Characterizing the Crystallographic Orientation of Snow Crystals with
Electron Backscatter Diffraction
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.