3. Crystal
Crystal is a solid particle in which the
constituent molecules, atoms, or ions
are arranged in some fixed and rigid,
repeating three-dimensional pattern
orlattice.
8. Mechanismof
Crystallization
• A solution must be saturated before any solid matter can
becrystallizeout.Ifthetemperatureofsaturatedsolution
is lowered, or solvent allowed to evaporate from such a
solution
• Then
• Theexcessofsolidmaterialseparatesout
• Although
• Lowering the temperature carefully, it is often possible to
coolitconsiderably withouttheseparationofcrystals.
Such a solution is called as super-saturated solution
11. Nucleation
• The first step in the crystallization process is nucleation. The first
atoms in the mass to form a crystal structure become a center, and
more atoms organize around this nucleus. As this happens, more
unit cells assemble around the nucleus, a small seed crystal is
formed.
• The process of nucleation is extremely important in crystallization,
is the nucleus of a crystal will determine the structure of the entire
crystal.
• Imperfections in the nucleus and seed crystal can lead to drastic
rearrangements asthecrystalcontinuestoform.
• Nucleation happens in a supercooled liquid or a super-saturated
solution.
12. Crystalgrowth
• As other molecules and atoms surround the nucleus, they branch of
from the symmetry which has already been set up, adding to the seed
crystal.Thisprocesscanhappenveryquickly,orveryslowly,depending
ontheconditions.
• Water can crystalize into ice in a matter of minutes, while it takes
millennia to form “typical” geological crystals like quartz and
diamonds. The basic formation set up around the nucleus determines
theentiretyofthecrystalstructure.
• This difference in formation accounts for the differences in crystals,
fromtheuniquenessofasnowflaketotheclarityofadiamond.
13. Crystal
Growth There are only a handful of geometric shapes that crystals can take. These are
determinedbythebondsandinteractionsofthemoleculesinvolved.
The different shapes are caused by the different bond angles of atoms, based on
theoriginalnucleus.
An impurities in the solution or material will lead to diversion from the typical
pattern.
As seen in snowflakes, even tiny impurities in the nucleus lead to completely
newanduniquedesigns.
15. Sizeofcrystal
• In crystallization process, the nuclei formation should
be under control since the number of nuclei will
controlthesizeofcrystal.
• Large crystals may be obtained as a result of slow
cooling of solution, just above saturation point due to
reduction innumber of spontaneous nucleiformation.
Crystal growth from these few nuclei & grow to
desiredsizebeforenewnucleiappear.
• Rapid cooling inversely cause increase in degree of
supersaturation and resulting in large number of
nucleianda“crop”ofsmallcrystals.
