Crystals Can Grow What is a Crystal?
Cochise College
Crystals Can Grow Roger Weller,
geology instructor
(wellerr@cochise.edu)
last edited: 12/22/15
What is a Crystal?
There are many definitions of what is a crystal. Most of the definitions
overlap,
but others are drastically different.
The oldest definition is probably the simplest. Minerals were found that
looked
surprisingly different than common rocks. These minerals were rare,
had flat,
often shiny surfaces arranged in interesting geometrical patterns.
In addition
these specimens were quite colorful compared to rocks and were often
transparent.
quartz
calcite
fluorite
A more advanced definition stems from the definition of
crystalline structure.
Within a solid that possesses crystalline
structure, the atoms are arranged in a
repeating geometrical pattern. A
crystal is a piece of solid material (element or
chemical compound)
in which the geometrical pattern is coherent
throughout the
entire object. The external shape is of no significance
because crystals often form
in crowded conditions and their outer shape is
deformed.
geometrical arrangement of sodium and chlorine atoms in a salt molecular
structure
geometrical arrangement of carbon atoms in the molecular structure of graphite
A third definition is tied to the external shape of crystals. If a small
object
(mineral, chemical or element) with a coherent repeating geometrical
pattern of
atoms is allowed to grow larger from a melt, solution or cooling gas
in an
environment in which there are no physical barriers to its growth, layers
of atoms
will selectively be added to the structure in such a way that the
object will develop
flat surfaces. These flat surfaces, called faces, will
reflect the symmetry of the
underlying layers of atoms.
Calcite and halite are good examples. Note how
the shape of both calcite and
halite reflect their internal arrangement of atoms
at the molecular level.
calcite molecular model showing rhombic arrangement of atoms
calcite crystals showing rhombic shape
halite molecular model showing cubic arrangement of atoms
halite crystals with cubic shapes
A fourth definition of
the word crystal has nothing at all to do with real crystals
but stems from
history. Before the story can be told, it is necessary to explain
what
glass is. Glass is almost the exact opposite of the definition of crystal.
In a
glass atoms are arranged at random with respect to each other; their are no
nice
neat geometrical patterns. Glass starts out as a melt in which the
atoms are moving
around and not attached to each other because of the tremendous
amount of
thermal energy (heat). The liquid is quickly cooled, not
allowing the atoms enough
time to arrange themselves into geometrical patterns.
It is ironic,
therefore, that a variety of glass with a high lead content is referred to
as
"crystal". Many, many years ago in Europe, people found nice shiny
crystals
of the mineral quartz.
quartz crystals
They
took the quartz crystals home, tied strings to them and hung them where
candle
light could make them sparkle.
At a later time, the tradition continued and
the crystals were hung from
chandeliers with candles.
When the supply of crystals
diminished, people took and cut pieces of glass to hang in place of the
crystals.
They continued to call the cut glass crystals. Eventually
it was discovered that if a
large quantity of lead oxide was added to the glass
melt, a type of glass could be
created that would produce strong rainbows of
color when cut and polished and
then hung below chandeliers. Today, all
forms of glass bowls , vases, and
decanters made from this leaded glass are
referred to as "crystal".
This definition
of crystal
does not apply to the scientific definition of crystal.
copyright 2015-R.Weller
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