Morphology and mechanical properties of fossil diatom frustules from genera of Ellerbeckia and Melosira

Fossil frustules of Ellerbeckia and Melosira are studied using laboratory-based nano X-ray tomog-raphy (nano-XCT), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Three-dimensional (3D) morphology characterization using nondestructive nano-XCT reveals continuous connection of fultoportulae, tube processes and protrusions. The study confirms that Ellerbeckia is different from Melosira, and it shows that both genera are heavily silicified frustules with valve faces linking together and forming cylindrical chains. For this cylindrical architecture that is characteristic for both genera, valve face thickness, mantle wall thickness and copulae thickness are changing with the cylinder diameter. Furthermore, EDS reveals that these fossil frustules are made of Si and O only, with no other elements in the percentage concentration range. Nanopores with about 15 nm diameter are detected inside the biosilica of both genera using TEM. In-situ micromechanical experiments with uniaxial loading are carried out within the nano-XCT on these fossil frustules to determine the maximal loading force under compression and to describe the fracture behavior. The fracture force for both genera is correlated to the fossil frustules' dimensions. The results from in-situ mechanical tests show, that the crack initiation starts either at very thin or at linking structures of the frustules.