Rhombohedral calcite in ostrea shells: the influence of biopolymers on biocrystal morphology, crystal face generation, and crystal assembly patterns

Abstract

Ostreoidea form most of their shell from foliated calcite, consisting of an intricate arrangement of lath-shaped nanocrystals. The Ostreoidea-foliated microstructure tolerates the incorporation of other biocalcite microstructures. It intercalates the meshwork of chalk blades and sequences of organic layers patterned with rhombohedra. Rhombohedral and foliated calcite in Ostrea edulis and Ostrea stentina shells were studied using electron backscatter diffraction, energy-dispersive spectroscopy, laser confocal optical microscopy, scanning secondary electron microscopy, backscattered electron microscopy, and transmission electron microscopy. The foliated shell contains parallel arrangements of organic substance layers. These layers comprise a multitude of isolated calcite rhombohedra and occur within the shell and on its inner surface. With shell thickness growth, the organic substance-covered rhombohedra become overgrown by foliated calcite and form foliated-calcite-rimmed pillars. These pillars merge, generate a continuous layer, and lead into the proximal foliated shell. The calcites of the rhombohedra and the foliated pillar rim are never in direct contact; an organic substance separates them. Rhombohedral crystal and foliated pillar rim/shell crystal morphology and microstructure are distinct, while rhombohedral crystal and foliated shell preferred crystallographic orientation (crystal texture) are similar. For the rhombohedra and the foliated shell, a turbostratic-like texture was observed, characterized by ring-shaped c- and a*-axis orientation distributions in 2D pole figures. The rhombohedral crystal c-axis orientation of the c-axis ring is clustered: one cluster coincides with the c-axis orientation of the sagittal foliated shell, while the other coincides with that of the proximal foliated shell. Rhombohedral calcite formation is regarded as an accessory phenomenon, generated through a metabolically induced/guided precipitation-related process within a biological hydrogel. Rhombohedral microstructure and texture are controlled by the organic substance that hosts the rhombohedra.