Biological control of crystallographic architecture: hierarchy and co-alignment parameters

Area of Science:

• Biomineralization
• Materials Science
• Crystallography

Background:

• Mytilus edulis (blue mussel) shells are composed of calcite and nacre layers.
• These biomaterials possess a hierarchical structure that differs between morphology and crystallography.
• Understanding their crystallographic hierarchy is key to understanding their material properties.

Purpose of the Study:

• To quantitatively analyze the crystallographic hierarchy in Mytilus edulis prismatic calcite and nacre.
• To investigate the propagation of crystallographic co-orientation across organic matrix membranes.
• To determine the extent of biological control over crystallographic texture in these biomaterials.

Main Methods:

• Electron backscatter diffraction (EBSD) was employed to quantitatively measure crystallographic co-orientation.
• Analysis focused on the propagation of orientation across matrix membranes and the formation of composite-crystal grains.
• Misorientation angles and distributions within calcite and aragonite layers were analyzed.

Main Results:

• Crystallographic co-orientation forms low-mosaic composite-crystal grains (calcite fiber bundles, nacre towergrains) and larger supergrains.
• Nacre layers exhibit a twin-domain structure with specific misorientation angles around the c-axis (63.8°).
• Calcite layers form a single supergrain with misorientations around the c-axis (9.4°), indicating significant biological control.

Conclusions:

• The crystallographic hierarchy in Mytilus edulis shells is fundamentally different from classical crystalline materials.
• Biological control over crystallization is evident in the log-normal distribution of misorientation angles.
• The study proposes numerical measures (geometric mean, multiplicative standard deviation) for biological control over crystallographic texture.