Chemistry Reference
In-Depth Information
crystals (Berman et al. 1993). These are usually glycoproteins, and it has been
shown by single-crystal, high-resolution X-ray diffraction analyses that they are
often located along specific crystal planes. Despite the fact that they are very
large compared to the atomic order within the crystal, the glycoproteins do not
cause major dislocations. Some disorder does result, as can be detected by line
width broadening. These sites (probably indistinguishable from grain boundaries)
within the crystal bulk are also possible locations for trace metals. The presence
of occluded macromolecules and ions influences the solubility of the mineral
phase, and this may directly bear on the mechanisms of differential dissolution of
different genera of foraminifera and coccolithophoridae observed in deep-sea
sediments.
Crystal shape.
The shape of a crystal reflects the nature of the environment in which
it grows. In biology this environment, and hence crystal shapes, are generally very well
controlled. Detailed analyses of crystal shapes and, in particular, the exact crystal faces
expressed can shed much light on the mechanisms of growth. If a very unstable face is
expressed, then some process must be responsible for its stabilization. The unusual face
may be a nucleating plane, as has been suggested for the formation of certain types of
calcium oxalate monohydrate crystals in plants (Bouropolous et al. 2001) and calcareous
sponge spicules (Aizenberg et al. 1995). Another example of unstable faces being
expressed occurs in the coccoliths (Young et al. 1999; Young and Henriksen 2003).
These faces could in part explain different solubilities of coccolith calcite between
species, as unstable faces are more soluble than stable ones. Curved surfaces of calcite
are quite common, especially in cases where the crystal grows in a vesicle. These
surfaces may be curved down to the atomic level. How such a curved surface is stabilized
is not known, nor are its solubility characteristics. In general, detailed characterizations of
crystal surfaces may be most helpful in understanding modes of formation and, in turn,
vital effects, as well as solubility properties.
Ontogenetic variations.
Variations in mineralization are known to occur in many
different genera, including foraminifera. These too need to be understood in order to
determine whether or not they can contribute to the complexities of equilibrium and non-
equilibrium deposition. It may also be possible that the change in mineralization
mechanism during growth is not a switch from one mechanism to another, but the
addition of another mechanism in the juvenile and adult stages. Lowenstam and Weiner
(1989) did raise this possibility for the scleractinian corals vis-a-vis the trabeculae centers
of calcification.
Crystal maturation.
Once formed, crystals may continue to change over long periods
of time, especially if their surface to volume ratios are very high. This is the case in bone,
where a sintering process occurs even within the lifetime of the animal and continues
after death (Legeros et al. 1987). Thus diagenetic processes, even in the deep sea may not
only involve dissolution, but also sintering. Obviously if in a closed system, this should
have little effect on proxies. It can, however, complicate efforts to understand the basic
mechanisms of deposition vis a vis proxies, as the mature mineral may not faithfully
reflect the mineral phase formed initially. Chemical change may also occur during
diagenesis. It has been shown, for example, that aragonite formed secondarily in coral
skeletons even while the coral was still submerged (Spiro 1971).
Concluding comment
Given our present state of knowledge, the prospects for finding simple, robust
explanations for different vital effects in terms of biomineralization mechanisms may
seem hopeless at worst, and challenging at best. This, in our opinion, is most likely not
