Page 279 - Microtectonics

272 10 · Special Techniques
EBSD is a very powerful method to investigate microstruc-
ture in rocks, and presently more popular than EC patterns
since resolution is higher and patterns are easier to deter-
mine (×Photo 10.3a–e). The interpretation of the diffraction
patterns through indexing of lines in the diffraction patterns
is now highly automatised and can be carried out by a number
of computer programs (Fig. 10.8c, ×Photo 10.3d,e).
Combinations of atomic number contrast, OC and
EBSD-pattern information for a sample provide a power-
ful tool in microtectonics. It is standard practice to make
atomic number contrast and OC images of a microstruc-
ture, and then select a large number of points where EBSD
patterns are determined and translated into orientation
data. In this way, the complete crystallographic preferred
orientation of a microstructure can be obtained quickly,
and linked to a spatial image (×Photo 10.3a–e). A useful
way to present such data is through a grain boundary
misorientation map, where grain boundaries are marked
by the angle of misorientation separating neighbouring
grains and subgrains (Trimby and Prior 1999). Attempts
are being made to automatise the process completely, to
make complete maps of the atomic number contrast and
the orientation of the material over a fine, regular grid.

10.2.4.4
Sample Preparation

Samples used in the SEM are studied under vacuum, and
should therefore be dry. This is no problem with most
rock samples, but clay samples may have to be dried be-
fore use. One specific problem of the SEM is that the elec-
tron beam, which hits the specimen, tends to cause local
electrostatic charging which can cause beam deflection,
thereby distorting the image. It is therefore necessary that
samples are conductive. This is a problem in most rock
samples, which have to be coated with a thin conductive
layer of a metal such as gold (secondary electron-mode
images) or carbon (back-scatter electron-mode images).

Fig. 10.8. a Electron channelling (EC) pattern from quartz; the white
cross defines the pole to a positive rhomb (1011) plane (Lloyd 1987).
The diameter of the pattern covers ~20°. b Electron backscattered dif-
fraction (EBSD) pattern from quartz. The pattern extends over a sig-
nificantly larger angular area (~80° square in this case) than the EC
pattern, indicated by a white circle, and is therefore much easier to
index. c EBSD pattern indexing (e.g. quartz). A number of commer-
cial computer-based systems are available to index EBSD patterns. They
are all based on a comparison between the configuration of Bragg
lines/bands in the observed pattern (b) with those predicted by
theory (c) and stored in a database of crystal diffraction characteris-
tics for each mineral phase (see Prior et al. 1999). Indexing can be ei-
ther manual or automatic and can be used to define the complete crys-
tal orientation (via three spherical Euler angles), provide mineral iden-
tification (i.e. quartz), the spatial coordinates of the pattern within
the sample, a numerical indication of pattern quality (i.e. ‘band con-
trast’) and the mean angular deviation (‘goodness-of-fit’) between
observed and predicted patterns. (Images courtesy Geoffrey Lloyd)