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Atoms and Minerals Magnet and Iron and slide

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1 Atoms and Minerals Magnet and Iron and slide
Quartz SiO2 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 Electrons
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 Column shows # electrons in outermost shell
Periodic Table of the Elements # protons (+) equals # electrons (-) Electrons in shells Number of outermost electrons determine types of bonding 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 electrons In that state they are called IONS They 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
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 small Na+ large Cl-
Halite (NaCl)- An Example of Ionic Bonding Table Salt Na+ Cl- Na+ Cl- Na+ Cl- Na+ Na+ Halite small Na+ large Cl- 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
Cl2 Chlorine gas Sharing Electrons in Outermost Shell

13 Covalent Bonds in Water
H2O Water is polar

14 Other Bond Types Metallic bonding Intermolecular bonding
Valence electrons are free to migrate among atoms Weaker and less common than ionic or covalent bonds Intermolecular bonding Hydrogen bonds- charged regions in water attract Van der Waals bonds- electrons momentarily grouped on same side of nucleus

15 Isotopes Isotopes and radioactive decay
Atomic mass is the total mass of neutrons plus protons in an atom An isotope is an atom that exhibits variation in its atomic mass, i.e. different numbers of neutrons Some isotopes have unstable nuclei that emit particles and energy in a process known as radioactive decay. 12C 13C stable 14C radioactive

16 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

17 Diamond and graphite – polymorphs of carbon

18 Physical properties of minerals
Crystal Form External expression of the orderly internal arrangement of atoms The mineral garnet often exhibits good crystal form

19 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)

20 Galena PbS displays metallic luster
Valuable ore of Lead

21 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

22 Physical properties of minerals
Streak Streak Color of a mineral in its powdered form Helpful in distinguishing different minerals with similar composition Hardness Resistance of a mineral to abrasion or scratching All minerals are compared to a standard scale called the Mohs scale of hardness


24 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 perfect cleavage Biotite Mica

25 Three directions of perfect cleavage – fluorite, halite, and calcite
Each Cleavage Plane is paired

26 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.

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

28 Is it calcite or dolomite?

29 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

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

31 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

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

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

34 Silicate Molecule The Silicon-Oxygen O2 - Tetrahedron Si4+ O2 - O2 -
2_25 O2 - Si4+ O2 - O2 - The basis of most rock-forming minerals, charge - 4 O2 -

35 Silicate Bonding I Oxygen O atoms 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

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

37 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

38 Single chains weakly paired
A Pyroxene Single chains weakly paired

39 An Amphibole Cleavages 56 and 124 deg Positive ion Double chains (c)

40 Example: Mica Sheet silicates (d)

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

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

43 Classification of Minerals
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

44 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’

45 Summary

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