Structural aspects of biogenic silica.

The objectives of this paper are to discuss the characterization of biogenic silica in terms of structural properties, and to elucidate the mechanisms of structural organization within biological systems. The scale of organization is a critical factor in the characterization of biosilicification processes, and order at the nanometre, micrometre and macroscopic levels is described. Molecular order is discussed in the light of high-resolution transmission electron microscopy and solid-state NMR results obtained from samples of biogenic silica. Microscopic organization is expressed in a range of structural motifs, e.g. gels, sheets, fibres, tubes and globular assemblies, and reflects the infinitely adaptive morphology of biogenic silica. Macroscopic structures such as curved rods, spicules, perforated plates, teeth and reticular frameworks can be assembled from these microscopic motifs. The mechanisms of structural organization involve spatial (scalar and vectorial) constraints, ordered particle aggregation and chemical regulation. The possible importance of organic surfaces is discussed.