1. Earth Materials: Silicate Minerals & Igneous Rocks

2. Introduction to Rocks and Minerals (Continued):
Today’s Lecture:
Introduction to Rocks and Minerals (Continued):
Common rock-forming “silicate” minerals
(Chapter 5)
Introduction to rocks & the rock cycle (Prelude A)
Igneous Rocks (Chapter 6)

3. Average composition of the Earth’s crust.
Question: What minerals would
you expect to be most abundant on Earth?
Percent of elements by WEIGHT
Average composition of the Earth’s crust.

4. The Common Rock-forming Minerals
Earth’s Crust
Primarily Si & O followed in abundance by
Fe, Mg, Ca, Na, K, etc.
Dark-colored silicates (mantle and oceanic crust)
Olivine (Si, O, Fe, Mg)
Pyroxene (Si, O, Fe, Mg, Ca)
Amphibole (Si, O, Fe, Mg)
Light-colored silicates (crust, esp. continental crust)
Quartz (SiO2) - Hard, transparent
Feldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink
Clay (Mostly come from weathering feldspar)
Calcite (CaCO3, shells) Limestone - Used for cement

5. SiO Basic Building Block of Silicate Minerals:
The Silicon-Oxygen Tetrahedron
An anion with charge of -4
1 silicon (Si) atom
4 oxygen (O) atoms Si 4+ O 2-
SiO 4 4-
Silicon tetrahedron has
An overall charge of -4

6. The Silicon-Oxygen Tetrahedron
Silicates: The Common Rock-forming Minerals
Basic Building Block:
The Silicon-Oxygen Tetrahedron
Tetrahedra link up by forming
covalent bonds between oxygen atoms:
Two tetrahedra can join
by sharing an electron
between adjacent oxygen
atoms
Single silicon tetrahedron:
A silicon atom covalently-
bonded to four oxygens.
Oxygen atom
Silicon atom

7. Double chains: Amphibole Single chains: Pyroxene
The Common rock-forming minerals
Silicates
Silicon-oxygen tetrahedra can be arranged into:
Double chains: Amphibole
Sheets: Micas
Single chains: Pyroxene

8. Balancing Charges in Silicates: Role of Metal Cations
Silicate chains and sheets
Not electrically neutral!
Unsatisfied negative charges on oxygen atoms
located at the edges of chains, or between sheets, are
neutralized by coordinating
metallic ions at those sites.
Iron (Fe)
Magnesium (Mg)
Potassium (K)
Sodium (Na)
Aluminum (Al)
Calcium (Ca)

9. Ionic Substitution
Ions of similar size (ionic radius) and charge can substitute for one another in a mineral.

10. 1) Comprised of one or more minerals
Prelude Chapter: Rocks
Definition of a rock:
A rock is:
1) Comprised of one or more minerals
2) Naturally occurring
There are three types of rocks:
Igneous (formed by cooling from magma)
Sedimentary (formed by the breakdown of other rocks)
Metamorphic (formed when preexisting rocks
are heated under pressure.

11. Prelude Chapter: Rocks
Rocks and minerals
Some rocks composed entirely of one mineral
limestone (calcite)
Most rocks have more than one kind of mineral
granite
Some rocks contain non-mineral matter
coal (has organic debris)
obsidian (volcanic glassy rock -> not crystalline)

12. Prelude Chapter: Rocks

13. Prelude Chapter: Rocks
collection of
one or more
minerals
rock

14. Prelude Chapter: Rocks
minerals
rock
mineral

15. minerals rock mineral minerals collection of one or more A collection
Prelude Chapter: Rocks
So far we have:
minerals
rock
mineral
collection of
one or more
minerals
A collection
of one or more
types of atoms

16. Granite & its constituent minerals:
Prelude Chapter: Rocks
Example:
Granite & its constituent minerals:
Quartz
Amphibole (hornblende)
Feldspar

17. The Rock Cycle Rocks may be classified into three types: Igneous:
Formed by the crystallization of
molten rock material called magma
Sedimentary:
Formed from pre-existing rocks by
weathering (chemical and physical breakup)
and erosion (transport).
Metamorphic:
Formed by textural and compositional changes
that occur when pre-existing rocks are buried
and subjected to increased temperatures and
pressures.
Rock Cycle (see accompanying slide/MOV):
Connects the three rock groups to each other by process.
The rock cycle is embedded within the hydrological and
the plate cycles discussed previously,

18. Molten rock Focus: Interlude A & Chapter 6. Igneous Rocks
In the Earth is called magma.
Magma is buoyant, rises to surface,
& sometimes breaks through as
volcanic eruptions.
When magma reaches the surface it
is called lava.

20. Igneous Rocks An igneous rock is formed when magma or
lava cools and solidifies.

21. Igneous rocks Why care? Igneous rocks make up bulk of Earth’s crust.
Earth’s mantle is composed entirely of igneous rock!
Igneous rocks are important economically as
building stones and as host rocks for a variety of
mineral (ore) deposits.
Volcanic activity is a well-known geological hazard,
and the associated igneous rocks hold the secrets
for understanding both the nature of past volcanic
eruptions and the potential for future eruption hazards.

22. Volcanic Igneous Rocks
Igneous rocks that form by the eruption of magma at the surface are called volcanic (or extrusive).
Magma erupted at the surface is called lava.
Fragmented materials are called pyroclastic and consist of ash & cinders.

23. Cooling Rate Crystal Size
In igneous rocks, texture is controlled by the cooling rate of the magma.
Cooling Rate
Crystal Size
Slow cooling
larger crystals
Fast cooling
small or no crystals

26. Plutonic Igneous Rocks
Igneous rocks that form deep below the surface are called
plutonic (intrusive) igneous rocks.
To see them, they must be uplifted to surface
and the overlying rock eroded away.

27. Crystallization As a magma cools, atoms arrange themselves
into orderly crystalline structures called
minerals. This process is called:
Crystallization

28. Again, the rate of cooling controls the grain size of the rock formed.
Plutonic igneous rocks cool slowly at
depth and are therefore coarser grained!
Microscopic views of plutonic igneous rocks

30. Subsurface
intrusion called
a dike

31. TEXTURE AND MINERAL COMPOSITION
ALL ROCKS ARE CLASSIFIED ACCORDING TO THEIR:
TEXTURE AND MINERAL COMPOSITION
Texture involves
a consideration of :
a. Size
b. Shape
c. Arrangement
of the minerals
making up a rock.

32. Types of Igneous Textures
Fine-grained
Coarse-grained
Glassy
Porphyritic

33. Types of Igneous Textures
fast cooling magma/lava
forms at or near surface
sometimes gas holes present
hard to see individual crystals
forms deep
below the surface
slow cooling
crystals are corase and intergrown
Fine-grained
Coarse-grained
magma cooled slowly for a
while then erupted
minerals crystallized at
different temperatures and
or rates over a period of time
rapid cooling (quenching) at surface
amorphous:
atoms unable to
form orderly crystalline
structures
Porphyritic
Glassy

34. Devil’s Postpile

35. Plutonic or volcanic? A C B D