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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