Geology Reference
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Fig. 4. SEM observation of polished sections of the Satonda stromatolites. (a) Image of an aragonite lamina collected in
the low angle backscattered electron mode and revealing crystal directions. The fibrous texture is visible and the
aragonite crystals are mostly oriented NNE - SSW. (b) Image of the same area collected in the secondary electron mode
and showing the aragonite fibres. (c) At higher magnification, these fibres show a rough surface and seem to consist of an
aggregation of tiny crystals.
to map the orientation of the in-plane projection of
the c-axis of aragonite (Fig. 6). Interestingly,
although TEM provides a better spatial resolution
for imaging than STXM, it is limited for electron
diffraction by the size of the smallest aperture, that
is, in this case, 100 nm. This is larger than the spatial
resolution achieved by STXM for the mapping
of the orientation of aragonite c-axis as shown
hereafter. Finally, STXM polarization-dependent
imaging contrast is much less sensitive to variations
of the orientation of aragonite than electron diffrac-
tion, in particular out-of-plane variations, and can
thus be more efficient in providing an average
view of the crystallographic texture of aragonite in
the whole FIB foil. From this dataset, several obser-
vations can be made. First, the aragonite laminae
are composed of micrometre-sized domains that
respond homogeneously to the polarization changes
(see areas 1, 2, 3 and 4 in Fig. 6). This response
means that the in-plane projections of the c-axes
of the grains present within these areas share a
common orientation. Area 4 corresponds to the
bundle of aragonite fibres observed by TEM in the
aragonite laminae underlying the Mg - Si-rich
laminae. The whole area is more absorbing at 08
than at 908. The fit of the variation of absorption
in A cos 2 (u2 f) gives a single value for the orien-
tation of the c-axis of aragonite (f) which is
around 1558 (equivalent to 2258). The STXM data
thus confirm that the c-axes of these crystals (which
are also the growth axes of aragonite) are roughly
parallel to each other and parallel to the growth axis
of the fibres as observed by TEM. Area 3 corre-
sponds to the massive cluster of nanocrystals
observed in Figure 5 at the bottom of the underlying
aragonite layer. Interestingly, this area also shows a
homogeneous response to the variations of polariz-
ation direction, with a f value of 1078. This
observation suggests that the c-axes of these nano-
crystals also show a preferential orientation. It is
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