Rocks and Minerals

Text Chapter 7

KEY POINTS: ATOMS & ELEMENTS (AtomsElementsMineralsRocks)

• Rocks are made up of one or more minerals.• Minerals are composed of one or more

elements.• Elements such as oxygen or iron are made up of

groups of like atoms.• Atoms are composed of combinations of

protons, neutrons, and electrons.• Atoms of elements are joined together by

bonds.

Minerals•Rocks are made up of one or more minerals.•Geologists have identified more than 4,000 different minerals.•But only about 20 minerals are commonly found in rocks.•Typically, individual rocks are composed of just a handful of minerals. •Granite (R) is composed of 4: quartz, feldspar, hornblende and biotite (a type of mica).

Most Common Elements in Granite Minerals

• Granite is an igneous rock that is composed primarily of four minerals. Magma cooling very slowly far under the earth's surface, allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. These minerals are quartz, feldspar, mica, and usually hornblende.

•Quartz (oxygen & silicon).•Feldspar (oxygen, silicon, aluminum, calcium, sodium & potassium)•Mica (oxygen, silicon, aluminum, iron, calcium, magnesium & hydrogen).•Hornblende [a.k.a. amphibole] (oxygen, silicon, aluminum, iron, calcium, magnesium & hydrogen).

Quartz vs. Hornblende (a.k.a. amphibole)The mineral quartz (top R) is composed of just two elements: silicon and oxygen.Other minerals may contain many elements.Hornblende or amphibole (bottom R) is made up of calcium, magnesium, iron, aluminum, silicon, and oxygen.

Four of the Most Common Minerals

• Quartzquartz is a common mineral that is found in many different types of rocks. The chemical formula is Silicon oxide (SiO2). Impurities in the rock at the time of formation causes the quartz crystal to produce different colors. Quartz can be colored yellow, milky white, rose, brown to black, blue and the best known color purple amethyst.

FeldsparFeldspar is the most abundant mineral in rocks that are located at or near the earth's surface. Feldspar can have a glassy white, blue, green, pink or red crystals. When feldspars are exposed to the atmosphere they weather easily. Clays are formed by weathered feldspar.Kaolinite is the highest quality of the feldspar clays used by potters.

• Hornblende:Hornblende is a mineral that contains magnesium, iron, silica and aluminum. Hornblende is black, brown and green in color. It occurs in crystals of many igneous rocks.

• Mica: Mica can be clear, black, green, red, yellow, brown, purple lapidolite and other colors too. Clear mica was named Muscovite because it was found near Moscow, Russia. It was used as window glass in the Muscovite's homes. Muscovite contains water which helps to make it clear. Biotite mica is dark green to black in color because it contains metals such as iron and magnesium.

What are Minerals Made of? Elements!•Minerals can be categorized by the chemical elements they contain.•There are 92 naturally occurring elements on Earth.•98% of the rocks in the crust below our feet are mainly composed of just 8 elements.•These eight elements are common ingredients in everyday objects or in the foods we eat.

Eight Most Common Elements in Continental Crust(The most common minerals in Earth’s crust are composed of the most common elements.)

Certain elements are vital for human health.•Human food contains many of the common elements that are naturally extracted from rocks as minerals break down at Earth’s surface to form soil.•These elements are absorbed by crops growing in soils and find their way to our bodies via the food we eat.•Many of the elements are essential for good human health.

Elements and Atoms

• Elements can’t be subdivided into other materials.• Elements can be separated into individual atoms.• An atom is the smallest particle that retains the

characteristics of an element.• Atoms are composed of one or more of the same

three basic components: electrons, neutrons and protons.

• The number of protons in an atom is unique for each element and is called the element’s atomic number, e.g. oxygen = 8; silicon = 14 (protons).

Helium Atom (not to scale). Note: charges of electrons, protons, & neutrons.

Helium and Neon: In all atoms electrons lie in energy levels around the nucleus. There are only 2 electrons in the first level, and maximum of 8 electrons in the second level. In these two

elements the number of electrons = protons, so they don’t form ions and don’t bond with other elements.

Ions and BondsThe outer energy levels of many elements are incomplete.These elements may gain electrons from other elements or may lose electrons.Atoms that lose or gain electrons are known as ions and have a positive or negative electrical charge.Positive and negative Ions of individual elements are attracted to each other and bond together to form minerals (note sodium chloride, R).

How salt is formed from two ions (negatively and positively charged) seeking stability via an ionic bond.

Covalent Bonding Water (H20), one of the most common substances on the planet is composed of two atoms of hydrogen (H) and one of oxygen (O).Each hydrogen atom has an electron that is shared with oxygen to give it a full energy level.Likewise, oxygen shares an electron with each of the hydrogen atoms.This type of sharing is called covalent bonding and typically occurs where atoms share electrons to achieve a stable structure.

Together oxygen and silicon account for 70% of the atoms in the Earth’s continental crust. Many crustal

minerals are mainly combinations of these two elements.

Common Silicate Structures (tetrahedron combinations that form 1. olivine, 2. pyroxene group, 3. amphibole or

hornblende group, 4. mica group, and 5. quartz/feldspar group).

The SilicatesThe bonding of silicon and oxygen atoms forms a pyramid-like shape known as the silica tetrahedron.Minerals made of silicon and oxygen are known as silicates.Silicates feature atomic structures that are characterized by different arrangements of the silica tetrahedron.

Tetrahedra = Basic Building Blocks

• These basic building blocks, the tetrahedra, can be joined together in combination with other elements (using ionic or covalent bonds or both) to form different silicate minerals such as those found in granite.

• The amount of silica present in magma is especially significant in controlling magma viscosity.

• Silica combines with other elements in magma (e.g. iron, magnesium, potassium).

Silica content determines magma viscosity.

• In magmas with low silica content atoms combine to form minerals such as olivine and pyroxene with simple forms like pairs and single chains.

• With more silica, the tetrahedra form complex double chairs, sheets, and three-dimensional frameworks.

• These larger, more complex forms get tangled together in the magma, making flow difficult and resulting in higher viscosity

Bond Strength/Weakness

• Ionic bonds formed by electrical attraction are typically weaker than covalent bonds where electrons are shared.

• Minerals composed of ions that form covalent bonds are typically stronger, and therefore more likely to be preserved on the Earth’s surface, than those with only ionic bonds.

• The diamond, the hardest mineral, is formed of carbon atoms that share covalent bonds among four neighboring atoms.

Character of Rocks

• Rocks’ character depends to a large degree on the atomic structures of their minerals and the way the atoms are bonded together.

• Some minerals are strong, like quartz, and appear in a variety of roles in different rock types.

• Other minerals have weak bonds and will dissolve in water.

• The combination of elements combining to form minerals will determine the melting temperature of rocks and behavior of the magma produced.

MINERALS: KEY POINTS• A mineral is a naturally occurring, inorganic solid with a definite

chemical composition and uniform atomic structural elements.• Minerals can be identified on the basis of characteristic physical

properties such as:(The four features used most often to characterize minerals)1. Crystal form2. Cleavage3. Hardness 4. Color(The two less common features used)5. Luster, and6. Streak

Crystal FormsThe shape of a mineral crystal is related to the way the chemical bonds form.Atoms or molecules of a mineral align in a uniform manner to form a crystal structure.Common shapes are prisms, pyramids, needles, cubes and sheets.

CleavageDepending on how their constituent atoms are arranged, minerals may brake along specific surfaces of weakness called cleavage planes.Cleavage planes mark those parts of the mineral where ions are connected by relatively weak ionic bonds.

Cleavage PlanesCovalent bonds are strong and less likely to form cleavage planes. Mica separates into sheets (R).Other minerals may have multiple cleavage planes: sheets, cubes or other shapes.Quartz has none.

HardnessTen minerals make up the Mohs hardness scale (R).Minerals not in the table are ranked relative to these.For example, a mineral that could scratch feldspar by not quartz would have a hardness of about 6.5.A copper penny ranks around 3; glass 6.

Mohs Hardness Scale

ColorUsed imperfectly to describe minerals.Minerals come in a variety of colors.However, some minerals exist in a wide range of colors.Quartz, for instance, occurs in over a dozen different colors.And, minerals can change color when they are exposed to changing natural conditions (rain, heat) on or near the Earth’s surface.

Luster and StreakLuster = how light is reflected from a mineral.Streak = the mark formed when a mineral is scratched across an unglazed piece of porcelain.Gold creates a yellow streak; iron sulfide makes a black streak.Other properties: calcite creates bubbles of CO2 when exposed to acids; and sulfur smells bad.

Summary Flow Chart

Three Classifications of Rocks: Igneous, Sedimentary and Metamorphic (differentiated on the basis of their chemical and physical properties)

Igneous Rocks: KEY POINTS

• Igneous rocks form from the cooling of melted rock (magma).

• When magma reaches the surface it cools quickly and solidifies into small mineral crystals in volcanic igneous rock.

• If magma solidifies below the surface, it cools slowly to form large mineral crystals in plutonic igneous rock.

• Igneous rocks can be classified by texture and color (indicating mineral composition)

• Types of igneous rocks vary with plate tectonic settings.

Plutonic vs. Volcanic Igneous RocksIgneous rock is formed through the cooling and solidification of magma or lava.Igneous rock may form either below the surface from magma as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks from lava.

Igneous landforms: Volcanic process form volcanoes, lava flows and ash falls. Plutonic

process lead to batholiths, laccoliths, dikes, sills and crystallized magma chambers called “stock.”

Another Version of Figure 7.18

Batholith:a great mass of intruded igneous rock that for the most part stopped in its rise a considerable distance below the surface (Stone Mt. Georgia

& its confederate memorial).

Half Dome, a granite monolith in Yosemite National Park is part of the Sierra Nevada batholith.

Laccolith = a mass of igneous rock that is intruded between sedimentary beds and produces a domical bulging of the overlying strata

Sedimentary Rocks: KEY POINTS

• Clastic sedimentary rocks: composed of sediments –rock and mineral fragments that form when rocks break apart at or near Earth’s surface. Clastic sedimentary rocks make up the majority of all sedimentary rocks.

• Chemical sedimentary rocks: crystallized from a solution, e.g. seawater, as a result of changing conditions.

• Biochemical sedimentary rocks: form by the actions of living organisms or are composed of the remains of dead organisms.

• Often form in a series of layers called beds or strata.

Clastic Sedimentary Rocks

• Clastic sedimentary rocks are formed from rock and mineral fragments (clasts).

• This process occurs in a series of steps:1. Generation of clasts due to the breakdown of

an original rock by weathering;2. Transportation of the eroded material from the

source area; and3. Lithification—the deposition and subsequent

conversion of the material to rock.

Sedimentary Rocks: Typically are composed of beds or strata.

Limestone (a sedimentary rock deposit) permits water to seep through it and form caverns.

Sedimentary Process

Sedimentary Process (Cont’d)

Metamorphic Rocks: KEY POINTS

• Metamorphism relates to changes in mineral composition or texture that occur in solid rocks as a result of increasing pressure or temperature.

• The temperature range for rock metamorphism is approx. 200 to 1,100 degrees C (390-2,010 degrees F).

• Contact metamorphism: occur when rocks come in contact with a heat source (usually magma).

• Regional metamorphism occurs when rocks undergo increased temperatures and pressure typically associated with the plate tectonic processes that form mountains.

Igneous Rocks Relative abundance of in the earth's crust (basalt and gabbro dominate the igneous category).

Metamorphic RocksRelative abundance of metamorphic rocks in the earth's crust (gneiss dominates).

The Rock Cycle and Minerals: KEY POINTS

• The rock cycle links the principal igneous, sedimentary, and metamorphic rocks together in an idealized view of the formation of rocks in the Earth’s crust.

• The rock cycle serves as an example the Earth operates as a system and constantly changes.

• Minerals form in a range of geologic settings associated with the formation of the most common rock types.

Sedimentary RocksRelative abundance of sedimentary rocks in the earth's crust (shale dominates).

The Rock Cycle

The Rock Cycle

The Rock Cycle (takes millions upon millions of years for Louie or Billy to go around just once).