Earth Materials: Silicate Minerals & Igneous Rocks.

Earth Materials: Silicate Minerals & Igneous Rocks

Earth Materials: Silicate Minerals & Igneous Rocks

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)

Today’s Lecture:

Average composition of the Earth’s crust.Average composition of the Earth’s crust.

Percent of elements by WEIGHT

Question: What minerals would you expect to be most abundant on Earth?

Earth’s CrustPrimarily 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, transparentFeldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink Clay (Mostly come from weathering feldspar) Calcite (CaCO3, shells) Limestone - Used for cement

The Common Rock-forming Minerals

Si4+

O2-

O2-

O2-

O2-

SiO44-

Silicon tetrahedron hasAn overall charge of -4

1 silicon (Si) atom4 oxygen (O) atoms

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

An anion with charge of -4

Tetrahedra link up by forming covalent bonds between oxygen atoms:

Single silicon tetrahedron:A silicon atom covalently-bonded to four oxygens.

Two tetrahedra can joinby sharing an electronbetween adjacent oxygenatoms

Silicates: The Common Rock-forming Minerals

Basic Building Block: The Silicon-Oxygen Tetrahedron

Oxygen atom

Silicon atom

Silicates

The Common rock-forming minerals

Silicon-oxygen tetrahedra can be arranged into:

Single chains: PyroxeneDouble chains: Amphibole

Sheets: Micas

Silicate chains and sheets

Not electrically neutral! Unsatisfied negative charges on oxygen atomslocated at the edges of chains, or between sheets, are neutralized by coordinating metallic ions at those sites.

Balancing Charges in Silicates: Role of Metal Cations

Iron (Fe)Magnesium (Mg)

Potassium (K)Sodium (Na)

Aluminum (Al)Calcium (Ca)

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

Definition of a rock:

A rock is:

Prelude Chapter: Rocks

2) Naturally occurring

1) Comprised of one or more minerals

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.

Rocks and minerals

Some rocks composed entirely of one minerallimestone (calcite)

Prelude Chapter: Rocks

Most rocks have more than one kind of mineralgranite

Some rocks contain non-mineral mattercoal (has organic debris)obsidian (volcanic glassy rock -> not crystalline)

rockrock

Prelude Chapter: RocksPrelude Chapter: Rocks

rockrock

collection ofone or moreminerals

Prelude Chapter: RocksPrelude Chapter: Rocks

rockrock minerals mineral

Prelude Chapter: RocksPrelude Chapter: Rocks

So far we have:

rockrock

collection ofone or moreminerals

mineral

A collectionof one or moretypes of atoms

minerals

Prelude Chapter: RocksPrelude Chapter: Rocks

Example:

Granite & its constituent minerals:

QuartzAmphibole (hornblende) Feldspar

Prelude Chapter: RocksPrelude Chapter: Rocks

The Rock CycleRocks may be classified into three types:

Igneous: Formed by the crystallization of molten rock material called magmaSedimentary: 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 RocksFocus: Interlude A & Chapter 6. Igneous Rocks

Molten rockMolten rock

When magma reaches the surface it is called lava.lava.

In the Earth is called magma.magma.

Magma is buoyant, rises to surface,& sometimes breaks through as volcanic eruptions.

Igneous RocksIgneous Rocks

An igneousigneous rock is formed when magma orlava cools and solidifies.

Igneous rocksIgneous rocksWhy 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 ofmineral (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 RocksVolcanic Igneous Rocks

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

In igneous rocks, texture is controlled by the cooling rate

of the magma.

Crystal SizeCrystal Size

Slow cooling

Cooling RateCooling Rate

larger crystals

Fast cooling small or no crystals

Plutonic Igneous RocksPlutonic Igneous Rocks

Igneous rocks that form deep below the surface are called plutonicplutonic (intrusive) (intrusive) igneous rocks.

To see them, they must be uplifted to surfaceand the overlying rock eroded away.

As a magma cools, atoms arrange themselves into orderly crystalline structures called

minerals. This process is called:

CrystallizationCrystallization

Again, the rate of cooling controls Again, the rate of cooling controls the grain size of the rock formedthe grain size of the rock formed.

Plutonic igneous rocks cool slowly at depth and are therefore coarser grained!

Microscopic views of plutonic igneous rocks

Subsurface

intrusion called

a dike

ALL ROCKS ARE CLASSIFIED ACCORDING TO THEIR:

TEXTURE AND MINERAL COMPOSITION

Texture involves Texture involves a consideration of :a consideration of :

a. Size

b. Shape

c. Arrangement

of the minerals making up a rock.

Types of Igneous Textures

Fine-grained Coarse-grained

Porphyritic Glassy

Fine-grained Coarse-grained

Porphyritic Glassy

fast cooling magma/lavafast cooling magma/lava forms at or near surfaceforms at or near surface sometimes gas holes presentsometimes gas holes present hard to see individual crystalshard to see individual crystals

forms deep forms deep below the surfacebelow the surface slow coolingslow coolingcrystals are corase crystals are corase and intergrownand intergrown

magma cooled slowly for a magma cooled slowly for a while then erupted while then erupted minerals crystallized at minerals crystallized at different temperatures anddifferent temperatures and or rates over a period of timeor rates over a period of time

rapid cooling rapid cooling (quenching) at (quenching) at surfacesurface amorphous: amorphous: atoms unable to atoms unable to form orderly form orderly crystalline crystalline structuresstructures

Types of Igneous Textures

Devil’s Postpile

Plutonic or volcanic?

A C

B D