Crystals Can Grow 
What is a Crystal?
    Cochise College   

    Geology Home Page               

    Roger Weller, geology instructor
    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.




        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