BACKGROUND:
Minerals can be identified by various properties.
Students will become familiar with mineral characteristics the more they
study and examine minerals. Characteristics commonly used to identify
minerals include color, hardness, crystal shape, texture, cleavage, density
(or specific gravity) and magnetism. The students should be familiar with at
least some of these terms.
| Mohs HARDNESS
SCALE
(1) TALC (SOFTEST)
(2) GYPSUM
(3) CALCITE
(4) FLUORITE
(5) APATITE
(6) ORTHOCLASE
(7) QUARTZ
(8) TOPAZ
(9) CORUNDUM
(10) DIAMOND (HARDEST) |
Cleavage is defined as the breakage of a mineral
along planes of atomic weakness. Specific gravity is the ratio of the mass
of a given volume of a mineral to the mass of an equal volume of water.
Simply, specific gravity refers to how heavy a mineral feels (density).
Magnetism is a property of two common minerals,
magnetite and pyrrhotite (both minerals contain iron.) This property is
unique, and therefore, easily recognizable. Lodestone, which is the mineral
magnetite, was used as a compass by early navigators.
Hardness refers to the resistance of a substance to
being scratched. The Mohs Hardness Scale (named after Austrian mineralogist
Friedrich Mohs) lists 10 reference minerals that are arranged in increasing
order of hardness. Note that this is a relative hardness scale. Diamond is
actually over four hundred times harder than talc.
The Mohs scale is only one way to classify mineral
hardness. A simpler method, often used by geologists during field work, is
to classify minerals as harder or softer than a steel knife or a fingernail.
For example, if you scratch the mineral with the knife and nothing happens,
the specimen is likely quartz or feldspar. If your fingernail can scratch
the mineral, it may be gypsum or less commonly talc.
PROCEDURE:
- Use the Gem Placemats to help students discover the hardness of
different gems, especially those gems that are minerals. Review the Gem
Placemat with the students. Go over which gems are minerals. Remember a
mineral must be inorganic, for example amber is not a mineral. You
may want to show students different types of gems using the Gem Kit.
- Write the Mohs hardness scale on the board. Talk about the gems
in the kit and have them determine the hardness. They should
observe that most gems are very hard. Ask them why. One reason is that
durability makes minerals valuable. Since harder minerals tend to be
more durable, most gems are hard. For example, diamond is highly priced
because it is the hardest mineral.
- You may want students to find out the hardness of the minerals that
they used during the lab.
- You may want the students to research more information about minerals.
The recommended reference books explain many mineral characteristics,
and have many excellent mineral images. Other reference books from the
library can be used so students can discover properties of minerals.
You may also want to visit the web site: http://mineral.galleries.com,
which has abundant information on minerals especially gems.
