The specific gravity, or relative density, of a
mineral is a comparison of the weight of a given volume of a mineral to the
weight of an equal volume of water. If a mineral has a specific gravity of
2, this means a given specimen of that mineral weighs twice as much as the
same volume of water. The specific gravity of a mineral depends on the
elements which compose it and how closely the atoms of those elements are
packed together. For example, the specific gravity of lead is much higher
than the specific gravity of silicon because lead has a higher atomic
weight. Likewise although diamond and graphite are both composed of carbon,
diamond has a higher specific gravity because its carbon atoms are packed
very closely together.
Specific gravity is very useful in distinguishing
minerals. If a mineral has a high specific gravity it will feel heavier than
another mineral of the same size. Gold, for example, has a higher specific
gravity than pyrite or "fools’ gold."
Density and specific gravity are sometimes used
interchangeably. However, density is a measurement of the weight of a
substance per some unit volume grams per cubic centimeter, whereas specific
gravity is a ratio with no units. Specific gravity is a relative
measurement. Weight is also different from specific gravity. Weight is the
heaviness of something, measured in pounds or grams, with no relationship to
a given volume.
- Explain the differences between specific gravity, density, and weight.
- Have students compare the specific gravity of mineral samples. This is
accomplished by comparing two samples that are roughly the same size.
Have the students hold one sample in each hand; the one that feels
"heavier" has a higher specific gravity. Have the students
rank the samples from highest to lowest specific gravity.
- You may want students to bring 10 small items that are all about the
same size from home, such as like nails, screws, or buttons. Have them
arrange the objects in order from highest to lowest specific gravity. It
may help to have students work together. The more a student compares and
contrasts the specific gravity of different objects, the easier it will
be for them to determine the specific gravity of minerals. Geologist
develop a "feel" for the specific gravity. Early miners knew
that gold was heavier, and could easily make the comparison as they
learned to distinguish gold from pyrite. Your students need to be
miners, and acquire the technique.