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Chapter 17

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.

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Chapter 17

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  1. Chapter 17 Rocks and Minerals

  2. 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.

  3. Fig. 17.2

  4. 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.

  5. 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.

  6. 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.

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

  8. http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html#anchor8http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html#anchor8

  9. 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.

  10. 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.

  11. Basic crystalline systems

  12. Quartz crystals are hexagonal Fig. 17.5

  13. 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.

  14. Silicates • Si and 4 Oxygen atoms from a tetrahedral ionic structure, SiO42-. • All silicates contain SiO42- and itcancombine 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)

  15. 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.

  16. 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.

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

  18. Ferromagnesian Silicates Olivine Biotite Hornblende Augite

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

  20. Nonferromagnesian Silicates Orthoclase Quartz Mica (muscovite)

  21. 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.

  22. Non-silicates hematite dolomite apatite halite gypsum galena gold

  23. 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)

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

  25. 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.

  26. 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.

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

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

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

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

  31. 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.

  32. 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.

  33. 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.

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

  35. 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.

  36. 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

  37. 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).

  38. 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.

  39. 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.

  40. 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.

  41. 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.

  42. 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.

  43. 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

  44. 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

  45. 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.

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

  47. Gypsum is calcium sulfate (CaSO4)

  48. 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.

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