Structure and Properties of Albite Melt at High Pressures

Abstract

We have used first-principles molecular dynamics simulation to examine the high-pressure behavior of an aluminosilicate melt with albite stoichiometry (NaAlSi3O8). We have explored the equation of state and transport properties up to a pressure of 25 GPa and over a range of temperatures (2500–4000 K). Our results show that upon compression of up to 5 GPa, the initial densification in an albite melt occurs by the reduction of the T–O–T bond angle; however, the Si–O coordination remains virtually unchanged. Upon compression beyond 5 GPa, the densification occurs via changes in the T–O coordination. We also find that at lower isotherms, i.e., 2500–3000 K, the viscosity decreases upon compression and there is a concomitant enhancement of the diffusivity of the network-forming tetrahedral cations (silicon/aluminum) and oxygen anions. However, there is a reversal in the trend of viscosity and diffusivity upon further compression. For all of the temperatures explored in this study, at higher pressures, i.e., >10 GPa, the viscosity increases with increasing pressure, whereas the diffusivity decreases with increasing pressures. This behavior of the melt transport property at high pressures is expected, contrary to the observed behavior of melt transport at low pressures and low temperatures. The pressure-dependent anomalous transport properties are very likely related to the formation of the 5-fold coordinated aluminum ions that have a shorter lifetime compared to those of the more stable 4-fold and 6-fold coordinated units.

Insights into magma ocean dynamics from the transport properties of basaltic melt

“…1). This is consistent with the systematic difference between LDA and GGA, which has been well documented in silicate melts 45,56 . Along the 2200 K isotherm and up to 35 GPa, pressures calculated in our study are larger than the recent FPMD simulation which used the GGA method 46 .…”

“…However, the diffusivity of Al, Si, and O at the lower temperature of 2200 K show positive pressure dependence at lower pressure up to ~8 GPa. This behavior is similar to previous work on polymerized aluminosilicate melt 56,62,63 as well as depolymerized melt such as MgSiO 3 at lower temperatures 64 . A comparison of our results with previous LDA results…”

“…where A = a + bP + cP 2 and B = d + eP + fP 2 with a = −6.8 ± 0.2, b = 0.03 ± 0.006, c = −0.0012 ± 0.00013, d = 2241 ± 621, e = −33.5 ± 15, f = 3.5 ± 0.5, and T VFT = 1527 ± 121 K. The decrease in viscosity with compression at low pressures and low temperatures has also been reported in other polymerized silicate melts in prior computational 43,45,56,[66][67][68] and experimental 29,31,36 studies (Fig. 3).…”

The Critical Point and the Supercritical State of Alkali Feldspars: Implications for the Behavior of the Crust During Impacts

“…They are shown in Figures 2c and 2d. Table 1 shows the results and the comparison to existing data in the literature, both experimental (Lange, 2007; Tenner et al, 2007) and calculated (Bajgain & Mookherjee, 2020). The ρ 0 values extrapolated from our simulations using a second‐order polynomial are 2.26 g cm −3 at 1373 K for NaAlSi 3 O 8 , that is, about 2.7% smaller than the experimental values (Lange, 2007).…”

Structures and transport properties of supercritical SiO2-H2O and NaAlSi3O8-H2O fluids

“…Our results also show that when the water content exceeds 20 wt%, the viscosities of the SiO 2 -H 2 O systems at 3000 K are very similar to those of the NaAlSi 3 O 8 -H 2 O systems at the same temperature (FigureS12in Supporting Information). Although the pressures of the two systems are not exactly the same, a pressure difference of less than 1 GPa has a weak effect on the viscosity under high temperature conditions(Bajgain and Mookherjee, 2020;Karki et al, 2011). Therefore, the viscosity of supercritical fluids with different compositions at the same temperature and pressure may be similar.…”