Minerals and Rocks

[Pages:5]Minerals and Rocks

Minerals

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you've read this lesson, reread the statements to see if you have changed your mind.

Before

Statement

After

1. Minerals generally are identified by observing their color.

2. Minerals are made of crystals.

Key Concepts

? How do minerals form?

? What properties can be used to identify minerals?

? What are some uses of minerals in everyday life?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

What is a mineral?

Do you ever drink mineral water? Maybe you take vitamins and minerals to stay healthy. The word mineral has many common meanings. For geologists, scientists who study Earth and the materials of which it is made, the word mineral has a very specific definition.

A mineral is a naturally occurring, inorganic solid that has a crystal structure and a definite chemical composition. In order for a substance to be classified as a mineral, it must have all five of the characteristics listed in this definition. Both coal and pyrite are shiny, hard substances that form deep inside Earth. But only one is a mineral. Coal formed from ancient plant material. Pyrite crystals are made of the elements iron and sulfur. One substance is a mineral, and one is not.

Characteristics of Minerals

How can you determine which substance is the mineral? Consider each of the five characteristics of minerals.

Naturally Occurring To be classified as a mineral, a substance must form naturally. Materials made by people are not minerals. Diamonds that form deep beneath Earth's surface are minerals. Diamonds that are made in a laboratory are not minerals. However, manufactured diamonds may look similar to naturally occurring diamonds.

Reading Essentials

Identify the Main Ideas Write notes next to each paragraph to summarize the main ideas. On a separate piece of paper, organize these notes into two columns. Place each main idea in the left column. List the details to support it in the right column. Use your table to review the lesson.

Reading Check 1. Name the five characteristics that define a mineral.

Minerals and Rocks 227

Make a five-tab book to record your notes on mineral characteristics.

ONcactuurrarilnlyg Inorganic

Solid StCrurycsttuarle CComhepmosiictaioln

Math Skills

A ratio compares numbers. For example, in the chemical formula for water, H2O, the number 2 is called a subscript. The subscript tells you how many atoms of that element are in the formula. A symbol with no subscript means that element has one atom. So, the ratio of hydrogen (H) atoms to oxygen (O) atoms in H2O is 2:1. This is read two to one.

2. Use Ratios Quartz has

the formula SiO2. What is the ratio of silicon (Si) atoms to oxygen (O) atoms in quartz?

Key Concept Check 3. Describe How do minerals form?

228 Minerals and Rocks

Inorganic A material that contains carbon and was once alive is organic. A mineral cannot be organic. This means that a mineral cannot have once been alive. Also, a mineral cannot contain anything that was once alive, such as plants.

Solid A mineral must be solid. Liquids and gases are not considered minerals. Solid ice is a mineral, but water is not.

Crystal Structure A mineral must have a crystal structure. The atoms in a crystal are arranged in an orderly, repeating pattern called a crystal structure. This organized structure produces smooth faces and sharp edges on a crystal.

Definite Chemical Composition A mineral is made of specific amounts of elements. A chemical formula shows the amount of each element in a mineral. For example, pyrite is made of the elements iron (Fe) and sulfur (S). There always must be one iron atom for every two sulfur atoms. Therefore, the chemical formula for pyrite is FeS2.

Think again about coal and pyrite. The plants that turned into coal were once alive. Coal cannot be a mineral. Pyrite has all five characteristics of a mineral, so it is a mineral.

Mineral Formation

How do atoms form minerals? Atoms within a liquid join together and form a solid. Crystallization is the process by which atoms form a solid with an orderly, repeating pattern. Crystallization can happen in two main ways.

Crystallization from Magma Melted rock material is called magma. As magma cools, some of the atoms join together and form solid crystals. As the liquid magma continues to cool, more atoms are added to the surface of the crystals. The longer it takes the magma to cool, the larger the crystals become because atoms continue to be added to the crystals. Crystals grow large when the magma cools slowly. When magma cools quickly, the crystals that form remain small.

Crystallization from Water Many substances, such as salt, dissolve in water--especially if the water is warm. When water cools or evaporates, the particles of the dissolved substances come together again and crystallize. Gold crystals form this way. The orderly arrangement of atoms in the mineral gold is visible using a very powerful microscope.

Mineral Identification

Every mineral has a unique set of physical properties, or characteristics. These properties are used to identify minerals. By testing several properties, scientists can distinguish between similar minerals.

Reading Essentials

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Density

If you pick up two mineral samples that are about the same size, one might feel heavier than the other. The heavier mineral has a higher density. It has more mass in the same volume. The densities of many minerals are similar, but a very high or a very low density can help identify a mineral.

Hardness

Scientists measure the hardness of a mineral by observing how easily it is scratched or how easily it scratches something else. The Mohs hardness scale, shown in the table below, ranks hardness from 1 to 10. On this scale, diamond is the hardest mineral, with a hardness value of 10. The softest mineral is talc, with a hardness of 1.

Mohs Hardness Scale for Minerals

Mineral

Hardness

Hardness of Common Objects

talc

1 (softest)

gypsum

2

fingernail (2.5)

calcite

3

copper wire or penny (3.5)

fluorite

4

wire nail (4.5)

apatite

5

glass, steel knife blade (5.5)

feldspar

6

streak plate (unglazed porcelain) (6.5)

quartz

7

topaz

8

corundum

9

diamond

10 (hardest)

4. Apply A sample of

mineral A and a sample of mineral B are about the same size. The mineral B sample is heavier than the mineral A sample. Which mineral has the higher density? How do you know?

Visual Check 5. Interpreting Tables Circle the minerals that are harder than glass.

Color and Streak

Some minerals can be identified by their unique color. The mineral malachite always has a certain green color. But the colors of most minerals vary. Quartz is a common mineral that has many different colors.

The colors of most minerals vary from sample to sample, but the color of a mineral's powder does not vary. The color of a mineral's powder is called its streak. You can observe streak by scratching the mineral across a tile of unglazed porcelain. Sometimes, the color of a mineral and the color of its streak are different. For example, the mineral hematite can have a red, brown, or black color, but its streak is always a dark, rusty red.

6. Analyze Can you

identify a mineral by its color alone? Explain your answer.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Minerals and Rocks 229

Visual Check

7. Contrast Highlight the

surfaces formed by the breaks in the minerals. How does cleavage differ from fracture?

Luster

Minerals reflect light in different ways. Luster describes the way that a mineral's surface reflects light. Some terms used to describe mineral luster are metallic, glassy, earthy, or pearly. Hematite can have either a metallic luster or a dull luster. Muscovite mica has a pearly luster. Quartz has a glassy luster.

Cleavage and Fracture

Sometimes the way a mineral breaks helps identify it. Minerals break in two ways. If a mineral breaks along smooth, flat surfaces, it displays cleavage. The mineral on the left in the figure below illustrates the property cleavage. It forms a flat surface where it breaks. A mineral can break along a single cleavage direction or along several directions. Muscovite mica has one cleavage direction and peels off in sheets. Halite has three cleavage directions and breaks into cubes.

A mineral that breaks along rough or irregular surfaces displays fracture. The mineral on the right in the figure below illustrates the property fracture.

Cleavage and Fracture

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

ACADEMIC VOCABULARY

exhibit (verb) to display, to present for the public to see

Key Concept Check 8. Identify What are the common properties used to identify minerals?

Cleavage

Fracture

Crystal Shape

Minerals exhibit many different crystal shapes. A mineral's atomic structure determines its crystal shape.

Crystal shapes can vary greatly. Crystals of hematite have no definite shape, or are shapeless. They are described as massive. Muscovite mica has diamond-shaped or six-sided crystals, but muscovite commonly occurs in flat, sheetlike layers. Amethyst, a type of quartz, has crystals shaped like pyramids.

Sometimes crystals grow so close to each other that the crystal shape is too small to see. If there is room for large crystals to grow, the crystal shape can be used to help identify the mineral.

230 Minerals and Rocks

Reading Essentials

Unusual Properties

Some minerals have unusual properties that make them easy to identify. For example, halite tastes salty. Magnetite is magnetic and attracts steel objects. Calcite fizzes when acid touches it. A type of calcite called Iceland spar has a property called double refraction. Images viewed through a crystal of Iceland spar appear doubled.

Quartz crystals can produce an electric current when compressed. This property makes quartz crystals useful in radios, microphones, and watches. Several minerals display the property of fluorescence. Calcite and quartz glow under ultraviolet light.

Minerals in Everyday Life

From the moment you wake in the morning until you fall asleep at night, you use materials made from minerals. For example, table salt contains the mineral halite. Toothpaste contains calcite or silica. Some cosmetics contain mica. Some minerals are valuable because we use them every day. We appreciate others simply for their beauty.

Did you know that beverage cans and car batteries are made from minerals? These items are made of metals. Most metals combine with other elements in the formation of a mineral. For example, aluminum can be removed from the mineral bauxite. The minerals must be processed to remove the metals from them. Deposits of metallic or non-metallic minerals that can be produced at a profit are called ores.

Some minerals, such as gemstones, are valuable because of their appearance. Gemstones have physical properties that make them valuable. They are usually harder than quartz. Gemstones often have intense colors and brilliant luster. The natural crystals are cut and polished. Emeralds are green gemstones often used in jewelry.

Reading Check 9. Name one unusual property of a mineral.

Key Concept Check 10. Describe How are minerals used in everyday life?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Minerals and Rocks 231

Mini Glossary

cleavage: a break in a mineral along a smooth, flat surface

crystal structure: an orderly, repeating pattern of atoms in a crystal

crystallization: the process by which atoms form a solid with an orderly, repeating pattern

fracture: a break in a mineral along a rough or irregular surface

luster: the way that a mineral's surface reflects light

mineral: a naturally occurring, inorganic solid that has a crystal structure and a definite chemical composition

ore: a deposit of metallic or non-metallic minerals that can be produced at a profit

streak: the color of a mineral's powder

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes two characteristics of a specific mineral discussed in the lesson.

2. Write the five characteristics of a mineral in the diagram below.

Characteristics of a Mineral

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

3. Write a sentence to explain why crystals formed from cooling magma vary in size.

What do you think

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

232 Minerals and Rocks

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