Chapter 17 Rocks and Minerals

Composition of the Earth During the early molten stage of the Earth the heavier abundant elements, such as iron and nickel, sank to the deep interior of the earth, leaving the lighter elements on the surface. This thin layer on top of the earth is called the crust. Only 8 elements make up about 98.6% of the crust. All other elements make up the remaining 1.4%. Oxygen is the most abundant element, about 50% of the crust as part of compounds. Silicon makes up over 25%. The solid materials of the earth’s crust are known as minerals and rocks.

Fig. 17.2

Composition of the Earth About 2,500 minerals exist but only about 20 are common in the crust, for example quartz, calcite, and gypsum Minerals are the fundamental building blocks of the rocks that make the earth’s crust. A rock is a solid aggregation of one or more minerals that have been cohesively brought together by a rock-forming process. Examples of common rocks are sandstone, limestone, and granite.

Minerals A mineral is defined as a naturally occurring, inorganic solid element or compound with a crystalline structure. The element or compound: 1. cannot be synthetic (must be naturally occurring) 2. must not be produced by a living organism (must be inorganic) 3. must have atoms arranged in a regular, repeating pattern (a crystal structure). The crystal structure of a mineral can be present on the microscopic scale and it is not necessarily obvious to the unaided eye.

A crystal is composed of a structural unit that is repeated in 3 dimensions. This is the basic structural unit of a crystal of sodium chloride, the mineral halite.

The structural unit for a crystal of table salt, sodium chloride, is cubic, as you can see in the individual grains.

http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html#anchor8

Crystal Structures The crystal structure of a mineral can be made up of atoms of one or more kinds of elements. Diamond is a mineral with only carbon atoms in a strong crystal structure. Quartz is a mineral with atoms of silicon and oxygen in a different crystal structure. Minerals have well defined chemical compositions or a range of chemical compositions.

Crystals Crystals can be classified and identified on the basis of the symmetry of their surfaces. The symmetry is an outward expression of the internal symmetry in the arrangement of the atoms making up the crystal. On the basis of symmetry crystalline substances are classified into 6 major systems which in turn are subdivided into smaller groups.

Basic crystalline systems

Quartz crystals are hexagonal Fig. 17.5

Silicates and Nonsilicates The most abundant minerals contain silicon and oxygen, since they are the most abundant elements on Earth. All minerals are classified on the basis of whether they contain these two elements or not. The two main groups are thus the silicates and nonsilicates. The silicates can contain other elements in addition to silicon and oxygen. The silicate minerals are by far the most abundant, making up about 92% of the earth’s crust.

Silicates Si and 4 Oxygen atoms from a tetrahedral ionic structure, SiO42-. All silicates contain SiO42- and it can combine with metallic ions, for example ions of iron or magnesium. (ferromagnesian silicates) The SiO42- can also combine with the silicon atoms of other tetrahedral units. (nonferromagnesian silicates)

a tetrahedron, which has four equal sides. The geometric shape of a tetrahedron, which has four equal sides. A silicon with four oxygen Atoms are arranged in the shape of a tetrahedron with The silicon in the center. This is the basic building block of all silicate minerals.

Silicate Minerals Silicate minerals can be divided into: 1. ferromagnesian silicates-The basic tetrahedral structure joins with ions of iron, magnesium, calcium, and other metals. They have a darker color and greater density than the other silicates because of the presence of the metal ions. 2. nonferromagnesian silicates-Do not contain iron or magnesium ions. These minerals have a light color and a low density compared to the ferromagnesians. Quartz, a very well known mineral belongs here.

Dark colored ferromagnesian silicates: Augite is on the right and hornblende is on the left.

Ferromagnesian Silicates Olivine Biotite Hornblende Augite

Light colored nonferromagnesian silicates mica (front center), white and pink orthoclase (top center), and quartz (left).

Nonferromagnesian Silicates Orthoclase Quartz Mica (muscovite)

Nonsilicate Minerals There are 8 subgroups: carbonates 2. sulfates 3. oxides 4. sulfides 5. halides 6. phosphates 7. hydroxides 8. native elements The carbonates are the most abundant of the nonsilicates but others are important as fertilizers, sources of metals and sources of industrial chemicals.

Non-silicates hematite dolomite apatite halite gypsum galena gold

Minerals to Know: Ferromagnesian Silicates: augite, hornblende, olivine Nonferromagnesian Silicates: quartz, mica, orthoclase, talc Nonsilicates: All native elements, also gypsum (CaSO4), galena, halite (NaCl), apatite, dolomite (MgCO3), hematite, calcite (CaCO3)

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density Often misleading, quartz for example comes in many colors but its true color is clear.

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density Although the color may be misleading, when rubbed across a tile, the streak often reveals the ‘true’ color of the mineral.

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density Talc, hardness of 1

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density Quartz, hardness of 7

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density The most useful clue.

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density How the mineral breaks along smooth planes. This is when they have zones of weakness.

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density How the mineral fractures. This occurs when they do not have areas of weakness. The broken surface is irregular and not in the flat plane of a cleavage.

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density The sheen. Examples are metallic, pearly, vitreous (glassy) and earthy.

Physical properties of minerals 8 characteristics to determine the mineral. Color Streak Hardness Crystal form Cleavage Fracture Luster density Mass divided by volume

Mineral Forming Processes Mineral crystals usually form in a liquid environment, but they can also form from gases or in solids under the right conditions. Two liquid environments where minerals can form are water solutions and solutions of a hot, molten mass of melted rock materials. The molten rock material from which minerals crystallize is known as magma. It can cool and crystallize to solid minerals either below or on the surface of the earth. Magma which is forced out to the surface of the earth is also called lava, the molten material associated with volcanoes.

Mineral Forming Processes When minerals form from solutions the dissolved ions must be very concentrated. Their charges can pull them together and crystals can form. Mineral forming processes are influenced by: Temperature Pressure Time Availability and concentrations of ions in solution

Mineral Forming Processes Large crystals result from slow cooling of magma or from high water content in the magma (very dilute solutions). Small or even microscopic crystals result from rapid cooling and low water content (very concentrated solutions). Sudden chilling can prevent crystal growth altogether, resulting in glass, a solid that cooled too quickly for its atoms to move into ordered crystal structures (amorphous solid).

Ore Minerals If mineral deposits have an economic value they are called ore minerals. They are left over from crystallizing magna and crystallize in rock fractures to form thin, flat bodies of mineral material called veins. Some native elements can occur as ore minerals. They include copper, diamond, gold, silver, and sulfur.

Rocks A rock is defined as an aggregation of one or more minerals and perhaps other materials that have been brought together into a cohesive solid. They include volcanic glass, a silicate that is not considered a mineral because it lacks a crystalline structure. A rock can consist of one or more kinds of minerals that are somewhat “glued” together by other materials such as glass. Granite is a rock that is primarily three silicate minerals: quartz, mica, and feldspar. You can see the grains of the 3 minerals in the freshly broken surface of most granites.

Granite is a coarse-grained igneous rock composed mostly of light-colored, light-density Nonferromagnesian minerals. The earth’s continental areas are dominated by granite and by rocks with the same mineral composition as granite.

Rocks There are 3 main groups of rocks that are based on the way that rocks form: 1. Igneous rocks are formed as a hot, molten mass of rock materials cooled and solidified. 2. Sedimentary rocks are formed from particles of dissolved materials from previously existing rocks. 3. Metamorphic rocks are from rocks that were subjected to high temperatures and pressures that deformed or recrystallized the rock without complete melting.

Igneous Rocks Ignis means “fire”. Temperatures hot enough to melt the rocks are required. Magma may cool and crystallize to solid igneous rock either below or on the surface of the earth. All rocks at one time were igneous rocks. Today about two thirds of the outer layer, or crust, is made up of igneous rocks. The texture of the igneous rocks depends on how rapidly the cooling of the magma takes place. The more rapidly the magma cools the more coarse the texture is. Very rapidly cooling magma results in volcanic glass.

Igneous Rocks Formed as hot, molten mass of rock materials. Rhyolite and obsidian are the chemical equivalents of granite, except they are different in texture. Rhyolite is fine grained and obsidian is a translucent volcanic glass. Granite Obsidian Rhyolite

Igneous Rocks Formed as hot, molten mass of rock materials. Granite less dense than basalt, makes up most of continental crust. Basalt makes up most of oceanic crust. Granite Basalt

Sedimentary Rocks Sedimentary rocks are rocks that formed from particles or dissolved materials from previously existing rocks. They are transported by moving water and are deposited as sediments. Sediments are accumulations of silt, sand, or other materials that settle out of water.

Sedimentary Rocks There are two types of sedimentary rocks: 1. Clastic sediments-Weathered rock fragments. 2. Chemical sediments-Dissolved rock material.

Gypsum is calcium sulfate (CaSO4)

Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris. It is used to make drywall. Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. Gypsum is a very common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. The largest deposits known occur in strata from the Permian age. Gypsum is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins.

This is a sample of breccia, a coarse grained sedimentary rock with coarse, angular fragments.

This is a sample of sandstone, a sedimentary rock that formed from sand grains in a matrix of very fine grained silt, clay, or other materials. The grains in this sample are mostly feldspar and quartz minerals, which probably accumulated near the granite from which they were eroded.

Chemical Sedimentary Rocks Carbonates and evaporates are the most common sedimentary rocks. The carbonates are limestone and dolomite. Limestone is composed of calcium carbonate (CaCO3). This is also the composition of the mineral called calcite. It is precipitated directly from freshwater or salt water or indirectly by the actions of plants and animals that form shells of calcium carbonate. Dolomite is magnesium carbonate (MgCO3).

This is a sample of limestone, a sedimentary rock made of calcium carbonate that formed under water directly or indirectly from the actions of plants and animals. This fine grained limestone formed indirectly from the remains of tiny marine organisms.

Lithification of sand grains to become sandstone. Lithification means rock formation. Loose sand grains are deposited with open pore space between the grains. The weight of overburden compacts the sand into a tighter arrangement, reducing the pore space. C. Precipitation of cement in the pores by groundwater binds the sand into the rock sandstone, which has a clastic texture.

Metamorphic Rocks Metamorphic Rocks are previously existing rocks that have been changed by heat, pressure, or hot solutions into a distinctly different rock. The heat, pressure, or hot solutions that produced the changes are associated with geologic events, such as movement of the crust and heating and hot solutions from intrusion of a magma. Pressures from movement of the crust can change the rock texture by flattening, deforming, or realigning mineral grains. Temperatures from an intruded magma must be just right to produce a metamorphic rock They must be high enough to disrupt the crystal structures to cause them to recrystallize, but not high enough to melt the rocks and form igneous rocks.

Increasing metamorphic change occurs with increasing temperature and pressures. If the melting point is reached, the change is no longer metamorphic and igneous rocks are formed.

This banded metamorphic rock is very old; at an age of 3 This banded metamorphic rock is very old; at an age of 3.8 billion years, it is probably the oldest rock on the surface of the earth. It was formed in Greenland.

This is sample of marble, a coarse-grained metamorphic rock with interlocking calcite crystals. The calcite crystals were recrystallized from limestone during metamorphism.

Rocks to remember: Igneous: granite, obsidian, rhyolite Sedimentary: Clastic: breccia, sandstone (made from sand), shale (made from silt and clay) Chemical: limestone (made from calcite), dolomite, gypsum, salt (made from halite) Metamorphic: Marble, slate, schist, gneiss

The Rock Cycle Earth is a dynamic planet with constantly changing surface and interior. Internal changes alter the earth’s surface by moving the continents and building mountains that are eventually worn away by weathering and erosion. Seas advance and retreat over the continents as materials that are cycled from the atmosphere to the land and from the surface to the interior of the earth and then back again. Rocks are transformed from one type to another through this continental change. There is not a single rock on the earth’s surface today that has remained unchanged through the earth’s long history. The concept of continually changing rocks through time is called the rock cycle.

The Rock Cycle -Rock forming Process. A schematic diagram of the rock cycle concept, which states that geologic processes act continuously to produce new rocks from old ones.

Exercises Applying the concepts. p 441: # 1, 2, 3, 4, 6, 7, 8, 9, 11. New Book: p. 483-485 # 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 27, 28, 29, 32, 33, 34, 36, 39, 40, 41, 42, 43, 44, 45.

Summary Chapter 17 Most common elements on earth’s crust: 1. oxygen and 2. silicon. Earth’s crust is made up of minerals and rocks. Rocks are made up of minerals. What is a mineral. What is a crystal-they have 6 different symmetries and the simplest is the cubic. e.g. NaCl. Silicates and nonsilicates (presence of Si and O) Ferromagnesian (dark color and high density) and nonferromagnesian silicates (light color and low density) differ by presence of Fe and Mg in addition to Si and O. Carbonates are the most abundant of the nonsilicates but others are important as fertilizers, sources of metals and sources of industrial chemicals. Know the specific minerals indicated on slide. What each of the 8 physical properties of minerals represent. The most useful is the crystal structure. What are magma and lava. Minerals form from solutions of dissolved ions in water or from magma that has cooled and solidified. The crystal structure is affected by T, P, and ion concentration. The slower the crystallization process and the more dilute the larger the crystals formed. Ore minerals include native elements: copper, diamond, gold, silver, and sulfur and occur in veins. Glass is a solid that cooled too quickly for its atoms to move into ordered crystal structures. 3 types of rocks: igneous, sedimentary and metamorphic. Sedimentary can be clastic or chemical. Granite and basalt are the most abundant igneous rocks. All rocks at one time were igneous rocks. Know the specific rocks indicated on slide. The rock cycle. It originates with the igneous rocks.