The crystal shape of a mineral may not be helpful in
identifying that mineral in a rock. Most minerals in rocks grow in a
confined space, in competition with other minerals, so they do not develop
full crystal forms. You may see "faces" of the crystal that can
give clues to its form, but not the entire crystal. For example, calcite may
show some parts of its rhombohedral shape. When crystal shape is absent,
other characteristic of minerals can help you identify them.
One such characteristic is cleavage. Cleavage is the
tendency of a mineral crystal to split in definite directions (when a force
is applied) producing more or less smooth surfaces. Cleavage is caused by
weaknesses in the orderly placement of the atoms within a crystal. It is
strictly a directional property, which can only occur in crystalline
substances. Different minerals may have one, two, three, four, or six
cleavages. The smooth surfaces in minerals within rock samples often
indicate cleavage. Note that not all minerals show cleavage. For example,
quartz breaks irregularly.
Two other useful characteristics are hardness and
luster. Hardness is the ability of the mineral to resist scratching or
abrasion. Luster is the way the mineral reflects light. There are two types
of luster. Metallic minerals look like shiny or rusted metal. Nonmetallic
elements reflect light like glass or pearls or glue. For example pyrite is
metallic, quartz and rubies are vitreous or glass looking, turquoise is
waxy, and feldspar is pearly looking.
- Set up minerals and rocks that are associated on the worksheet.
- Make sure students know the geometric shapes covered in the Pre Lab.
Remind students that the internal arrangement of the atoms reflects the
outside appearance of the mineral.
- Explain that minerals make up rocks. Tell the students that many of
the minerals that they have seen in the lab can thus be found in rocks.
- Have the students examine the minerals in the kit. Instruct them to
draw the crystals or cleavage planes on the worksheet.
- Next, have the students look at the rock specimens, and try to
recognize the mineral samples they have just sketched. Place the rock
specimens under the microscopes or have the students use hand lenses.
- The students should discover that crystal shapes can aid in
determining the minerals in a rock. They may also learn that some
minerals in rocks, like quartz, do not always make those crystal shapes.
Other characteristics, such as hardness or luster, can be used to
identify these minerals.