Lecture 3 Physical Geology PDF

Minerals The importance of minerals to human life as we know it is immeasurable. Minerals are the source of many of the resources we use in everyday life such as lead, copper, iron, or gold. They are also the source of many of our dietary supplements such as magnesium, iron, or calcium. Some mineral...

Minerals The importance of minerals to human life as we know it is immeasurable. Minerals are the source of many of the resources we use in everyday life such as lead, copper, iron, or gold. They are also the source of many of our dietary supplements such as magnesium, iron, or calcium. Some minerals are sought after because of their shape, color, or rarity. To geologists, minerals are important because they are the building blocks of the rocks that make up the Earth. The minerals in rocks tell a very important story about the origin of our world and, indeed, about all Earth-like planets. The study of minerals is called mineralogy. Each type of mineral is distinguished by a combination of properties, some of which we can see with the unaided eye, others of which are discernable only at the microscopic and atomic levels. Examples of these properties include color, luster, hardness, chemical composition, and the transmission of light under a microscope. Minerals are so important and so easily distinguishable that geologists use them as the basis for classifying almost all rocks. Nearly all rocks are made of minerals. Therefore, to be ready to learn about rocks, you must f irst understand what minerals are as well as the characteristics of some of the most common minerals. You will discover that each mineral is composed of specif ic chemical elements, the atoms of which are in a remarkably orderly arrangement. A mineral’s chemistry and the architecture of its i nt e rnal st ruc t ure d e t e rmi ne t he physi c al properties used to distinguish it from other mine rals. Yo u sho uld le arn ho w to re ad ily determine physical properties and use them to identify common minerals The difference between Minerals and A mineral is a naturally occurring, rocks inorganic, crystalline solid that has a Fig.: Each of the differently colored grains in this specific chemical composition. granite is a different mineral. The pink grains are feldspar, the black ones are biotite, and the glassy- white ones are quartz. A rock is an aggregate of minerals. Some rocks are made of only one mineral, but most contain two to five abundant minerals plus minor amounts of several others. NATURAL OCCURRENCE A synthetic diamond can be identical to a natural one, but it is not a true mineral because a mineral must form by natural processes. Like diamond, most gems that occur naturally can also be manufactured by industrial processes. Natural gems are valued more highly than manufactured ones. For this reason, jewelers should always tell their customers whether a gem is natural or artif icial, and they usually preface the name of a manufactured gem with the term synthetic. INORGANIC SOLID Organic substances are made up mostly of carbon that is chemically bonded to hydrogen or other elements. Although organic compounds can be produced in laboratories and by industrial processes, plants and animals create most of the Earth’s organic material. In contrast, inorganic compounds do not contain carbon- hydrogen bonds and generally are not produced by living organisms. All minerals are inorganic and most form independently of life. An exception is the calcite that forms limestone. Limestone is commonly composed of the shells of dead corals, clams, and similar marine organisms. Shells, in turn, are made of the mineral calcite or a similar mineral called aragonite. Although produced by organisms and containing ELEMENTS,ATOMS,AND THE CHEMICAL COMPOSITION OF MINERALS To consider the chemical composition and crystalline structure of minerals, we must understand the nature of chemical elements—the fundamental components of matter. An element cannot be broken into simpler particles by ordinary chemical processes. Most common minerals consist of a small number—usually two to five—of different chemical elements. A total of 88 elements occur naturally in the Earth’s crust. However, eight elements—oxygen, silicon, aluminum, iron, calcium, magnesium, potassium, and sodium—make up more than 98 percent of the crust (Table). A complete list of all elements is given in the Table. Each element is represented by a one- or two-letter symbol, such as O for oxygen and Si for silicon. The table shows a total of 108 elements, not 88, because 20 elements are produced in nuclear reactors but do not occur naturally. :  An atom is the basic unit of an element. CRYSTALS:THE CRYSTALLINE NATURE OF MINERALS A crystal is any substance whose atoms are arranged in a regular, periodically repeated pattern. All minerals are crystalline. The mineral halite has the composition NaCl: one sodium ion (Na) for every chlorine ion (Cl). Figure is an “exploded” view of the ions in halite. Figure is more realistic, showing the ions in contact. In both sketches the sodium and chlorine ions alternate in orderly rows and columns intersecting at right angles. This arrangement is the crystalline structure of halite. (a and b) The orderly arrangement of sodium and chlorine ions in halite. (c) Halite crystals. The crystal model in (a) is exploded so that you can see into it; the ions are actually closely packed as in (b). Note that ions in (a) and (b) form a cube, and the crystals in (c) are also cubes.  A crystal face is a planar surface that develops if a crystal grows freely in an uncrowded environment. The sample of halite in Figure has well-developed crystal faces. In nature, the growth of crystals is often impeded by adjacent minerals that are growing simultaneously or that have formed previously. For this reason, minerals rarely show perfect development of crystal faces Some physical properties of Minerals CLEAVAGE Cleavage is the tendency of some minerals to break along f la t surfaces. The surfaces are planes of weak bonds in the crystal. Some minerals, such as mica and graphite, have one set of parallel cleavage planes (Fig.). Others have two, three, or even four different sets. Some minerals, like the micas, have excellent cleavage. You Cleavage in mica. This large crystal is can peel sheet after sheet from a mica crystal as if you were peeling the variety of mica called muscovite. layers from an onion. Others have poor cleavage. Many minerals have no cleavage at all because they have no planes of weak bonds. The number of cleavage planes, the quality of cleavage, and the angles between cleavage planes all help in mineral identification. FRACTURE Fracture is the pattern in which a mineral breaks other than along planes of cleavage. Many minerals fracture into characteristic shapes. Conchoidal fracture creates smooth, curved surfaces. It is characteristic of quartz and olivine. Glass, although not a mineral because it has no crystalline structure, also typically fractures in a conchoidal pattern. Some minerals break into splintery or f ibrous fragments. Most fracture into irregular shapes. HARDNESS Hardness is the resistance of a mineral to scratching. It is easily measured and is a fundamental property of each mineral because it is controlled by bond strength between the atoms in the mineral. Geologists commonly gauge hardness by attempting to scratch a mineral with a knife or other object of known hardness. If the blade scratches the mineral, the mineral is softer than the knife. If the knife cannot scratch the mineral, the mineral is harder. To measure hardness more accurately, geologists use a scale based on ten minerals, numbered 1 through 10. Each mineral is harder than those with lower numbers on the scale, so 10 (diamond) is the hardest and 1 (talc) is the softest. The scale is known as the Mohs hardness scale after F. Mohs, the Austrian mineralogist who developed it in the early nineteenth century. The Mohs hardness scale shows, for example, that a mineral scratched by quartz but not by orthoclase has a hardness between 6 and 7 (Table). Because the minerals of the Mohs scale are not always handy, it is useful to know the hardness values of common materials. A fingernail has a hardness of slightly more than 2, a copper penny about 3, a pocketknife blade slightly more than 5, window glass about 5.5, and a steel file about 6.5. If you practice with a knife and the minerals of the Mohs scale, you can develop a “feel” for minerals with hardness of 5 and under by how easily the blade scratches them. When testing hardness, it is important to determine whether the mineral has actually been scratched by the object, or whether the object has simply left a trail of its own powder on the surface of the mineral. SPECIFIC GRAVITY Specific gravity is the weight of a substance relative to that of an equal volume of water. If a mineral weighs 2.5 times as much as an equal volume of water, its specific gravity is 2.5. You can estimate a mineral’s specific gravity simply by lifting a sample in your hand. If you practice with known minerals, you can develop a feel for specific gravity. Most common minerals have specific gravities of about 2.7. Metals have much greater specific gravities; for example, gold has the highest specific gravity of all minerals, 19. Lead is 11.3, silver is 10.5, and copper is 8.9. COLOR Color is the most obvious property of a mineral, but it is commonly unreliable for identification. Color would be a reliable identification tool if all minerals were pure and had perfect crystal structures. However, both small amounts of chemical impurities and imperfections in crystal structure can dramatically alter color. For example, corundum (Al2O3) is normally a cloudy, translucent, brown or blue mineral. Addition of a small amount of chromium can convert corundum to the beautiful, clear, red gem known as ruby. A small quantity of iron or titanium turns corundum into the striking blue gem called sapphire.  STREAK Streak is the color of a fine powder of a mineral. It is observed by rubbing the mineral across a piece of unglazed porcelain known as a streak plate. Many minerals leave a streak of powder with a diagnostic color on the plate. Streak is commonly more reliable than the color of the mineral itself for identification. LUSTER Luster is the manner in which a mineral reflects light. A mineral with a metallic look, irrespective of color, has a metallic luster. The luster of nonmetallic minerals is usually described by self-explanatory words such as glassy, pearly, earthy, and resinous. OTHER PROPERTIES Properties such as reaction to acid, magnetism, radioactivity, fluorescence, and phosphorescence can be characteristic of specific minerals. Calcite and some other carbonate minerals dissolve rapidly in acid, releasing visible bubbles of carbon dioxide gas. Minerals containing radioactive elements such as uranium emit radioactivity that can be detected with a scintillometer. Fluorescent materials emit visible light when they are exposed to ultraviolet light. Phosphorescent minerals continue to emit light after the external stimulus ceases.