Why are crystals different shapes?
The shape of a crystal is determined by a number of variables:
How the atoms/molecules pack together as they form the crystal. This may
be suggested by the structure of the individual molecule in some cases; in
other cases the relation between the structure of an individual molecule and
the shape of its molecular crystal is not obvious. An example: For a "flat"
molecule like naphthalene you might expect (correctly) that the crystal will
form flat platelets.
In the case of ionic solids, the shape of the crystal depends upon how
the atoms pack together. This is affected by the relative size and charge on
the cations and anions. It also depends upon the "shape" of the ions. Not
all ions can be considered to be charged spheres.
Another more subtle factor is the relative speed at which the various
faces of the crystal grow. Sometimes it is easier for a molecule/atom to fit
into one face compared to another, so the crystal grows faster in one
direction than another. The relative growth rates of the faces can be
affected by the growth conditions -- solvent, temperature, etc. This part of
crystal growth becomes more art than science!!
Many substances have more than one crystal structure. How many, and how
they are produced is difficult to predict. Examples that come to mind are
sulfur and phosphorous, but there are many more. Which crystal modification
exists often affects the chemistry, physical properties and reactivity of
If one introduces the applied pressure as a variable, it is safe to say
the MOST all substances have more than one crystal structure (hence crystal
shape). The most studied example is water. At last count there are from 11
to 13 crystal structures of ice, depending upon the temperature and
pressure. These structures of water are important in astrophysics because on
other planets, moons, comets, etc. there are various conditions of
temperature and pressure that are not encountered on earth.
The bottom line is this. Many variables affect crystal shapes, and how
they do is difficult to isolate and predict.
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Update: June 2012