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DOE OSTI2020

Materials Data on Na2SiO9 by Materials Project

Na2SiO9 crystallizes in the monoclinic P2_1 space group. The structure is two-dimensional and consists of one Na2SiO9 sheet oriented in the (0, 0, 1) direction. there are two inequivalent Na sites. In the first Na site, Na is bonded in a 2-coordinate geometry to seven O atoms. There are a spread of Na–O bond distances ranging from 2.23–3.04 Å. In the second Na site, Na is bonded in a trigonal planar geometry to three O atoms. There are a spread of Na–O bond distances ranging from 2.27–2.32 Å. Si is bonded in a tetrahedral geometry to four O atoms. There are a spread of Si–O bond distances ranging from 1.59–1.86 Å. There are nine inequivalent O sites. In the first O site, O is bonded in a bent 120 degrees geometry to one Na and one Si atom. In the second O site, O is bonded in a distorted single-bond geometry to one Si and one O atom. The O–O bond length is 1.33 Å. In the third O site, O is bonded in a distorted trigonal planar geometry to two equivalent Na and one Si atom. In the fourth O site, O is bonded in a distorted trigonal planar geometry to two Na and one Si atom. In the fifth O site, O is bonded in a linear geometry to one Na and one O atom. The O–O bond length is 1.25 Å. In the sixth O site, O is bonded in a bent 120 degrees geometry to one Na and one O atom. The O–O bond length is 1.23 Å. In the seventh O site, O is bonded in a distorted single-bond geometry to one Na and one O atom. In the eighth O site, O is bonded in a 1-coordinate geometry to one Na and two O atoms. The O–O bond length is 1.95 Å. In the ninth O site, O is bonded in a 2-coordinate geometry to one Na and two O atoms.

36 MATERIALS SCIENCE↗

NASA NTRS1992

Estimation of the viscosities of silicate liquids at high pressure from measurements of oxygen diffusivity

The dynamics and evolution of a magma ocean depend significantly on the structure and physical properties of silicate liquids at high pressure. Viscosity is particularly important because of its effect on the dynamics of convection and crystal setting. Traditionally, the viscosities of silicate liquids have been measured at high pressure by falling sphere viscometry but, due to some limitations of this technique, few measurements have been made at pressures exceeding 2.5 GPa. An alternative approach, which has previously been proven up to 2 GPa, is to use high pressure measurements of oxygen self-diffusivity (D) to estimate viscosity (eta) from the Eyring relationship eta = kT/D lambda (where k is the Boltzmann constant, T is absolute temperature, and lambda is the diffusive jump distance). We have performed preliminary experiments on Na2Si4O9 liquid in order to explore the feasibility of this method up to pressures in excess of 10 GPa. Diffusion couples were prepared from glass starting materials with one half of the sample enriched in O-18 and the other half containing the natural abundance (0.2 wt percent). Diffusion experiments were performed at 1600 to 1800 C at pressures in the range of 2.5 to 10 GPa for times up to 6 minutes using a 1200 ton multianvil apparatus. Oxygen diffusivities were derived from the resulting O-18 concentration profiles, which were analyzed using an ion microprobe. At 1800 C, oxygen diffusivities increase continuously from 1 x 10(exp -10) m(exp -2) s(exp -1) at 2.5 Gpa to 5 x 10(exp -10) m(exp -2) s(exp -1) at 10 GPa. From the Eyring relationship, the viscosity of Na2SiO9 liquid is predicted to decrease by approx. 0.7 log units over this pressure range at 1800 C. These trends, which can be related to changes in the structure of the liquid at high pressure, are in agreement with results of molecular dynamics calculations. These results suggest that it should be possible to estimate the viscosities of silicate liquids up to at least 15 GPa and 2200 C by this method.

Rubie, D. C.↗