Visualizing frozen point defect tracks in Fe-containing olivines

Using a combination of first-principles calculations and Monte Carlo simulations, we show that Fe-containing silicates such as olivines naturally offer a way for visualizing tracks left by diffusing vacancies. Fe in its 2+ and 3+ valency states prefers two distinct cation sites in the olivine structure. Vacancies formed at the cationic M sites, cause neighboring Fe ions in their normally occurring Fe2+ state to change valency to Fe3+, compensating for the charge imbalance and reducing energy costs, consequently altering the local site preference of Fe. Once the vacancy diffuses away, Fe atoms remain stuck in their metastable location producing a microscopic record of the vacancy's trajectory. Our results may be verified using high-resolution transmission electron microscopy, combined with electron energy-loss spectroscopy. Copyright (C) EPLA, 2012