Transcript minerals & magma

Origins of Rocks:
Where do rocks come from?
Why do they form?
How do they form?
(Mineralology and
Petrology)
(L2 & L3)
What is a rock?
A naturally-occurring
aggregate of minerals
 CD: EM/A,B
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What is a Mineral?
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A naturally occurring, inorganic,
crystalline solid with a specific chemical
composition
A specific chemical composition means
that the composition of a mineral can be
expressed as a chemical formula
Example: halite (salt) is NaCl
Identify Minerals by Their Physical Properties
[See CD: Earth Materials]
Crystal Habit
Cleavage &
Fracture
* Striations
Hardness
Specific Gravity
Color
Streak
Luster
Acid Test
What is a Crystal?
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A form of matter which has a
regular, repeating framework of
atoms
Halite (salt) crystals (NaCl)
What are atoms?
[CD:Slides 1094-1105 in Geo Time]
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The smallest unit of an element that retains the
element’s physical and chemical properties
 An element is a substance that cannot be
broken down into a simpler substance
 Made up of protons, neutrons and electrons
 examples: gold, iron, hydrogen, oxygen,
sodium, chlorine, carbon, silicon, helium
Protons, Neutrons, Electrons
Protons have positive charge & mass
of 1
 Neutrons have no charge & mass of 1
 Protons and neutrons make up the
nucleus of an atom
 Electrons have a negative charge, NO
mass and are most important for
chemical reactions
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Structure
of an
Atom
Protons +/1
Neutrons 0/1
Electrons -/0
IONS & ISOTOPES
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IONS
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+/- CHARGED ATOMS
FORMERLY NEUTRAL
ATOMS WHICH HAVE
GIVEN UP (+) OR
TAKEN ON AN
ELECTRON (-)
INVOLVED IN IONIC
BONDING
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ISOTOPES
ELEMENTS WITH
VARIABLE ATOMIC
WEIGHT DUE TO
VARYING
NUMBERS OF
NEUTRONS IN THE
NUCLEUS.
Hydrogen atom:
Mass = 2
Carbon atom: Mass = 12
6 protons, 6 neutrons, 6 electrons
1 proton, 1 electron
Electrons move in orbitals (not “orbits!”) forming
shell-like spheres around the nucleus.
** If atoms loose or gain electrons they have an
electrical charge (-/+) and are called ions; also, a
group of atoms (molecule) can behave as an ion if
they have too many or too few electrons.
What holds the atoms together
in crystals?
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Chemical bonds:
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Ionic Bonds
 atoms (ions) held together by their
electrical charges
 weaker kind of chemical bond
Covalent Bonds
 atoms held together because they share
electrons
 stronger kind of chemical bond
Example of an ionic bond:
NaCl
Example of covalent
bonding: Carbon
Naturally occurring
crystalline carbon is
the mineral diamond
Covalent bonding
between the carbon
atoms make this the
hardest known
substance
WHAT FACTORS DETERMINE WHAT
MINERAL WILL FORM?
1 WHAT ELEMENTS ARE AVAILABLE
2 ABUNDANCE OF EACH ELEMENT
3 SIZE & CHARGE OF EACH ELEMENT
4 ENVIRONMENT - SPACE, PRESSURE,
TEMPERATURE
Ionic Radii &
charge of some
geologically
important ions
allows “ionic
substitution” or
SOLID
SOLUTION
Average Composition of the
Continental Crust
O
O
Si
Weight Percent
Volume Percent
COMMON ROCK-FORMING
MINERAL FAMILIES
NATIVE ELEMENTS - GOLD,
COPPER
HALIDES - PRECIPITATES (salts)
OXIDES - 1 METAL + OXYGEN
SULFIDES - 1 METAL + SULFUR
SULFATES - (SO4)2CARBONATES - (CO3)2SILICATES - SUPER STARS!!!
Silicates
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The largest group of minerals are the
silicates
Silicates are distinguished by being
composed of the silicate ion: (SiO4)4
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an ion of Silicon has a charge of 4+
four ions of Oxygen have a charge of 8-
Silicate ions have a tetrahedral
shape
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In other words, they’re four-sided
pyramids
Tetrahedrons can link together
to make complex crystals
isolated
ring
single chain
double chain
sheet
framework--like sheet but in
three dimensions
Where do rocks come from?
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All rocks are made from other rocks!
Three basic families of rocks based on their
origin:
 Igneous
 Sedimentary
 Metamorphic
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CD: EM/C.
Igneous Rocks
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Igneous rocks formed when their constituent
minerals crystallized out of molten rock as it
cooled from a high temperature
Three ways to melt rocks:
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increase in temperature - hot spots
decrease in pressure - mid-ocean ridges
add water - subduction zones
Where do Igneous
rocks form?
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Inside the earth (intrusive/Plutonic)
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in cracks: dikes
in-between rock layers: sills, laccoliths
in magma chambers
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magma is molten rock beneath the surface of the
Earth
Surface of the earth (extrusive/Volcanic):
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lava flows
pyroclastics: pumice and ash
Sedimentary Rocks?
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Formed from sediments settling out of a fluid
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sediments are:
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particles worn off of other rocks
tiny mineral grains precipitated out of a fluid
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for example, salt, gypsum, limestone
created by biologic activity
Fluid can be either air or water
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mudstones form in water
petrified sand dunes form in air
Metamorphic rocks?
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Formed when rocks are exposed for a long time
to a different (usually higher) pressure /
temperature than which they formed in
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most minerals are only stable at a particular
pressure and temperature range
Examples:
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high pressure: downgoing slab in subduction zones
high temperature: rocks next to magma/lava
high pressure and temperature: roots of mountain
ranges
high pressure
high temperature
The Rock Cycle
CD: Earth Materials, C.
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Since every rock is formed out of preexisting rocks, the evolution of rocks
can be shown to be a rock cycle,
which illustrates how every rock type
can be formed out of the other two
types.
Igneous Rocks
CD:EM/D
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There are a wide variety of igneous
rocks, and they can be better
understood if we sort them by their:
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Texture
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size distribution of mineral grains
Chemical composition
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types of minerals present
Igneous Rock Textures
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Aphanitic (Fine) Texture
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Very tiny crystals
Indicates rapid cooling: extrusive or volcanic rocks
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Phaneritic (Coarse) Texture
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Large crystals
Indicated slow cooling: intrusive or plutonic rocks
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volcanic rocks form on the surface or shallow subsurface
plutonic rocks form underground
Porphyritic (Mixed) Texture
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Large crystals set in a matrix of tiny crystals
Indicates slow then fast cooling
Aphanitic Texture (Fine)
Phaneritic Texture (Large)
Porphyritic Texture (Mixed)
Igneous Rock Chemistry
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Most igneous rocks are made of:
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Oxygen (O)
Silicon (Si)
Aluminum (Al)
Iron (Fe)
Magnesium (Mg)
Calcium (Ca)
Sodium (Na)
Potassium (K)
Igneous Rock Chemistry
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All igneous rocks can be described as
mafic, intermediate or felsic
Mafic Igneous Rocks
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Dominated by dark-colored mineral grains
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High Fe, Mg, Ca
Low Si
Felsic Igneous Rocks
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Dominated by light-colored mineral grains
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Low Fe, Mg
High Si
Igneous Rock Chemistry
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Finally, igneous rocks can be further
subdivided on the basis of how much
Sodium (Na) and Potassium (K) they
contain
If they have more Na and K than Ca,
they are considered to be alkaline
igneous rocks (Felsic)
Chemistry and Minerals
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The kind of minerals an igneous rock
contains results from the chemical
composition of the parent magma, the
molten rock from which the rock formed
The chemistry of the parent magma is the
result of two processes:
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Partial melting of a source rock
Fractional crystallization of the magma as it
cools
Partial Melting
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Any given rock is made of several different
minerals with different melting
temperatures
When a rock begins to melt, some minerals
will melt before others
This process results in a melt which has a
different chemical composition than the
rock from which it is forming
Fractional Crystallization
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As a magma cools, crystals will start to
grow within it
These crystals remove elements from
the magma, changing the chemical
composition of the remaining melt
This remaining melt can then grow
crystals different from those that have
already grown from it
Classification of Igneous
Rocks
volcanic
felsic
plutonic
intermediate
mafic
ultramafic
Hawaiian Igneous Rocks
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It takes a great many steps of fractional
crystallization to make a felsic rock out
of a mafic one
In Hawaii, most of the magma was
erupted too quickly for it to be anything
other than a mafic, aphanitic rock: a
basalt
Therefore, the most common minerals
found in Hawaiian igneous rocks are
olivine and Ca-rich plagioclase
Another way of looking at it...
Magma & Plate Tectonics
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Ultramafic magmas - mantle source
Mafic magmas - partial melt of mantle
Intermediate magmas - “ “ of oceanic
crust
Felsic magma - partial melt of
continental crust
Don’t forget that the “mother of all rock”
is PERIDOTITE.
Assignment: V6