Rock Cycle - Minerals (4B)

  • Analyzing different mineral shapes.
  • Interpreting the shapes of the minerals in a rock.
  • crystal
  • geometry
  • shape

Students determine the shapes of minerals in rocks.

Gold has neither a crystal form nor cleavage 


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.

  1. Set up minerals and rocks that are associated on the worksheet. 
  2. 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.
  3. 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.
  4. Have the students examine the minerals in the kit. Instruct them to draw the crystals or cleavage planes on the worksheet.
  5. 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.
  6. 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.

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