Functional cooperation of calcite and aragonite in clam shells
The exoskeletons of clams contain both calcite and aragonite. The hardness and robustness of the shells is guaranteed by the functional cooperation of the CaCO3 isomorphs together with organic layers.
Calcite (hexagonal) |
Aragonite (orthorhombic) |
||
|
|
|
|
|
|
large, prismatic Calcite crystal |
layers of plates of Aragonite crystals |
||
Between the two compartments there is a separating layer with high water content.
The crystallites in the nacre layer are separated by an organic matrix (polysaccharide-protein layer) which significantly limits their thickness (influence on the three-dimensional structure).
The structural cooperation of the two CaCO3 modifications with organic separating layers leads to a drastic change in the material properties. (Increase in the mechanical stability by a factor of ca. 3000 compared to "inorganic" crystalline aragonite).