Microstructure and Texture of Foraminiferal Ca-Carbonate: The Different Biomineralization Strategies of Rotaliida, Robertinida, and Miliolida

Abstract

We report differences for shell calcite and aragonite crystallography and crystal organization for Neogloboquadrina dutertrei (Rotaliida), Hoeglundina elegans (Robertinida), Pyrgo murrhina, Triloculina sp., and Quinqueloculina sp. (Miliolida). Crystals were investigated with electron backscatter diffraction (EBSD) and high-resolution field-emission SEM (FE-SEM) imaging. Rotaliid and robertinid crystals have dendritic-fractal morphologies, interdigitate strongly, and are twinned. First-formed N. dutertrei calcite crystallites are fibrils. Arrays of these form bundles and evolve into densely mineralized crystal entities. First-formed H. elegans aragonite crystallites are granules. These nucleate onto a membranous template and evolve into laths and undulated laminae. The latter are stacked in parallel and generate round-shaped crystal units. H. elegans aragonite and N. dutertrei calcite have an axial-crystal-texture at nucleation onto the template. For H. elegans, the latter is maintained for the entire shell. For N. dutertrei, the axial-crystal-texture transforms to a single-crystal-texture toward distal shell surface. For N. dutertrei, the change in crystal texture is controlled by the crystal growth process and growth competition. Crystal growth controlled by growth competition is not observed for H. elegans aragonite or miliolid calcite. Miliolid calcite is not twinned. It is a meshwork of nanometer-sized single-crystal rods, interspersed by nanometer-sized single-crystal rhombohedra. At the proximal shell surface, the rods do not have preferred orientation. At the distal shell surface, the calcite is rather granular, co-oriented, and textured. For all investigated species, calcite/aragonite c-axis rotates with shell curvature. Despite distinct foraminiferal shell crystallographic-structural differences, we find similarity for crystal nucleation. Nonetheless, for the investigated species, crystal growth is modulated by different growth determinants.