Section 1: Minerals

• Common Elements

  • The crust is the outermost layer of Earth. It includes all continental material and the material that forms the ocean bottom.

  • Mineral: a naturally occurring element or compound that is inorganic, solid, and has a crystalline structure.

    • Inorganic means that minerals are materials that are not produced by living organisms.

    • The composition of minerals are indicated by their chemical formulas.

• Physical Properties

  • A mineral has a particular chemical composition.

  • Different amounts of the chemical impurity chromium in the crystalline structure of corundum cause the difference in color.

  • Some physical properties are controlled by the orderly arrangement of atoms in a mineral’s structure.

    • This orderly pattern is what makes a mineral crystalline.

  • When minerals break along planes that cut across relatively weak chemical bonds, a smooth, flat surface is created.

  • Cleavage: The ability of a mineral to do this is the physical property

    • All parallel cleavage planes define a single direction of cleavage.

    • Because mica has one direction of cleavage, it can be separated in layers.

    • Feldspar is an example of a mineral with two planes of cleavage.

  • Fracture: When a mineral breaks unevenly

  • Bonds connecting atoms in materials often have different strengths.

  • Hardness: The physical property that measures resistance to scratching

    • When a hardness test is performed by rubbing two objects together, the softer of the two will wear away.

  • The way a mineral reflects light is the physical property known as luster.

    • Two main types of luster, metallic and nonmetallic, help subdivide minerals and often give a clue about their compositions.

  • Streak: The color of a mineral in powdered form

    • The streak of a mineral may be the same color as the mineral specimen. When a mineral shows different colors, the streak powder color generally stays the same, which helps identify the mineral.

    • A streak test is performed by rubbing a mineral on an unglazed, white porcelain tile.

  • The orderly internal arrangement of atoms in a mineral often is related to its external crystal shape.

    • Minerals can be classified using six basic crystal forms.

    • The types of symmetry shown by the crystal are key elements in determining the crystal system to which a mineral belongs.

• Mineral Formation

  • A mineral crystal grows as atoms are added to its surfaces, edges, or corners.

  • The types of atoms that are added depend on the atoms in the growing crystal’s surroundings.

    • Growth also is controlled by how fast atoms can migrate to the crystal and by the temperature and pressure of the surroundings.

  • Mineral crystals can form in different ways.

    • One way is by precipitation from hot, water-rich fluids.

    • Another way is by solidification from molten rock.

    • A third way is by the evaporation of water rich in dissolved salts at low temperatures near Earth’s surface.

  • Some minerals are produced from hot-water solutions containing dissolved mineral matter.

  • Hydrothermal minerals form on the rims of hot springs.

  • When hot water passes through cracks in cooler rock, minerals may form within the cracks

  • Magma: Molten rock material found inside Earth

    • When the temperature of magma drops well below the solidification temperature of a mineral, crystals of that particular mineral may form and grow.

    • When water slowly evaporates, dissolved mineral material may be left behind to form crystals.

• Mineral Groups

  • About 3,800 minerals have been identified in nature.

  • The atomic arrangement and composition of minerals allow them to be sorted into groups.

  • Most minerals contain silica.

    • Silica is a common term for a compound that contains silicon plus oxygen or silicon dioxide (SiO2).

    • In silicate minerals, the elements silicon and oxygen bond together to form a geometric structure called a tetrahedron.

  • The simplest silicate structures have silicon-oxygen tetrahedrons that are not linked together.

  • Several important silicate groups form most of Earth’s crust.

  • Earth’s oceanic crust is denser and contains a larger percentage of silicates whose tetrahedrons are linked together as single chains or are not linked.

  • Some important mineral groups are not silicates. These include the carbonates, oxides, halides, sulfides, sulfates, and native metals. The non-silicate groups are a source of many valuable ore minerals and building materials.

• Mineral Uses

  • People use minerals either directly as objects of wealth, or as raw materials to make things.

  • Not all minerals need to provide metals to be valuable. Nonmetallic minerals are valuable as well.

Section 3: Igneous Rocks

• What’s A Rock?

  • Rock: a naturally formed mixture containing minerals, rock fragments, or volcanic glass bound together.

  • Texture: describes the size, shape, and arrangement of the rock’s
    components.

  • The rock-making process is a continuous cycle.

• Intrusive Igneous Rocks: form within, or push into, regions of Earth’s crust

  • Igneous rocks are those that form from molten magma.

  • Minerals have different melting temperatures.

  • This interaction between magma and the rock it pushes into can cause changes in the rock, changes in the magma, or both.

  • As magma cools, different minerals crystallize at different temperatures.

  • When the temperature of the magma is high, olivine, pyroxene group minerals, and plagioclase feldspars crystallize first.

  • The Bowen’s reaction series, shown below, illustrates the sequence in which minerals crystallize from magma at different temperatures.

  • Igneous rocks form from three types of magma—granitic magma, basaltic magma, or andesitic magma.

  • Granitic rocks include rocks, such as granite, that contain the minerals quartz, potassium feldspar, mica, and hornblende.

  • Basaltic rocks contain the minerals plagioclase feldspar, pyroxene, and olivine.

  • Finally, andesitic rocks have compositions intermediate between granitic and basaltic rocks.

  • The size of the mineral crystals in a rock is called the grain size.

  • Grain size depends on how quickly the magma cooled that formed the rocks.

• Extrusive Igneous Rocks: rocks that form from lava erupted at Earth’s surface.

  • If a volcanic eruption is on land, lava pours out into the air.

  • If a volcanic eruption is on the ocean floor, lava flows into water.

  • When magma cools inside Earth’s crust and forms intrusive igneous rocks, the crustal rock surrounding the magma can be hot.

  • Extrusive igneous rocks have different textures than intrusive igneous rocks.

  • Rocks with small grain sizes are called fine-grained.

  • Fine-grained, extrusive igneous rocks often have grain sizes that are too small to be seen without magnification.

  • The difference between extrusive igneous rocks and intrusive igneous rocks is due mainly to the difference in their textures.

  • Extrusive igneous rocks are fine-grained with small crystals.

  • Intrusive igneous rocks are coarse-grained with large crystals.

  • Different rocks form from granitic or basaltic magmas, depending on how quickly the magma cools. Coarse-grained granite and fine-grained rhyolite both form from granitic magmas. Coarse-grained gabbro and fine- grained basalt both form from basaltic magmas.

  • When lava erupts at Earth’s surface, other types of extrusive igneous rocks can be formed.

Section 3: Sedimentary Rocks

• Rocks from Surface Materials

  • Clasts: small bits and pieces

  • Rocks that are tumbled more than 3,000 km along the bottom of the Colorado River, shown below, can be broken into fine particles before they reach the Gulf of California.

  • Mechanical weathering occurs when physical forces break rocks into smaller clasts.

  • As clasts are transported they grind against each other and other hard objects in their environment.

  • Sandstone can for when sand grains are deposited, compacted, and cemented together.

  • Pore Space: The empty space between clasts

  • Water, oil, and natural gas found beneath Earth’s surface are stored in the pore spaces of sedimentary rocks.

  • The process by which clasts stick together by being pushed together is called compaction.

  • Cementation: When minerals slowly precipitate out of water and fill spaces between clasts

• Detrital Sedimentary Rocks

  • Clasts have different sizes and geologists classify clasts according to their size. In order of decreasing size, clasts ar e classified as gravel, sand, silt, or clay.

  • Sand is defined by size, not by composition. Sand doesn’t even have to be made from rock material; it can be made from shells.

  • The size of a clast determines how the clast can be transported.

  • The separation of clasts according to size by wind or water is called sorting.

  • Sorting can also occur as clasts are deposited. Deposition of clasts occurs when the clasts are no longer being transported.

  • Detrital sedimentary rock composition depends on sources of rock material that were eroded, transported, and eventually deposited.

  • The number of possible combinations of different kinds of clasts is large.

  • Some minerals tend to be more common in detrital sediments because they are harder or more resistant to being dissolved.

  • Geologists examine sedimentary rock compositions and try to reconstruct what happened to form them.

  • Just as igneous rocks are classified according to composition and texture, similar observations are used to classify detrital sedimentary rocks.

  • Mineral composition is extremely variable, so adjectives are used to modify the general name of the rock.

  • Clast size also provides clues to help determine the depositional environment of the sediment that formed the detrital rock.

• Biochemical Sedimentary Rocks

  • If sedimentary rocks contain the remains of living organisms they are called biochemical sedimentary rocks.

  • Most of Earth’s limestone is composed, at least partially, of the remains of marine organisms that had hard parts made of calcium carbonate.

  • Another common rock that originates from the remains of organisms is coal.

  • Coal usually develops from peat, a brown, lightweight deposit of moss and other plant matter. Peat forms shallow swamps or bogs in a temperate or tropical climate.

  • As sediment accumulates above a layer of peat the peat becomes more compressed. Continual compression drives out water and other compounds, leaving behind a form of carbon called coal.

Section 4: Metamorphic Rocks and the Rock Cycle

• Metamorphic Rocks

  • Sharp folds sometimes display intense transformations in metamorphic rocks.

  • Any igneous, sedimentary, or metamorphic rock can be changed through metamorphism.

  • Metamorphic rocks form under conditions that are between the conditions that form igneous and sedimentary rocks.

• Metamorphic Rock Composition

  • Metamorphic changes in rocks are caused by thermal energy, pressure, and chemical reactions.

  • Clay minerals, micas, and amphiboles are examples of minerals that contain water in their crystal structures.

  • Regional movements of Earth’s tectonic plates can cause rocks to be buried deeply, producing large increases in the temperature of the rocks.

  • Metamorphic changes in rocks that occur over large areas are called regional metamorphism.

• Metamorphic Rock Textures

  • Metamorphic processes produce rocks with different textures.

  • Folio means “leaf.” Foliated texture has the appearance of layered leaves or pages of a book.

  • Foliated: crystals are arranged in layers and bands.

    • Foliated textures are formed under high pressure.

  • Metamorphic textures can also be nonfoliated where crystals are in more random orientations.

  • The most common sedimentary rocks in Earth’s crust are rocks, such as shale and siltstone, that are formed from mud.

  • Mineral grains are randomly oriented when no directed force is involved.

  • Orientation of mineral grains is perpendicular to the direction of pressure caused by compression.

  • Mineral grains are parallel to the direction of shearing force.

  • The smallest-grain sizes in foliated textures occur in slate, which forms thin layers and exhibits rock cleavage.

  • Gneiss rock textures often are banded, and gneisses generally represent the limit between metamorphic and igneous conditions.

  • Similar in texture to intrusive igneous rocks, nonfoliated metamorphic rocks tend to have random crystal orientation and uniform grain size.

  • Regional and contact metamorphism cause changes that can occur over millions of years.

• Classifying Metamorphic Rocks

  • Metamorphic rocks can be classified by their texture.

    • Metamorphic rocks can be foliated or nonfoliated.

  • A rock with a schist-like texture made of garnet and mica is a garnet-mica schist.

• The Rock Cycle

  • Rocks above and below Earth’s surface are continually being changed into other types of rocks.

    • Sedimentary and metamorphic rocks can be melted to form igneous rocks.

    • Weathering, compaction and cementation can change igneous and metamorphic rocks into sedimentary rocks. A rock can even be changed into a different rock of the same type.

  • Rock Cycle: The continual changing of rocks into different types.

  • As rocks move through the various stages of the rock cycle, matter is always conserved