Naturally Occurring and Biomimetic Synthesized Calcite Spherulites

Naturally occurring calcite spherulitic particles were collected from East Kirkton Quarry in Bathgate, Scotland. Their microstructure has been revealed, by using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy, to consist of a low crystallinity core with deposition of multilayer radially oriented calcite microrods. The surrounding materials of the spherulites are mainly Ca-free silicates. To understand the formation mechanism of this construction of calcite crystals, biomimetic synthesis of similar spherulites have been carried out by using alginate and stevensite as structure directing agents. It is found that alginate is essential for growth of the spherulites, since the spherulites are developed only when they are embedded in the alginate network, a soft matter substrate. Stevensite also plays an important role of adjusting the hardness of the alginate substrate, offering a suitable network for the self-aggregation and self-orientation of the calcite nanocrystals. Possible interparticle interactions or the driving force of the particle aggregation and self-orientation are discussed.