CHAPTER 5 2 Identifying Minerals SECTION Minerals of Earth s Crust KEY IDEAS As you read this section, keep these questions in mind: What are seven physical properties scientists use to identify minerals? What are five special properties some minerals have? How Do Scientists Identify Minerals? Earth scientists who study minerals are called mineralogists. Mineralogists identify and classify minerals. The main way a mineralogist identifies a mineral is by studying its properties. The seven main physical properties that mineralogists can use to identify minerals are shown below. Property Color Streak Luster Cleavage and fracture Hardness Crystal shape Density Description what color the mineral has in white light the color of a mineral when it is a powder how the mineral reflects light how the mineral breaks how difficult it is to scratch the mineral what kind of crystal the mineral forms the mass of a given volume of a mineral COLOR A mineral s color is one of the easiest properties to observe. However, the color of a mineral sample is generally not enough to identify the mineral. Different minerals can have the same color. For example, many different minerals are clear or white. In addition, different samples of the same mineral can have different colors. For example, quartz can be clear, purple, pink, or even black. STREAK If you grind a mineral into a fine powder, its color may seem to change. The color of a mineral in powdered form is known as the mineral s streak. You can see the color of a mineral s streak by rubbing the mineral against a tool called a streak plate. No matter what color they are, all samples of a given mineral have the same streak. READING TOOLBOX Summarize As you read this section, make two-column notes to summarize the mineral properties described in the section. Write the name of the property in the first column. Write a description of the property in the second column. If you wish, use drawings to help you remember the different properties. READING CHECK 1. Explain Why is color usually not enough to identify a mineral? Holt McDougal Earth Science 63 Minerals of Earth s Crust
Talk About It Research Use the Internet or library to find pictures of minerals with different lusters. Describe the luster of each mineral in your own words. Share your descriptions with a partner. READING CHECK 2. Describe What do minerals with a metallic luster look like? LUSTER Some minerals, such as pyrite, look shiny when light shines on them. Others, such as talc, look dull. These minerals look different because they reflect light differently. The way a mineral reflects light is called its luster. Some minerals look like polished metal. These minerals have a metallic luster. Galena, gold, and pyrite are examples of minerals with a metallic luster. Minerals that do not look like polished metal have a nonmetallic luster. Talc, quartz, and calcite are examples of minerals with a nonmetallic luster. Minerals with a nonmetallic luster may look greasy, like talc does. They may look bright and glassy, like quartz does. They may look pearly, like mica does. They may even look dull, as if they do not reflect light at all. Critical Thinking 3. Explain Why do people who cut minerals for jewelry need to know about a mineral s cleavage or fracture? CLEAVAGE AND FRACTURE If you hit a diamond in the right way, it will break along a smooth, flat surface. People who cut minerals to make jewelry must know how different minerals break. Minerals such as diamond that break along smooth, flat surfaces show the property of cleavage. Mica is another example of a mineral that shows cleavage. Mica breaks easily into thin, flat sheets. Some minerals, such as quartz, do not break along flat surfaces. Instead, they break unevenly. A mineral that breaks unevenly shows the property of fracture. Minerals that show fracture may form different shapes when they break. If the mineral breaks to form a rough surface, it has an uneven or irregular fracture. If the mineral looks like broken wood, it has a splintery or fibrous fracture. If the mineral forms a curved surface, it has a conchoidal fracture. Quartz shows conchoidal fracture. It breaks along a curved surface. Holt McDougal Earth Science 64 Minerals of Earth s Crust
HARDNESS AND THE MOHS SCALE You may have heard that diamonds are the hardest substance on Earth. This does not mean that diamonds are hard to break. When mineralogists talk about a mineral s hardness, they are talking about how difficult it is to scratch the mineral. Diamonds are the hardest minerals because no other mineral can scratch them. Mineralogists use a scale called the Mohs hardness scale to describe the hardness of a mineral. The Mohs scale is a list of 10 minerals. Each mineral is harder than the minerals below it on the scale. Therefore, each mineral will scratch all the minerals below it on the scale. You can use the Mohs scale to find the hardness of a mineral not on the scale. To do this, scratch the unknown mineral with each of the minerals on the scale. The mineral s hardness is higher than the hardest mineral it will scratch. It is lower than the softest mineral it will not scratch. For example, galena will scratch gypsum, but it will not scratch calcite. Gypsum s hardness is 2. Calcite s hardness is 3. Therefore, galena s hardness is between 2 and 3. The table below shows the Mohs hardness scale. It also describes some common tests you can do to identify the hardness of a mineral. READING CHECK 4. Define What do mineralogists mean when they talk about the hardness of a mineral? Mineral Hardness Common Test Talc 1 easily scratched by a fingernail Gypsum 2 can barely be scratched by a fingernail Calcite 3 can barely be scratched by a copper penny Fluorite 4 easily scratched by glass or steel Apatite 5 can barely be scratched by glass or steel Feldspar 6 scratches glass, but does not scratch steel Quartz 7 easily scratches glass and steel Topaz 8 scratches quartz Corundum 9 scratches topaz Diamond 10 scratches everything LOOKING CLOSER 5. Apply Concepts A particular mineral will scratch apatite, but not feldspar. What is the hardness of the mineral? CRYSTAL SHAPE Remember that a mineral always forms crystals of the same shape. There are six main crystal systems, or shapes. The table on the next page shows the crystal systems. Holt McDougal Earth Science 65 Minerals of Earth s Crust
Talk About It Make Connections Use a dictionary to look up the meaning of the prefix iso-. With a partner, discuss why cubic crystals are sometimes also called isometric crystals. Crystal System Isometric (also called cubic) Description The crystals have three axes. All three axes are the same length. They form 90 angles where they meet. axis Orthorhombic The crystals have three axes. All the axes are different lengths. They form 90 angles where they meet. Tetragonal The crystals have three axes. Two of the axes are the same length. The third axis is longer or shorter than the other two. The axes form 90 angles where they meet. LOOKING CLOSER 6. Compare What is the main difference between cubic crystals and orthorhombic crystals? Hexagonal The crystals have four axes. Three of the axes are the same length. They form 120 angles where they meet. The fourth axis is longer or shorter than the other three. Monoclinic The crystals have three axes. All three are different lengths. Two of the axes form 90 angles where they meet. 7. Compare What is the main difference between monoclinic crystals and triclinic crystals? Triclinic The crystals have three axes. All three are different lengths. None of the axes form 90 angles where they meet. Holt McDougal Earth Science 66 Minerals of Earth s Crust
DENSITY A mineral s density is the ratio of the mineral s mass to its volume. You can calculate density using the formula below. The most common units of density are grams per cubic centimeter (g/cm 3 ). Water has a density of about 1 g/cm 3. Math Skills 8. Calculate A mineral sample has a mass (m) of 85 g and a volume (V) of 34 cm 3. What is its density? density mass, or D m volume V The density of a specific mineral is always the same. Most minerals on Earth have densities between 2 g/cm 3 and 3 g/cm 3. Minerals that contain heavy metals, such as lead or gold, may have much higher densities. What Other Properties Can Minerals Have? Some minerals have special properties. Mineralogists can use these properties to identify the minerals more easily. The table below describes five examples of special properties. Property Fluorescence or phosphorescence Chatoyancy or asterism Double refraction Magnetism Radioactivity Description Minerals that glow under ultraviolet light have the property of fluorescence. Some samples of calcite show the property of fluorescence. If a mineral keeps glowing after the light is shut off, it has the property of phosphorescence. Some minerals look silky when light reflects off of them. These minerals have the property of chatoyancy. Asterism is a similar property. When light shines on a mineral with asterism, a six-sided star shape appears on the mineral. When light passes through some minerals, it is split into two rays. If you look at a picture or line through one of these minerals, it will seem to be doubled. This property is called double refraction. Calcite is an example of a mineral that shows double refraction. Some minerals will stick to magnets. Other minerals can act like magnets. These minerals have the property of magnetism. Lodestone is an example of a magnetic mineral. The atoms of some elements, such as uranium, are unstable. Over time, they break down into atoms of other elements. When they break down, they give off energy, such as X rays. Minerals that contain these elements also give off radiation. A mineral that gives off radiation has the property of radioactivity. Pitchblende is an example of a mineral that shows radioactivity. Talk About It Discuss Talk with a partner about other words you have heard that include the root aster. Make a prediction of what you think that root means. Use a dictionary to test your prediction. LOOKING CLOSER 9. Compare How is fluorescence different from phosphorescence? Holt McDougal Earth Science 67 Minerals of Earth s Crust
Section 2 Review SECTION VOCABULARY cleavage the tendency of a mineral to split along specific planes of weakness to form smooth, flat surfaces density the ratio of the mass of a substance to the volume of the substance; commonly expressed as grams per cubic centimeter for solids and liquids and as grams per liter for gases fracture the manner in which a mineral breaks along either curved or irregular surfaces luster the way in which a mineral reflects light mineralogist a person who examines, analyzes, and classifies minerals Mohs hardness scale the standard scale against which the hardness of minerals is rated streak the color of a mineral in powdered form 1. Compare Complete the Venn diagram below to compare cleavage and fracture. Cleavage Fracture 2. Apply Concepts Gold has a hardness between 2 and 3. Describe what would happen if you scratched a piece of gold with a piece of talc, a piece of calcite, and a piece of quartz. 3. Explain Why is streak generally a more useful property than color in identifying a mineral? 4. Infer A scientist places a sample of a mineral on top of a drawing. When he looks through the mineral at the drawing, each line in the drawing is doubled. What property does the mineral show? Holt McDougal Earth Science 68 Minerals of Earth s Crust