Crystals in Minerals
The shape of the crystals of a mineral is determined by the way the atoms are arranged.
When crystals grow large, you can see how the arrangement of atoms influences the shape.
Notice how the large halite crystal inFigure3.1has square shapes. This shape is the result
of the pattern of sodium and chlorine atoms in crystal. Now, compare the crystal inFigure
3.1with the grains of salt magnified under a microscope shown inFigure3.4. These small
crystals have similar shapes to the large crystal. You can see that the shapes of the crystals
are made up of squares. You can try this at home. If you sprinkle salt into your hand and
look carefully at each grain of salt, you will see that it is perfect little cube.
Figure 3.4: When you look at grains of table salt under a microscope, you can see that the
crystals are made of square shapes. ( 32 )
Large crystals only form when they have room to grow. Often, crystals are very small. Even
if you cannot see the individual crystals in a mineral sample, the atoms are still ordered in
a regular, repeating pattern. This pattern can be used to help identify an unknown mineral
sample. A trained scientist may be able to determine the crystal structure by the shape of
a large crystal. If they cannot figure out the crystal structure by looking at the mineral,
scientists use an instrument that uses X rays to find out how the atoms are arranged in a
mineral sample.
A mineral has both a characteristic chemical composition and a characteristic crystal struc-
ture. Sometimes, minerals have the same chemical composition, but different crystal struc-
tures. What do you know about diamond and graphite? Diamonds are valued as gems for