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
Genre
Nonfiction
Comprehension Skill
Compare and
Contrast
Text Features
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•
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Captions
Diagrams
Table
Glossary
Science Content
Rocks and
Minerals
Scott Foresman Science 4.6
ISBN 0-328-23556-3
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Vocabulary
cleavage
igneous
luster
metamorphic
mineral
ore
rock cycle
sedimentary
streak
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3 4 5 6 7 8 9 10 11 12 V010 15 14 13 12 11 10 09 08 07
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 Earth’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
Galena
2
3
Hardness
Streak
The hardness of minerals varies. Scientists find
out a mineral’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 fingernail’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.
Cleavage
1
5
10
Talc
Apatite
Diamond
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.
4
5
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.
6
7
Hematite
Galena
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.
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9
Lead
Copper
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.
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11
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.
Mineral
Color
Luster
Hardness
Streak
Calcite
colorless
or white,
pink, yellow,
greenish,
or red
ranges
from glassy
to dull
3
white
Feldspar
colorless or
many colors,
such as beige,
pink, gray,
and bluish
green
glassy or
pearly
6-6.5
white
Hornblende
dark green
to black
silky
5-6
none
Magnetite
black
shiny or
metallic
5.5-6
black
This mineral’s hardness is
much less than that of the
other minerals listed.
This mineral’s
luster is shiny and
metallic.
This mineral does
not make a streak.
This mineral can
have a beige color.
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13
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
Pegmatite
Gabbro
Obsidian forms from lava that
cools quickly. It looks like glass.
14
Basalt is a very common
igneous rock. It makes up
most of the ocean floor.
15
Sedimentary Rocks
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.
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17
Metamorphic Rock
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
Slate
Gneiss
18
19
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.
20
Heat and pressure can change shale into
a much harder rock called slate. The heat
and pressure destroy the shale’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.
21
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 Earth’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.
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23
Glossary
What did you learn?
1. What kind of material makes up rocks?
cleavage
property of minerals that break along
smooth, flat surfaces
igneous
type of rock that forms from molten
rock
luster
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
ore
a rock rich in valuable minerals that
can be removed from Earth’s crust
rock cycle
the process that recycles rock into
new types of rock
sedimentary
type of rock that forms when layers
of sediments settle on top of one
another and harden
streak
the color of the powder that a
mineral leaves when it is scratched
across a special plate
24
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.
5.
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?