Chapter: Rocks and Minerals

Chapter: Rocks and Minerals

Table of Contents

Section 3: Metamorphic Rocks and the Rock Cycle

Section 1: Minerals—Earth’s Jewels

Section 2: Igneous and Sedimentary Rocks

• Minerals are inorganic, solid materials found in nature. Inorganic means they usually are not formed by plants or animals.

• X-ray patterns of a mineral show an orderly arrangement of atoms that looks something like a garden trellis.

What is a mineral?— Minerals Defined

Minerals—Earth’s Jewels

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• The particular chemical makeup and arrangement of the atoms in the crystal is unique to each mineral.

• Rocks usually are made of two or more minerals.

What is a mineral?— Minerals Defined

Minerals—Earth’s Jewels

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• Each mineral has unique characteristics you can use to identify it.

• Minerals form in several ways. One way is from melted rock material inside Earth called magma.

• As magma cools, atoms combine in orderly patterns to form minerals.

How do minerals form?Minerals—Earth’s Jewels

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• Evaporation can form minerals.

• Just as salt crystals appear when seawater evaporates, other dissolved minerals, such as gypsum, can crystallize.


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• A process called precipitation (prih sih puh TAY shun) can form minerals, too.

• Water can hold only so much dissolved material. Any extra separates and falls out as a solid.


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• Large mineral grains that fit together like a puzzle seem to show up in rocks formed from slow-cooling magma.

Formation CluesMinerals—Earth’s Jewels

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• If you see large, perfectly formed crystals, it means the mineral had plenty of space in which to grow.

Formation CluesMinerals—Earth’s Jewels

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• This is a sign they may have formed in open pockets within the rock.

• Each mineral has a set of physical properties that can be used to identify it.

• Most common minerals can be identified with items you have around the house and can carry in your pocket, such as a penny or a steel file.

Properties of MineralsMinerals—Earth’s Jewels

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• All minerals have an orderly pattern of atoms. • The atoms making

up the mineral are arranged in a repeating pattern.

CrystalsMinerals—Earth’s Jewels

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• Solid materials that have such a pattern of atoms are called crystals.

• Minerals that split into pieces with smooth, regular planes that reflect light are said to have cleavage (KLEE vihj).

Cleavage and FractureMinerals—Earth’s Jewels

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• Cleavage is caused by weaknesses within the arrangement of atoms that make up the mineral.

• Not all minerals have cleavage. Some break into pieces with jagged or rough edges.

Cleavage and FractureMinerals—Earth’s Jewels

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• Materials that break this way, such as quartz, have what is called fracture (FRAK chur).

• Sometimes a mineral’s color can help you figure out what it is. But color also can fool you.

ColorMinerals—Earth’s Jewels

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• The common mineral pyrite (PI rite) has a shiny, gold color similar to real gold.

• Because of this, pyrite also is called fool’s gold.

• Scraping a mineral sample across an unglazed, white tile, called a streak plate, produces a streak of color.

Streak and LusterMinerals—Earth’s Jewels

1


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• The streak is not necessarily the same color as the mineral itself. This streak of powdered mineral is more useful for identification than the mineral’s color.

• Luster describes how light reflects from a mineral’s surface.


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• If it shines like a metal, the mineral has metallic (muh TA lihk) luster.

• Nonmetallic minerals can be described as having pearly, glassy, dull, or earthy luster.

• Friedrich Mohs developed a way to classify minerals by their hardness.

HardnessMinerals—Earth’s Jewels

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• The Mohs scale classifies minerals from 1 (softest) to 10 (hardest).

HardnessMinerals—Earth’s Jewels

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• You can determine hardness by trying to scratch one mineral with another to see which is harder.

• Specific gravity compares the weight of a mineral with the weight of an equal volume of water.

Specific GravityMinerals—Earth’s Jewels

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• Pyrite—or fool’s gold—is about five times heavier than water. Pure gold is more than 19 times heavier than water.

• Measuring specific gravity is another way you can identify minerals.

• The mineral magnetite will attract a magnet.

Other PropertiesMinerals—Earth’s Jewels

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• The mineral calcite has two unusual properties. It will fizz when it comes into contact with an acid like dilute HCl.

• Also, if you look through a clear calcite crystal, you will see a double image.

• Only a small number of the more than 4,000 minerals make up most rocks.

Common MineralsMinerals—Earth’s Jewels

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• Most of the rock-forming mineralsare silicates (SIH luh kaytz), which contain the elements silicon and oxygen.


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• More than half of the minerals in Earth’s crust are types of a silicate mineral called feldspar.

• Other important rock-forming minerals are carbonates.


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• The carbonate mineral calcite makes up most of the common rock limestone.

• Gems are minerals that are rare and can be cut and polished, giving them a beautiful appearance.

GemsMinerals—Earth’s Jewels

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• To be gem quality, most minerals must be clear with few or no blemishes or cracks.

• A gem also must have a beautiful luster or color.

• One reason why gems are so rare is that they are formed under special conditions.

The Making of a GemMinerals—Earth’s Jewels

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• Diamond, for instance, is a form of the element carbon.

• Scientists suggest that diamond forms deep in Earth’s mantle. It takes a certain kind of volcanic eruption to bring a diamond close to Earth’s surface, where miners can find it.

• A mineral is called an ore if it contains enough of a useful substance that it can be sold for a profit.

OresMinerals—Earth’s Jewels

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• The iron used to make steel comes from the mineral hematite, lead for batteries is produced from galena, and the magnesium used in vitamins comes from dolomite.

• Ores of these useful metals must be extracted from Earth in a process called mining.

• After an ore has been mined, it must be processed to extract the desired mineral or element.

Ore ProcessingMinerals—Earth’s Jewels

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• Smelting melts the ore and thenseparates and removes most of the unwanted materials.

Ore ProcessingMinerals—Earth’s Jewels

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• After this smelting process, it can be refined, which means that it is purified.

Section Check

1Question 1

What does inorganic mean?

Answer

Inorganic means not formed by plants or animals.

Section Check

1Question 2

List some places you might find minerals in your home.

Answer

You can find minerals in salt shakers, pencils, glasses, and ceramic dishes.

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Section Check

1Question 3

Explain the difference between a rock and a gem.

Answer

Gems are rare minerals that can be cut and polished. They have a beautiful color and lack cracks or blemishes. Rocks are often cloudy and when they are cut, they crack.

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Igneous Rock• Igneous (IHG nee us) rocks form when

melted rock material from inside Earth cools.

• When melted rock material cools on Earth’s surface, it makes an extrusive (ehk STREW sihv) igneous rock.

• When the melt cools below Earth’s surface, intrusive (ihn TREW sihv) igneous rock forms.

Igneous and Sedimentary Rocks

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Chemical Composition• The chemicals in

the melted rock material determine the color of the resulting rock.


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• If it contains a high percentageof silica and little iron, magnesium, or calcium, the rock generally will be light in color.

Chemical Composition• Light-colored

igneous rocks are called granitic (gra NIH tihk) rocks.


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Chemical Composition• If the silica content

is far less, but it contains more iron, magnesium, or calcium, a dark-colored or basaltic (buh SAWL tihk) rock will result.

• Intrusive igneous rocks often are granitic, and extrusive igneous rocks often are basaltic.


2

Rocks from Lava• Extrusive igneous rocks form when melted

rock material cools on Earth’s surface.


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Rocks from Lava• When the melt reaches Earth’s surface, it is

called lava.


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Rocks from Lava


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• Lava cools quickly before large mineral crystals have time to form.

Rocks from Lava• Extrusive igneous rocks can form in two

ways.


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Rocks from Lava• In one way, volcanoes erupt and shoot out

lava and ash.


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Rocks from Lava


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• Also, large cracks in Earth’s crust, called fissures (FIH shurz), can open up.

Rocks from Lava• Oozing lava from a

fissure or a volcano is called a lava flow.

• The fastest cooling lava forms no grains at all. This is how obsidian, a type of volcanic glass, forms.


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Rocks from Magma• Intrusive igneous

rocks are produced when magma cools below the surface of Earth.


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Rocks from Magma• Intrusive igneous

rocks form when a huge glob of magma from inside Earth is forced upward toward the surface but never reaches it.


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Rocks from Magma

• Intrusive igneous rocks generally have large crystals that are easy to see.

• Some extrusive igneous rocks do not have large crystals that you can see easily.


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• Others are a mixture of small crystals and larger, visible crystals.

Sedimentary Rocks• Pieces of broken rock,

shells, mineral grains, and other materials make up what is called sediment (SE duh munt).

• Sediment can collect in layers to form rocks. These are called sedimentary (sed uh MEN tuh ree) rocks.


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Sedimentary Rocks

• When sediment is dropped, or deposited, by wind, ice, gravity, or water, it collects in layers.

• After sediment is deposited, it begins the long process of becoming rock.


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• Most sedimentary rocks take thousands to millions of years to form.

Detrital Rocks

• Detrital rocks are made of grains of minerals or other rocks that have moved and been deposited in layers by water, ice, gravity, or wind.

• Other minerals dissolved in water act to cement these particles together.


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Identifying Detrital Rocks• To identify a detrital

sedimentary rock, you use the size of the grains that make up the rock.


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Identifying Detrital Rocks• The smallest, clay-

sized grains feel slippery when wet and make up a rock called shale.


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• Silt-sized grains make up the rougher-feeling siltstone.

Identifying Detrital Rocks• Sandstone is

made of yet larger, sand-sized grains.


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sandstone

• Pebbles mixed and cemented together with other sedimentup rocks called

make

conglomerates (kun GLAHM ruts).

Chemical Rocks

• Chemical sedimentary rock forms when mineral-rich water from geysers, hot springs, or salty lakes evaporates.

• As the water evaporates, layers of the minerals are left behind.


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• Chemical rocks form from evaporation or other chemical processes.

Organic Rocks• Chalk and coal

are examples of the group of sedimentary rocks called organic rocks.


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• Living matterdies, piles up, and then is compressed into rock.

Organic Rocks


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• If the rock is produced from layers of plants piled on top of one another, it is called coal.

Fossils

• Chalk and other types of fossiliferous limestone are made from the fossils of millions of tiny organisms.

• A fossil is the remains or trace of a once-living plant or animal.


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Section Check

2Question 1

What type of rock forms when melted rock material from inside Earth cools?

A. igneousB. lavaC. oreD. sedimentary

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Section Check

2Answer

The correct answer is A. Igneous rock forms when melted rock, called magma, cools.

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Section Check

2Question 2

What determines the color of igneous rock?

Answer

The chemicals in the melted rock determine the color of igneous rock.

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Section Check

2Question 3

The photo shows _______ rock.

A. igneousB. metamorphicC. sedimentaryD. volcanic

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Section Check

2Answer

The answer is C. Sedimentary rocks are formed by layers of different sediments over time.

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New Rock from Old Rock• Many physical changes on and within Earth

are at work, constantly changing rocks. • It can take millions of years for rock to

change.

Metamorphic Rocks and the Rock Cycle

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• Sometimes existing rocks are cooked when magma is forced upward into Earth’s crust, changing their mineral crystals.

• All these events can make new rocks out of old rocks.

Metamorphic Rocks• New rocks that form when existing rocks

are heated or squeezed but are not melted are called metamorphic (me tuh MOR fihk) rocks.


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Metamorphic Rocks


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• Granite can change to gneiss. • The sedimentary rock sandstone can

become quartzite, and limestone can change to marble.

Types of Changed Rocks

• A physical characteristic helpful for classifying all rocks is the texture of the rocks.


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• Texture differences in metamorphic rocks divide them into two main groups— foliated (FOH lee ay tud) and nonfoliated.

Types of Changed Rocks• Foliated rocks

have visible layers or elongated grains of minerals.


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• These minerals have been heated and squeezed into parallel layers, or leaves.

• Many foliated rocks have bands of different-colored minerals.

Types of Changed Rocks• Nonfoliated rocks do not have distinct

layers or bands.


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• These rocks, such as quartzite, marble, and soapstone, often are more even in color than foliated rocks.

The Rock Cycle• Scientists have created a model called the

rock cycle to describe how different kinds of rock are related to one another and how rocks change from one type to another.


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Click image to view movie.

The Journey of a Rock• A blob of lava that oozes to the surface and

cools forms an igneous rock.


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• Wind, rain, and ice wear away at the rock, breaking off small pieces. These pieces are called sediment.

The Journey of a Rock• Mineral-rich water seeps through the

sediment and glues, or cements, it together. It becomes a sedimentary rock.


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• Pressure and heat inside Earth can change it into a metamorphic rock.

• Metamorphic rock deep inside Earth can melt and begin the cycle again.

Section Check

3Question 1

What type of rock results when an existing rock changes form?

A. igneous rockB. metamorphic rockC. a mineralD. sedimentary rock

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Section Check

3Answer

The answer is B. A metamorphic rock is formed when an existing rock is heated by Earth’s temperature or squeezed by intense pressure.

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Section Check

3Question 2

Compare and contrast the two groups of metamorphic rocks.

Answer

Foliated rocks have many layers. Nonfoliated rocks do not have layers and the individual mineral grains can be difficult to see.

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Section Check

3Question 3

Would you expect to find a well-preserved artifact in a metamorphic rock such as quartzite? Explain.

Section Check

3Answer

No; quartzite is an example of a nonfoliated metamorphic rock. These rocks do not have distinct layers. Quartzite forms when quartz sand grains recrystallize during intense heating and pressure. An artifact would get destroyed during this process.

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End of Chapter Summary File

Chapter: Rocks and Minerals

Documents

minerals form

minerals color

common minerals

dissolved minerals

mineral sample

streak of powdered mineral

streak of color

orderly arrangement