Igneous rocks

Earth Materials: Silicate
Minerals & Igneous Rocks


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)


Question: What minerals would
you expect to be most abundant on Earth?
Percent of elements by WEIGHT

Average composition of the Earth’s crust.


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


Basic Building Block of Silicate Minerals:
The Silicon-Oxygen Tetrahedron

An anion with charge of -4
O

1 silicon (Si) atom
4 oxygen (O) atoms

2-

4+

Si
O

O

2-

O

2-

2-

4SiO4
Silicon tetrahedron has
An overall charge of -4


Silicates: The Common Rock-forming Minerals

Basic Building Block:
The Silicon-Oxygen Tetrahedron
Tetrahedra link up by forming
covalent bonds between oxygen atoms:
Single silicon tetrahedron:
A silicon atom covalentlybonded to four oxygens.
Oxygen atom

Silicon atom

Two tetrahedra can join
by sharing an electron
between adjacent oxygen
atoms


The Common rock-forming minerals
Silicates

Silicon-oxygen tetrahedra can be arranged into:

Single chains: Pyroxene

Double chains: Amphibole

Sheets: Micas


Balancing Charges in Silicates: Role of Metal Cations

Silicate chains and sheets
Unsatisfied
Not electrically neutral!

Iron (Fe)
Magnesium (Mg)
Potassium (K)
Sodium (Na)
Aluminum (Al)
Calcium (Ca)

negative charges
on oxygen atoms
located at the
edges of chains,
or between
sheets, are
neutralized by
coordinating

metallic ions at
those sites.


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


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.


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)


Prelude Chapter: Rocks

rock


Prelude Chapter: Rocks

collection of
one or more

rock

minerals


Prelude Chapter: Rocks

rock

minerals

mineral

Prelude Chapter: Rocks

So far we have:

rock

minerals

mineral

collection of
one or more

minerals

A collection
of one or more
types of atoms


Prelude Chapter: Rocks

Example:

Granite & its
constituent
minerals:
Quartz
Amphibole (hornblende)
Feldspar

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,


Focus: Interlude A & Chapter 6. Igneous Rocks

Molten rock
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.



Igneous Rocks

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


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.


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.


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