- Grew, E.S., Yates, M.G., Beane, R.J., Floss, C. and Gerbi, C., accepted. Chopinite-sarcopside solid solution, [(Mg,Fe)_3 ](PO_4 )_2 ,
in GRA95209, a transitional acapulcoite: Implications for phosphate genesis in meteorites. American Mineralogist. - Johnson, S.E., Lenferink, H.J., Price, N.A., Marsh, J.H., Koons, P.O.,
West, D.P., and Beane, R.J., accepted. Clast-based kinematic vorticity gauges: the effects of slip at matrix/clast interfaces. Journal of Structural Geology - Beane, R.J. and Field, C., 2007. Kyanite deformation in whiteschist of the UHPM Kokchetav Massif, Kazakhstan. Journal of Metamorphic Geology v. 25, p. 117-128.
- Hawkins, A.T., Selverstone, J., Brearley, A.J., Beane, R.J., Carlson, W.D. and Ketcham, R., 2007. Origin and mechanical significance of honeycomb garnets in high-pressure metasedimentary rocks from the Tauern Window, Eastern Alps. Journal of Metamorphic Geology v 25, p. 565-583.
- Beane, R. J., 2004. Using the scanning electron microscope for discovery based learning in undergraduate courses. Journal of Geoscience Education, v. 52, p. 250-253.
- Msall, M.E., Dietsche, W., Beane, R., Wichard, R. and Carpenter, J.*, 2004. Examination of an unusual grain boundary in CaF2. Physica Status Solidi (c), v. 1, no. 11, p. 2983-2986.
- Beane, R. J. and Prior, D.J. in revision. Crystallographic orientation evidence for the formation of atoll garnet in eclogite. Geology.
Electron Backscatter Diffraction (EBSD)
In geology, electron backscatter diffraction (EBSD) is a powerful tool for the observation and analysis of microstructures and for phase identification.
Overview
Bowdoin College's electron backscatter diffraction (EBSD) system was purchased with funds awarded through the NSF Major Research Instrumentation program (proposal 0320871 funded to Rachel Beane).
Features
The EBSD system, by HKL Technology Inc., includes a Nordlys II EBSD Detector, forescatter detectors and software for orientation mapping (stage and beam control), texture determination, and phase identification (using the American Mineralogist Geological Phase database).
The system is attached to a LEO 1450VP SEM (variable pressure scanning electron microscope) with an EDAX energy dispersive spectrometer (EDS) for mineral chemistry.
How it Works
The EBSD system uses backscattered electrons (BSE) emitted from a specimen in a SEM to form a diffraction pattern that is imaged on a phosphor screen.
Analysis of the diffraction pattern allows identification of the phase and its crystal lattice orientation.
The scanning and mapping capabilities of the system permit rapid acquisition of data, from polished rock thin sections, at sub-micron resolutions.
Among other uses, these data may be applied to evaluate crystallographic preferred orientations (CPO) of mineral fabrics, and to examine misorientation axes and angles that may signify processes such as subgrain development and dislocation creep.
Researchers interested in applying EBSD methods to textural problems in rocks are encouraged to contact Professor Rachel Beane for possible collaborations.
Methods
The typical methods this lab uses for collection and processing of EBSD data follow. Specific projects will vary from these methods.
Electron backscatter diffraction (EBSD) analyses are conducted at Bowdoin College on a LEO 1450VP SEM outfitted with an HKL Nordlys II detector and Channel 5 software. Samples are prepared by taking standard polished thin sections weighted with halved brass rods (M. Cheadle, personal communication) and polishing an additional six hours in a non-crystallizing colloidal silica suspension on a Buehler Vibromet2 vibratory polisher (SYTON method of Fynn and Powell, 1979).
Thin sections are not carbon coated; charging is minimized by using a chamber pressure of 10-15 Pa, combined with the 70° tilt. Operating parameters for collecting EBSD patterns are an accelerating voltage of 20kV, working distance of 25 mm, and probe current of 2.2nA.
Channel 5 acquisition and indexing settings vary by phase, but typical values are 2x2 or 4x4 binning, high gain, Hough resolution=75, 7 bands, and 80 reflectors. Mean angular deviations between the detected Kikuchi bands and the simulations are less than 1.3 degrees (and often less than 0.8 degrees). Data are post-processed by removing wild spikes, removing observed systematic misindexing, and by extrapolating zero solutions based on 4 neighbors (if required).