Chapter 3: Atoms, Elements, Minerals, Rocks: Earth s Building Materials. Introduction : What Is A Mineral? Introduction : What Is A Mineral?

Transcription

1 Chapter 3: Atoms, Elements, Minerals, Rocks: Earth s Building Materials Introduction : What Is A Mineral? The four components of our planet: Atom - The smallest individual particle that retains the distinctive properties of a chemical element. Element - Any of the 92 naturally occurring fundamental substances into which matter can be broken down chemically (for example, hydrogen, oxygen, carbon, silicon, lead). Introduction : What Is A Mineral? (2) Mineral - Naturally formed, inorganic, solid material with a specific chemical composition and a characteristic crystalline structure. Rock - Naturally formed, coherent mass of one or more minerals, sometimes including organic debris.

2 Mineraloids Mineraloid Some naturally occurring solid compounds do not meet the definition of a mineral because they lack: - a definite composition, or - a characteristic crystal structure, or - both. Key Characteristics of Minerals Minerals have two key characteristics. Composition: - The chemical elements that compose a mineral, and their proportions. Crystal structure: - The organized way in which the atoms of the elements are packed together in a mineral. Composition of Minerals A few minerals are composed of a single element (examples are diamond, graphite, gold, copper, and sulfur). Most minerals are compounds, containing more than one element. Chemical elements are the most fundamental substances into which matter can be separated by chemical means.

3 SALT: Mineral or Element? Salt (NaCl) is not an element, because it can be separated into sodium and chlorine. Sodium (Na) and chlorine (Cl) cannot be broken down further chemically, so each is an element. Each element is identified by a symbol. Atoms: Elementary Structure Protons and neutrons are dense, and form the nucleus (core) of an atom. Protons have positive electric charges. Neutrons have no charge. The nuclei of atoms always have a positive charge. Electrons, which have negative electrical charges that balance exactly the positive charges of protons, move in orbitals around the nucleus. Figure 3.1

4 Atomic Number The number of protons in the nucleus gives each atom its special chemical characteristics. Elements are catalogued by atomic number. Uranium, with 92 protons in its nucleus, has the highest atomic number of the naturally occurring elements. Ununquadium,the heaviest synthesized element, reported early in 1999, has an atomic number of 114. Isotopes Mass number: the sum of the numbers of neutrons and protons in an atom. Isotopes: atoms with the same atomic number but different mass numbers (for example, carbon-12, carbon-13, and carbon-14 all have six protons per atom, and thus have the same atomic number). Energy-Level Shells Electrons move around the nucleus of an atom in complex three-dimensional patterns called orbitals. Groupings of orbitals are called energy-level shells. Electrons require different amounts of energy to orbit in different energy-level shells.

5 Ions Ions: Atoms that have lost or gained an electron. Cation: An atom that has lost an electron and thus has a positive charge. Anion: An atom that has gained an electron and thus has a negative charge. Compounds Chemical compounds form when atoms of different elements combine in a specific ratio. Properties of compounds are quite different from the properties of their constituent elements. Figure 3.2

6 Bonding A molecule is the smallest unit that has the distinctive chemical properties of a compound. A molecular compound always consists of two or more kinds of atoms held together. The force that holds the atoms together in a compound is called bonding. Bonding determines the physical and chemical properties of a compound. Four Types of Bonding (1) There are four important kinds of bonds: Ionic bonding: electron transfers between atoms produce cations and anions. Covalent bonding: some atoms share electrons rather than transferring them, creating a strong bond. - Elements and compounds with covalent bonding tend to be strong and hard. - The sparkle that makes diamonds attractive gems is due to covalent bonding. Four Types of Bonding (2) Metallic bonding: closely packed atoms share electrons in higher energy-level shells among several atoms. - Because the electrons are loosely held, they can drift from one atom to another. Van der Waals bonding: weak secondary attraction between certain molecules formed by transferring electrons. - Much weaker than ionic, covalent,or metallic bonding. - Graphite and talc.

7 Figure 3.3 A Figure 3.3 B Figure 3.4

8 Figure 3.5 Complex Ions Two or more kinds of ions form such strong covalent bonds that the combined atoms act as if they were a single entity. Such a strongly bonded unit is called a complex ion. Calcite (CaCO 3 ) Gypsum (CaSO 4 2H 2 O) Periodic Table of Chemical Elements (1) Dmitri Mendeleev ( ) developed the Periodic Table. Within rows, elements increase in atomic number from left to right. Elements within each column have the same number of electrons in their outermost energylevel shell.

9 Periodic Table of Chemical Elements (2) All elements in the first column easily give up the lone outer-shell electron to form cations (H +, Li +,etc.). The farthest columns to the right contains the six elements that have full energy-level shells. These are called noble gases because they have no tendency to gain or lose electrons and thus no tendency to form compounds. Figure 3.6 Crystal Structure of Minerals The atoms in most solids are organized in regular, geometric patterns, called the crystal structure. Solids that have a crystal structure are said to be crystalline. Ice in a glacier meets the definition of a mineral. Solids that lack crystal structures are amorphous. glass and amber.

10 Ionic Substitution Ionic substitution is the substitution of one ion for another in a compound. The bonding in most common minerals is ionic. Ionic substitution depends upon: Crystal structure; Ion size; - commonly expressed as ionic radius (distance from the center of the nucleus to the outermost shell of orbital electrons); Ion electrical charge. Figure 3.7 Figure 3.8

11 Figure B3.1 Figure B3.2 Crystal Form Crystal form Crystal: any solid body that grows with planar surfaces. The interfacial angle in any crystalline structure remains constant.

12 Figure 3.10 Figure 3.12 Growth Habit and Polymorphism Growth habit: - The characteristic crystal form of each mineral. Polymorphism: - Some elements and compounds form two or more different minerals: - C Graphite, Diamond - CaCO 3 Calcite, Aragonite - FeS 2 Pyrite, Marcasite - SiO 2 Quartz, Cristobalite

13 Cleavage Cleavage is the tendency to break in preferred directions along bright, reflective planar surfaces. A cleavage surface is a breakage surface, whereas a crystal face is a growth surface. The planar directions along which cleavage occurs are governed by the crystal structure. They are planes along which the bonding between atoms is relatively weak. Figure 3.13 Luster Luster is the quality and intensity of light reflected from a mineral. The most important lusters are: Metallic (polished metal surface). Vitreous (glass). Resinous (resin): the look of dried glue or amber. Pearly (pearl): the iridescent look of a pearl. Greasy (as if the surface were covered by a film of oil).

14 Color and Streak Color is determined by several factors, but its main cause is chemical composition. Unreliable for identification. Streak is the thin layer of powdered material left when a specimen is rubbed on an unglazed ceramic plate. Much more reliable than color for identification. Hardness and the Mohs Scale Hardness is a mineral s relative resistance to scratching. The Mohs relative hardness scale uses ten minerals, each with its distinctive hardness: scale indicate relative hardness. Figure 3.16 A

15 Figure 3.16 B Figure 3.16 C Figure 3.18

16 Hardness and the Mohs Scale (2) We test relative hardness by using common objects: copper penny,equivalent to fluorite s hardness of 4. steel knife blade, equivalent to feldspar s hardness of 6. Density and Specific Gravity Density is mass per unit volume. Minerals with a high density, such as gold, contain atoms with high mass numbers that are closely packed. Minerals with a low density, such as ice have loosely packed atoms. The unit of density is gram per cubic centimeter (g/cm 3 ). Specific Gravity Density is easily measured using the property called specific gravity. Specific gravity is the weight of a substance in air divided by the weight of an equal volume of pure water. Specific gravity is a ratio of weight.

17 Mineral Properties and Bond Types Minerals properties depend strongly on the kinds of bonds present. Ionic and covalent bonds are strong, making minerals hard and strong. Metallic and van der Waals bonds are much weaker. Common Minerals in Earth s Crust Only 12 elements occur in the continental crust in amounts greater than 0.1 percent by weight. These 12 elements make up percent of the crustal mass. The crust, therefore, is constructed mostly of a limited number of minerals. Approximately 4,000 minerals have been identified, but only about 30 are commonly encountered. Figure 3.19

18 Three Mineral Groups Silicate minerals (SiO 4 ) 4-, the most abundant in Earth s crust. Carbonate (CO 3 ) 2-, phosphate (PO 4 ) 3-, and sulfate (SO 4 ) 2- minerals. Ore minerals, sulfides (S 2- ) and oxides (O 2- ) that contain valuable metals. Silicates: The Largest Mineral Group Two elements, oxygen and silicon, make up more than 70 percent of the weight of the continental crust. Polymerization is the creation of compounds by accepting or sharing electrons. Linking silicate tetrahedra by oxygen sharing results in huge anions. It produces endless chains. Figure 3.20

19 Figure 3.21 Figure 3.22 Olivines and Garnets Two very important rock-forming mineral groups, the olivines and the garnets, have crystal structures in which the silicate tetrahedra are isolated. Olivine is among Earth s most abundant mineral groups, a very common constituent of igneous rocks in oceanic crust and the upper part of the mantle.

20 Figure A Olivines and Garnets (2) Olivine occurs in such flawless and beautiful crystals that is used as a gem, peridot. Chains: Pyroxenes and Amphiboles One of the most important mineral groups, the pyroxenes, contains single-chain linkages. The most common pyroxene is called augite. A very common and important family of minerals, the amphiboles, contains double chains. The most common of the amphiboles is called hornblende.

21 Chains: Pyroxenes and Amphiboles The pyroxenes and the amphiboles are hard to tell apart. The cleavages in pyroxene are right angles (90 o ). The cleavages in amphibole are at 120 o. Figure 3.23 Sheets: Clays, Micas, Chlorites, and Serpentines Kaolinite, Al 4 Si 4 O 10(OH) 8, is one of the most common clays. Muscovite, KAl 2 (Si 3 Al)O 10 (OH) 2, is a common mica. Chlorite, which contains Mg 2+ and Fe 2+ cations, is usually greenish in color.

22 Figure Sheets: Clays, Micas, Chlorites, and Serpentines (2) The serpentine group consists of three polymorphs with the formula Mg 6 Si 4 O 10 (OH) 8. Chrysotile is the white asbestos of commerce. Figure 3.24

23 Quartz Quartz is pure SiO 2. Forms six-sided crystals. Found in many colors. The colors come from minute amounts of iron, aluminum,titanium, and other elements present by ionic substitution. Fine grain forms of quartz are called chalcedony: - Agate - Flint (gray) - Jasper (red) The Feldspar Group Most Common Minerals in Earth s Crust Feldspar: The most common mineral group in Earth s crust. Accounts for about 60 percent of all minerals in the continental crust. Feldspar and quartz constitute 75 percent of the volume of the continental crust. Feldspar has a structure formed by polymerization. Figure 3. 26

24 Figure The Carbonates Group Carbonates: The carbonate anion, (CO 3 ) 2-, forms three common minerals: - Calcite. - Aragonite. - Dolomite. Calcite reacts vigorously to HCl. Figure 3.29

25 The Phosphate and Sulfate Mineral Groups Phosphates: Apatite is the most important phosphate mineral. - Contains the complex anion ((PO 4) Common mineral in many varieties of igneous and sedimentary rocks. - Main source of the phosphorus used for making phosphate fertilizers. The Phosphate and Sulfate Mineral Groups (2) Sulfates: All sulfate minerals contain the sulfate anion, (SO 4 ) 2- Only two are common: - Anhydrite(CaSO 4); - Gypsum (CaSO 4.2H 2O). Gypsum is the raw material used for making plaster. The Ore Mineral Group Our Source for Metals Sulfides: Pyrite (FeS 2 ) and pyrrhotite (FeS) are the most common. Galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS 2 ). Familiar metals extracted from sulfide ore minerals are cobalt, mercury, molybdenum, and silver.

26 Oxides Oxides; The iron oxides, magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ), are the two most common oxide minerals. - Hematite is red when powdered. Other oxide ore minerals are: - Rutile (TiO 2), the principal source of titanium; - Cassiterite (SnO 2), the main ore mineral for tin; - Uraninite (U 3O 8), the main source of uranium. Oxides (2) Other metals extracted from oxide ore minerals are chromium, manganese, niobium, and tantalum. Minerals Give Clues To Their Environment Of Formation (1) Scientists are able to determine the temperature and pressures at which carbon will form a diamond or form graphite, its polymorph. Diamonds were at one time subjected to pressures and temperatures equivalent to those in the mantle at least 150 km below Earth s surface.

27 Figure 3.31 Minerals Give Clues to Their Environment of Formation (2) Clues to climate: - Regolith is the blanket of loose rock particles that covers Earth. - Some minerals form in regolith during the weathering process. - We can decipher past climates from the kinds of minerals preserved in sedimentary rocks. Clues to seawater composition: - The content of past seawater can be determined from minerals formed when the seawater evaporated and deposited its salts. Rocks: Mixtures of Minerals Igneous rocks Formed by solidification of magma. Sedimentary rocks Formed by sedimentation of materials transported in solution or suspension. Metamorphic rocks Formed by the alteration of preexisting sedimentary or igneous rocks in response to increased pressure and temperature.

28 Distinguishing The Three Rock Types The differences among rock types are identified by two features. Texture: - The overall appearance of a rock due to the size, shape, and arrangement of its constituent mineral grain. Mineral assemblage: - The type and abundance of the minerals making up a rock. Texture and Mineral Assemblage A systematic description of a rock includes both texture and mineral assemblage. Megascopic textural features of rocks are those that we can see with the unaided eye. Microscopic textural features of rocks are those that require high magnification to be viewed. Figure 3.32 A

29 Figure 3.32 B Figure 3.32 C Figure 3.32 D

30 Mineral Concentration The two most common processes of concentration of a mineral are: Vapors are released by a cooling body of magma. A hot saline solution, such as heated seawater, reacts with and alters a rock, and in the process extracts the scarce metals. - As such a solution cools the metals are deposited in veins. Figure 3.33 Figure 3.34