Rocks and Minerals

What is a Rock?(And why do we study them?)

At first this seems like an easy question. We see rocks every day. But have you really thought about the answer?

Each rock is like reading a story. By studying them, observing them, really looking into them, we are “reading” about the history of the earth.

A rock is really a lot of different things. Some rocks are shiny, some are flat and dull, and others are very compact. Analyzing these differences can help us understand how they were formed.

For us to get the ball rolling, we’ll start by saying that a rock is a mixture or combination of minerals.

For Example:

GRANITE

Minerals

Every rock, regardless of it’s shape or size, is made up of smaller parts called minerals. A mineral is a naturally occurring chemical compound that has a specific chemical formula.

Examples of Minerals

The next few pages will provide details about some common minerals found in rocks.

Fill in your data table with as much information as you get from these slides and extra credit will be given for additional information you get through researching on your own. Make your table as complete as possible.

Mineral Data Table

Shape Color Luster Texture Hardness Streak Reactivity Magnetism Other

Quartz

Mica

Feldspar

Talc

Etc.

Common Mineral Features

Your data table also has a brief explanation of the features you will be looking for.

Describing MineralsWhen studying minerals, the following features

should be observed:

Shape – intersection of the crystal faces; many crystals lacked time or room to develop adequately for visual inspection

Color – most obvious; often least useful Luster – how light reflects off mineral, includes: metallic and nonmetallic (vitreous,

pearly, glassy, dull, earthy...)Texture – is the surface smooth or bumpy, sharp, dull, etc.Hardness – measured using Moh's scale; 1 (talc - softest mineral) to 10 (diamond-

hardest mineral)Streak – color of minerals powder left on a streak plate Reactivity – calcite found in limestone and marble reacts readily to acid by fizzing

(produces carbon dioxide gas)Magnetism – attracted to magnet (magnetite) or even acts as a magnet (lodestone)

SilicatesMinerals composed of a base of Silicon and Oxygen

Quartz- hardest common mineral- highly abundant in Earth’s crust- glassy luster- found in sand, sandstone, granite

Feldspar- most abundant rock-forming mineral- many different colors

Mica- flaky mineral- non-metallic luster- peels into thin layers

Talc- softest common mineral- white, gray or green color- soapy feel

CarbonatesMinerals composed of a base of Carbon and Oxygen

Calcite- fairly soft mineral- white or clear when pure- reacts to acid-found in caves, sea shells, egg shells and bones

Dolomite- does not react as readily to acid- found in some limestones and marbles

Iron OxidesMinerals composed of a base of Iron and Oxygen

Hematite- most common iron oxide- medium hardness- red or silver luster- streak is always red-brown

Magnetite- dark iron oxide- medium hardness- attracted to a magnet- forms small grains or crystals in many rocks

Iron SulfidesMinerals composed of a base of Iron and Sulfur

Pyrite- most common sulfide- medium hardness- yellow or gold luster- may form crystals- known as fools gold

Chalcopyrite- contains copper- tarnishes to iridescent blue, green or purple- medium soft mineral- the major source of copper

A Few Others

Sulfur- yellow- very soft- earthy luster- smells like rotten eggs- burns easily- lets off poisonous gas

Galena- very high lead content- bluish grey color- metallic luster- very dense- leading ore for silver

A Few Others, ContinuedGypsum- clear, white, yellow or red- very soft- many household uses

Halite- rock salt (NaCl)- forms cubes- can be identified by taste (although not recommended)

Diamond- made of carbon- hardest mineral

And Now … It’s Time to RockWe have a basic understanding of what rocks are made of, now let’s look at HOW they are made.

And HOW rocks are formed is the basis for our three rock groups.

Igneous

Sedimentary

Metamorphic

Igneous Rocks

Igneous rocks make up 95% of the rocks in the Earth’s crust. They are also some of the oldest rocks that are found at the surface of Earth. Igneous rocks form from molten rock, which is either ejected at the surface of the Earth, or that cools off underground. When volcanoes erupt, they can eject molten rock, as shown in the photo.

Igneous rocks are sub divided into two types of classifications (based on where they form): Extrusive and Intrusive.

Extrusive Igneous RocksExtrusive igneous rocks form when molten rock reaches the earth's surface and cools. Air and moisture cool the lava rapidly. The quick cooling doesn't allow the formation of large crystals, so most extrusive rocks have small crystals or none at all. In some extrusive rocks, like pumice and scoria, air and other gases are trapped in the lava as it cools. We can see holes remaining in the rock where the bubbles of gas were located.

Extrusive ExamplesBasalt- the most common extrusive- eroded basalt creates the black sand beaches

Obsidian- cools EXTREMELY quickly- no crystals- has a glassy surface

Pumice- cooled off VERY quickly- associated with gaseous eruptions- lightweight- no crystals- some forms can float

Intrusive Igneous RocksIntrusive igneous rocks form from magma that cools before it even reaches the Earth’s surface. Molten rock that is still underground is called magma. Molten rock that cools before it reaches the surface hardens to become intrusive igneous rock. Because it forms deep beneath Earth's surface, it has more time to cool and it develops large crystals.

Intrusive ExamplesDiorite-Medium to coarse grained- quite rare

Gabbro- medium to coarse grained- exceedingly variable- found on the moon

Granite- medium to coarse grained- light to dark in color- made of feldspar and quartz

Extrusive vs. Intrusive Formation

Sedimentary RocksSedimentary rocks were formed from sediment that came from the weathering and erosion of other rocks. They can be formed from igneous, sedimentary and metamorphic rocks that have been broken down by weathering. Small rocks, fragments and organic remains that have been moved by water, wind or other agents of erosion are called sediment. Over a period of time, sediment is cemented together to form a solid mass, now known as sedimentary rock. In nature sedimentary rocks are usually found in layers with the oldest layer on the bottom.

Sedimentary rocks are sub divided into three types of classifications: Clastic, Chemical and Organic.

Sedimentary Clastic Rocks

Clastic sedimentary rocks were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere. The word clastic comes from the Greek word klastos which means "broken."

Clastic ExamplesConglomerates- large rock pieces that are cemented together- can break apart- easy to see what it is made of

Shale- made of very fine grains of clay- very smooth- breaks easily

Sandstone- made of fine grains of sand cemented together- sand can rub off of the rock

Sedimentary Chemical RocksChemical sedimentary rocks are the result of chemical deposits, usually formed when water evaporates and leaves minerals behind. Sedimentary rocks that result from chemical deposits are called chemical rocks.

Chemical Examples

Cave Formations- formed by water dripping and carrying minerals downward

Halite- often found where lakes or seas have dried up- can be found in various colors

Sedimentary Organic RocksThe third form of sedimentary rocks are known as Organic rocks. These are formed from the life processes of living organisms or the remains of the organisms. These remains settle over time and become cemented together forming a new rock.

Organic Examples

Coal- formed from plants that died in swamps years ago- a very important fossil fuel

Some Limestones- formed from the shells of clams- can be from ancient sources

Note: some limestones are considered Chemical Rocks

Metamorphic RocksRocks that have changed shape or composition due to intense heat and pressure are called Metamorphic rocks.

Metamorphic rocks can be created from sedimentary, igneous or other metamorphic rocks. They are formed deep inside the earth where sit under intense heat and pressure from the rock above.

The texture of metamorphic rocks

creates two categories:

Foliated and Non-foliated.

Foliated Metamorphic Rocks

In foliated metamorphic rocks, the composition of the original rock is not apparent. This is because the heat and pressure have, in a sense, recombined the contents of the rock to form different minerals.

The individual mineral grains may recrystallize to form larger crystals that make parallel bands or lines across the rock. Notice the foliation (or lines) in the examples that follow.

Foliated Examples

Slate- made from heat and pressure on shale

Schist- made from heat and pressure on basalt

Gneiss- made from heat and pressure on granite

The crystals in non-foliated rocks combine and rearrange so they don’t form any bands.

Marble is an example

But Weeping Angels are aliens and not made of marble … not a very good example, after all!

Non-foliated Metamorphic Rocks

Marble is formed from limestone, which is formed from calcite. Even though limestone and marble are made of the same thing, a sculptor would never use limestone for a project. Marble is used because it is compact and has bigger crystals from all of the heat and pressure. It is going to hold together for a lot longer than the limestone would.

Non-foliated Examples

Quartzite- made from heat and pressure on sandstone

Marble- made from heat and pressure on limestone

Now, Did you Get All of That?Take THIS QUIZ to see how you do in distinguishing between the different rock types.

If you do not score at least 10, review over the rocks and try again.

Do not move on until you can get at least 10 correct answers!