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Characterizing the Crystallographic Orientation of Snow Crystals with Electron Backscatter Diffraction
  • Evan Schehrer,
  • Kevin Hammonds
Evan Schehrer
Montana State University

Corresponding Author:[email protected]

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Kevin Hammonds
Montana State University
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Abstract

Electron backscatter diffraction (EBSD) is an advanced materials characterization technique that has been used to determine the crystallographic orientation of glacial ice specimens, along with a plethora of other inorganic crystalline materials. However, due to the specific sample preparation requirements necessary for performing EBSD (i.e., flat, smooth, clean surfaces), characterizing porous and ephemeral materials such as seasonal snow and/or firn has always been a challenge. In this work, we demonstrate a methodology for performing EBSD on naturally-collected and laboratory grown snow specimens of depth hoar and surface hoar snow grains. These grain types were chosen because characterizing the primary growth orientation of these snow crystal types may allow for greater insight into many fundamental physical snow processes including mechanical behavior, metamorphism, and radiative transfer properties. Thus far, our results show that plate-shaped surface hoar grains grow preferentially along the primary prism axis, the most favorable and lowest energy face in the Ice 1h crystal structure. Depth hoar crystals were found to be more complex, such that growth may vary greatly depending on the temperature gradient, vapor flux, and supersaturation within the snowpack. EBSD results show that depth hoar can grow along each of the growth planes (basal, primary, and secondary planes), while secondary electron images and optical microscopy reveals the complex step-like features associated with depth hoar crystals grown along the basal growth plane due to vapor deposition. Continued research aims at growing depth hoar crystals in a controlled laboratory environment from ice substrates with known crystal orientations to further investigate the growth rates along each of the different growth planes.