Structure and Speciation in Hydrous Silica Melts. 1. Temperature and Composition Effects

Abstract

Monte Carlo simulations were used to investigate the phase behavior of hydrated liquid silica as a function of temperature and overall water mole fraction, x w. Simulations using the Feuston-Garofalini potential were performed in the isobaric-isothermal ensemble at p = 1 GPa for 15 temperatures (2000 < or = T < or = 9000 K) and 25 compositions (0.0 < or = x w < or = 0.4). The unusual volume minimum exhibited by tetrahedrally coordinated liquid silica is found to persist up to x w approximately 0.267, although the temperature of the volume minimum decreases with increasing water content. Structural properties of the pure and hydrated systems are compared and the addition of water to liquid silica disrupts the silica network more dramatically than temperature alone. The simulations yield very low concentrations of molecular water, e.g. only about 1.2% of the oxygen atoms are bound to exactly two hydrogen atoms at x w = 0.4 and T = 3000 K.

Structural and dynamical properties of supercritical H2O–SiO2 fluids studied by ab initio molecular dynamics

“…8). In addition, the [22,23,24] Comparing to the ln K of several studies, both as a function of temperature and as a function of H 2 O concentration (Fig. 8) shows that our results with ln K = 1.99 at 50 mol% H 2 O are in good agreement to other modeling studies, given that conditions vary between all these studies.…”

“…Indications for a shift in the dissolution mechanism have been found in several studies. [24] found in a Reverse Monte-Carlo study using the classical Garofalini-potential, [25] that in a supercritical fluid of 40 mol% H 2 O in SiO 2 at 3000 K and 1 GPa, only 1% of the nominal water was molecular H 2 O.…”

Structure and Speciation in Hydrous Silica Melts. 2. Pressure Effects

“…A neighbor is defined as an atom within 2.35 Å of the atom of interest. 15 First, examining the coordination of Si at the lower three pressures, more than 95% of Si atoms have four O neighbors at T e 4500 K for all three compositions, and f SiO4 decreases to ≈70% for the stable melts at 9000 K. The fraction of overcoordinated Si atoms, f SiO5 , increases for T e 4500 K but then reaches a plateau. In contrast, the fraction of undercoordinated Si atoms, f SiO3 , remains near zero for T e 4500 K, and then increases for the higher temperatures.…”

“…In the region 3000 < T < 9000 K, pure FG silica exhibits a volume depression. 15,20 This depression region persists up to 2.5 GPa, with the temperature of the volume minimum shifting to lower values with pressure. At 10 GPa, this depression is no longer present and V m increases steadily by 9% from 2000 to 9000 K.…”

“…In this paper, we focus on the pressure range 0.25 GPa e p e 10 GPa and study the effect of pressure on structure and water speciation in silica-rich liquids using Monte Carlo simulation. The companion to this paper 15 provides a detailed investigation to the temperature and composition effects at 1.0 GPa.…”