Calcite crystal orientation patterns in the bilayers of laminated shells of benthic rotaliid foraminifera

Shells of calcifying foraminifera play a major role in marine biogeochemical cycles; fossil shells form important archives for paleoenvironment reconstruction. Despite their importance in many Earth science disciplines, there is still little consensus on foraminiferal shell mineralization. Geochemical, biochemical, and physiological studies showed that foraminiferal shell formation might take place through various and diverse mineralization mechanisms. In this study, we contribute to benthic foraminiferal shell calcification through deciphering crystallite organization within the shells. We base our conclusions on results gained from electron backscattered diffraction (EBSD) measurements and describe microstructure/texture characteristics within the laminated shell walls of the benthic, symbiontic foraminifera: Ammonia tepida, Amphistegina lobifera, Amphistegina lessonii. We highlight crystallite assembly patterns obtained on differently oriented cuts and discuss crystallite sizes, morphologies, interlinkages, orientations, and co-orientation strengths. We show that: (i) crystals within benthic foraminiferal shells are mesocrystals, (ii) have dendritic-fractal morphologies and (iii) interdigitate strongly. Based on crystal size, we (iv) differentiate between the two layers that comprise the shells and demonstrate that (v) crystals in the septa have different assemblies relative to those in the shell walls. We highlight that (vi) at junctions of different shell elements the axis of crystal orientation jumps abruptly such that their assembly in EBSD maps has a bimodal distribution. We prove (vii) extensive twin-formation within foraminiferal calcite; we demonstrate (viii) the presence of two twin modes: 60°/[0 0 1] and 77°/~[6 –6 1] and visualize their distributions within the shells. In a broader perspective, we draw conclusions on processes that lead to the observed microstructure/texture patterns.We thank Prof. A. Altenbach, Department of Earth and Environmental Sciences, LMU Munich, Germany and Prof. S. Redfern, Department of Earth Sciences University of Cambridge, UK for providing the foraminifera samples. We thank the staff of Central Facility for Electron Microscopy at Ulm University for their technical support at sample preparation for EBSD. This project was partly funded by the European Union’s Horizon 2020 research and innovation program under grant agreement No. 643084 and supported by Deutsche Forschungsgemeinschaft, DFG Grant GR-1235/9-1. A.C. and I. S.-A. were funded by Project CGL2017-85118-P, Spanish Ministry of Education and Science