Earth and Planetary Materials

Earth's core formed very early in the planet's history, and is thought to have carried with it atomic species other than iron. Although it is isolated from the surface, it influences our environment through the production of Earth's magnetic field. To understand the process by which the field is generated and how it changes with time, several properties of iron and iron alloys must be understood first. Along with major advances in experimental methods, first principles studies of iron have contributed substantially to our understanding of Earth's core over the last several years. Future progress demands a consideration of iron with other alloying elements. We know light elements must be present in the solid inner core and liquid outer core from seismic density measurements. The major candidates are O, Si, and S, although, in principle their identity is virtually unconstrained. Important recent work has shown one possible way to calculate the concentration of individual light elements in the inner and outer parts. But major questions remain regarding: a) phase relations in the solid. Is the amount of light element in the inner core sufficient to stabilize new crystalline phases? b) What is the liquidus temperature of the outer core alloy (and therefore the temperature at the inner core boundary)? c) How does simultaneous consideration of multiple alloying elements change the partitioning determined with a single element?

In addition to examining the deep interior of the Earth, we are interested in the phase properties of hydrated silica (SiO₂) liquids. Silica plays an important role in the chemistry and geology of the Earth, accounting for more than 50 wt.% of the crust and 40 wt.% of the mantle. The physical and chemical properties of silicate melts are known to be sensitive to the presence of dissolved volatiles, such as water and carbon dioxide. Many experimental studies have sought to better understand the role of water, including its speciation, within silica-rich liquids, but these techniques are limited by the difficulty of performing in situ measurements under conditions similar to those of the Earth's interior. We use molecular simulation to model these systems and examine reactivity, structure, and speciation on an atomic level. Unanswered questions include: a) What is the speciation of water in silica-rich fluids (i.e. does molecular water - H₂O - persist or does water dissociate to form OH-)? b) What are the PVT properties of silica-water mixtures? c) Where does the miscibility gap in water-silica mixtures lie?

This project is one part of the Virtual Laboratory for Earth and Planetary Materials at the University of Minnesota. For more information on the Virtual Laboratory, please visit their homepage, www.vlab.msi.umn.edu.

Recent Publications on Earth and Planetary Materials:

K.E. Anderson, L.C. Grauvilardell, M.M. Hirschmann, and J.I. Siepmann,

'Structure and speciation in hydrous silica melts. 2. Pressure effects,'

J. Phys. Chem. B, submitted for publication.

K.E. Anderson, M.M. Hirschmann, and J.I. Siepmann,

'Structure and speciation in hydrous silica melts. 1. Temperature and composition effects,'

J. Phys. Chem. B, submitted for publication.

D. Bhatt, N.E. Schultz, A.W. Jasper, J.I. Siepmann, and D.G. Truhlar,

'Phase behavior of elemental aluminum using Monte Carlo simulations,'

J. Phys. Chem. B, 110, 26135-26142 (2006).