Site preference of Fe atoms in the olivine (Fe$_x$Mg$_{2-x}$)SiO$_4$ and its surface

Olivine is involved in many natural reactions and industrial reactions as a catalyst. The catalytic ability is highly possible rely on the Fe$^{2+}$ in olivine. We use density functional theory calculation and thermodynamics to investigate the site preference of Fe atom in olivine which composition from iron-rich to iron-poor and its surfaces. The Fe$^{2+}$ always shows its high spin (quintet) state which has larger ion radius than Mg$^{2+}$ in olivine crystal and surfaces. The Fe$^{2+}$ inside the surface slab prefers the smaller M1 site than M2 site by enlarging the metal-oxygen octahedra when occupied the metal site as in the bulk system. Energy contribution of entropies accumulation caused temperature raise stops this preference at the temperature where a cation order-disorder distribution energy crossover happen in olivine. Surface exposed site provide Fe$^{2+}$ large space due its unsaturated nature. This lead a higher level of preference of Fe$^{2+}$ to the surface site than any metal site inside the crystal no matter M1 or M2 site is exposed. This indicate the Fe$^{2+}$ in the bulk system can diffuse to a metal site exposed on the surface driven by the energy difference. Many reactions can use the on surface Fe$^{2+}$ as a catalyst because of the active chemical behavior of Fe. Meanwhile this energetics preference should be considered in the future model to explain the natural observed zoning olivine have a high Fe edge and low Fe center. These microscopic understanding can be essential to many olivine related geochemical and astrochemical reactions.
Comment: 8 figures