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Earth Materials: Minerals Today’s Lecture: Chapter 5. Patterns in Nature: Minerals & Prelude A: Rock Groups Chemical bonding: Focus on covalent bonds Mineral polymorphs Physical properties of minerals Common rock-forming “silicate” minerals Introduction to rocks & the rock cycle Atomic Bonding Ionic Bonding Example: Table Salt: Sodium (Na) and Chlorine (Cl) Sodium gives up an electron becoming a positively-charged charged cation. Chlorine picks up an electron becoming a negatively charged anion. Bonding between sodium and chlorine in halite is based on these charge differences. Sharing Electrons: Covalent Bonding Nucleus Shared electrons Factors that determine the internal structure of minerals: 1) Composition of magma or fluids from which the minerals form. 2) Conditions under which the mineral forms: Temperature Pressure Minerals comprised of the same elements in the same proportions can possess markedly different internal structures. For example: Higher pressure -> Denser packing of atoms -> Different mineral Mineral Structure & Conditions of Formation Different minerals w/ same chemical composition , but differing structures, are called “polymorphs” Graphite (a form of pure carbon) Soft gray material, e.g., pencil lead Crystal structure: sheets of carbon Diamond (also pure carbon) Forms deep in Earth at high pressures, & is hardest substance known to humans Crystal structure: dense & compact Identifying Minerals To identify minerals, we use their physical and optical properties. Some properties are more diagnostic than others, so we try to use a combination when making a determination. Useful properties include: Color Luster Hardness Streak Crystal form Cleavage Fracture Reaction to acid Taste Smell Magnetization Optical properties Elasticity Specific gravity Physical properties of minerals Color Obvious, but often misleading. Slight impurities in a mineral can change its color. Example: Quartz (when pure it is colorless), but there are many color varieties which result from small amounts of other elements. Physical properties of minerals Luster The appearance of light reflected from minerals. Examples: Metallic luster vs. nonmetallic luster Glassy (vitreous) luster Resinous luster Physical properties of minerals Hardness Very useful! Measures a mineral’s resistance to scratching. We use Moh’s hardness scale (below) for comparisons. Crystal Form Reflects the Internal Arrangement of Atoms Crystal form in halite (salt; NaCl) is cubic A law of mineralogy: Constancy of angles between crystal faces Crystal Form The shape of a well-formed crystal reflects directly the orderly internal arrangement of Its constituent atoms. Well-formed crystals that grow without interference are called “euhedral”. Quartz (SiO2) Anhedral crystals form when crystals don’t have room to grow and bump into each other feldspar in an igneous rock Irregular boundaries between crystals due to interference during growth Anhedral crystals formed by crowding during growth Quartz geode Crystal terminations of euhedral quartz Anhedral quartz crystals formed by crowding during growth Euhedral crystals of amphibole in a volcanic rock Cleavage: Tendency to break along preferred planes of weakness. Cleavages represent directions of weaker bonding between atoms. 2-directional cleavage in mica In mica, atoms are arranged in weakly-connected sheets Asbestos Cleaves into long flexible fibers Asbestos Asbestos -group of silicate minerals that readily separate into fibers that are: thin, flexible, heat resistant, chemically inert => many uses - mainly three types: chrysotile (“white asbestos”) crocidolite (“blue asbestos”) amosite (“brown asbestos”) 3-directional cleavage Cleavage in both Halite (salt) & calcite (lime) is in three directions. But the angles between cleavages are different for these minerals. Halite has a cubic cleavage. Calcite cleaves into rhombohedra. Can you spot which is which in the samples to the left? In Summary…. Cleavage: The tendency of a mineral to break along planes of weak bonding in the crystal structure. The number and angles between cleavgae faces are very useful properties for identification. Calcite (rhombs) Halite Mica (sheets) (cubes) Conchoidal Fracture in Quartz Bond strengths are equal in all directions. No preferred directions of weakness. Quartz does not cleave, but breaks along smooth, curved, glassy surfaces. Called “conchoidal” (glassy) fracture Conchoidal fracture in volcanic glass Streak: Color of mineral in its powdered form Hematite: Iron oxide Carbonate minerals, like calcite, dissolve in acid and release carbon dioxide “The Acid Test” CO2 bubbles Important Non-silicate Minerals Halides Halite (Na, Cl: NaCl) -> common table salt Sulfates Gypsum (Ca,S,O,H: CaSO4-H2O) -> calcium sulfate + water, main ingredient of plaster & other building materials Oxides Hematite (Fe, O: Fe2O3) -> steel Important Non-silicate Minerals Carbonates Calcite (Ca, C, O: CaCO3) Dolomite (Ca, Mg,C, O: CaMg(CO3)2 Found together in sedimentary rock limestone. Main ingredient to cement, roads & building stones. The Common Rock-forming Minerals Over 4000 minerals: only few dozen are abundant, making up most rocks of Earth’s crust => rock-forming minerals Only 8 elements make up most of crust’s minerals & represent over 98% of the continental crust The two most abundant elements: Silicon (Si) Oxygen (O) Question: What minerals would you expect to be most abundant on Earth? Percent of elements by WEIGHT Average composition of the Earth’s crust. The Common Rock-forming Minerals Earth’s Crust Primarily Si & O followed in abundance by Fe, Mg, Ca, Na, K, etc. Dark colored (mantle and oceanic crust) Olivine (Si, O, Fe, Mg) Pyroxene (Si, O, Fe, Mg, Ca) Amphibole (Si, O, Fe, Mg) Light colored (crust, esp. continental crust) Quartz (SiO2) - Hard, transparent Feldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink Clay (Mostly come from weathering feldspar) Calcite (CaCO3, shells) Limestone - Used for cement Basic Building Block of Silicate Minerals: The Silicon-Oxygen Tetrahedron An anion with charge of -4 O 1 silicon (Si) atom 4 oxygen (O) atoms 2- 4SiO4 4+ Si O O 2- O 2- 2- Silicon tetrahedron has An overall charge of -4 Silicates: The Common Rock-forming Minerals Basic Building Block: The Silicon-Oxygen Tetrahedron Tetrahedra link up by forming covalent bonds between oxygen atoms: Single silicon tetrahedron: A silicon atom covalentlybonded to four oxygens. Oxygen atom Silicon atom Two tetrahedra can join by sharing an electron between adjacent oxygen atoms The Common rock-forming minerals Silicates Silicon-oxygen tetrahedra can be arranged into: Double chains: Amphibole Single chains: Pyroxene Sheets: Micas Balancing Charges in Silicates: Role of Metal Cations Silicate chains and sheets Unsatisfied Not electrically neutral! Iron (Fe) Magnesium (Mg) Potassium (K) Sodium (Na) Aluminum (Al) Calcium (Ca) negative charges of oxygens located at the edges of chains, or between sheets are neutralized by coordinating metallic ions at those sites. Ionic Substitution Ions of similar size (ionic radius) and charge can substitute for one another in a mineral. Prelude Chapter: Rocks Definition of a rock: A rock is: 1) Comprised of one or more minerals 2) Naturally occurring There are three types of rocks: Igneous (formed by cooling from magma) Sedimentary (formed by the breakdown of other rocks) Metamorphic (formed when preexisting rocks are heated under pressure. Prelude Chapter: Rocks rock Prelude Chapter: Rocks collection of one or more rock minerals Prelude Chapter: Rocks rock minerals mineral Prelude Chapter: Rocks So far we have: rock minerals mineral collection of one or more minerals A collection of one or more types of atoms Prelude Chapter: Rocks Example: Granite & its constituent minerals: Quartz Amphibole (hornblende) Feldspar Prelude Chapter: Rocks Rocks and minerals Some rocks composed entirely of one mineral limestone (calcite) Most rocks have more than one kind of mineral granite Some rocks contain non-mineral matter coal (has organic debris) obsidian (volcanic glassy rock -> not crystalline)