Chapter 3.1

Earth’s Minerals and Rocks

Minerals and Mineral Groups

Lesson Objectives

  • Describe the characteristics that all minerals share.
  • Identify the groups in which minerals are classified and their characteristics.

Introduction

Minerals are categorized based on their chemical composition. Owing to similarities in composition, minerals within a same group may have similar characteristics.

What is a Mineral?

Mineralsare everywhere!The salt you sprinkle on food is the mineral halite. Silver in jewelry is also a mineral. Baseball bats and bicycle frames both contain minerals. Although glass is not a mineral, it is produced from the mineral quartz. Scientists have identified more than 4,000 minerals in Earth’s crust. A few are common, but many are uncommon.

Geologists have a very specific definition for minerals. A material is characterized as a mineral if it meets all of the following traits. A mineral is an inorganic, crystalline solid. A mineral is formed through natural processes and has a definite chemical composition. Minerals can be identified by their characteristic physical properties such as crystalline structure, hardness, density, flammability, and color.

Crystalline Solid

Minerals are crystalline solids. Acrystalis a solid in which the atoms are arranged in a regular, repeating pattern. The pattern of atoms in different samples of the same mineral is the same. Is glass a mineral? Without a crystalline structure, even natural glass is not a mineral.

Inorganic Substances

Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils.Inorganicsubstances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock but is not a mineral.

Natural Processes

Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earth’s crust is a mineral. Is a diamond created in a laboratory by placing carbon under high pressures a mineral?No.Do not buy a laboratory-made “diamond” for jewelry without realizing it is not technically a mineral.

Chemical Composition

Nearly all (98.5%) of Earth’s crust is made up of only eight elements – oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium – and these are the elements that make up most minerals.

All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up ofchemical compounds. Each mineral has its own chemical formula. Halite, pictured above, is NaCl (sodium chloride). Quartz is always made of two oxygen atoms bonded to a silicon atom, SiO2. If a mineral contains any other elements in its crystal structure, it's not quartz.

Physical Properties

The physical properties of minerals include:

  • Color: the color of the mineral.
  • Streak: the color of the mineral’s powder.
  • Luster: the way light reflects off the mineral’s surface.
  • Specific gravity: how heavy the mineral is relative to the same volume of water.
  • Cleavage: the mineral’s tendency to break along flat surfaces.
  • Fracture: the pattern in which a mineral breaks.
  • Hardness: what minerals it can scratch and what minerals can scratch it.

Mineral Formation

Lesson Objectives

  • Describe how melted rock produces minerals.
  • Describe how hot rock produces different minerals.
  • Explain how minerals form from solutions.

Introduction

Minerals form under an enormous range of geologic conditions. There are probably more ways to form minerals than there are types of minerals themselves. Minerals can form from volcanic gases, sediment formation, oxidation, crystallization from magma, or deposition from a saline fluid, to list a few. Some of these methods of mineral formation will be discussed below.

Formation from Hot Material

Arockis a collection of minerals. Imagine a rock that becomes so hot it melts. Many minerals start out in liquids that are hot enough to melt rocks.Magmais melted rock inside Earth, a molten mixture of substances that can be hotter than 1,000oC. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to be seen clearly.

Granite is rock that forms from slowly cooled magma, containing the minerals quartz (clear), plagioclase feldspar (shiny white), potassium feldspar (pink), and biotite (black).

When magma erupts onto Earth's surface, it is calledlava. Lava cools much more rapidly than magma. Mineral crystals do not have time to form and are very small. The chemical composition will be the same as if the magma cooled slowly.

Existing rocks may be heated enough so that the molecules are released from their structure and can move around. The molecules may match up with different molecules to form new minerals as the rock cools. This occurs during metamorphism, which will be discussed in the chapter “Rocks.”

Formation from Solutions

Water on Earth, such as the water in the oceans, contains chemical elements mixed into a solution. Various processes can cause these elements to combine to form solid mineral deposits.

Minerals from Salt Water

When water evaporates, it leaves behind a solid precipitate of minerals.Water can only hold a certain amount of dissolved minerals and salts. When the amount is too great to stay dissolved in the water, the particles come together to form mineral solids, which sink. Halite easily precipitates out of water, as does calcite. Some lakes, such as Mono Lake in California, the Great Salt Lake in Utah, or the Dead Sea contain many mineral precipitates. When the water level in these bodies of water drops, towers of halide minerals (salt) are left behind. This may have been the basis for the biblical story of Lot’s wife turning to a pillar of salt.

Minerals from Hot Underground Water

Hot water can hold more minerals than cold water. Magma heats nearby underground water, which reacts with the rocks around it to pick up dissolved particles. As the water flows through open spaces in the rock and cools, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are calledveins.

When minerals are deposited in open spaces, large crystals can form. This is how geodes form.

Mining and Mineral Use

Lesson Objectives

  • Explain how minerals are mined.
  • Describe how metals are made from mineral ores.
  • Summarize the ways in which gemstones are used.
  • Identify some useful minerals.

Introduction

Some minerals are very useful. Anoreis a rock that contains minerals with useful elements. Aluminum in bauxite ore is extracted from the ground and refined to be used in aluminum foil and many other products. The cost of creating a product from a mineral depends on how abundant the mineral is and how much the extraction and refining processes cost. Environmental damage from these processes is often not figured into a product’s cost. It is important to use mineral resources wisely.

Finding and Mining Minerals

Geologic processes create and concentrate minerals that are valuable natural resources. Geologists study geological formations and then test the physical and chemical properties of soil and rocks to locate possible ores and determine their size and concentration.

A mineral deposit will only be mined if it is profitable. A concentration of minerals is only called anore depositif it is profitable to mine. There are many ways to mine ores.

Surface Mining

Surface mining allows extraction of ores that are close to Earth’s surface. Overlying rock is blasted and the rock that contains the valuable minerals is placed in a truck and taken to a refinery. Surface mining includes open-pit mining and mountaintop removal. Other methods of surface mining include strip mining, placer mining, and dredging. Strip mining is like open pit mining but with material removed along a strip.

Placersare valuable minerals found in stream gravels. California’s nickname, the Golden State, can be traced back to the discovery of placer deposits of gold in 1848. The gold weathered out of hard metamorphic rock in the western Sierra Nevada, which also contains deposits of copper, lead, zinc, silver, chromite, and other valuable minerals. The gold traveled down rivers and then settled in gravel deposits. Currently, California has active mines for gold and silver and for non-metal minerals such as sand and gravel, which are used for construction. Before California had its gold rush, North Carolina was a major producer of gold. The gold was found in stream running down from the Appalachians. North Carolina is still a major producer of many valuable gems. North Carolina produces more mica, feldspar, and pyrophyllite than any other state. It is also one of the leading producers of emeralds. Mining in North Carolina accounts for around $500,000,000 of economic activity each year.

Underground Mining

Underground mining is used to recover ores that are deeper into Earth’s surface. Miners blast and tunnel into rock to gain access to the ores. How underground mining is approached - from above, below, or sideways - depends on the placement of the ore body, its depth, concentration of ore, and the strength of the surrounding rock.

Underground mining is very expensive and dangerous. Fresh air and lights must also be brought into the tunnels for the miners, and accidents are far too common.

Ore Extraction

The ore’s journey to becoming a useable material is only just beginning when the ore leaves the mine. Rocks are crushed so that the valuable minerals can be separated from the waste rock. Then the minerals are separated out of the ore. A few methods for extracting ore are:

  • heap leaching: the addition of chemicals, such as cyanide or acid, to remove ore.
  • flotation: the addition of a compound that attaches to the valuable mineral and floats.
  • smelting: roasting rock, causing it to segregate into layers so the mineral can be extracted.

To extract the metal from copper ore, the rock is melted at a temperature greater than 900oC, which requires a lot of energy. Extracting metal from rock is so energy intensive that if you recycle just 40 aluminum cans, you will save the energy equivalent of one gallon of gasoline.

Mining and the Environment

Although mining provides people with many needed resources, the environmental costs can be high. Surface mining clears the landscape of trees and soil, and nearby streams and lakes are inundated with sediment. Pollutants from the mined rock, such as heavy metals, enter the sediment and water system. Acids flow from some mine sites, changing the composition of nearby waterways. Coal mining is especially bad for releasing acid into local waterways.

U.S. law has changed so that in recent decades a mine region must be restored to its natural state, a process calledreclamation. This is not true of older mines. Pits may be refilled or reshaped and vegetation planted. Pits may be allowed to fill with water and become lakes or may be turned into landfills. Underground mines may be sealed off or left open as homes for bats.

Valuable Minerals

Some minerals are valuable because they are beautiful. Jade has been used for thousands of years in China. Diamonds sparkle on many engagement rings. Minerals like jade, turquoise, diamonds, and emeralds are gemstones. Agemstone, or gem, is a material that is cut and polished for jewelry. Many gemstones are minerals.

Gemstones are usually rare and do not break or scratch easily. Gems in nature do not sparkle or shine. Most are cut along cleavage faces and then polished so that light bounces back off the cleavage planes. Light does not pass through gemstones that are opaque, such as turquoise.

Gemstones are not just used in jewelry. Diamonds are used to cut and polish other materials, such as glass and metals, because they are so hard. The mineral corundum, of which ruby and sapphire are varieties, is used in products such as sandpaper.

Minerals are used in much less obvious places. The mineral gypsum is used for the sheetrock in homes. Window glass is made from sand, which is mostly quartz. Halite is mined for rock salt. Copper is used in electrical wiring, and bauxite is the source for the aluminum used in soda cans.

Chapter 3.2

Types of Rocks

Lesson Objectives

  • Define rock and describe what rocks are made of.
  • Know how to classify and describe rocks.
  • Explain how each of the three main rock types formed.
  • Describe the rock cycle.

Introduction

There are three types of rocks: igneous, sedimentary and metamorphic. Each of these types is part of the rock cycle. Through changes in conditions one rock type can become another rock type. Or it can become a different rock of the same type.

What Are Rocks?

A rock is a naturally formed, non-living earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass.

How is a rock different from a mineral? Rocks are made of minerals. The mineral grains in a rock may be so tiny that you can only see them with a microscope, or they may be as big as your fingernail or even your finger.

Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has a distinctive set of minerals. A rock may be made of grains of all one mineral type, such as quartzite. Much more commonly, rocks are made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains.

Different textures indicate different histories. Rocks that cooled slowly from magma (molten rock) underground have large crystals. Rocks that cooled rapidly from a very similar magma that erupted onto Earth’s surface will have much smaller crystals.

Three Main Categories of Rocks

Rocks are classified into three major groups according to how they form. Rocks can be studied in hand samples that can be moved from their original location. Rocks can also be studied inoutcrop, exposed rock formations that are attached to the ground, at the location where they are found.

Igneous Rocks

Igneous rocksform from cooling magma. Magma that erupts onto Earth’s surface is lava. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.

Sedimentary Rocks

Sedimentary rocksform by the compaction and cementing together ofsediments, broken pieces of rock-like gravel, sand, silt, or clay. Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemicalprecipitates, the solid materials left behind after a liquid evaporates.

Metamorphic Rocks

Metamorphic rocksform when the minerals in an existing rock of any kind are changed by heat or pressure within the Earth.

The Rock Cycle

Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly; many take place below the Earth’s surface, so we may not even notice the changes. Although we may not see the changes, the physical and chemical properties of rocks are constantly changing in a natural, never-ending cycle called therock cycle.

The concept of the rock cycle was first developed by James Hutton, an eighteenth century scientist often called the “Father of Geology”. Hutton recognized that geologic processes have “no [sign] of a beginning, and no prospect of an end.” The processes involved in the rock cycle often take place over millions of years. So on the scale of a human lifetime, rocks appear to be “rock solid” and unchanging, but in the longer term, change is always taking place.

In the rock cyclethe three main rock types—igneous, sedimentary, and metamorphic—change from one to another. Arrows connecting the three rock types show the processes that change one rock type into another. The cycle has no beginning and no end. Rocks deep within the Earth are right now becoming other types of rocks. Rocks at the surface are lying in place before they are next exposed to a process that will change them.

Processes of the Rock Cycle

Several processes can turn one type of rock into another type of rock. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism.

Crystallization

Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoingcrystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals.