A toms and Minerals

1. Magnet and Iron and slide Atoms and Minerals Quartz SiO2 a common mineral

2. Minerals: Building blocks of rocks • Definition of a mineral: • Naturally occurring • Inorganic solid • Ordered internal molecular structure • Definite chemical composition • Definition of a rock: • A solid aggregate or mass of minerals

3. Atomic structure • Central region called the nucleus • Consists of protons (positive charges) and neutrons (neutral charges) • Electrons • Negatively charged particles that orbit around the nucleus • Located in discrete energy levels called shells

4. Flattened structure of an atom # protons (+) equals # electrons (-) Electrons in shells Number of outermost electrons determine types of bonding Argon Outermost (Valence) shell

5. Some definitions: • Atomic number:number of protons in the nucleus • Atomic Mass: total mass of protons and neutrons within an atom’s nucleus • We can see these on a Periodic Table

6. Periodic Table of the Elements # protons (+) equals # electrons (-) Electrons in shells Number of outermost electrons determine types of bonding 1 2 3 4 5 6 7 8 Shows atomic number (# protons) and atomic mass (# protons + neutrons). Column shows # electrons in outermost shell

7. Electrons are in shells. Octet Rule: Atoms larger than Hydrogen and Helium need 8 electrons in their outer shell for stability

8. Neutral Atoms have #protons = # electrons Silicon has 4 electrons in Its outer shell Oxygen has 6 electrons in its valence shell

9. To satisfy the octet rule atoms can gain or lose electronsIn that state they are called IONSThey can combine with oppositely charged ions to form neutral molecules Ions Oxygen, normally 6 valence electrons, wants 2 extra Silicon, normally 4 valence electrons, would like to be rid of, or share, 4

10. Chemical Bonding 1: Ionic • Chemical bonding • Formation of a compound by combining two or more atoms • Ionic bonding • Atoms gain or lose outermost (valence) electrons to form ions • Ionic compounds consist of an orderly arrangement of oppositely charged ions • Usually Columns I (alkali metals e.g. Na) and VII (halogens e.g. Cl)

11. Halite (NaCl)- An Example of Ionic Bonding Halite small Na+ large Cl- Table Salt Na+ Cl- Na+ Cl- Na+ Cl- Na+ Na+ Crystalline structure of Internal atomic arrangement is primarily determined by the size of ions involved NaCl Small Sodium ions between large Chlorine ions (a)

12. Covalent bonding – sharing of valence electrons Cl2Chlorine gas Sharing Electrons in Outermost Shell

13. Covalent Bonds in Water H2O Water is polar

14. Metallic Bonds • Metallic bonding • Valence electrons are free to migrate among atoms • Weaker than ionic or covalent bonds

15. Intermolecular Bonds 1 • Intermolecular bonding • Hydrogen bonds- charged regions in water

16. Intermolecular Bonds 2 • Van der Waals bonds- opposite charges near gap between close atoms With close approach, the electron clouds repel each other. When one electron cloud (2) is further than the opposite nucleus (1), the other electron cloud (1) is attracted to the opposite nucleus (2). A weak momentary bond is present. 2 1

17. Isotopes • Atomic mass is the total mass of neutrons plus protons in an atom • An elements isotopes are all atoms with the same # protons but different #neutrons • Some isotopes have unstable nuclei, emit particles: “radioactive” decay. • 12C 13C stable 14C radioactive all 6 protons

18. Structure of minerals • Polymorphs • Two or more minerals with the same chemical composition but different crystalline structures • Diamond and graphite (both carbon) are good examples of polymorphs • The transformation of one polymorph to another is called a phase change • Example: Graphite in a High Pressure Cell Makes Diamond • Some polymorphs make good PT* indicators *We can measure the Temperature and Pressure a mineral experienced when it formed in the past

19. Diamond and graphite – polymorphs of carbon

20. Physical properties of minerals • Crystal Form • External expression of the orderly internal arrangement of atoms The mineral garnet sometimesexhibits good crystal form:dodecahedra

21. Physical properties of minerals • Luster • Appearance of a mineral in reflected light • Two basic categories • Metallic • Nonmetallic • Terms are used to further describe nonmetallic luster are vitreous (glassy), pearly, silky, earthy (like dirt), adamantine (greasy)

22. Galena PbS displays metallic luster Valuable ore of Lead

23. Physical properties of minerals • Color • Generally an unreliable diagnostic property to use for mineral identification • Often highly variable for a given mineral due to slight changes in mineral chemistry • Exotic colorations of some minerals produce gemstones • But we use it anyway Quartz (SiO2) exhibits a variety of colors

24. Physical properties of minerals • Streak • Color of a mineral in its powdered form • Helpful in distinguishing different minerals with similar appearance • Hardness • Resistance of a mineral to abrasion or scratching • All minerals are compared to a standard scale called the Mohs scale of hardness Example: Hematite has a reddish streak Streak

26. Cleavage • Tendency to break along planes of weak bonding • Produces flat, shiny surfaces • Described by resulting geometric shapes • Number of planes • Angles between adjacent planes Micas have one perfect cleavage Biotite Mica

27. Three directions of perfect cleavage – fluorite, halite, and calcite Fluorite, CaF2 4 planes of cleavage, octahedron Calcite, CaCO3, 3 planes of cleavage not at 90o Salt, NaCl, 3 planes of cleavage, 90o Each Cleavage Plane is paired

28. Cleavage This Feldspar has two planes of good cleavage about 90o apart. Notice the other surface is rough, not a plane. non metallic, usually light in color, streak clear to white, hardness ~6, usually scratches glass

29. Physical properties of minerals • Fracture • Absence of cleavage when a mineral is broken. Shown: conchoidal fracture in Quartz • Specific Gravity • Ratio of the weight of a mineral to the weight of an equal volume of water • Average value is approximately 2.7 • Simply hefting a mineral works too.

30. Physical properties of minerals • Other properties • Magnetism • Reaction to hydrochloric acid • Malleability • Double refraction • Taste • Smell • Elasticity

31. Is it calcite or dolomite?

32. Classification of Minerals • Nearly 4000 minerals have been identified on Earth (We discuss a few) • Rock-forming minerals • Common minerals that make up most of the rocks of Earth’s crust • Only a few dozen members • Composed mainly of the 8 elements that make up 98% of the continental crust

33. Commonly formed Ion chargesoften called “oxidation state” Metals can form more than one Ion. Fe+2 is name Ferrous, Fe+3 is named Ferric

34. Classification of Minerals • Silicates • Most important mineral group • Comprise most of the rock-forming minerals • Very abundant due to large amounts of silicon and oxygen in Earth’s crust • Basic building block is the silicon-oxygen tetrahedron molecule • Four oxygen ions surrounding a much smaller silicon ion

35. The Component Atoms Silicon has 4 electrons in Its outer shell Oxygen has 6 electrons in its valence shell

36. Remember: atoms can gain or lose electronsThey then combine with oppositely charged ions to form neutral molecules Ions Anion (negative) Cation (positive)

37. 2_25 The Silicate Tetrahedron O2 - Si4+ O2 - O2 - The basis of most rock-forming minerals, charge - 4 O2 -

38. Silicate Bonding I • Oxygen Oatoms may obtain electrons from Si atoms, producing the SiO4 -4 Ion. • The negative charge is balanced by positive metal ions. • This occurs in Olivine, (Fe,Mg)2SiO4, a high temperature Fe-Mg silicate. Forms of this mineral are stable 100’s of kilometers below Earth’s surface. • Sort of Ionic Bond

39. Positive ion Tetrahedron facing down Tetrahedron facing up Fe and Mg Example OLIVINE SiO4-4 Ion Independent tetrahedra

40. Silicate Bonding II • Alternately, the oxygen atoms may complete their outer electron shells by sharing electrons with two Silicon atoms in nearby silicon tetrahedra. • A sort of covalent bond

41. A Pyroxene Single chains weakly paired Two good cleavages at ~90o, non metallic, dark green to black, H ~ 6

42. 2_26c An Amphibole Positive ion Cleavages 56 and 124 deg Double chains (c)

43. Example: Mica One excellent cleavage plane Sheet silicates (d)

44. note sheet structure Clay Minerals(at high magnification) Very small crystals Kaolinite(hand specimen) Clays are also Sheet Silicates, just as Micas are Vietnam Anecdote

45. Example: Quartz SiO2 2_26e Framework silicates (e) (3-D, also the Feldspars)

46. Classification of Minerals More 3-D Framework Minerals: Feldspars • Common Silicate minerals • Feldspar Group • Most common mineral group • two directions of perfect cleavage at 90 degrees • In Feldspars, some of the Silicon atoms (oxidation state +4) are replaced by Aluminum (oxidation state +3) • Ion is not symmetrical • Pearly Luster A Potassium Feldspar

47. A Potassium Feldspar Here is a flattened model of a 3-D framework Potassium Feldspar, similar to Orthoclase KAlSi3O8 . In some of the Tetrahedra an Aluminum ion Al+3, is in the center instead of Silicon. An additional Potassium, K+, completes the charge balance.

48. Feldspars that use Calcium (Ca) or Sodium (Na) metals to balance the SiO4- 4 and AlO4-5 charges are called: Plagioclase feldspar Note the Twinning, seems to have ‘stripes’

49. Summary

50. Rocks are made of minerals. Many are silicate minerals. This granite, an igneous rock, has Quartz, an amphibole called Hornblende, a pink potassium feldspar, and a white Plagioclase feldspar