lecture 02 atoms and minerals u.ppt
TRANSCRIPT
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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 shellsNumber of outermost electrons determine types of bonding
ArgonOutermost (Valence) shell
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Some definitions:Some definitions:
• Atomic number:Atomic number: number of number of protons in the nucleusprotons in the nucleus
• Atomic Mass: Atomic Mass: total mass of protons and neutrons within an atom’s nucleus
• We can see these on a Periodic Table
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Periodic Table of the Elements
# protons (+) equals # electrons (-)Electrons in shellsNumber 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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Atoms larger than Hydrogen and Helium need 8 electrons in their outer shell for stabilityOctet Rule:
Electrons are in shells.
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Neutral Atoms have #protons = # electrons
Oxygen has6 electrons in its valence shell
Silicon has 4 electrons inIts outer shell
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To satisfy the octet rule atoms can gain or lose electronsIn that state they are called IONS
They can combine with oppositely charged ions to form neutral molecules
IonsOxygen, 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-
Na+
Na+
Na+
Na+
Na+
Cl-
Cl-Cl-
Crystalline structure ofNaCl
(a)Small Sodium ions between large Chlorine ions
Internal atomic arrangement is primarily determined by the size of
ions involved
Table Salt
Halite (NaCl)- An Example of Ionic Bonding
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Covalent bonding – sharing of valence electrons
Cl2 Chlorine gasSharing Electrons in Outermost Shell
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Covalent Bonds in Water
H2OWater 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• 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
Streak
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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
BiotiteMica
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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 Earth’s 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. Fe+2 is name Ferrous, Fe+3 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 Earth’s 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
Oxygen has6 electrons in its valence shell
Silicon has 4 electrons inIts outer 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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O2 -
O2 -
O2 -
O2 -
Si4+
2_25The Silicon-Oxygen Tetrahedron
The basis of most rock-forming minerals, charge - 4
Silicate Molecule
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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 100’s of kilometers below Earth’s surface.
• Sort of Ionic Bond
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Tetrahedronfacing down
Tetrahedronfacing up
Positive ionExample OLIVINE
Independent tetrahedra
Fe and Mg
SiO4 -4 Ion
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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
Positiveion
Double chains(c)
An Amphibole
Cleavages 56 and 124 deg
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Sheet silicates(d)
Example: Mica
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Clay Minerals(at high magnification)
Kaolinite(hand specimen)
note sheet structure
Clays are also Sheet Silicates, just as Micas are
Vietnam Anecdote
Very small crystals
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2_26e
Framework silicates(e)
Example: Quartz SiO2
(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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Plagioclase feldsparNote the Twinning, seems to have ‘stripes’
Feldspars that use Calcium (Ca) or Sodium (Na) metals to balance the SiO4 - 4 and AlO4 -5 charges are called:
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Summary