Minerals introduction to earth science

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Earth Materials: Minerals Today’s Lecture: Chapter 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 cation Chlorine picks up an electron becoming a negatively charged anion 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 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 silicon (Si) atom oxygen (O) atoms 2- 4+ Si O O 2- O 2- 2- 4SiO4 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: Single chains: Pyroxene Double chains: Amphibole 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)
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