Rocks and Minerals. Tillery, Chapter 19. Solid Earth Materials

Transcription

1 Rocks and Minerals Tillery, Chapter 19 Science 330 Summer 2007 No other planet in the solar system has the unique combination of fluids of Earth. Earth has a surface that is mostly covered with liquid water, water vapor in the atmosphere, and both frozen and liquid water on the land. Solid Earth Materials Earth s Molten Stage During the early formation of the Earth it was molten During this stage the heavier elements such as iron and nickel, sank to the deeper interior i of the Earth. This left a thin layer of lighter materials on the surface that is mow called the crust. The majority of the Earth s mass lies below the crust Chemical Analysis 8 elements make up 98.6% of the crust These 8 elements make up the solid materials of the Earth s crust and are known as rocks and minerals. A mineral is solid inorganic material of the Earth that has both a known chemical composition and a crystalline structure that is unique to that mineral A rock is a solid aggregate of one or more minerals that have been cohesively brought together by a rock-forming process. (A)The percentage by weight of the elements that make up Earth's crust. (B) The percentage by weight of the elements that make up the whole Earth. 1

2 Minerals Introduction Minerals A mineral is solid inorganic material of the Earth that has both a known chemical composition and a crystalline structure that is unique to that mineral Rocks A rock is a solid aggregate of one or more minerals that have been cohesively brought together by a rock-forming process. Crystal Structures Can be made up of atoms of one or more kinds of elements. Crystals are classified according to six major groups, with subdivisions i i of each. A crystal is composed of a structural unit that is repeated in three 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. These quartz crystals are hexagonal prisms. 2

3 Crystalline substances are classified into six major systems: isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. The six systems are based on the arrangement of crystal axes, which reflect how the atoms or molecules are arranged inside. For example, crystals in the orthorhombic system have three axes of different lengths intersecting at 90 O angles while crystals in the hexagonal system have three horizontal axes intersecting at 60 O and one vertical axis. Some common examples in each system are illustrated here. (A)The geometric shape of a tetrahedron with four equal sides. (B) A silicon and 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. (A)Isolated silicon-oxygen oxygen tetrahedra do not share oxygens. This structure occurs in the mineral olivine. (B) Single chains of tetrahedra are formed by each silicon ion having two oxygens all to itself and sharing two with other silicons at the same time. This structure occurs in augite. (C) Double chains of tetrahedra are formed by silicon ions sharing either two or three oxygens. This structure occurs in hornblende. (D) The sheet structure in which each silicon shares three oxygens occurs in the micas, resulting in layers that pull off easily because of cleavage between the sheets. Silicates and Nonsilicates Silicates made of silicon and oxygen and make up 92 % of Earth s crust. Ferromagnesian Silicates made of iron, magnesium, and silicates Form a basic tetrahederal structure. Higher density and darker color than other silicates due to the presence of iron and magnesium Nonferromagnesiam Silicates silicates that do not contain either iron or magnesium. Lower density and lighter color than the ferromagnesian silicates. Compare the dark colors of the ferromagnesian silicates augite (right), hornblende (left), and biotite to the light-colored nonferromagnesian silicates. 3

4 Structure of silicates Isolated tetrahedrons Chain silicates Sheet silicates Framework silicates Compare the light colors of the nonferromagnesian silicates mica (front center), white and pink orthoclase (top and center), and quartz, to the dark-colored ferromagnesian silicates. Nonsilicates make up 8% of Earth s crust Carbonates Sulfates Oxides Sulfides Halides Phosphates Hydroxides Native elements Physical Properties of Minerals Color A visual measure. Not very useful for identification as color of minerals varies considerably. Streak This is the color of the mineral when it is finely ypowdered. Rubbed across a piece of tile, leaving a fine powder of the mineral on the tile. Hardness Resistance of the material to being scratched. Measured using the Mohs hardness scale, which compares the hardness of the mineral to 10 reference minerals. (A)Gypsum, with a hardness of 2, is easily scratched by a fingernail. (B) Quartz, with a hardness of 7, is so hard that even a metal file will not scratch it. Crystal form Related to the internal geometric arrangement of the atoms that make up the crystal structure. Cleavage the tendency of mineral to break along smooth planes. Depends upon zones of weakness in the crystal structure. Fracture The broken surface is irregular and not in a flat plane. 4

5 Luster Surface sheen Metallic like metal Pearly like pearl Vitreous like glass Earthy Density ratio of the mass of a mineral to its volume. Specific gravity ratio of mineral density to the density of water Depends on: Kind of atoms which make up the mineral How the atoms are arranged in the crystal lattice. Mineral-forming Processes Introduction Magma Molten rock from which minerals are formed Lava Magma that is forced to the surface Influences on the mineral forming process Temperature Pressure Time Availability and concentration of ions that are in solution Minerals Formed at High Temperatures Bowen s Reaction Series Arranged with minerals at the top that crystallize at higher temperature and minerals at the bottom that crystallize out at lower temperature. Minerals Formed at Normal Temperatures These form at normal temperatures and pressures and in contact with atmospheric gases such as oxygen, carbon dioxide, and water. There are most of the non-silicates; carbonates, sulfates, oxides, halides, and sulfides. Bowen's reaction series. Minerals at the top of the series (olivine, augite, and calcium-rich plagioclase) crystallize at higher temperatures, leaving the magma enriched in silica. Later, the residual magma cools and lighter-colored, less dense minerals (orthoclase feldspar, quartz, and white mica) crystallize. Thus, granite-like rocks can form from a magma that would have produced basaltic rocks had it cooled quickly. 5

6 Altered Minerals These minerals undergo changes in chemistry or crystal structure as a result of pressure, temperature, or chemical solutions Similar to minerals that form under high temperatures with similar physical properties. Ore Minerals Some minerals are left over after the crystallizing of magma These elements are flushed away in hot water solutions as the magma crystallizes. Usually crystallize in rock fractures to form thin, flat bodies of mineral material called veins. If these minerals have some economic value they are called ore minerals. Introduction Rocks Elements are chemically combined to form minerals Minerals are physically combined to form rocks. Igneous Rocks Form from molten rock material Intrusive igneous rock Formed when magma cools deep within the Earth s surface Cools very slowly as it is in contact with molten rock. Produces course-grained igneous rock. Extrusive igneous rock Magma that cools above the Earth s surface. Produces fine-grained igneous rocks. This rapid cooling does not allow time for crystals to form. Igneous rock classification scheme based on mineral composition and texture. There are other blends of minerals with various textures, many of which have specific names. 6

7 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 of granite. This is a piece of obsidian, which has the same chemical composition as the granite. Obsidian has a different texture because it does not have crystals and is a volcanic glass. The curved fracture surface is common in non-crystalline substances such as glass. Sedimentary Rocks Form from material from previously existing rock Material is provided by weathering of previously existing rock Sediments Weathered rock materials Dissolved rock materials Clastic sediments Another name for weathered rock materials This is a sample of breccia, a coarse-grained sedimentary rock with coarse, angular fragments. Compare the grain sizes to the centimeter scale. 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 the feldspar and quartz minerals, which probably accumulated near the granite from which they were eroded. 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. 7

8 Chemical sediments Another name for dissolved rock material. The dissolved materials are ions from mineral and rocks that have been completely broken down. Removed from solution by: Chemical precipitation from the solution Crystallization from evaporating water. Biological sediments. Compaction As sediments are laid down grain by grain, the mass becomes greater. The increasing mass of the sediment layer above creates pressure on the layers below. Eventually this pressure becomes great enough to compact the existing layers into a cohesive rock layer. Cementation After, or during, the compaction process, the spaces between the sediment particles become filled with a chemical deposit. This deposit holds the compacted layers into a cohesive mass of sedimentary rock. (A) In compaction, the sediment grains are packed more tightly together, often by overlying sediments, as represented by the bricks. (B) In cementation, ti fluids contain dissolved minerals that are precipitated in the space between the grains, cementing them together into a rigid, solid mass. Metamorphic Rocks Rocks changed by heat, pressure, or hot solutions due to: Movement of the Earth s crust Heat generated by intrusion of hot magma Pressure can change rock by flattening, deforming, or realigning mineral grains. Foliation When the pressure on flat crystal flakes tends to align the flakes into parallel sheets. Gives the rock the property of breaking along the planes between the aligned mineral grains in what is known as rock cleavage. Increasing metamorphic change occurs with increasing temperatures and pressures. If the melting point is reached, the change is no longer metamorphic, and igneous rocks are formed. This is a sample of marble, a coarse-grained metamorphic rock with interlocking calcite crystals. The calcite crystals were recrystallized from limestone during metamorphism. 8

9 This banded metamorphic rock is very old; at an age of 3.8 billion years, it is probably among the oldest rocks on the surface of the earth. The Rock Cycle Earth is a dynamic planet with the surface and interior in a constant state of flux. Internal changes alter the surface by moving the Earth s plates, building mountains. Seas advance and retreat over the continents brining in new materials and taking other materials away. Rocks are continually being changed by Earth s forces. The Rock Cycle describes the continually changing structure of rocks. Igneous, sedimentary, or metamorphic rock are just temporary stages in the continuing changes that all rocks undergo. A schematic diagram of the rock cycle concept, which states that geologic processes act continuously to produce new rocks from old ones. 9