Atoms and Minerals

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Atoms and Minerals
Magnet and Iron and slide

Quartz SiO2 common mineral
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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

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• 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

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Flattened structure of an atom
protons () equals electrons (-) Electrons in
shells Number of outermost electrons determine
types of bonding
Argon
Outermost (Valence) shell
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Some definitions

• Atomic number number of protons in the nucleus
• Atomic Mass total mass of protons and neutrons
  within an atoms nucleus
• We can see these on a Periodic Table

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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
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Electrons are in shells.
Octet Rule
Atoms larger than Hydrogen and Helium need 8
electrons in their outer shell for stability
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Neutral Atoms have protons electrons
Silicon has 4 electrons in Its outer shell
Oxygen has 6 electrons in its valence shell
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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
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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)

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Halite small Na large Cl-
Halite (NaCl)- An Example of Ionic Bonding
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)
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Covalent bonding sharing of valence electrons
Cl2 Chlorine gas
Sharing Electrons in Outermost Shell
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Covalent Bonds in Water
H2O
Water is polar
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Other Bond Types

• Metallic 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

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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

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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

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Diamond and graphite polymorphs of carbon
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Physical properties of minerals

• Crystal Form
• External expression of the orderly internal
  arrangement of atoms

The mineral garnet often exhibits good crystal
form
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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)

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Galena PbS displays metallic luster
Valuable ore of Lead
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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
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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

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• 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
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Three directions of perfect cleavage fluorite,
halite, and calcite
Each Cleavage Plane is paired
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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.

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Physical properties of minerals

• Other properties
• Magnetism
• Reaction to hydrochloric acid
• Malleability
• Double refraction
• Taste
• Smell
• Elasticity

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Is it calcite or dolomite?
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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
  Earths crust
• Only a few dozen members
• Composed mainly of the 8 elements that make up
  98 of the continental crust

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Commonly formed Ion chargesoften called
oxidation state
Metals can form more than one Ion. Fe2 is name
Ferrous, Fe3 is named Ferric
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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 Earths crust
• Basic building block is the silicon-oxygen
  tetrahedron molecule
• Four oxygen ions surrounding a much smaller
  silicon ion

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The Component Atoms
Silicon has 4 electrons in Its outer shell
Oxygen has 6 electrons in its valence shell
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Remember atoms can gain or lose electronsThey
then combine with oppositely charged ions to form
neutral molecules
Ions
Anion (negative)
Cation (positive)
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Silicate Molecule
2_25
The Silicon-Oxygen Tetrahedron
O2 -
Si4
O2 -
O2 -
The basis of most rock-forming minerals, charge
- 4
O2 -
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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 100s of kilometers below Earths
  surface.
• Sort of Ionic Bond

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Fe and Mg
Example OLIVINE
SiO4 -4 Ion
Independent tetrahedra
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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

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A Pyroxene
Single chains weakly paired
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2_26c
An Amphibole
Positive ion
Cleavages 56 and 124 deg
Double chains (c)
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Example Mica
Sheet silicates (d)
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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
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2_26e
Example Quartz SiO2
Framework silicates (e)
(3-D, also the Feldspars)
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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
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Feldspar
Aluminum atoms (yellow) with nearby Sodium atoms
(green) to balance charge
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Plagioclase feldspar
Feldspars that use Calcium (Ca) or Sodium (Na)
metals to balance the SiO4 - 4 and AlO4 -5
charges are called
Note the Twinning, seems to have stripes
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Summary
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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
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End of Minerals Lecture

• The remainder of the slides following this one
  comprise a picture gallery of minerals. Read them
  at home.
• You will find these extra slides a useful
  supplement to the lecture slides, as a review of
  the minerals we will learn.
• The identification of common minerals is an
  important skill for the field geologist. Extra
  credit questions after each test will include
  rock and mineral identification, and map skills.

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End Minerals

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Silicate Mineral Examples
Mica
Feldspar
Olivine
Quartz
Pyroxene
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Classification of Minerals

• Common Silicate minerals
• Olivine
• High temperature Fe-Mg silicate (typical mantle
  mineral - formed 100s km in Earth
• Individual tetrahedra linked together by iron and
  magnesium ions
• Forms small, rounded crystals with no cleavage

(Mg,Fe)2SiO4
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Classification of Minerals

• Common Silicate minerals
• Pyroxene Group for example (Mg,Fe)SiO3
• Single chain structures involving iron and
  magnesium, chains weakly paired
• Two distinctive cleavages at nearly 90 degrees
• Augite is the most common mineral in the pyroxene
  group

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Classification of Minerals

• Common Silicate minerals
• Amphibole Group Ca2(Fe,Mg)5Si8O22(OH)2
• Double chain structures involving a variety of
  ions
• Two perfect cleavages exhibiting angles of 124
  and 56 degrees
• Hornblende is the most common mineral in the
  amphibole group

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Hornblende Crystal56 and 124 degreeCleavages
Distinguish Hornblende from Pyroxene Group by
cleavage
Pyroxene CrystalTwo Cleavage Faces at about 90
degrees
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Classification of Minerals

• Common Silicate minerals
• Mica Group
• Sheet structures that result in one direction of
  perfect cleavage
• Biotite is the common dark colored mica mineral
• Muscovite is the common light colored mica
  mineral

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Muscovite
KAl3Si3O10(OH)2
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Classification of Minerals

• Common Silicate minerals
• Feldspar Group
• Most common mineral group
• 3-dimensional framework of tetrahedra exhibit two
  directions of perfect cleavage at 90 degrees
• Orthoclase (potassium feldspar) and Plagioclase
  (sodium and calcium feldspar) are the two most
  common members
• Pearly Luster

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Potassium feldspar
KAlSi3O8
Note Pearly Luster
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Plagioclase feldspar
(Ca,Na)AlSi3O8
Note the Twinning, seems to have stripes
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Classification of Minerals

• Common Silicate minerals
• Clay minerals
• Clay is a general term used to describe a variety
  of complex minerals
• Clay minerals all have a sheet or layered
  structure
• Most originate as products of chemical weathering

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Clay Minerals(at high magnification)
note sheet structure

Kaolinite(hand specimen)
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Classification of Minerals

• Important non-silicate minerals
• Many non-silicate minerals have economic value
• Examples
• Hematite (oxide mined for iron ore)
• Halite (halide mined for salt)
• Sphalerite (sulfide mined for zinc ore)
• Native Copper (native element mined for copper)

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Nonsilicate Mineral Examples
Spinel (Oxide)
Halite (Halide)
Gypsum (Sulfate)
Hematite (Oxide)
Calcite (Carbonate)
Pyrite (Sulfide)
Galena (Sulfide)
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Classification of Minerals

• Important non-silicate minerals
• Carbonates
• Primary constituents in limestone and dolostone
• Calcite (calcium carbonate) and Dolomite
  (calcium-magnesium carbonate) are the two most
  important carbonate minerals

Calcite showing cleavage faces
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Classification of Minerals

• Important non-silicate minerals
• Several major groups exist including
• Oxides
• Sulfides
• Sulfates
• Native Elements
• Carbonates
• Halides
• Phosphates

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Hematite, an oxide, Fe2O3
An important Iron ore
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Galena, PbS, a Sulfide

• An important ore of Lead

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Gypsum, a SulfateCaSO4.2H2O

• An important evaporite mineral

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Native Copper Cu