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

Abstract

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 μm. 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°/[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.

Biomineral crystallographic preferred orientation in Solenogastres molluscs (Aplacophora) is controlled by organic templating

“…These are: Growth of crystals by growth competition ; e.g. formation of argonauta prismatic calcite 30 , 31 , brachiopod columnar calcite 32 , bivalve myostracal aragonite 33 , 34 , and rotaliid foraminifera calcite 35 , 36 . Growth of crystals between biopolymer membranes that form through self-organization prior to mineralization; e.g.…”

“…Growth of crystals by growth competition ; e.g. formation of argonauta prismatic calcite 30 , 31 , brachiopod columnar calcite 32 , bivalve myostracal aragonite 33 , 34 , and rotaliid foraminifera calcite 35 , 36 .…”

“…Hence, the different textures that we measure for Solenogastres sclerite aragonite can only be induced by specific ultrastructural characteristics of epithelial, papilla and cuticle cells and their invaginations. The aragonite of Solenogastres spicules and the calcite of rotaliid foraminifera 35 , 36 are the only examples so far where it can be shown that crystallographic axes orientation of the biocrystals is already determined at the onset of the mineral formation process, at nucleation, and that it is solely influenced by organic substance and the associated biopolymers.…”

Molluscs generate preferred crystallographic orientation of biominerals by organic templates, the texture and microstructure of Caudofoveata (Aplacophora) shells

“…E.g., in the common fibrous microstructure of brachiopod shells, calcite c-axis is perpendicular to the fibres 53 , 54 , which is the slowest growth direction of calcite. In foraminifera 25 – 27 and bivalve myostraca 35 , 38 , 55 the initial fine-grained layer of seed crystals already has a strong axial texture. For the “single crystalline” sea urchin spines, an initial polycrystalline untextured layer has never been observed.…”

A Molecular‐Scale Understanding of Misorientation Toughening in Corals and Seashells

“…Misorientation was observed before in various bio minerals. [3,[13][14][15][16][17][18][24][25][26][28][29][30][32][33][34]42,44,[49][50][51][52][53][54][55][56][57] The nanoscale misorientation of pairs of adjacent crystals, however, was never measured directly and quantitatively, nor compared across diverse biominerals as done here. Note that coral skeletons are formed by so-called plumose spherulites, that is, radial distribution of acicular aragonite crystals starting from a line and radiating in three dimensions as the fibers in a feather duster or a bottle brush.…”