2 Identifying Minerals

Transcription

1 CHAPTER 1 2 Identifying Minerals SECTION Minerals of the Earth s Crust BEFORE YOU READ After you read this section, you should be able to answer these questions: What seven properties can be used to identify a mineral? What are some special properties of minerals? How Can You Identify Minerals? If you close your eyes and taste different foods, you can usually figure out what the foods are. You can identify foods by noting their properties, such as texture and flavor. Minerals also have properties that you can use to identify them. COLOR The same mineral can have many different colors. For example, the mineral quartz can be clear, white, pink, or purple. Minerals can also change colors when they react with air or water. For example, pyrite ( fool s gold ) has a golden color. If pyrite is exposed to air and water, it can turn brown or black. Because the color of a mineral can vary a lot, color is not the best way to identify a mineral. LUSTER The way a surface reflects light is called luster. When you say that something looks shiny, you are describing its luster. A mineral can have a metallic, submetallic, or nonmetallic luster. The table below gives some examples of different kinds of luster. STUDY TIP Reading Organizer As you read this section, create an outline of the section. Use the properties of minerals to form the headings of your outline. 1. Explain How can the color of a mineral change? Luster Description Examples Metallic bright and shiny, like metal gold, copper wire Submetallic dull, but reflective graphite (pencil lead ) Nonmetallic Vitreous glassy, brilliant glass, quartz Waxy greasy, oily wax, halite Silky looks like light is reflecting satin fabric, asbestos off long fibers Pearly creamy pearls, talc Resinous looks like plastic plastic, sulfur Earthy rough, dull concrete, clay Say It Apply Ideas In a small group, think of a list of 10 to 15 everyday materials. Together, try to describe the luster of each material using the terms in the table. Interactive Textbook 5 Minerals of the Earth s Crust

2 SECTION 2 Identifying Minerals continued 2. Explain Why is streak more useful than color in identifying a mineral? STREAK The color of a mineral in powdered form is called its streak. You can find a mineral s streak by rubbing the mineral against a piece of unglazed porcelain. The piece of unglazed porcelain is called a streak plate. The mark left on the streak plate is the streak. Streak is a more useful property than color for identifying minerals. This is because the color of a mineral s streak is always the same. For example, the color of the mineral hematite may vary, but its streak will always be red-brown. CLEAVAGE AND FRACTURE Different minerals break in different ways. The way that a mineral breaks depends on how its atoms are arranged. When some minerals break, the surfaces that form are smooth and flat. These minerals show the property of cleavage. Other minerals break unevenly, along curved or rough surfaces. These minerals show the property of fracture. The mineral biotite, a type of mica, shows the property of cleavage. It breaks easily into thin, flat sheets. The mineral halite also shows the property of cleavage. Its crystals break into cubes. TAKE A LOOK 3. Identify What kind of fracture does quartz show? The mineral quartz shows the property of fracture. It breaks along a curved surface. This kind of fracture is called conchoidal fracture. Math Focus 4. Calculate How many times denser is gold than water? DENSITY Density is a measure of how much matter is in a given amount of space. Density is usually measured in grams per cubic centimeter (g/cm 3 ). For example, the density of water is 1 g/cm 3. Geologists often use specific gravity to describe the density of a mineral. A mineral s specific gravity is the density of the mineral divided by the density of water. For example, gold has a density of 19 g/cm 3. Its specific gravity is 19 g/cm 3 1 g/cm Interactive Textbook 6 Minerals of the Earth s Crust

3 SECTION 2 Identifying Minerals continued HARDNESS A mineral s resistance to being scratched is its hardness. Scientists use the Mohs hardness scale to describe the hardness of minerals. The harder a mineral is to scratch, the higher its rating on the Mohs scale. Talc, one of the softest minerals, has a rating of 1. Diamond, the hardest mineral, has a rating of 10. Scientists use reference minerals to find the hardness of unknown minerals. They try to scratch the surface of the unknown mineral with the edge of a reference mineral. If the reference mineral scratches the unknown mineral, the reference mineral is harder than the unknown mineral. You probably don t have pieces of these reference minerals. However, you can find the hardness of a mineral using common objects. For example, your fingernail has a hardness of about 2 on the Mohs scale. A piece of window glass has a hardness of about 5.5. Hardness Mineral Hardness Mineral 1 Talc 6 Orthoclase 2 Gypsum 7 Quartz 3 Calcite 8 Topaz 4 Fluorite 9 Corundum 5 Apatite 10 Diamond 5. Define What is hardness? Critical Thinking 6. Apply Concepts A scientist tries to scratch a sample of orthoclase with a sample of apatite. Will he be able to scratch the orthoclase? Explain your answer. SPECIAL PROPERTIES Some minerals have special properties. These properties can be useful in identifying the minerals. Special Properties of Some Minerals Calcite and fluorite show the property of fluorescence. This means that they glow under ultraviolet light. Calcite produces a chemical reaction when a drop of weak acid is placed on it. It fizzes and produces gas bubbles. Some minerals, such as this calcite, show optical properties. Images look doubled when they are viewed through calcite. Magnetite shows the property of magnetism. It is a natural magnet. Halite has a salty taste. You should not taste a mineral unless your teacher tells you to. Minerals that contain radioactive elements may show the property of radioactivity. The radiation they give off can be detected by a Geiger counter. TAKE A LOOK 7. Describe Under ultraviolet light, what happens to minerals that show the property of fluorescence? Interactive Textbook 7 Minerals of the Earth s Crust

4 Section 2 SECTION VOCABULARY cleavage in geology, 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 fracture the manner in which a mineral breaks along either curved or irregular surfaces hardness a measure of the ability of a mineral to resist scratching luster the way in which a mineral reflects light streak the color of a mineral in powdered form 1. Compare How are cleavage and fracture different? 2. Explain Why is color not the best property to use to identify a mineral? 3. Identify Give five properties that you can use to identify a mineral. 4. Apply Concepts A geologist has found an unknown mineral. She finds that a sample of calcite will not scratch the unknown mineral. She also finds that a sample of apatite will scratch the unknown mineral. About what is the unknown mineral s hardness? Explain your answer. 5. Calculate The density of a mineral is 2.6 g/cm 3. What is its specific gravity? Interactive Textbook 8 Minerals of the Earth s Crust

5 F Inside the Restless Earth Answer Key Chapter 1 Minerals of the Earth s Crust SECTION 1 WHAT IS A MINERAL? 1. Minerals form naturally. 2. It is not a mineral, because it is not inorganic. 3. a substance made of two or more elements bound together 4. the regular pattern of atoms that make them up 5. cubes 6. All are silicate minerals, so all contain silicon and oxygen. 7. Sulfate minerals contain compounds of sulfur and oxygen; sulfide minerals contain compounds of sulfur and another element. 1. solid, inorganic, crystalline, naturally formed 2. An atom is the smallest part of an element that has all the qualities of that element. Elements are made up of only one kind of atom. 3. The crystal s shape is determined by the arrangement of atoms or molecules in the crystal. 4. Water is not a solid. 5. Silicate minerals contain compounds of silicon and oxygen. Nonsilicate minerals do not contain compounds of silicon and oxygen. 6. native elements, oxides, sulfides, sulfates, halides, carbonates SECTION 2 IDENTIFYING MINERALS 1. It can react with air or water. 2. A mineral s color may change, but its streak is always the same. 3. conchoidal times 5. the resistance of a mineral to being scratched 6. No, because orthoclase is harder than apatite. 7. They glow. 1. Minerals with cleavage break along smooth, flat surfaces. Minerals with fracture break along curved or irregular surfaces. 2. The same mineral can have many different colors. 3. hardness, streak, cleavage or fracture, luster, density 4. The mineral s hardness is probably about 4. Calcite has a hardness of 3. Apatite has a hardness of 5. Since apatite scratches the mineral but calcite doesn t, the mineral s hardness must be somewhere between 3 and SECTION 3 THE FORMATION, MINING, AND USE OF MINERALS 1. Metamorphism: garnet, graphite, talc Reaction: gold, copper, pyrite 2. gold, copper 3. iron, coal, salt 4. horizontal, vertical, angled 5. pollution and habitat destruction 6. When mineral materials are recycled, less of the minerals have to be mined from the Earth. 7. Silver: electronics, jewelry Bauxite: aluminum cans, utensils 8. ilmenite, magnetite, bauxite, beryl 9. These appliances run on electricity. If electricity couldn t move through them easily, they might not work correctly. 10. calcite, quartz 11. impurities 1. An ore is a rock or mineral that has enough useful material in it to be mined at a profit. 2. Type of material Metal Nonmetal Main features has shiny surfaces, does not transmit light, transmits heat and electricity easily, can be rolled into sheets or stretched into wires has shiny or dull surfaces, does not transmit heat or electricity easily, transmits light Common objects made from it wires, cars, electronics cement, computer chips, glass 3. evaporation, metamorphism, deposition 4. Open-pit mines: gold Quarries: gravel Strip mines: coal Interactive Textbook Answer Key 35 Inside the Restless Earth