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Fig. 5. TEM imaging of the FIB foil. (a) Negative image (bright field mode) of the whole FIB foil. The Si - Mg-rich
lamina appears as a smooth dark grey area on the left hand side of the foil (area delimited by the dashed curves).
Three electron diffraction patterns were obtained from different areas in the fibre area with a 100-nm wide aperture
showing a common crystallographic orientation. One electron diffraction pattern was measured on the right hand side
of the Mg - Si-rich lamina using a 1-mm wide aperture. On this powder pattern, the clustering of diffraction spots
shows a common crystallographic orientation of different grains within the probed area. On the left hand side of the
Mg - Si-rich lamina aragonite appears as fibres (see magnified image). (b) Arrows indicate the general direction of the
fibres. On the right hand side, aragonite appears as clustered small domains [some grains are indicated by arrows in
the magnified image (c)].
possible that these nanocrystals form larger-scale
fibres by clustering, as supported by SEM images,
but this cannot be ascertained based on TEM and
the STXM images. STXM offers, however, a view
differing from that of SEM. While SEM images
show a general fibrous texture within the carbonate
laminae with fibres perpendicular on average to the
laminae surfaces, STXM suggests that the orien-
tation of the in-plane projection of the c-axis of
aragonite varies significantly within a lamina from
the bottom to the top.
features have been previously observed using
more classical methods and include the hetero-
geneous microfabrics with either micritic (fine-
grained) or sparitic (fibres) textures (e.g. Riding
2000) that have been related to various biotic or
abiotic mineralization processes. Yet, some of
these features are interesting in the light of their
formation mechanism. Stromatolites have often
been considered as biologically induced formations,
that is, resulting from chemical variations (e.g. pH,
alkalinity) indirectly induced by the metabolic
activity of diverse microorganisms such as cyano-
bacteria and/or sulphate-reducing bacteria (e.g.
Dupraz & Visscher 2005; Altermann et al. 2006).
In addition to these metabolic processes, it has
been shown that extracellular polymeric substances
(EPS) may play a complex but significant role in the
Discussion
The present study provides a basic description at
the submicrometre-scale of the mineral texture of
aragonite in Satonda stromatolites. Some textural
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