Crystal Growth in the Foliated Aragonite of Monoplacophorans (Mollusca)

Crystal-growth features of the foliated aragonite from two species of the rare monoplacophoran molluscs have been analyzed. The crystals have unique morphologies. They are very thin along the c axis and elongated along the a axis, and their arrangement varies depending oil the species. Surface energy minimization in the crystal arrangement observed in Micropilina leads to a discrete number of constant angular relationships, which is explained by twin laws and epitaxy. Textural analysis shows that crystals form oriented aggregates with their c axes perpendicular to the shell surface. Close to the shell margin, crystals compete so as to orient their a axes nearly perpendicular to the growth front of the lamellae, although the scattering of the a axis soon increases toward the shell interior. In contrast to inorganic crystals, growth along the c axis is inhibited by organic molecules. Their incorporation may be related to the existence of weak intermolecular interactions between CO3 groups along this axis. Conversely, there is no chemical affinity to incorporate organic molecules along the a axis, where particularly short CO3-Ca ionic bonds occur. These structural factors explain the formation of crystals which are elongated and free of organic inclusions along the a axis.