Mesoscopic crystallographic textures on shells of a hyaline radial foraminifer Ammonia beccarii

Abstract

Most foraminifera, simple unicellular protists, create calcareous shells with precisely controlled architectures. We investigated the crystal fabric of porous shells of a hyaline radial foraminifer, Ammonia beccarii. The complex structures of the calcitic shell include micrometric single-crystalline domains around a straight pore ∼2 μm in diameter which are divided by strained intermediate regions. The c axes of calcite in the domains were almost perpendicular to the shell surface. The mesoscopic textures, such as periodic lateral lines with a ∼200–450 nm interval and vertical cracks with a ∼150 nm interval, were observed throughout the single-crystalline domains in the shell wall. Specific mechanical properties, such as a lower Young's modulus and a higher hardness of the shells than those of Iceland spar, are ascribed to the presence of the specific mesoscopic textures.