Improving Phyllosilicate Electron Backscatter Diffraction Data Using Ion Milling
Abstract
Phyllosilicates are difficult phases to characterize using electron backscatter diffraction (EBSD), as they tend to yield poor diffraction patterns. The underlying cause of this problem is under debate, however, prior studies point to poor sample surface preparation and problems inherent in the phyllosilicate structure. Traditional protocol for sample surface preparation for EBSD involves stepwise mechanical polishing followed by chemical mechanical polishing. Although variations of this procedure have proven relatively successful, problems still exist when preparing polyphase materials that pair soft and hard phases, i.e. mica and quartz. Ion milling is a technique ideally suited for EBSD sample preparation as it offers the ability to smooth sample surface topography and remove subsurface damage induced by mechanical polishing. We explore the viability of this method as a final step in preparing polyphase, mica-bearing geological materials for EBSD analysis. We prepared a suite of samples using a combination of mechanical and chemical mechanical polishing. The samples were collected from the Northern Mylonite Zone of the Cheyenne Belt in southern Wyoming and include mylonitic muscovite-kyanite quartzite and garnet-muscovite-biotite schist. Three thin sections of each lithology were cut in different orientations according to a kinematic reference frame in an effort to introduce orientational variability to our data and to determine whether an optimal orientation exists for acquiring electron backscatter diffraction patterns (EBSPs) from mica. EBSD data were collected on each sample using an FEI Nova 600i scanning electron microscope with an EDAX-TSL EBSD attachment. All working conditions and data collection areas were carefully noted for reproducibility. Sample surface topography, likely caused by preferential mechanical polishing of softer phases, was exaggerated in our sections and is responsible for relatively poor initial EBSD data. Ion milling, using the South Bay Technology, Inc. IBS/e ion beam sputter deposition and etching system, at high energy (600eV, 5.0mA) and low incident angle (4 degrees) while oscillating our sample for 20 minutes was used in an attempt to reduce topography. This was followed by a low energy step (250eV, 2.0mA, 20 minutes) to remove any damage imparted by prior polishing. Samples undergoing this treatment show an increase in surface topography and a decrease in the overall image quality of EBSPs. Sample surface topography was a limiting factor in our experiment, forcing us to examine before and after EBSD data within individual phases. Samples showing overall image quality improvement are those that were continuously rotated at low incident angles (4 degrees) using short, high and low energy steps regardless of orientation. Our results suggest that improvement of image quality of phyllosilicate EBSPs is possible with an ion mill but analytical limitations remain, possibly owing to phyllosilicate crystal structure and lattice defects.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V51C2213W
- Keywords:
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- 5194 PHYSICAL PROPERTIES OF ROCKS / Instruments and techniques