In the case of biomineralization, the first mineral formed is encountered in intracellular vesicles. In cases involving a intracellular phase, non-crystalline or highly disordered phases were observed like ferrihydrite [(Fe3+)2O3•0.5H2O], amorphous calcium carbonate (ACC) and amorphous calcium phosphate (ACP). These unordered phases are subsequently transformed into crystalline phases – a transformation that generally occurs in a preformed extracellular matrix.
One of the best studied cases is that of the spicule of sea urchin larva. Nano spherical particles of ACC, initially formed in intracellular vesicles, are deposited on the surface of the growing spicule, that subsequently crystallizes on the deposit site. An accepted phase conversion mechanism is a solid state transformation that propagates in three dimensions by the amorphous phase by secondary nucleation starting at the contact surface between the amorphous phase and the crystal (as earlier described).
The solid state transformation of ACC to aragonite and from aragonite to calcite was presented in videos by De Yoreo et al (2003) that did not observe a direct transformation from ACC to calcite.
In general, the amorphous solid to crystalline transformation is known as devitrification and happens at higher temperatures than the glass-transition temperature (Tg), with the reduction of viscosity and the diffusion of the amorphous matrix, making the creation and growth of polycrystalline areas possible.
Figure 1.17: Hidroxiapatite (HA) deposition on nacre matrix.
Is There Really a Difference Between Abiotic Crystal Growth and Biomineralization?
Biologically formed crystals are generally characterized by extravagant forms some, surprisingly, circinate, with no relation in crystalline symmetry and don’t have stable crystalline faces. Many biogenic crystals – at their final size – present textures comprised of nonspherical particles, like what happens in mollusks shells, coral skeletons, cephalopod shells, echinoderm skeletons, sponge spicules, brachiopod shells, crustaceans cuticles, etc.
In live organisms, the cellular environment intervenes in the mineralization pathways. The magnitude of this control is still in discussion and the possibility of richness of form in the abiotic processes are known as “biomorphs” minerals – there would be no boundary separating abiotic crystallization and biomineralization.
