NANOSCALE STRAIN MAPPING: A NEW LOOK AT DEFECT STRUCTURES IN MINERALS

Many mineral properties can be best understood in terms of structural perturbations at or near the atomic level. Transmission electron microscopy (TEM), especially high-resolution TEM, is unmatched for direct observation of structural perturbations such as point defects, dislocations, grain boundaries, and precipitates. However, TEM observations have largely been illustrative rather than quantitative, and most insights into physical phenomena at the atomic scale depend on existing theories and computational models.

Recent advances in digital imaging and image-processing techniques have opened the way for quantitative analysis of TEM images. One such technique is geometric phase analysis, which was developed to map strain in semiconductors and alloys at the nanoscale from high-resolution TEM images. We adapted the phase technique to analyze strain around defects in olivine. Displacements and strain fields were measured for a [100] dislocation with an accuracy better than 0.01 nm. The technique was also employed to analyze the rotation across a low-angle grain boundary in olivine. The rotation maps reveal a boundary plane that is wavy rather than flat at the nanoscale. Furthermore, analytical elastic theory indicates that this waviness is intrinsic to boundaries, independent of the host material. Finally, maps of the strain-energy distribution around dislocations provide information in the region close (within 3 nm) to dislocation cores where classical elastic theory is only considered approximate. Our results are important for understanding dislocation dynamics, interactions, and patterning in olivine and serve as an experimental check on computational methods such as finite-element and molecular modeling.

We will present an overview of the geometric phase technique with specific examples from our research on olivine and suggest several additional applications of the phase technique that could provide exciting new information about minerals at the nanoscale.