Extended Defects in Olivine and their Impact on Diffusive Reaction Kinetics

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This project comprises an experimental study examining (a) the effects of thermodynamic potentials?temperature, pressure and, particularly, chemical potentials of oxygen, oxides and halides?on the formation and persistence of extended defects in olivine and (b) the effect of such extended defects on transport coefficients of rare earth cations and halide anions. The analytical techniques to be employed for characterization of the defects emphasize high-resolution, energy-filtered transmission electron microscopy (TEM; HRTEM) and (local-electrode) atom-probe tomography (APT; LEAPTM). The proposed APT work is developmental: we are among the earliest investigators in the petrology community attempting to apply this promising technique?in which one can characterize the location within a crystal of specific atoms (and potentially isotopes)?to the characterization of minerals (which are, to first order, electronic insulators) and of mineral reactions.The capability of the Earth and planetary sciences community to understand the thermomechanical and thermochemical evolution of the terrestrial planets requires a deep and supple understanding of ionic diffusive dynamics in silicate and oxide mineral and their melts. Because of the number of chemical components in geological systems, there are a variety of kinetic paths available for diffusion-effected reactions, particularly under conditions of a high driving potential, i.e., where phases out of equilibrium with one another are suddenly placed in contact. This situation happens frequently in nature; it happens almost always in experiments used to understand mineral dynamics. For example, this research addresses directly a current controversy in petrology where the transport coefficient for rare-earth cations in ferromagnesian olivine has been measured in different experiments?both considered authoritative and ?good??and yet indicate a kinetics difference of almost 104! Metastable (and perhaps transient) defect structures in olivine in the faster case are anticipated; we will characterize these structures, their thermochemical persistence and their impact on dynamics. This project will (a) educate a graduate student in these particular aspects of petrology, mineral physics and ionic materials science and (b) discern the possibilities and develop the approaches of APT/LEAPTM for application to minerals and synthetic, multicomponent oxides. The research has application, also, to the design and characterization of new materials for use as oxide electrodes in batteries and fuel cells. Our work will emphasize petrology; we are, however, aware of the potential of the research for societal needs with regards to energy technology.