Structure and properties of grain and phase boundaries in rocks

• Applicant: Professor Dr. Georg Dresen
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2007 to 2012
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 22341081

Grain boundaries and phase boundaries are important structural defects in rocks. In polycrystalline materials grain interfaces are present in many different configurations forming extended three-dimensional networks very much like the networks of liquid films that constitute foams. Even though grain boundary structures are measured on the nanometer scale, the macroscopic properties of rocks such as elasticity, strength, electrical conductivity, and the efficiency of diffusive mass transport depend on the physical and chemical properties of grain boundaries. For example, variation of interfacial energy and wettability of grain and phase boundaries at different thermodynamic conditions and fluid chemistry are of basic importance for a broad range of geosciences problems that encompass melt transport at ocean ridges, interseismic strength recovery, fluid transport in geothermal and hydrocarbon reservoirs and nuclear waste disposal sites. Our conceptual view of the grain boundary structure stems largely from observations on metals and ceramics, but we simply do not know to what extent existing grain boundary models are representative for rocks. In particular, structural properties of grain and phase boundaries in rocks as a function of orientation and energy and their relation to transport properties remain largely elusive. Recent advances in experimental and analytic techniques (bicrystal synthesis, High- Resolution Transmission Electron Microscope (HREM), analytical TEM, Focussed Ion Beam (FIB), Atomic Force Microscopy (AFM), Secondary Ion Mass Spectrometer (SIMS) depth profiling etc.) as well as computational methods (ab initio electronic structure calculations (DFT), molecular dynamics (MD) simulations) allow investigating the grain boundary structure with unprecedented spatial resolution and analytical precision. In this project we propose studying the structural properties of grain and phase boundaries of an important rock-forming mineral, such as olivine. We plan to investigate the grain boundary structure using synthetic bicrystal samples. This gives the unique possibility to vary grain boundary structural parameters systematically on intact, unaltered specimens. To resolve the grain boundary structure on the atomic scale we will compare the TEM observations with predictions from atomistic models of the boundary. This approach will provide crucial input parameters for the required image simulation of the experimentally observed grain boundary.

• DFG Programme: Research Units
• Subproject of: FOR 741: Nanoscale Processes and Geomaterials Properties
• Participating Persons: Professor Dr. Sandro Jahn; Dr. Richard Wirth