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Monument Valley Unit 1-CO, p.21 Quartz Crystals Fig. 2-CO, p.22 Vermiculite mine, Libby Montana. Discovered 1881, its properties make it valuable for packing, insulation and as a soil additive (when heated it expands to form a lightweight, fireproof, inexpensive material). Contaminated with asbestos: a mineral that is a known to cause Fig. 2-1, p.23 cancer and lung disease. What is a mineral? naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure. Fig. 2-2b, p.25 naturally occurring: many minerals can be manufactured (e.g., synthetic diamonds). inorganic: do not contain carbon-hydrogen bonds (organic…plants and animals create most of Earth’s organic material)…what about coal and oil? Shells of marine animals? solid: all minerals are solids. Is water a mineral? Is ice a mineral? chemical composition and crystalline structure covered next. Granitic Rock, Bugaboo Mtns, BC Canada Fig. 2-2a, p.25 Elements, Atoms, and the Chemical Composition of Minerals. Minerals are the building blocks of rocks, composed of chemical elements. Element: fundamental component of matter, cannot be broken down by ordinary processes. Most minerals are composed of 2-5 elements. Of the 88 naturally occurring elements in Earth’s crust, 8 make up 98% of the crust: O, Si, Al, Fe, Ca, Mg, K and Na (see slide). Atom: basic unit of an element, and it consists of a nucleus (+) surrounded by electrons (-). The nucleus of composed of protons (+) and neutrons (no charge) (see slide). Ion: positively (cation) or negatively (anion) charged atoms. Oxygen is commonly an anion and Earth’s abundant minerals are commonly cations, so form compounds (of elements). Held together by chemical bonds. So, minerals are compounds, usually 2-5 essential elements, expressed as a chemical formula: SiO2 for quartz, contains one silicon (+4) for every two oxygen (-2) atoms. Can you think of an mineral that consists of only one element? The 88 elements that occur naturally in the Earth’s crust can combine many different ways to form the more than 3,500 minerals known. However, the 8 abundant minerals generally combine in only a few ways, resulting in 9 rock-forming minerals (or mineral groups) that make up most of the Earth’s crust. Table 2-1, p.28 Electrons concentrate in layers or shells around the nucleus of an atom. Fig. 2-3, p.28 When sodium (Na) and chlorine (Cl) atoms combine, sodium loses one electron, becoming a cation (Na+); chlorine gains the electron to become an anion (Cl-). p.26 p.27 Every mineral is a crystal (has a crystalline structure). It means the atoms are arranged in a regular, periodically repeated pattern. The mineral halite (common table salt; NaCl) has one Na for every Cl atom. They alternate in orderly rows and columns that intersect at right angles. This is its crystalline structure. Unit Cell: a small group of atoms, like a brick in a wall, repeats itself over and over. Repeating bricks produce a rectangular shaped wall (or some modification of a rectangle), similar to the unit cell in crystals. Halite’s unit cell shown above. Fig. 2-4, p.29 Orderly arrangement of sodium and chlorine ions in halite (shows the unit cell). Exploded view. Fig. 2-4a, p.29 Show ions in contact (halite crystal). Fig. 2-4b, p.29 Shape of wellformed crystal determined by the shape of the unit cell and the manner in which the crystal grows. Stacking of small cubic unit cells could produce the cube (top) or octahedron (8sided crystal) to right. Not all unit cells are cubic. Fig. 2-5, p.29 Halite crystals showing crystalline structure from regular repeated pattern of unit cells. These crystals probably grew during evaporation of salty seawater. This halite (to right) shows welldeveloped crystal faces (flat surface that develops if a crystal grows freely in an uncrowded environment). Fig. 2-4c, p.29 In nature the growth of crystals is often impeded by adjacent mineral grains. Minerals rarely show perfect development of crystal faces. This is a thin section of granite, showing that crystals fit together like a jigsaw puzzle. Crystals grew around others as molten magma solidified. Fig. 2-6, p.29 Physical Properties of Minerals How do geologists identify minerals in the field? Chemical and crystal structure analysis’ aren’t practical in the field, so geologists rely on physical properties to identify minerals, and can use simple tests to confirm identification. These physical properties include crystal habit, cleavage and fracture, hardness, color, luster, specific gravity, streak and a few other properties such as magnetism and reaction to acid. Crystal Habit Characteristic shape of a mineral, and the manner in which aggregates of crystals grow. If it grows freely, shape is controlled by arrangement of atoms (like halite cubes). Above are equant crystals of garnet, Fig. 2-7a, p.30 with same dimensions in all directions. Fibrous appearance of asbestos. Fig. 2-7b, p.30 Bladed crystals of kyanite. Fig. 2-7c, p.30 Some minerals occur in more than one habit. Quartz grows as elongated crystals (left) and as massive crystals with no characteristic shape. Fig. 2-8, p.30 Cleavage Tendency of some minerals to break along flat surfaces. These surfaces are planes of weak bonds in the crystal. Mica (for example; above) has one set of parallel cleavage planes. Minerals can have 1, 2, 3 or even four different sets of cleavage Fig. 2-9, p.31 planes that can be described as excellent to poor and none. A flat surface created by cleavage, and a flat surface that is a crystal face can appear identical. Cleavage surface is duplicated when a crystal is broken, a crystal face is not. Above: feldspar has two cleavages at right angles; calcite has three sets of cleavage, not a right angles (deformed box look); fluorite has four cleavage planes (double pyramid look); quartz has no cleavage (right), but fractures. Fig. 2-10, p.31 Feldspar Fig. 2-10a, p.31 Calcite Fig. 2-10b, p.31 Fluorite Fig. 2-10c, p.31 Fracture: the manner in which a mineral breaks, not along cleavage planes. Conchoidal fracture, shown above (smoky quartz) creates smooth, curved surfaces. Fig. 2-11, p.32 Hardness: resistance of a mineral to scratching Table 2-11, p.32 Specific Gravity Weight of a substance relative to an equal volume of water. Most common minerals have a specific gravity of 2.7 (the mineral weighs 2.7 times that of an equal volume of water). Gold’s specific gravity is 19. Use “heft” to determine a mineral’s approximate specific gravity. Color Color is the most obvious property of a mineral, but can be the most unreliable for identification. Chemical impurities and imperfections in the crystal structure can dramatically alter the color. Quartz, for example, can occur in many colors including white, clear, black, purple and red as a result of tiny amounts of impurities and minor defects in the ordering of atoms. Streak Streak is the color of the fine powder of a mineral. Rub the mineral across a porcelain plate called a streak plate. Many minerals leave a diagnostic color. Hematite (iron oxide), for example, can be dull red to shiny black in color, but it always leaves a red powder on a streak plate (so can be more reliable than color for identification). Luster: This is the manner in which a mineral reflects light. A mineral with a metallic look has a metallic luster. Pyrite (to right) has a metallic luster. Nonmetallic luster can be described by terms such as glassy, pearly, earthy, resinous and vitreous. Fig. 2-12, p.13 Other Properties These include reaction to acid (calcite and other carbonates liberate Co2), magnetism (magnetite), radioactivity (uranium), fluorescence (when exposed to utraviolet light), phosphorescence and double refraction. Mineral Classes and the Rock-Forming Minerals Minerals are classified by their abundant chemical elements (Table 2.3). Of all the minerals, nine are the most common rockforming minerals in the crust. Seven are silicates (containing silicon and oxygen) and the other two are carbonates (containing carbon and oxygen). Silicates: make up 92% of the Earth’s crust. Most abundant because oxygen and silicon are the most abundant elements in the Earth’s crust (and they readily combine). Feldspars are the most common silicates, then pyroxene and quartz. Silicate tetrahedron commonly link together by sharing oxygen atoms. Fig. 2-13, p.13 Every silicon atom in the Earth’s crust surrounds itself with four oxygen atoms. The bonds are strong. This pyramid shaped structure is called the silicate tetrahedron, and is the fundamental building block of all silicate minerals. Fig. 2-13a, p.13 Most silicate tetrahedra combine with additional elements (cations such as aluminum, iron, calcium, potassium, sodium and magnesium). Quartz is the only common silicate that contains only Si and O. Silicate tetrahedra commonly link by sharing oxygen atoms to form chains, sheets, or other threedimensional networks (Figure 2.14). Fig. 2-13b, p.13 Table 2-3, p.34 Rockforming silicates fall into five main groups, based on the way the tetrahedra link together. Fig. 2-14, p.15 Feldspar alone makes up about half of the Earth’s crust. Quartz composes 12% of the Earth’s crust, and is widespread and abundant in continental rocks. Fig. 2-16, p.36 Olivine Fig. 2-15a, p.36 Pyroxene Fig. 2-15b, p.36 Amphibole Fig. 2-15c, p.36 Biotite Fig. 2-15d, p.36 Clay Fig. 2-15e, p.36 Feldspar Fig. 2-15f, p.36 Quartz Fig. 2-15g, p.36 Calcite: CaCO3 A carbonate mineral much less common than silicates in the Earth’s crust, but important rock-forming minerals because found in sedimentary rocks across the continents. Fig. 2-15h, p.36 Dolomite: CaMg (C03)2 Another carbonate. Shells and other hard parts of many marine organisms are made of carbonate minerals, and accumulate to form limestone. Fig. 2-15i, p.36 Commercially Important Minerals Many minerals, although not abundant, are commercially important. Our society depends on metals such as iron, copper, lead, zinc, gold and silver. Ore minerals: minerals from which metals or other elements can be profitably recovered. Gold and silver occur as single elements; iron is bonded to oxygen; copper, lead and zinc commonly bonded to sulfur. Industrial Minerals: commercially important, but not considered an ore (not mined for metals). Examples are halite mined for table salt; gypsum for ?; limestone for ?; sulfur for? Gems: minerals of beauty (diamonds also used industrially). Galena is the most important ore of lead Fig. 2-17, p.37 Native sulfur occurs at vents of dormant and active volcanoes. Fig. 2-18, p.37 Sapphire is one of the most costly precious gems Fig. 2-19, p.38 Topaz is a popular semiprecious gem Fig. 2-20, p.38 Environmentally Hazardous Rocks and Minerals Usually in their natural states, most rocks and minerals are environmentally benign and harmless to humans (or we wouldn’t survive). A few are harmful and dangerous (asbestos for example is cancer-causing). Most hazardous rocks and minerals are buried and through natural processes are exposed so slowly that they release toxic materials in low concentrations. Irresponsible mining can concentrate and release hazardous materials. One form of asbestos occurs as long, curly fibers. Fig. 2-21a, p.39 One form of asbestos occurs as short, sharply pointed needles. Fig. 2-21b, p.39 Acid mine drainage can poison aquatic life and stain stream beds and banks. Fig. 2-22, p.41 p.42