Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells.

Area of Science:

• Biomineralization
• Marine Biology
• Materials Science

Background:

• Marine organisms, such as brachiopods, biomineralize calcite for shell formation.
• The primary layer of brachiopod shells was previously characterized as nanocrystalline.

Purpose of the Study:

• To investigate the microstructure of the primary layer in the modern brachiopod Gryphus vitreus.
• To characterize the crystallographic orientation and morphology of biocalcite grains.
• To propose a growth model for the observed dendritic biocalcite structure.

Main Methods:

• High spatial resolution electron backscatter diffraction (EBSD) at 15 and 10 kV.
• Analysis of the primary shell layer of the modern brachiopod Gryphus vitreus.
• Microstructural and crystallographic analysis of calcite grains.

Main Results:

• Discovery of a dense, dendrite-like biocalcite microstructure in the primary layer.
• Identification of concave/convex calcite grains with interdigitated recesses and protrusions, forming a nanoscale structure.
• Observation of dendritic mesocrystals with a spread of crystallographic orientation, some exceeding 20 μm.
• Similar preferred crystallographic orientation between the primary and fibrous shell layers despite different morphologies.

Conclusions:

• The primary layer of Gryphus vitreus exhibits a unique dendritic mesocrystal structure.
• Two distinct control mechanisms govern the formation of the primary and fibrous shell layers.
• A growth model involving amorphous calcium carbonate (ACC) precursor vesicles is proposed for the dendritic structure.