Modelling Biomineralization: Studies on the Morphology of Synthetic Calcite

Abstract

Many organisms possess the remarkable ability to deposit single crystals of calcite (CaCCO3) with morphologies not normally observed in the inorganic world [1]. Whilst it is true that single geological crystals of calcite can exhibit an enormous range of different habits, all these forms have common interfacial angles and symmetry as described by the R̄3c space group. By contrast, the external forms of some biological single crystals of calcite have symmetries that are non-crystallographic. The coccolith segments deposited by the unicellular marine alga Emiliania huxleyi illustrate this phenomenon particularly well:- Current theories of biomineralization suggest that calcifying organisms have adopted strategies for controlling morphology based on the deployment of functional organic molecules. For example, proteins rich in aspartate and glutamate residues and also phosphoserine, are common for molluscs [3] whilst coccoliths of E. huxleyi are deposited along with sulphated and carboxylated polysaccharides [4]. Thus, carboxylate groups and, to a lesser extent, sulphates and phosphates play an important role in the biomineralization of calcite. Fig. 1. Single crystal segment of E. huxleyi [2] (left) compared to a similarly oriented geological calcite crystal (right)