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Atoms_ Elements_ _ Minerals


									       Atoms, Elements, & Minerals

Tourmaline – elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4
Atoms, Elements, & Bonding
Elemental Composition of the Earth’s Crust
Crystallinity – the Silicon-Oxygen Tetrahedron
  • Crystalline Solids
  • The Important Minerals
  • Physical Properties of Minerals
Rocks versus Minerals
                             Rock: an aggregate of one or
                             more minerals; in this case there
                        Fig. 2.1
                             are four minerals present.


          Mineral Structures and Atoms
Atomic scale

1 angstrom = 1.0 × 10-10 meters

      Mineral: A naturally occurring inorganic element
      or compound having orderly internal structure and
      characteristic chemical composition, crystal form,
      and physical properties.
                                                   From: Plummer et al.
   Atoms, Elements, & The Periodic Table

• Atoms are the smallest division of matter that
  retain the characteristics of the elements.
• There are 92 naturally occurring elements.
• The modern Periodic Table was devised in 1869
  by Julius Meyer and Dmitri Mendeleev. It
  organizes the elements into groups and families
  with similar chemical and physical properties.

          Groups or Families

                Periodic Table
                 Box 2.2 Tbl. 01

Crustal volume:<1% of Earth      Crustal mass:<1% of Earth
Mantle volume: 83% of Earth      Mantle mass: 68% of Earth
Core volume: 16% of Earth        Core mass: 31% of Earth
  What element is most abundant by mass for the entire Earth?
          Atomic Particles: Basics

• Atoms are composed of electrons and two large
  nuclear particles called protons and neutrons.
• Protons and neutrons are approximately equal in
  mass and are ~1800 times more massive than the
  electron. Both nuclear particles are composed of
  quarks, smaller fundamental particles.
• Protons have unit positive charge (+1), while
  electrons have unit negative charge (-1). Neutrons
  carry no charge.
• Atoms are electrically neutral and thus the
  number of electrons must equal the number of
                Basic Terminology
• Atomic number (Z): The atomic number represents the
  number of unit positive charges on the nucleus and is
  equal to the number of protons within the nucleus, since
  each proton carries one unit positive charge. In electrically
  neutral atoms, it also represents the number of electrons,
  which carry unit negative charge.

• Mass number (A): The mass number is equal to the total
  number of nucleons, which is the sum of the number of
  protons and neutrons. A does not equal the total mass of
  the atom; rather, it represents a whole number
  approximation of the mass, as expressed in amu.

• The number of neutrons is given by A - Z.
• Isotopes are atoms of an element with different
  numbers of neutrons
• chemical properties are the same
• may be stable or unstable
   – stable isotopes retain all of their protons and
      neutrons through time (e.g. 18O & 16O)
   – unstable or radioactive isotopes
      spontaneously lose subatomic particles from
      their nuclei over time (e.g. 238U & 235U)
• stable isotopes can be used to track climate
  change over time
• unstable isotopes can be used to date the ages
  of rocks
                  Atomic Weight

• Atomic weight is the weighted average of the atomic
  masses of the naturally occurring isotopes. For example,
  a natural sample of the element chlorine contains a mixture
  of 75.53% 35Cl and 24.47% 37Cl. Thus the atomic weight is
  obtained by multiplying the mass of each isotope (in amu)
  times its fractional abundance:

• 0.7553 (34.97 amu) + 0.2447 (36.95 amu) = 35.45 amu
               Atomic Models

• Bohr Model
  – Electron shells

• Quantum Mechanics
  – Energy Levels
  – Orbitals
  – Aufbau Filling Order
                         Bohr Atomic Model

                                                          Oxygen Atom

The Bohr model for the atom envisioned these electrons in stable orbits of specified
radius and energy, where we could exactly pinpoint the position of any individual
electron. Each energy level was permitted to have a specified number of electrons, and
was called a shell. We know now that this simple view is not correct; it is impossible
to simultaneously determine the position and velocity of an electron accurately.
         The Quantum Mechanical View
•   Using the theory of quantum or wave mechanics we can calculate the
    probabilities of various electron configurations, and thus show that
    specified regions near the nucleus have higher probabilities for finding
    an electron than others. Each electron does, however, have a specific

•   The combination of the energy and probability gives rise to the current
    understanding for electron distributions, which are referred to as
    electron orbitals; these orbitals are referred to as s (sharp), p
    (principal), d (diffuse), and f (fundamental).

•   With increasing atomic number, each new element has an additional
    electron also added to its extra-nuclear cloud. From theory and
    experiment, we know that these electrons are added in a systematic
    fashion, with the lowest energy orbitals being filled first.

•   This process is called aufbau filling (1s -> 2s -> 2p -> 3s -> 3p -> 4s -
    > 3d -> 4p -> 5s, etc. ).

           Compounds and Bonding
• Chemical compounds
• Ionic bonding
   – NaCl as a type example
   – Electron transfer and shell completion
• Covalent bonding
   – Diamond - pure C example
   – Electron sharing
• Silicon Tetrahedron
   – Structure (strong sp3 covalent bonds)
   – Building block of silicate minerals
 Chemical bonding
• controlled by outermost level                            1s2
  (valence) electrons
• elements want to have full
  outer energy levels and will
  seek to fill them
• atoms or groups of atoms with 1s22s22p6
  unequal numbers of protons and
  electrons have a non-zero
  charge, (ions)
• positive and negative ions are
  attracted to one another and
  may stick or chemically bond
                                     Outer Level Filling
Ionic Bonding and Electron Exchange

                             11p+ + 10e- = +1




                             17p+ + 18e- = -1
 Ionic Bonding:Electron Transfer

Cations (+) are always smaller than the neutral atom;
Anions (-) are always larger than the neutral atom.
 Chemical compound: Two or more elements joined together by
 a chemical bond. Most minerals are composed of at least two
                                   Chemical Formula: NaCl

Note Cubic Symmetry and Closest Packing
                                            Halite Atomic Structure
              Fig. 2.9

Ionic Radii
 Covalent Bonding: Electron Sharing

Diamond Example - Pure Carbon in complex 3D network
Covalent Bonding: Electron Sharing

                 Diamond Example - Pure Carbon
                 in complex 3D network
Silicon Tetrahedron: SiO4 (net -4 charge)

                         1 angstrom = 1.0 × 10-10 meters
                1.30 Å

                0.34 Å
             Silicate Mineral Structures
Silcon Tetrahedron:
strong sp3 hybrid covalent
bonds - 50% ionic; 50%
covalent character

These structures are the
basic building blocks of
silicate minerals.
      Bridging Oxygen (BO)
Silicate structures may be
Characterized by the number of
BO’s per Si. The higher the BO/Si
Ratio, the more complex and
polymerized the structure.
Silicate Structures
Olivine Structure: (Mg,Fe)2SiO4
Models of Chain Silicates: Pyroxenes
 Composition of the Earth’s crust: minerals
• over 4000 minerals have been
• only a few hundred are common
  (rock-forming minerals)
• over 90% of Earth’s crust is
  composed of minerals from only
  5 groups
Non-silicate minerals
•   carbonates
     – contain CO3 in their structures (e.g., calcite - CaCO3)
•   sulfates
     – contain SO4 in their structures (e.g., gypsum - CaSO4. 2H2O)
•   sulfides
     – contain S (but no O) in their structures (e.g., pyrite - FeS2)
•   oxides
     – contain O, but not bonded to Si, C or S (e.g., hematite - Fe2O3)
•   native elements
     – composed entirely of one element (e.g., diamond - C; gold - Au)

calcite                                  gypsum
Ore minerals
• minerals of commercial value cost of extraction vs. price of metals
• most are non-silicates (primary source of metals)
        • examples: magnetite and hematite (iron), chalcopyrite (copper),
          galena (lead), sphalerite (zinc)

              PbS                                 Sphalerite

                         Hematite                                Chalcopyrite
                         Fe2O3                                   CuFeS
Minerals of Arkansas
 quartz -- Ouachita Mtns.

 diamonds -- Murfreesboro: Crater of Diamonds State Park

 wavellite -- “Cat’s Eye” for radiating pattern in mineral
                         Saline and Montgomery Counties

                            (hydrated aluminum phosphate hydroxide)

Magnet Cove, Arkansas: over 40 minerals in one square mile
 (one of only a few sites like it in the world)
       --- compasses go haywire ---
    Minerals II: Physical Properties and Crystal Forms

Physical properties of minerals; use to identify
    linked to their atomic structures and compositions

             •   color
                  – visible hue of a mineral
             •   streak
                  – color left behind when mineral is
                     scraped on unglazed porcelain
             •   luster
                  – manner in which light reflects off
                     surface of a mineral (e.g. metallic,
             •   hardness
                  – scratch-resistance (scratched by
                     fingernail, knife, etc.)
             •   crystal form
                  – external geometric form
•   cleavage
     – planes of weakness in crystal
     – systematic and diagnostic
•   fracture
     – irregular breakage
•   specific gravity
     – density relative to that of water
     – mafic minerals: Mg, Fe, dark, 3.2-3.6 g/cc
                   (olivine, pyroxene, amphibole)
     - felsic minerals: K, Al, light, ~2.7 g/cc
                   (feldspars, quartz)
     - Galena – 7.5 Gold 19.3
•   magnetism
     – attracted to magnet
•   chemical reaction
     – calcite (CaCO3) “fizzes” in dilute HCl
                 Physical Properties
 Color    - Although an obvious feature, it is often unreliable
   to use to determine the type of mineral.
    – Color arises due to electronic transitions, often of trace
       constituents, in the visible range of the EM spectrum.
       For example, quartz is found in a variety of colors

Hope Diamond:
  44.5 carats

             Physical Properties
Streak - The color of a mineral in its
  powdered form; obtained by rubbing the
  mineral against an unglazed porcelain plate.
  – Streak is usually less variable than color.
  – Useful for distinguishing between minerals
    with metallic luster.
            Physical Properties

Luster - This property describes the
  appearance of reflected light from the
  mineral's surface.
• Metallic
• Nonmetallic: vitreous, pearly, silky,
  resinous, and earthy.
              Physical Properties

Hardness - This is the resistance of the mineral to
 abrasion or scratching. This property doesn't vary
 greatly from sample to sample of the same
 mineral, and thus is highly diagnostic. It also is a
 direct reflection of the bonding type and internal
 atomic arrangement. A value is obtained by
 comparing the mineral to a standard scale devised
 by Moh, which is comprised of 10 minerals
 ranging in hardness from talc (softest) to
 diamond (hardest).
Hardness Scale

  Fingernail Hardness (2.5)
   Scratches Gypsum (2)
      Polymorphism and polymorphs

• Substances having the same chemical composition
  but different crystal structures.

   – e.g. diamond and graphite

• Both minerals are composed of pure carbon, but
  diamond is the high pressure polymorph of
• This gives rise to extremely different physical

                         3 mm

           Natural Octahedral Diamond                 Graphite & Calcite
Diamond vs. Graphite Crystal Structures

  Hardness: 10                      Hardness: 1-2
             Physical Properties

Crystal form or habit - The external
  morphology of crystals generally reflect the
  internal arrangement of their constituent
  atoms. This can be obscured, however, if the
  mineral crystallized in an environment that did
  not allow it to grow without significant
  interaction with other crystals (even of the
  same mineral).
                                      Chrysotile Asbestos

Belongs to the Serpentine mineral family -
hydrated ferromagnesian silicate.
Crystal Forms: Quartz
Crystal Forms: Feldspar
Intergrown cubic crystals of fluorite
               Quartz Interfacial Angles

Perfectly                                                 Misshapen
Proportioned                                              Crystals
  Steno’s Law (1669): Crystal face internal angles remain constant!
  Macroscopic Forms and Microscopic Blocks


                                    Crystal Forms

         Unit Cells and Crystal Structure
Cubic unit cell:
smallest repeatable unit
              Physical Properties

Cleavage - Orientation and number of planes of
  weakness within a mineral. Directly reflects the
  orientation of weak bonds within the crystal
  structure. This feature is also highly diagnostic.

Fracture - This describes how a mineral breaks if it
  is not along well defined planes. In minerals with
  low symmetry and highly interconnected atomic
  networks, irregular fracture is common.
Planer Cleavage in Mica
Weak Bonding Yields Planer Cleavage
Amphibole Cleavage ~120/60°
Rhombohedral Cleavage in Calcite
Conchoidal Fracture in Glass
                       Color and Density
•   Two broad categories are ferromagnesian and nonferromagnesian
    silicates, which simply means iron and magnesium bearing or not. The
    presence or absence of Fe and Mg strongly affects the external appearance
    (color) and density of the minerals.
•    Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc
    – Olivine - high T, low silica rocks; comprises over 50% of upper mantle
    – Pyroxenes - high T, low silica rocks
    – Amphiboles - esp. hornblende; moderate T, higher silica rocks
    – Mica - esp. biotite; moderate T, higher silica rocks
    – Garnet - common metamorphic mineral
•    Nonferromagnesian silicates - light color, density close to 2.7 g/cc
    – Mica - exp. muscovite; moderate T, higher silica rocks
    – Feldspars - plagioclase and orthoclase; most common mineral in crust;
        form over a wide range of temperatures and melt compositions
    – Quartz - low T, high silica rocks; extremely stable at surface, hence it
        tends to be a major component in sedimentary rocks.
    – Clay - esp. kaolinite; different types found in different soils
    Special and Other Properties
Striations - Commonly found on plagioclase feldspar.
   Straight, parallel lines on one or more of the cleavage
   planes caused by mineral twinning.
Magnetism - Property of a substance such that it will
   spontaneous orient itself within a magnetic field.
   Magnetite (Fe3O4) has this property and it can be used to
   distinguish it from other non-magnetite iron oxides, such
   as hematite (Fe2O3).
Double Refraction - Seen in calcite crystals. Light is split
   or refracted into two components giving rise to two
   distinct images.
Chemical reactions – e.g. Calcite effervescence in HCL-
Calcite Double Refraction
               Water, Ice and Snow
•   Arguably the most important substance on Earth
•   Essential for biological life as we know it
•   Unique volumetric property
•   Molecular symmetry and its relationship to
    crystal morphology

          Chemical Formula: H2O

                        Water Atomic Structure
                  Snowflake Morphology
                                                    Hexagonal Symmetry

Oddly, ice is less dense than liquid                       1 - 5 mm
water, hence it floats and lakes
freeze from the top down!         From:
Snowflake Growth

LT-SEM Images of Snow Crystals

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