GPa and then more moderately by shrinking the size of Na-O
coordination polyhedra on further compression. Such anomalous compression behavior
might be related to the vibrational entropy, which we compute to decrease anomalously
on compression with a kink at 1–3 GPa.
With increasing water content, the depolymerization degree linearly increases and
becomes more insensitive to compression. Even though the Si-O-Si linkage is largely
modified by water, the interatomic distances, coordination number, and bond angle
distribution do not show any clear structural changes with water content. Such effects are
clarified by using an orientation order parameter for the Si-(O-)Si arrangement, showing
a tendency for disordering with increasing water content. ...