OBJECTIVES:

• Observing and describing key characteristics.
• Exploring observational skills

VOCABULARY:

• characteristic
• compound
• element
• observation

MATERIALS:

• specimens of calcite, halite, and quartz

Students use people to define "key characteristics."

Halite

BACKGROUND:

Minerals are composed of one or more elements. Since there are many elements, many combinations are possible; geologists have thus recognized thousands of different minerals. Minerals are important in a number of ways. Minerals are the building blocks of rocks. They are important to the world’s economy. Minerals such as gold and silver can determine the wealth of a nation.

Other minerals are admired because of their beauty, such as rubies and emeralds. Still other minerals are used in industry, such as chromite, a chromium ore, which is used to harden metals. Finally, minerals are very useful in everyday objects such as watches (quartz) and plaster (gypsum).

A mineral is "a naturally occurring, inorganic element or compound with a definite chemical composition, a characteristic crystalline structure, and distinct chemical properties." For example graphite, which is made up of the element carbon, is soft, so it easily writes on paper. The "lead" in pencils is really graphite. Mica is composed of silicon, oxygen, iron, and aluminum. It forms very thin crystals that flake off into sheets. Mica is used as glitter in some make up. Pyrite is an iron sulfide that has a metallic luster (shiny like metal) and brassy color. Many people mistake this for real gold, hence its common name "fools’ gold." Magnetite is an iron oxide is naturally magnetic and very dense.

Key characteristics, are unchanging properties of a substance. They can thus be reliably used to identify that substance. Minerals have key characteristics. These include:

1. CRYSTAL FORM - The natural growth (shape) of a mineral.
   
2. FRACTURE AND CLEAVAGE -The way a mineral breaks. Fracture is irregular breakage. For example, quartz has a conchoidal fracture; it breaks along hollowed and rounded, uneven surfaces. Cleavage is a regular breakage that follows the atomic structure of a mineral. Cleavage results in smooth, planar surfaces. Different minerals may have one, two, three, four, or six cleavages.
   
3. HARDNESS - The mineral’s resistance to scratching. It is controlled by the strength of atomic bonds within the mineral. Mineral hardness is rated from 1 (soft) to 10 (hard) on the Mohs hardness scale.
   
4. SPECIFIC GRAVITY - The density of a mineral relative to water.
   
5. STREAK - The color of a powdered mineral sample.
   
6. LUSTER - The way that a 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.
   
7. TASTE - Certain minerals like halite (salty) and sulfur (bitter) have characteristic "flavors."
   
8. MAGNETISM - The attraction of a mineral to a magnet.
   
9. REACTION TO ACID - The mineral reacts by "fizzing" with dilute HCl reacts with carbonate minerals.

PROCEDURE:

1. Discuss the difference between the words "characteristic" and "key characteristics" You can illustrate key characteristics by asking the students how they can recognize you. Use the following chart for guidance.

2. Illustrate key characteristics using quartz, calcite, and halite. These minerals have very distinct key characteristics that the students can easily observe.
       

   1. Quartz: Crystals always make a pointed pencil shape. It has 6 sides on terminal ends, resembling a prism. It is very hard (7 on the Mohs hardness scale). A steel knife will not scratch quartz. Quartz crystals are usually clear, but can occur in almost any color.
       

   2. Calcite: Shows a rhombohendral crystal shape. This can be described as a "drunken" or tilted 3-dimensional rectangle. Calcite fizzes readily with HCl. It is soft (about 3 on the hardness scale) and is easily scratched with a knife. Calcite may be clear to yellow in color. When a clear calcite crystal is placed over an image or word, it appears doubled when viewed through the crystal. This is called double refraction.
       

   3. Halite: Tastes like salt, because it is salt. Halite has a cubic crystal form, so it cleaves into perfect cubes. It has roughly the same hardness as calcite, and is colorless. Although taste is an important characteristic, caution students to avoid tasting minerals; some minerals have very unpleasant tastes.
       

3. Since these three minerals above can be clear in color, ask the students if color is a good key characteristic. Next, ask them how they would tell these minerals apart. Each of the minerals has its own key characteristics of hardness, shape, or taste. Color in this case is not a key characteristic.

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