LAB ACTIVITY
Look at the mineral samples you have been given and determine if there
are patterns in how the minerals break. Geologists use the word cleave
for how the minerals break. For each mineral you are given, record
some notes about how it cleaves.
BACKGROUND
The orderly arrangement of atoms in a mineral can be seen in its tendency
to break along smooth planes in specific directions -- a property known
as cleavage. The planes are known as cleavage planes. A mineral's ability
to resist breakage depends on the strength of chemical bonds between the
atoms in its crystal structure. The stronger the bonds, the more difficult
it is the break the mineral.
What makes a chemical bond "strong" or "weak?" In simple terms, the strength
of a bond depends on the intensity and the shape of the electrical field
(the complex cloud of electrons) between two atoms. Describing the electrical
field and all of the factors that control its intensity and shape, however,
is a very challenging task. To do it, chemists measure physical and electrical
properties of each atom and use those measurements to describe the bond
mathematically. One of the most important measurements is the distance between
atoms. As a rule of thumb, the closer two atoms are to each other, the more
tightly they bond together, all other factors being equal.
In most crystal structures, atoms are closer together in some directions
than in others. Thus, the bonds in those directions are stronger. Cleavage
takes place along planes that cut across the longest, weakest bonds in the
structure.
Some minerals, such as the micas, cleave in only one direction. Others,
in which more than one set of layers is weakly bonded, may cleave in two
or more directions. Geologists can use the number of cleavage directions
and the angles between them to distinguish one mineral from another. All
samples of the same mineral always exhibit the same cleavage because they
have the same molecular structure.
Some minerals do not exhibit cleavage at all, because there is little difference
in bond strength from one direction to another in their crystal structures.
Such minerals break to produce a rough, non-planar surface called a fracture
instead.
COMPUTER ACTIVITY
Rotate each molecular structure and search a plane along which this structure
might break (that is, a cleavage plane). The background information
above should give you some hints about what to look for. There is
a molecular structure for each of the mineral samples you were given.
Try to match the molecular structures with the mineral samples.
For
each of the molecules you are assigned describe the cleavage planes you predict
from these models. Determine the number of planes and try to describe
their directions by refering to the XYZ axes. Then tell why you think
these are the cleavage planes. There are good hints in the Background
section above.
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Muscovite |
Potassium aluminum silicate hydroxide fluoride Muscovite is composed of potassium, alumninum, silicon, oxygen, hydrogen and fluorine atoms. |
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Calcium carbonate (CaCO3) Calcite is composed of calcium, carbon and oxygen, atoms. |
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Calcium fluoride (CaF2) Calcite is composed of calcium and fluorine atoms. |
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Sodium Chloride (NaCl) Halite is composed of sodium and chlorine atoms |
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There are groupings of atoms in this structure that are shown without any bonds. This means there are weaker bonds between these groups of atoms. |
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Pyroxene is a class of minerals including augite. |
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Silicon Oxide SiO2 |
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Beryllium Aluminum Silicate (Be3Al2(SiO3)6) Beryl is composed of beryllium, alumninum, silicon, oxygen and sodium atoms. |