Ask A Scientist© Chemistry Archive

status       other
age          12

Question -   Is it possible that a coal field (such as those found in
Pennsylvania or West Virginia) could ever become a diamond field if put
under enough heat and pressure, or are coal and diamonds just "distant
cousins" to one another?  How does the formation of coal differ from the
formation of diamonds?
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First of all, coal is not just carbon.  Coal is a mixture of complicated
organic molecules derived from ancient plants, fungi, and bacteria.
However, in the natural "maturation" processes of coal, heat and pressure
change its chemical composition over time so that it becomes nearly pure
carbon in the form of graphite.

The difference between graphite and diamond is in the three-dimensional
arrangement of the carbon atoms in the material.  Graphite is made of flat
sheets of carbon atoms in a hexagonal arrangement.  The sheets stack one
over the other, with only weak interactions between sheets.  Diamond is not
arranged in sheets; instead, each carbon atom is connected to four others in
a tetrahedral arrangement.  It turns out that this arrangement is not as
different from the structure of graphite as it sounds.  If the sheets of
graphite are compressed close enough together, the carbon atoms will be in
just about the right position to make the bonds of diamond.  In fact, at
high temperatures and pressures, this indeed happens.

It goes the other way, too.  It turns out that graphite is actually more
stable than diamond at room temperature and atmospheric pressure.  Unless
the surface of a diamond is chemically stabilized, the diamond vill convert
to graphite!  Fortunately, fresh diamond surfaces are easily stabilized by
reacting with whatever touched them, so this isn't likely to happen unless
you carefully cut a diamond in a vacuum.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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