One of the hypothesis of biogenic growth of crystals suggests two mechanisms that compete between themselves: the first involves the dissolution of ACC in an aqueous solution followed by crystalline nucleation and ion-by-ion growth; in the second mechanism the ACC particles are partially stabilized in the presence of additives (ionic and or organic), either in solution or in aggregates. Temporarily stabilized, the ACC particles migrate to the surface of the crystal, where they are adsorbed and crystallized. When both of the processes run simultaneously, the necessary conditions would be the presence of a small quantity of water (possibly from the initial hydrated amorphous phase) and the stabilization of the particles in a micro-environment.
How does the transformation between an unordered phase and an ordered phase during the accession process of nanospherical particles? In the case of ACC and ACP, the hypothesis is that the amorphous phase is kinetically stable at room temperature and go through crystallization if maintained hydrated, because the presence of water in the micro-environment is the driving force of the phase transformation, contributing as a key factor in determining the equilibrium between processes mediated-by-ions and mediated-by-molecules. The reduction in participation by water (due to, for example, the formation of gel, or the dehydration of the micro-environment) presumably reduce the dissolution of the nanospheres dissolution of the unordered nanospheres, in a way the the particles continue unordered during the translocation for the surface of the growing crystal, where they finally crystallize after contact with the organized substrate.
We can speculate that the first crystal rhombohedral formed in the spicules of sea urchins(a) were possibly formed in a water rich environment. This would be a confined environment, and the nanospheres would be in direct contact with the envelope of the spicule(b).
Another observation made in adult urchin spines, like spicules in sponges, is that it is difficult to reconcile with one exclusive growth mechanism by accretion of nanoparticles, is that occluded macromolecules within the already grown crystal , tend to concentrate in certain crystalline planes, but not in others. If the macromolecules were initially present in the precursor phase, the only way they could line up in specific planes during the process of crystallization would be by a aqueous mediation.
