HR: 10:35h
AN: V21E-02    [PDF]
TI: First Principle Study of Olivine Solid Solutions
AU: * Mookherjee, M
EM: mainak@umich.edu
AF: Department of Geological Sciences, University of Michigan C.C. Little Building, Ann Arbor, MI 48109 United States
AU: Stixrude, L
EM: stixrude@umich.edu
AF: Department of Geological Sciences, University of Michigan C.C. Little Building, Ann Arbor, MI 48109 United States
AB: Intra-crystalline cation exchange between two octahedral M-sites in olivine [(Fe,Mg)$_{2}$SiO$_{4}$] has received considerable attention because of its importance on thermodynamics, transport and other physical properties of this major mantle mineral. Moreover, the pure Fe end-member, fayalite displays anti-ferromagnetic ordering below room temperature. The presence of magnetic ordering at such low temperatures may still have a significant impact on thermodynamic properties of solid solution at high temperatures. In order to gain further insight, olivine solid solution (Fe-Mg) is investigated using {\it ab initio} total energy calculations based on local density approximation (LDA) and generalized gradient approximation (GGA) of density functional theory. The nature of Fe-Mg order-disorder across crystallographically distinct M1 and M2 sites were studied. We performed spin-polarized calculations, treating Fe$^{2+}$ in a high-spin state. Initially various configurations of Mg and Fe in M1 and M2 sites were generated in order to construct an effective Hamiltonian and to obtain the atomic interaction parameters ({\it J}) between M1-M1, M1-M2 and M2-M2 sites. We used Monte Carlo simulation to obtain the equilibrium cation arrangements for various compositions across the Mg-Fe join. In agreement with experiments, our calculations show that at low temperature, Fe$^{2+}$, prefers the M1 site over the M2 site, and disorders at higher temperature. However, unlike neutron diffraction experiments, we do not find high temperature reversal (Fe in M1 at low temperature to Fe in M2 at high temperature). We are exploring the possible cause of such discrepancies. We intend to explore the effects of ordering of magnetic spins both at the Fe end-member and across the Fe-Mg join, and its interaction with the atomic ordering. We are also extending the present findings to higher pressures of geophysical relevance.
DE: 3620 Crystal chemistry
DE: 3900 MINERAL PHYSICS
DE: 3939 Physical thermodynamics
DE: 3999 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting