Biomineralization is the process of forming hard exo- or endoskeltons by biological organisms. The physical properties and the morphology of the composite material that is formed depends on the mode of crystal-growth, which depends on the diffusion of the constituents (metal cations, their anions, and additives).
We chose a model system, strontium carbonate / silica, because the precipitation of SrCO3 and SiO2 is pH dependent. A lower pH accelerates SiO2 formation while it slows down SrCO3 crystallization, and vice versa. Thus, the precipitation of the composite should occur in an alternating fashion, regulating the morphology of the carbonate precipitate. The solution of SrCl2 and water glass is poured into a shallow container and a glass slide is floated on top, eliminating the effects of bulk precipitation and convection. The pH is adjusted to a starting value of 12, and a small amount of dimethyl carbonate is added. This compound slowly hydrolyses and gives off carbon dioxide, the carbonate source for the crystal growth.
The crystal morphology is clearly dendritic and shows several distinct periods of growth. First, a few crystals grow from a common seed, and after a while, the blocking of surface sites on the carbonate crystals by water glass, leads to the start of a more needle-like growth with a significant increase of branching density. Finally, after 7 days, coral-like structures are formed.