Chemistry Reference
In-Depth Information
depicted in Figure 2 and it is difficult to obtain detailed structural information on the nature
of these particles and how they are arranged within the silicified structures. Detailed
structural studies of siliceous biominerals are, however, an extremely important starting
point in studies of biosilicification. The structure of a particular mineral provides, in its
make-up, information that may point to the path/mechanism by which it was formed. If we
could correctly “read” the information that structures provide we may proceed to a much
deeper understanding of the biosilicification process for the full range of organisms.
A wide range of techniques is now available to help us understand the structure of
these complex amorphous materials. Examples of these techniques and the information
they provide are described below.
Techniques for the study of biosilica structure
Small-angle and wide-angle X-ray diffraction techniques that can detect order at the
nm level (i.e., to look for small crystalline regions within a structure) have been applied to
both diatoms (Vrieling et al. 2000) and the primitive plant
Equisetum arvense
(Holzuter et
al. 2003) and have found no evidence for crystallinity although small differences in
instrument response to the technique have been observed by the authors for analysis along
the length and across the fibers of the siliceous sponge from the Hexactinellida family,
N.B. no crystallinity was detected. Other biosilicified materials should also be examined by
this technique. Recent studies of model silica-protein composites and diatoms by ultrasmall
angles (USAXS) have shown that structures on different length scales (including pore
structures) could be identified by this technique (Vrieling et al. 2002).
Electron microscopy has been extensively used to look at the sizes of fundamental
particles and their interconnection one to another. Figure 7 provides structural
information on particle size and orientation for small regions of silica that were removed
from plant hairs where a range of particle sizes and arrangements were observed. The
technique can be used to follow structural changes with time and gel-like and particulate
structures can be observed within the same organism at different development times,
Fibrous
Fibrous
Globular
Globular
Variable
Density
Variable
Density
200 nm
200 nm
Figure 7
. Transmission electron microscope image of silica from hairs on the lemma of
Phalaris
canariensis
L. Examples of fibrillar, globular and other structural arrangements all obtained from a
single cell hair sample.
