A phase transition should start on a very small scale. The growth can include the transfer of material to the interior of the new phase, by diffusion, by way of the old phase or by the grain contours (when growth occurs in the solid state).
After the formation of the new crystalline phase with r*, the grow rate will depend on the temperature and the degree of supersaturation.
Various situations are possible:
1 – The crystalline phase grows out of a gaseous phase.
2 – The crystalline phase grows within the liquid.
3 – The crystalline phase grows within a visco-elastic phase.
4 – The crystalline phase grows within another crystalline phase.
The first situation, by definition, isn’t encountered in biological systems. The second item is the most common construction of mineralized structures in diverse organisms and appears throughout this book. The third alternative is common in amorphous environments (glass and polymers) where, within highly viscous phases, they nucleate and crystalline phases grow, (devitrification) – can be responsible for the appearance of crystalline phases within supersaturated colloidal amorphous phases, like the transformations of amorphous calcium carbonate vesicles into aragonite and the transformation and formation of magnetite teeth from ferridrite in chitons; accounts for the formation of guanine crystals in the epidermis of fish and spiders. The fourth alternative is common in metals and ceramics, where a new crystalline phase nucleates and grows in the interior of another crystalline phase – like the nucleation of calcite from aragonite observed in mollusk shells and in in vitro tests. In this way, the nature of the interface between the crystalline phase and the other phase that takes over the new phase is decidedly important for the growth and in the morphology of the growing crystal.
When vapor atoms are adsorbed on a crystalline surface that grows because of this absorption, they spread over a considerable area. If this atom reaches an existing step on the solid’s surface, it tends to get stuck. On the step, the atom will have more direct neighbors than on the surface. While more atoms reach the step, they will continue until they reach the limit of the solid. At the moment that this happens, the step ceases to exist. At this point nucleation on the solid surface of a new bidimensional layer is needed. For gaseous phases, a high level of supersaturation is needed for an appreciable level of growth of the crystalline phase. The solid nuclei can grow from the region where the helical dislocation intercepts the crystal surface. In this case growth will occur even at low supersaturation levels. Due to the existence of a helical dislocation, the step will never progress until it disappears and growth will be spiral. Figure 2.1.16 is an illustration of an alkane crystal with spiral growth.
Figure 2.1.16 – Paraffin crystal (alkane, C36 H74 ) with spiral growth. The paraffin or alkanes are saturated aliphatic carbohydrates.
The speed at which the crystalline phase grows is determined by the rate of formation and the speed of the steps on the solids surface, by the concentration of impurities and by the concentration of defects like dislocation etc. With an increase in supersaturation, the number of possible sources of step formation on the surface also increases. Atoms of impurities strongly affect crystal growth; with lower levels of supersaturation and lower temperatures, the effect of impurities on growth intensifies.
