Earth Sciences Standards Preview 4.b. Students know how to identify common rock-forming minerals (including quartz, feldspar, mica, and hornblende) and ore minerals by using a table of diagnostic properties.
Standard Set 4. Earth Sciences 4. The properties of rocks and minerals reflect the processes that formed them. As a basis for understanding this concept: 4.a. Students know how to differentiate among igneous, sedimentary, and metamorphic rocks by referring to their properties and methods of formation (the rock cycle).
by Trish West
Compare and Contrast
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Captions Diagrams Table Glossary
Rocks and Minerals
Scott Foresman Science 4.6
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Vocabulary cleavage igneous luster metamorphic mineral ore rock cycle sedimentary streak
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by Trish West
What are minerals? Mineral Crystals You use minerals every day. Minerals are natural, nonliving solid crystals that make up rocks. Each mineral has the same chemical makeup. Quartz has the same chemicals whether it is found in Australia or California. Scientists have found more than 3,000 minerals. Just a few of them make up most of the rocks in Earthâ€™s crust. Most rocks contain combinations of minerals. Each type of rock always has the same combination. Granite is always made up of quartz and feldspar crystals.
Properties of Minerals Scientists identify a mineral by testing its properties. Now you will learn about several mineral properties.
Color A mineral can be different colors. Different minerals can be the same color. This makes it hard to identify a mineral by its color. Other properties are better for identifying minerals.
Luster Luster is the property of how a mineral reflects light. A glassy luster is shiny. A metallic luster looks like polished metal. A mineral with a soft shine can have a waxy, silky, or pearly luster. Some minerals have dull, chalky luster. Others have a greasy luster.
Satin spar gypsum Quartz
The hardness of minerals varies. Scientists find out a mineralâ€™s hardness by testing how hard it is to scratch. They use the Mohs scale to compare the hardness of different minerals. The Mohs scale ranks minerals from 1 to 10. Talc, with a hardness of 1, is the softest mineral. Diamond with a hardness of 10, is the hardest mineral. A mineral can scratch any other mineral that has a lower number on the Mohs scale. You can also use other objects to scratch minerals. A penny has a hardness of about 3. A fingernailâ€™s hardness is about 2.5. A steel file is about 6.5 on the scale.
Streak is the color of the powder that a mineral leaves when it is scratched across a special plate. Each sample of a mineral always has the same color streak.
The streak of cinnabar is bright red.
Crystal Shape You can identify a mineral by looking at its crystals. Scientists group crystals into six basic types. For example, the crystals of fluorite are cube-shaped.
Talc is not hard. Apatite is somewhat hard. Diamond is very hard.
Most minerals break into patterns. Those that break along smooth, flat surfaces have cleavage. Minerals that do not have cleavage may splinter like wood.
Hornblende breaks along flat, smooth surfaces. This is its cleavage.
Gold California’s state mineral is gold. Gold’s luster is shiny and metallic. Its color and streak are both yellow. Gold is soft, so it can be pounded into shapes or sheets.
How are minerals and ores sorted?
Other Properties of Minerals
Resources in Rocks
There are other ways to identify minerals. Some minerals may be attracted by a magnet. Minerals may feel sandy, sticky, powdery, or smooth. Some minerals can be shaped or cut. There are minerals that may smell like rotten eggs or garlic. And chemical tests can be used to identify some minerals.
Rocks can contain valuable resources. An ore is a rock rich in valuable minerals that can be removed from Earth’s crust. People mine ore for its minerals. Zinc and copper are two metals found in mineral ores. Iron ore contains the metal iron. Miners dig up the iron ore. They crush and heat it. This separates the iron. Iron mixed with other materials makes steel. Another mineral resource is gypsum. It is used in plaster and paint. Paint also contains the mineral mica. Minerals such as sylvite are used in fertilizers.
Vinegar fizzes when placed on limestone that has calcite. Carbon dioxide gas rises from the limestone.
Some ores lie deep under Earth’s surface. Workers use machines to dig them out.
Iron often comes from the mineral hematite. About seven-tenths of hematite is pure iron. The rest is oxygen. Hematite may have shiny six-sided crystals. These crystals can be gray. Hematite can also have coarse grains. These grains are reddish brown. Red hematite is used to make red paint. It is also used to polish glass. Hematite is found in California. It has been found in rocks near silver mines. Red soil can be a sign of hematite.
The mineral galena contains the metal lead. Galena is gray. It has a metallic luster. It is the most common mineral that contains lead. Galena is easy to mine. People have mined galena to get lead for about 5,000 years. California has two important sources for galena. There is a galena mine in Tuolumne County. There is also a source of galena near Darwin, in Inyo County.
Iron from the mineral hematite is used to make steel.
Galena is mined in parts of California. It is an important source of lead.
This park bench is iron.
Lead is a metal. It is dense and blue-gray. Lead seems heavy compared to similar amounts of other metals. Lead can be shaped and pulled into thin wires. It is used in building materials. Lead was also used to color paints and pottery in the past. The pieces of colored glass in stained-glass windows are held together with lead. Years ago, lead pipes carried drinking water to buildings. Lead was also used in glassware that held food. But scientists found that lead can poison people. So lead is no longer used for these purposes.
People first used copper thousands of years ago. Copper is a shiny metal. It is easy to shape and is often used to make wire. Copper is often found as a pure metal in rocks that formed from lava. Copper is also found in other minerals and ores, such as chalcocite. Copper is good at conducting heat and electricity. It is used to make motors and generators. Bronze and brass are made from a mix of copper and other metals. Copper is found in many parts of California. One of the largest copper mines is in Utah.
Lead aprons protect people while they are having X-ray pictures taken.
Copper is used for pipes.
The Statue of Liberty is covered with copper. Copper can turn green in damp weather.
Using Properties to Identify Minerals Scientists use a table like the one below to identify minerals. You can use it to find the names of the minerals pictured on the next page.
colorless or white, pink, yellow, greenish, or red
ranges from glassy to dull
colorless or many colors, such as beige, pink, gray, and bluish green
glassy or pearly
dark green to black
shiny or metallic
This mineralâ€™s hardness is much less than that of the other minerals listed.
This mineralâ€™s luster is shiny and metallic.
This mineral does not make a streak.
This mineral can have a beige color.
How are rocks classified? Igneous Rocks Rocks can be classified by the way they are formed. Igneous rocks form from molten rock. This molten rock is called magma. Igneous rocks don’t have layers. However, they may have crystals that interlock.
Lava Cooling Quickly When magma reaches Earth’s surface, it is called lava. Lava is cooled by water or air. Often, only very small crystals have time to form. Basalt is an igneous rock cooled in water. Many islands are made of basalt. Pumice is an igneous rock cooled in air.
Magma Cooling Slowly Some igneous rocks form slowly. As magma rises toward Earth’s surface, it can force open cracks. Sometimes it melts some of the rocks around it. This magma cools slowly. The slow cooling causes large crystals to form. Eventually the magma hardens into igneous rock. Sometimes this takes millions of years. Granite forms in the way you just read about. It has large crystals of quartz, feldspar, and mica. Pegmatite and gabbro are also rocks that form this way. An igneous rock such as granite is very hard. It can be used for buildings. Concrete in sidewalks might also contain pieces of igneous rocks. Slow-Cooling Igneous Rocks Granite
Gabbro Obsidian forms from lava that cools quickly. It looks like glass.
Basalt is a very common igneous rock. It makes up most of the ocean floor.
Types and Uses of Sedimentary Rocks
Sedimentary rocks form when layers of sediments settle on top of one another and harden. Sediments are made up of soil, shells, pieces of rock, and dead plants and animals. Water, wind, ice, and gravity move sediments around. The sediments settle in layers on land or on the bottoms of oceans, rivers, and lakes.
Limestone is one kind of sedimentary rock. It can form from the hard skeletons and shells of sea animals that died long ago. It is used to make cement and glassware. Sandstone is another kind of sedimentary rock. It can form from tiny pieces of mineral or rock. Houses and statues are often made from sandstone. There are also sedimentary rocks that form from tiny particles. The particles settle at the bottoms of lakes or oceans. Two examples are shale and mudstone. Shale is useful in making cement.
Layering Newer layers press down on older ones. Their weight and sticky clay minerals hold the rock particles together. Natural chemicals can also bind them. Each new layer of sediment settles on top of older layers. When scientists find fossils, they can tell that the fossils in lower layers are probably older.
Limestone is very heavy. The limestone you see below is in a building frame.
Layers of sedimentary rocks formed these cliffs.
Uses of Metamorphic Rock
Heat and pressure deep inside Earth can change rocks. When this happens, metamorphic rocks form. Metamorphic rock can form from sedimentary rocks or igneous rocks. It can even form from other metamorphic rock. When metamorphic rocks form, they might have crystals of a different shape or size. Sometimes new types of minerals are formed too. Metamorphic rock is usually hard. Heat and pressure can cause it to form in layers. These layers can chip into flat sheets and slabs. The picture on this page shows rough layers of gneiss formed by high pressure. The fine, thin layers of slate formed under low pressure.
Metamorphic rock is used in many buildings. Slate was once used to make waterproof roofs. Chalkboards were also made from slate. Today it is used in patios and walkways. Buildings and sculptures have been made from marble for thousands of years. Marble is a good material for carving or making statues. It does not break easily. Floor tiles, stone walls, and gravel also come from metamorphic rocks.
The metamorphic rocks in this picture were once limestone and sandstone. Phyllite
How do rocks change?
This shale is formed at the bottoms of oceans and lakes.
The Rock Cycle New rocks are always being formed. At the same time, other rocks are being destroyed. The recycling of old rock into new is an ongoing process called the rock cycle. Follow the arrows on these two pages to see how rocks can change. This granite is an igneous rock with large grains.
Granite forms when magma cools slowly. High heat and pressure change its particles. The granite turns into a metamorphic rock called gneiss.
This gneiss was once granite.
Rain and wind carry away tiny particles of the gneiss. The particles form layers that harden into sedimentary rock. Shale may be formed. Shale has thin layers of tiny particles.
Heat and pressure can change shale into a much harder rock called slate. The heat and pressure destroy the shaleâ€™s fossils. Slate splits easily into layers.
This slate was once shale.
Heat and pressure can change slate into phyllite. Phyllite can change again to a metamorphic rock called schist. Schist has larger grains than phyllite or shale.
This schist was once slate.
The rock cycle may not end here. Schist deep inside Earth can get hot and melt. It may cool and become a new igneous rock, such as granite.
How Rock Can Change
Igneous rock can wear down into sediment, which hardens into sedimentary rock.
All three types of rock can change into another type. But not all rocks complete the cycle. Rocks deep in Earthâ€™s crust may not get to the surface. Sedimentary rock can melt and change into igneous rock. But it might not become metamorphic rock. Because all three types of rock can change from one type to another, the rock cycle diagram can be drawn in different ways.
Igneous rock forms from magma. Sedimentary rock can change into metamorphic rock.
The rock cycle can start over when metamorphic rock melts into magma.
Metamorphic rock can melt into magma.
What did you learn? 1. What kind of material makes up rocks?
property of minerals that break along smooth, flat surfaces
type of rock that forms from molten rock
property of a mineral that describes how the mineral reflects light
metamorphic type of rock formed when heat and pressure change the properties of rock mineral
a natural, nonliving, solid crystal that makes up rocks
a rock rich in valuable minerals that can be removed from Earthâ€™s crust
the process that recycles rock into new types of rock
type of rock that forms when layers of sediments settle on top of one another and harden
the color of the powder that a mineral leaves when it is scratched across a special plate
2. What are three of the properties that a scientist can use to identify a mineral? 3. Why did people stop using lead for pipes and glassware? 4.
In your own words, write a summary of pages 20 and 21. First, write the most important idea from the two pages. Then write some important details.
Compare and Contrast Use the details on pages 8 and 9 to compare and contrast hematite and galena. How are they alike? How are they different?