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Fig. 3. TEM observation of powders of the Satonda stromatolites. (a) Bright field image showing aragonite fibres
as well as clusters of very small aragonite grains. The circles indicate the locations where electron diffraction patters
were measured (upper one on the cluster, lower one on a fibre). (b) The powder pattern shows spacings at 4.2, 3.4, 2.9,
2.7 and 2.5
˚
, which can be indexed as due to diffraction from the (110), (111), (002), (121) and (200) planes of
aragonite. (c) The single crystal diffraction pattern is consistent with aragonite and was measured along the [1 - 10]
zone axis.
We investigated the crystallographic orientation
of the aragonite in more detail by TEM and STXM
using a FIB-milled ultrathin foil, which was cut
across a Mg - Si-rich layer (Fig. 5). On both sides
of this layer, we observed the top of the underlying
aragonite laminae (left in Fig. 5) and the bottom of
the overlying aragonite laminae (right in Fig. 5).
The two distinct morphologies of aragonite
observed in the powdered samples were also
observed in this foil. In the top part of the underlying
aragonite laminae, aragonite appears as bundles of
single-crystal fibres, with their growth axes parallel
to each other and perpendicular to the laminae. In
contrast, aragonite in the overlying aragonite
laminae appears as a massive cluster of tiny arago-
nite crystals (c. 50 nm in size). The orientation of
the aragonite crystals in the FIB foil can be assessed
by electron diffraction. Single-crystal electron dif-
fraction patterns were obtained using a 100-nm
large aperture on the fibre area. These patterns
show unambiguously that aragonite fibres share a
common crystallographic orientation (Fig. 5).
Powder electron diffraction patterns were obtained
on the clusters of tiny aragonite crystals using a
1-mm large aperture. The clustering of some spots
on some rings suggests some orientation of the
nanocrystals. It is difficult, however, to get a more
comprehensive view of the crystallographic orien-
tation of aragonite in the foil. To obtain a more com-
plete view of the crystallographic orientation of
these very small aragonite domains, we thus
carried out STXM imaging and polarization-
dependent imaging contrast on the entire FIB-milled
foil. STXM has classically been used in the Earth
Sciences to characterize the speciation of diverse
elements such as C, N, O and heavy metals and met-
alloids such as at the nanoscale (e.g. Haberstroh
et al. 2006; Bernard et al. 2007; Benzerara et al.
2008; Lepot et al. 2008). Some organic carbon
could be detected by STXM in the FIB foil (data
not shown); however, it was not possible to decipher
whether this organic carbon was indigenous or was
an artifact resulting from impregnation of the
sample with epoxy. Here, we used the sensitivity
of X-ray absorption spectroscopy to the crystallo-
graphic orientation of aragonite, an effect com-
monly referred to as X-ray linear dichroism (e.g.
Metzler et al. 2008; Zhou et al. 2008). By varying
the direction of the polarization vector of the
X-ray beam from 0 to 908 in 108 steps and measur-
ing the absorption of X-rays at 290.3 eV for each
50-nm pixel of the FIB-milled foil, it was possible
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