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Transcript
Atoms, Elements, and Minerals
Chapter 2
Lauterbrunnen valley
Switzerland
Bryce Canyon
Utah
Rocks in the Earth's crust and mantle are made up of mineral assemblages with
chemical compounds, elements, molecular bonds which are formed from
ordered atomic structures. Where would you rather rock climb ? Why ?
Atoms and Elements
An element is a substance that can not be
broken down into others by ordinary chemical
reactions
An atom is a chemical unit that cannot be
broken down by chemical means composed of:
• Protons (positively charged)
• Neutrons (zero net charge)
• Electrons (negatively charged)
A molecule is made of 2 or
more atoms bonded together.
Atomic Structure
• Protons and neutrons form the nucleus
– Represents tiny fraction of the volume at the
center of an atom, but nearly all of the mass
• Electrons orbit the nucleus in discrete
shells or energy levels
– Shells represent nearly all of the volume of
an atom, but only a tiny fraction of the mass
– Numbers of electrons and protons are equal
in a neutral atom
– Chemical reactions involve outer shell
(valence) electrons
Chemical Bonding
Chemical bonding is controlled by outermost shell (valence) electrons.
Vacancies allow reactions.
• Ionic bonding
– Involves transfer of valence
electrons from one atom to
another
• Covalent bonding
– Involves sharing of valence
electrons among adjacent atoms
• Metallic bonding
– Electrons flow freely throughout
metals; results in high electrical
conductivity
Ionic bonding of NaCl (sodium chloride)
Four types of bonding
Ionic bonding
Halite
Figure 2.3
© 2008, John Wiley and Sons, Inc.
When one atom transfers an
electron to another.
Four types of bonding
Ionic bonding
Halite
Figure 2.3
© 2008, John Wiley and Sons, Inc.
When one atom transfers an
electron to another.
Halite
© 2008, John Wiley and Sons, Inc.
Four types of bonding
Covalent Bonding
Diamond
Figure 2.3
© 2008, John Wiley and Sons, Inc.
When electrons from different
atoms “pair up” or are shared.
Diamonds:
made of carbon atoms
connected by covalent bonds
© 2008, John Wiley and Sons, Inc.
Four types of bonding
Gold (Au)
Metallic Bonding
In metals, atoms are so tightly packed
that electrons can be shared among
several atoms. Here each atom is in
contact with 12 other gold atoms.
Outermost electrons are loosely held
and drift easily – allowing for high
heat and electrical conductivity.
Gold
This nugget was embedded in a rock, but weathering
and erosion removed most of the rock. (Discovery in
southern California.
Four types of bonding
Graphite
Van der Waals Bonds
A weak attraction that occurs between
neutral molecules that have asymetrica
charge distribution. (The positive end
of one molecule is attracted to the
negative end of another)
Graphite
© 2008, John Wiley and Sons, Inc.
Covalent Bonds in Diamonds and Graphite
Graphite
Diamonds
-Diamond and graphite are both made of carbon (C), but one is the
hardest substance on Earth and the other very soft.
Composition of Earth’s Crust
• Common elements
– Most common elements in Earth's
crust
• O, Si, Al, Fe, Ca, Na, K, Mg
• Common mineral types
– Most minerals are silicates
(contain Si and O)
Silicate Structures
• The Silicon-Oxygen tetrahedron
– Strongly bonded silicate ion
– Basic structure for silicate minerals
Silicate links
Silicate minerals consist of (SiO4)4anionic groupings linked together.
© 2008, John Wiley and Sons, Inc.
Atomic Structures Activity
- Organize into groups (10 or less)
- Write everyone's name on one piece of paper
- Examine your bag of molecules
- When you are told “GO”:
– Assemble each molecule below
– The first group to CORRECTLY assemble
All 3 molecules wins!
- Turn in the group sheets
(Hint: remember that molecules are 3-dimensional)
The molecules are........
1. Water
2. Table salt
3. Silica tetrahedra
Examples of Minerals
Halite (NaCl)
-Table salt
Gold (Au)
Olivine (MgSiO4)
-Minerals have a chemical formula.
What is a Mineral ?
A mineral is:
-naturally formed,
-inorganic substance,
-solid,
-crystalline,
-chemically distinct
Where do Minerals Form ?
•
•
•
•
Geosphere (most minerals)
Hydrosphere (e.g., halite)
Biosphere (e.g., calcite)
Atmosphere (water ice, snow)
foraminifera
© 2008, John Wiley and Sons, Inc.
The Mineral Ice
Ice occurs in nature and has a
specific chemical formula (H2O).
© 2008, John Wiley and Sons, Inc.
Is water a mineral?
Is this crocodile bone a mineral?
How about this crocodile skull
which has been fossilized?
© 2008, John Wiley and Sons, Inc.
Would you call coal a mineral?
Why or why not?
Steel in a processing plant.
Is steel a mineral?
© 2008, John Wiley and Sons, Inc.
How about quartz?
© 2008, John Wiley and Sons, Inc.
Mineral Properties
Physical and chemical properties of minerals are closely
linked to their atomic structures and compositions
• Hardness
- Scratch-resistance
• Cleavage
– Breakage planes
• Streak
– Color left behind when mineral
is scraped on unglazed porcelain
• Luster
– Manner in which light reflects
off surface of a mineral
• Color
– Visible hue of a mineral
? Mystery Mineral ?
Geologists have several “low tech” methods to
identify unknown minerals in the field...
© 2008, John Wiley and Sons, Inc.
Mohs' Hardness Scale
Gypsum
Quartz
Diamond
Crystal Faces and Angles
Crystal faces can have different surface area but will retain
angle
between
surfaces.
© 2008, the
John Wiley
and Sons,
Inc.
Crystal Faces and Angles
Asbestos fibers have distinct growth habits. Crystals grow in long,
cotton-like threads. Can be woven into fireproof fabric, also difficult
to
Mineral cleavage is different from crystal growth !
Cleavage:
- Planar surfaces left from a freshly broken surface
- Rocks break along defects or planes of weakness
Mineral cleavage
Halite has cubic cleavage
Try breaking it!
Color Streak
Hematite is a grayish silver mineral
but produces a red streak (high iron content)
© 2008, John Wiley and Sons, Inc.
Mineral Luster
Luster is how light
reflects off a surface
Vitreous
Quartz (SiO2)
Resinous
Sphalerite (ZnS)
Pearly
Talc (MgSi4O10(OH2))
What type of luster do you see in the mystery mineral ?
Our mystery mineral has a metallic luster.
Color Only can be Deceiving
Corundum (Al2O3) has different colors:
Ruby: red from substitution of Cr3 for Al3
High iron and titanium content
© 2008, John Wiley and Sons,Sapphire:
Inc.
Magnetite Characteristics