Crystal Growth in the Foliated Aragonite of Monoplacophorans (Mollusca)

Abstract

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 on 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. Co...