2.1 Geology 8th Grade Earth Materials: Minerals

2.1. Minerals & Prelude to 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 Volcano building Contest

Example: Table Salt: Sodium (Na) and Chlorine (Cl) Atomic Bonding Ionic Bonding Review 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 Review 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. - 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 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

Constancy of angles between crystal faces A law of mineralogy: Constancy of angles between crystal faces

Quartz (SiO2) The shape of a well-formed crystal reflects 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)

Euhedral crystals of quartz

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 Anhedral quartz crystals 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

Cleaves into long flexible fibers 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…. Calcite Mica (sheets) Cleavage: Halite 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 (cubes) Mica (sheets)

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

“The Acid Test” Carbonate minerals, like calcite, dissolve in acid and release carbon dioxide “The Acid Test” CO2 bubbles

Halides Sulfates Oxides Halite (Na, Cl: NaCl) -> common table salt 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

Carbonates Calcite (Ca, C, O: CaCO3) Dolomite (Ca, Mg,C, O: CaMg(CO3)2 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)

Average composition of the Earth’s crust. 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

SiO Basic Building Block of Silicate Minerals: The Silicon-Oxygen Tetrahedron An anion with charge of -4 1 silicon (Si) atom 4 oxygen (O) atoms Si 4+ O 2- SiO 4 4- Silicon tetrahedron has An overall charge of -4

The Silicon-Oxygen Tetrahedron Silicates: The Common Rock-forming Minerals Basic Building Block: The Silicon-Oxygen Tetrahedron Tetrahedra link up by forming covalent bonds between oxygen atoms: Two tetrahedra can join by sharing an electron between adjacent oxygen atoms Single silicon tetrahedron: A silicon atom covalently- bonded to four oxygens. Oxygen atom Silicon atom

Double chains: Amphibole Single chains: Pyroxene The Common rock-forming minerals Silicates Silicon-oxygen tetrahedra can be arranged into: Double chains: Amphibole Sheets: Micas Single chains: Pyroxene

Balancing Charges in Silicates: Role of Metal Cations Silicate chains and sheets Not electrically neutral! Unsatisfied negative charges of oxygens located at the edges of chains, or between sheets are neutralized by coordinating metallic ions at those sites. Iron (Fe) Magnesium (Mg) Potassium (K) Sodium (Na) Aluminum (Al) Calcium (Ca)

Ionic Substitution Ions of similar size (ionic radius) and charge can substitute for one another in a mineral.

1) Comprised of one or more minerals 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

Prelude Chapter: Rocks collection of one or more minerals rock

Prelude Chapter: Rocks minerals rock mineral

minerals rock mineral minerals collection of one or more A collection Prelude Chapter: Rocks So far we have: minerals rock mineral collection of one or more minerals A collection of one or more types of atoms

Granite & its constituent minerals: 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)

Sedimentary Rocks: types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution. Metamorphic Rocks: is a result of a transformation of a pre-existing rock. The original rock is subjected to very high heat and pressure, which cause obvious physical and/or chemical changes. Igneous Rocks: is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Sediment and Magma: In groups of two using the supplied materials design an experiment that demonstrates the five different forms of rock found in the rock cycle. You must have a written procedure and materials list that is detailed easily repeatable. An explanation of each step and which part of the rock cycle it is representing.