1. Earth Materials: Minerals

2. 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 Today’s Lecture:

3. F Ionic Bonding Example: Table Salt: Sodium (Na) and Chlorine (Cl) Sodium gives up an electron becoming cation a positively-charged charged cation. Chlorine picks up an electron becoming a negatively charged anion anion. Atomic Bonding Bonding between sodium and chlorine in halite is based on these charge differences.

4. Sharing Electrons: Covalent Bonding Shared electrons Nucleus

5. 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: u Temperature u 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

6. 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

7. F Color F Luster F Hardness F Streak F Crystal form F Cleavage F Fracture 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: F Reaction to acid F Taste F Smell F Magnetization F Optical properties F Elasticity F Specific gravity Identifying Minerals

8. F 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

9. F Luster The appearance of light reflected from minerals. Examples: Metallic luster vs. nonmetallic luster Glassy (vitreous) luster Resinous luster Physical properties of minerals

10. F Hardness Very useful! Measures a mineral’s resistance to scratching. We use Moh’s hardness scale (below) for comparisons. Physical properties of minerals

11. Crystal Form Reflects the Internal Arrangement of Atoms

12. Crystal form in halite (salt; NaCl) is cubic

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

14. F 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 (SiO 2 )

15. Euhedral crystals of quartz

16. 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

17. Quartz geode Anhedral crystals formed by crowding during growth

18. Anhedral quartz crystals formed by crowding during growth Crystal terminations of euhedral quartz

19. Euhedral crystals of amphibole in a volcanic rock

20. Cleavage: Tendency to break along preferred planes of weakness. Cleavages represent directions of weaker bonding between atoms.

21. 2-directional cleavage in mica

22. In mica, atoms are arranged in weakly-connected sheets

23. Asbestos Cleaves into long flexible fibers

24. -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”) Asbestos

25.  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? 3-directional cleavage

26. F 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. Mica (sheets) Calcite (rhombs) Halite (cubes) In Summary….

27.  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 Quartz

28. Conchoidal fracture in volcanic glass

29. Streak: Color of mineral in its powdered form Hematite: Iron oxide

30. Carbonate minerals, like calcite, dissolve in acid and release carbon dioxide “The Acid Test” CO 2 bubbles

31. Halides Halite (Na, Cl: NaCl) -> common table salt Sulfates Gypsum (Ca,S,O,H: CaSO 4 -H 2 O) -> calcium sulfate + water, main ingredient of plaster & other building materials Oxides Hematite (Fe, O: Fe 2 O 3 ) -> steel Important Non-silicate Minerals

32. Carbonates Calcite (Ca, C, O: CaCO 3 ) Dolomite (Ca, Mg,C, O: CaMg(CO 3 ) 2  Found together in sedimentary rock limestone.  Main ingredient to cement, roads & building stones. Important Non-silicate Minerals

33. 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: n Silicon (Si) n Oxygen (O) The Common Rock-forming Minerals

34. Average composition of the Earth’s crust. Percent of elements by WEIGHT Question: What minerals would you expect to be most abundant on Earth?

35. 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 (SiO 2 ) - Hard, transparent Feldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink Clay (Mostly come from weathering feldspar) Calcite (CaCO 3, shells) Limestone - Used for cement The Common Rock-forming Minerals

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

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

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

40. 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. Balancing Charges in Silicates: Role of Metal Cations Iron (Fe) Magnesium (Mg) Potassium (K) Sodium (Na) Aluminum (Al) Calcium (Ca)

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

42. Definition of a rock: A rock is: Prelude Chapter: Rocks 2) Naturally occurring 1) Comprised of one or more minerals 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.

43. rock Prelude Chapter: Rocks

44. rock collection of one or more minerals Prelude Chapter: Rocks

45. rock minerals mineral Prelude Chapter: Rocks

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

47. Example: Granite & its constituent minerals:  Quartz  Amphibole (hornblende)  Feldspar Prelude Chapter: Rocks

48. Rocks and minerals l Some rocks composed entirely of one mineral limestone (calcite) Prelude Chapter: Rocks l Most rocks have more than one kind of mineral granite l Some rocks contain non-mineral matter coal (has organic debris) obsidian (volcanic glassy rock -> not crystalline)