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Name CHAPTER 5 Class Date Minerals of Earth’s Crust 1 What Is a Mineral? SECTION KEY IDEAS As you read this section, keep these questions in mind: • • • • What is a mineral? What are the two main groups of minerals? What are the six types of silicate crystal structures? What are three nonsilicate crystal structures? What Is a Mineral? A gold nugget, a grain of road salt, and a ruby may seem very different, but they have one thing in common: they are all minerals. A mineral has these properties: • • • • • It forms naturally. It is not made by living things. It is solid. Its atoms and molecules are arranged in a pattern. READING TOOLBOX Make Connections As you read, make flash cards for the highlighted terms in this section. Add any other terms you think are important. After you read, work with a partner to practice using each term correctly in a sentence. It is made up of certain elements combined in specific ratios. • It has specific physical properties. All minerals form naturally. In other words, people do not make them. Therefore, steel is not a mineral, because people make it. Organic matter is matter that is found in living things, or that is made by living things. Inorganic matter is matter that is not made by living things. All minerals are inorganic. For example, coal is not a mineral, because it forms from the remains of plants. The atoms and molecules in a solid mineral are arranged in a pattern. This arrangement causes the mineral to form crystals. A crystal is a solid with a regular arrangement of atoms or molecules. Crystals generally have geometric shapes, such as cubes. Every sample of a mineral is made up of the same elements. For example, every sample of the mineral quartz is made up mainly of the elements silicon and oxygen. Those elements are combined in a ratio of 1:2 (SiO2). Scientists describe this property of minerals by saying that minerals have a consistent chemical composition. READING CHECK 1. Explain Why is steel not a mineral? Critical Thinking 2. Apply Concepts Glass is made up of silicon and oxygen atoms in a 1:2 ratio. The SiO2 molecules in glass are arranged in a random way. Is glass a mineral? Explain your answer. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 57 Minerals of Earth’s Crust Name SECTION 1 Class Date What Is a Mineral? continued Talk About It Research Use the Internet or library to find out the names of the main rockforming minerals. Learn more about one of the minerals. Share what you learn with a partner. What Are the Two Main Kinds of Minerals? There are thousands of different minerals. However, only about 20 minerals are common on Earth. These 20 common minerals are called rock-forming minerals because they make up most rocks on Earth. Scientists divide minerals into two main groups based on their chemical compositions. The two groups are silicate minerals and nonsilicate minerals. SILICATE MINERALS READING CHECK 3. List What two elements do all silicate minerals contain? Most minerals on Earth are silicate minerals. A silicate mineral contains combinations of silicon (Si) and oxygen (O) atoms. Quartz, feldspars, micas, and olivine are examples of silicate minerals. Quartz is made up of silicon and oxygen. All other silicate minerals also contain other elements. For example, feldspars contain sodium, calcium, or potassium in addition to silicon and oxygen. Olivine contains magnesium, iron, silicon, and oxygen. NONSILICATE MINERALS Only about 4% of Earth’s crust is made of nonsilicate minerals. A nonsilicate mineral does not contain combinations of silicon and oxygen. It may contain silicon or oxygen, but they are not bonded together. There are six main groups of nonsilicate minerals. The table below describes these six groups. LOOKING CLOSER 4. Compare Which element is found in all sulfate minerals, but not in sulfide minerals? Type of Nonsilicate Mineral Description Examples Carbonates contain a carbonate group (CO3) dolomite, CaMg(CO3)2; calcite, CaCO3 Halides contain chlorine or fluorine combined with sodium, potassium, or calcium fluorite, CaF2; halite, NaCl Native elements made of atoms of a single element silver, Ag; copper, Cu Oxides contain oxygen and an element other than silicon corundum, Al2O3; hematite, Fe2O3 Sulfates contain a sulfate group (SO4) anhydrite, CaSO4 Sulfides contain sulfur combined with galena, PbS; pyrite, FeS2 another element (a metal) Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 58 Minerals of Earth’s Crust Name Class SECTION 1 Date What Is a Mineral? continued What Types of Crystals Do Minerals Form? The atoms and molecules in different minerals are arranged in different patterns. The arrangement of the atoms and molecules affects the shape of the crystals the mineral forms. One way scientists identify minerals is by studying their crystal shapes. The atoms and molecules in a mineral are always arranged the same way. Therefore, a mineral always forms the same crystal shape. CRYSTALLINE STRUCTURE OF SILICATE MINERALS There are many different kinds of silicate minerals. They form many different kinds of crystals. However, the crystals of all silicate minerals have similar building blocks. The building blocks of silicate minerals are made of atoms of silicon and oxygen. Each building block contains one silicon atom and four oxygen atoms. The oxygen atoms form a pyramid. The silicon atom is in the center of the pyramid. This structure is called a silicon-oxygen tetrahedron. Oxygen Silicon READING CHECK 5. Explain Why does a mineral always form crystals with the same shape? Talk About It Make Connections Use a dictionary to look up the meaning of the word tetrahedron. With a partner, discuss why the building blocks of silicate minerals are called silicon-oxygen tetrahedrons. Oxygen Silicon These drawings show two different models of a silicon-oxygen tetrahedron. Silicon-oxygen tetrahedrons are arranged in different ways in different silicate minerals. There are two factors that affect how the tetrahedrons are arranged. First, the tetrahedrons may form bonds with atoms of other elements, such as potassium or magnesium. Those bonds change the way the tetrahedrons are arranged. Second, the tetrahedrons may share oxygen atoms. When tetrahedrons share oxygen atoms, they can form chains, rings, and networks, or webs. The table on the next page shows how tetrahedrons can share atoms. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 59 Minerals of Earth’s Crust Name SECTION 1 Class Date What Is a Mineral? continued Arrangement of SiliconOxygen Tetrahedrons Description Examples Isolated tetrahedrons The tetrahedrons in these minerals are not linked to other silicon or oxygen atoms. They may be linked to atoms of other elements. olivine Silicon-oxygen tetrahedron LOOKING CLOSER Rings of tetrahedrons The tetrahedrons in these minerals share oxygen atoms. They form rings with three, four, or six sides. Atoms of other elements can also bond to the rings. beryl, tourmaline Single chain of tetrahedrons The tetrahedrons in these minerals share oxygen atoms to form long chains. Each tetrahedron shares two oxygen atoms with other tetrahedrons. pyroxene Double chain of tetrahedrons amphibole These minerals have double chains of tetrahedrons. The double chains form when two single chains join together by sharing oxygen atoms. Sheets of tetrahedrons In a sheet of tetrahedrons, each tetrahedron shares three oxygen atoms with other tetrahedrons. mica Framework of tetrahedrons Every tetrahedron in these minerals shares four oxygen atoms with other tetrahedrons. The tetrahedrons form a framework, or web. quartz 6. Identify How are the silicon-oxygen tetrahedrons in beryl arranged? 7. Apply Concepts How many oxygen atoms does each silicon-oxygen tetrahedron in pyroxene share? 8. Compare Which mineral listed in the table contains silicon-oxygen tetrahedrons that share the most oxygen atoms with other tetrahedrons? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 60 Minerals of Earth’s Crust Name SECTION 1 Class Date What Is a Mineral? continued GROUPS OF SILICATE MINERALS There are six main groups of silicate minerals. The minerals in each group have the same arrangement of silicon-oxygen tetrahedrons. The names of the groups are related to the arrangement of the tetrahedrons. Name of Group Arrangement of Tetrahedrons Single-tetrahedron silicates isolated tetrahedrons Ring silicates rings of tetrahedrons Single-chain silicates single chains of tetrahedrons Double-chain silicates double chains of tetrahedrons Sheet silicates sheets of tetrahedrons Framework silicates frameworks of tetrahedrons LOOKING CLOSER 9. List What are the six main groups of silicate minerals? CRYSTAL STRUCTURES OF NONSILICATE MINERALS Remember that there are many different kinds of nonsilicate minerals. Each nonsilicate mineral contains different atoms and molecules. The atoms and molecules in nonsilicate minerals can be arranged in many different ways. Therefore, nonsilicate minerals can have a variety of crystal shapes. The three most common crystal structures for nonsilicate minerals are cubes, hexagonal (six-sided) prisms, and irregular shapes. Some nonsilicate minerals have similar arrangements of atoms and molecules. These minerals also have similar crystal structures. For example, many halide and sulfide minerals form cubes, because their atoms and molecules are arranged in similar ways. LOOKING CLOSER 10. Explain Why do galena and halite form crystals with similar shapes? Galena, a sulfide mineral Halite, a halide mineral Halite and galena both form cubic crystals. They form crystals with similar shapes because the atoms in the minerals are arranged in a similar way. The arrangement of atoms and molecules in a crystal affects the mineral’s properties. For example, many native elements are very dense. They are dense because the atoms in them are packed very close together. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 61 Minerals of Earth’s Crust Name Class Date Section 1 Review SECTION VOCABULARY crystal a solid whose atoms, ions, or molecules are arranged in a regular, repeating pattern mineral a natural, usually inorganic solid that has a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties nonsilicate mineral a mineral that does not contain compounds of silicon and oxygen silicate mineral a mineral that contains a combination of silicon and oxygen and that may also contain one or more metals silicon-oxygen tetrahedron the basic unit of the structure of silicate minerals; a silicon ion chemically bonded to and surrounded by four oxygen ions 1. Compare What is the difference between silicate minerals and nonsilicate miner- als? 2. Identify What are the three most common nonsilicate crystal structures? 3. Apply Concepts Give one example of a mineral. Describe at least three character- istics that show that it is a mineral. 4. Infer An Earth scientist discovers a new mineral. The mineral is made up of gold and sulfur atoms bonded together. Which group of nonsilicate mineral does this mineral belong in? 5. Compare Quartz is a framework silicate. Pyroxene is a single-chain silicate. Describe how these two minerals are different and how they are similar. 6. Explain Corundum (Al2O3) and hematite (Fe2O3) both contain oxygen, but they are not considered to be silicate minerals. Why is this? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 62 Minerals of Earth’s Crust