The calcite-aragonite transition in biological systems is still debated, seeing as the leading hypothesis doesn’t consider the hierarchical structure of the biomineralized aragonite tiles (orthorhombic, with a density of 2.93g/cm3, metastable at room temperature). Literature proposes the mineral bridges model: aragonite crystals grow through pores in the organic matrix, forming sequential layers. It’s through these mineral bridges (Figure 1.18 and 1.19) that these successive layers of nacre inherit the crystallographic orientation of the previous layers (as observed earlier in Gastropods).
Figure 1.18 – Scheme of growth of mineral bridges connecting successive layers of mineral matrix.
Figure 1.19 – Detail of minerals bridges observed between two tiles.
Aragonite nucleation begins in an organic substrate, with nucleant properties due to the chemical composition and also to its topography, with micrometric pools acting as reactors, stimulating nucleation is specific determined points and inhibiting nucleation in others. The mineralization of aragonite begins with the formation of three dimensional structures in the form of long polycrystalline “arrows” or needles not only on the surface of the pool walls but also immersed in their organic matrixes. At the beginning, the c axis is perpendicular to the surface of the shell, while the a and b axis grow randomly (Figure 1.20).
Figure 1.20 – Schematic showing the growth of crystals and the performance of geometric selection.
As the layers overlap, the b axis turns in the direction of the growth (expansion) of the layer, due to the competition between adjacent tiles. Simultaneously, organic matrix growth continues between the prisms, until they are covered. The needles form domes (convex curved structures), that are flattened due to the growth of a new organic matrix layer. The c axis of the nacreous tiles (the perpendicular axis to the shell surface) is inherited from the arrows of the dome, through these mineral bridges (Figure 1.21). Further, the theory of geometric selection, considering that the rate of growth of a crystal varies over different crystallographic orientations and the appearance of a preferred direction occurs during growth. In this way, only the organic processes wouldn’t be sufficient to explain the structural hierarchy of the biomineralized materials: inorganic processes like geometric selection and mineral bridges exerceriam roles fundamental in the structural development of the shells.
The organic matrix exercises control over nucleation, polymorph selection, and morphology of biomineralized crystals on a nano and macro scale, but there is no consensus on how, with essentially chemical, physical (structural, topographic) vias or both being discussed as options. It’s worth mentioning the fact that the percent of hidrossolúvel da matriz is capable of suppressing crystallization in solution, but promotes directional aragonite crystal growth on calcite surfaces. This supports the idea that the first layer mineralized, made up of calcite of a less defined morphology, by the golden mussel, would serve as a base for the start of nucleation of the nacreous layer, rich in aragonite and the principal constituent of the shell in this species.
Figure 1.21 -Crystallographic inheritance from minerals bridges.
The calcite-aragonite would be induced by proteins in the organic matrix: soluble proteins (present in the nacre) are responsible for the nucleation and growth of aragonite crystals in crystalline seeds of calcite nucleação e crescimento de cristais de aragonita em sementes cristalinas de calcita immersed in supersaturated solutions of calcium carbonate. The presence of crystalline calcite seeds can considerably reduce the necessity of an insoluble matrix for nucleation and growth of aragonite crystals.
