1.crystallization
The crystallization process consists of two major events, nucleation and crystal growth. Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometer scale (elevating solute concentration in a small region), that becomes stable under the current operating conditions. These stable clusters constitute the nuclei. However, when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (temperature, supersaturation, etc.). It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal structure --- note that 'crystal structure' is a special term that refers to the relative arrangement of the atoms, not the macroscopic properties of the crystal (size and shape), although those are a result of the internal crystal structure.
The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained (control of crystal size and shape constitutes one of the main challenges in industrial manufacturing). Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so as to supersaturate the solution again.
Many compounds have the ability to crystallize with different crystal structures, a phenomenon called polymorphism. Each poly morph is in fact a different thermodynamic solid state and crystal poly morphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products.
For crystallization to occur from a solution it must be supersaturated. This means that the solution has to contain more solute entities (molecules or ions) dissolved than it would contain under the equilibrium (saturated solution). This can be achieved by various methods. 1. solution cooling or heating. 2. addition of a second solvent to reduce the solubility of the solute. 3. chemical reaction. 4. change in PH being the most common methods used in industrial practice. Other methods, such as solvent evaporation, can also be used.
2. fossilization
The fossilization process: life---death---preservation---survival---discovery
Fossilization potential (FP --- the likelihood of being found as a fossil).
Life: high chance of burial + low in the food chain = high FP (with bonuses for already buried).
low chance of burial + high in the food chain = low FP
Death: smothering by storm or flood sedimentation = high FP
being eaten or rolled around = low FP
Preservation: rapid burial and hard parts = high FP
slow burial and no hard parts = low FP
Survival: burial below sea level and burial in sinking basin shallow burial = high FP
burial above sea level and deep burial = low FP
Discovery: a fossil cannot be discovered unless the sedimentary rock containing it is exposed at the Earth's surface.
