The unique fibrilar to platy nano- and microstructure of twinned rotaliid foraminiferal shell calcite

Diversification of biocrystal arrangements, incorporation of biopolymers at many scale levels and hierarchical architectures are keys for biomaterial optimization. The planktonic rotaliid foraminifer Pulleniatina obliquiloculata displays in its shell a new kind of mesocrystal architecture. Shell formation starts with crystallization of a rhizopodial network, the primary organic sheet (POS). On one side of the POS, crystals consist of blocky domains of 1 mu m. On the other side of the POS crystals have dendritic-fractal morphologies, interdigitate and reach sizes of tens of micrometers. The dendritic-fractal crystals are twinned. At the site of nucleation, twinned crystals consist of minute fibrils. With distance away from the nucleation-site, fibrils evolve to bundles of crystallographically well co-oriented nanofibrils and to, twinned, platy-blade-shaped crystals that seam outer shell surfaces. The morphological nanofibril axis is the crystallographic c-axis, both are perpendicular to shell vault. The nanofibrillar calcite is polysynthetically twinned according to the 60 degrees/[100] (= m/{001}) twin law. We demonstrate for the twinned, fractal-dendritic, crystals formation at high supersaturation and growth through crystal competition. We show also that c-axis-alignment is already induced by biopolymers of the POS and is not simply a consequence of growth competition. We discuss determinants that lead to rotaliid calcite formation.

Automated summary

Diversification and incorporation of biopolymers are important for optimizing biomaterials. The shell of Pulleniatina obliquiloculata, a planktonic foraminifer, exhibits a unique mesocrystal architecture. The shell formation involves the crystallization of a primary organic sheet, which consists of blocky and dendritic-fractal crystals with twinning. The formation of these crystals is influenced by high supersaturation and growth competition. Furthermore, alignment of the crystallographic c-axis is induced by biopolymers in the primary organic sheet. The factors contributing to the formation of calcite in Pulleniatina obliquiloculata are discussed.