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MineralsPart II

Courtesy of Katryn Wiese

SILICATESAmphibole family: Hornblende [Ca2(Fe,Mg)5Si8O22(OH)2]

Feldspar family•Plagioclase Feldspar: [CaAl2Si2O8] to [NaAlSi3O8]•Potassium Feldspar: [KAlSi3O8]

Garnet Fe,Mg,Ca, Al Silicate

Mica family: •Biotite [Silicate with K, Mg, Fe, Al, Ti, OH, F] •Muscovite [Silicate with K, Al, OH, F]

Olivine (Mg,Fe)2SiO4

Pyroxene family: Augite [Silicate with Fe, Mg]

Quartz SiO2

SerpentineMg6Si4O10(OH)8

Talc Mg3Si4O10(OH)2

CARBONATESCalcite CaCO3

SALTSHaliteNaCl

SULFATESGypsum CaSO4*2(H20)

SULFIDESGalena PbSPyrite FeS

OXIDESHematite Fe2O3

Magnetite Fe3O4

NATIVE ELEMENTSGraphite (C)

Elemental

Abundances

in Continental

Crust

Most minerals are silicates!

(silicate)

(silicate)

(silicate)

(silicate)

(silicate)

(silicate)(silicate)

Covalent Bond

between oxygen

and silicon.

Silicon-Oxygen Tetrahedra

The fundamental building block of all silicates.

SiO44-

The Silicon-Oxygen Tetrahedra is a complex anion.

4O2-+Si4+=SiO44-

Not electrically neutral thus to

balance the charge the complex

anion needs to bond with a

positively charged metal ion.

Thin-section

examples.

We are going to look at this more closely when we study igneous rocks.

Silicates Structures

Independent TetrahedronSilicate Structure

Example: Olivine

Si:O ratio 1:4

No Cleavage

Minerals with independent tetrahedra are hard and

usually have a hardness around 7 on the Mohs scale.

Strong ionic

bonds with

Fe and Mg

cations

interspersed.

100% Fe 100% Mg

Olivine

Single ChainSilicate Structure

Two cleavage planes at almost 90 degrees.

Si:O ratio 1:3

Example: Augite

of the Pyroxene Group

Single ChainSilicate Structure

Share two corner oxygens in tetrahedra,

thus the tetrahedra has a -2 electrical

charge. The accumulated charge make the

chain itself a negative ion complex

The bonds within the chains are strong but

the bonds between the chains are relatively

weak.

Pyroxenes are high in Fe and Mg and are

dark in color.

Si:O ratio 1:3

Double ChainSilicate Structure

Two cleavage planes forming angles

of about 60 and 120 degrees.

Si:O ratio 1:2.75

Example: Hornblende

of the Amphibole Group

Double ChainSilicate Structure

A double chain forms when a tetrahedra

shares two corner oxygens within a chain

and some tetrahedra share a third oxygen

with a neighboring chain resulting in over-all

Si:O ratio of 1:2.75

Double chain minerals are amphiboles,

with the most common amphibole is called

hornblende.

Both hornblende and single chain pyroxene

minerals are high in Fe and Mg, thus are

greenish black to black. The difference in

cleavage is a way to tell them apart.

Common ions Fe, Mg, Ca, Al, and Na

Si:O ratio 1:2.75

Sheet Silicate Structure

Micas (muscovite, biotite, etc)

muscovite

biotite

One perfect cleavage plane.

Si:O ratio

1:2.5

Biotite vs. Muscovite

Biotite’s darker color is due to

Fe and Mg content.

Muscovite has no significant Fe and Mg.

Minerals that have a lot of Fe and Mg are generally dark in color, thus rocks

formed from these minerals are also dark in color. These Fe and Mg rich

minerals have higher melting points so they are the 1st to crystallize in a melt.

Framework Silicates

Example: Orthoclase

(potassium feldspar)

Example: Quartz

Two cleavage planes

at ~90 degrees.

No cleavage planes

Si:O ratio 1:2

Quartzand it’s polymorphs

The second most abundant mineral in continental crust and the only mineral

to be composed entirely of oxygen and silicon.

Quartz has the highest degree of oxygen sharing thus the lowest

silicon to oxygen ratio, 1:2.

Quartz achieves electrical neutrality without other ions.

Strong covalent bonds makes quartz one of the hardest common minerals.

Quartz often forms

from hydrothermal

solutions like this gold

bearing quartz vein in

known as the “Mother Load”.

The Feldspars

• Feldspars are the most abundant mineral

in the Earth’s crust.

• Feldspars account for ~60% of the

Continental Crust by volume.

• Feldspars are separated into two main

groups: Plagioclase and Alkali (potassium)

Feldspars.

• Feldspars have the same silicon to oxygen ratio

as quartz (1:2), but feldspars unlike quartz,

aluminum often substitutes for silicon.

• Another difference from quartz is that Ca, Na or

K ions occupy the spaces between the Si-O

tetrahedron.

• Feldspars are hard (~6 on mohs scale).

The Feldspars

ColorCleavage (2 at 90 degrees)

Sub-parallel Exsolution Lamellae

Twinning Striations (cannot feel)

Plagioclase Feldspar

METALLIC MINERALS (listed in decreasing hardness) Review mineral formula to connect to family! H=Hardness; SG = specific gravity

MineralH S

GStreak •Color •Form •Cleavage/Fracture •Distinctive properties

Pyrite FeS2 6-6.5

5 Dark grey •Brass yellow; tarnishes brown.

•Cubes or octahedrons •Brittle. No cleavage. •Cubic form, brassy color, and SG=5.

MagnetiteFe3O4

6 5.2 Dark grey •Silvery grey to black. Tarnishes grey. Opaque.

•Octahedrons •No cleavage. •Attracted to a magnet. SG=5.2. No cleavage.

HematiteFe2O3

1.5-6

2.1-2.6

Red to red-brown

•Silvery grey, black, or brick red. Luster can also be nonmetallic.

•Thin tabular crystals or shapeless masses.

•No cleavage. •Red streak. Metallic + nonmetallic. Earthy red.

Galena PbS 2.5 7.6 Grey to dark grey

•Silvery grey. Tarnishes dull grey.

•Cubes and octahedrons

•Brittle. 3 good cleavage planes (cubes).

•SG=8. Dense! Silver cubes (form and cleavage).

Graphite C 1 2.1-2.3

Dark grey •Silvery grey to black. •Flakes, short hexagonal prisms, and masses.

•1 excellent cleavage plane.

•Dark grey. H=1. Greasy. Dark grey streak.

MineralH S

GStreak

•Color (and/or luster) •Form •Cleavage/Fracture •Distinctive properties

Garnet(Ca3,Mg3,Fe3Al2)n(SiO4)3

7 3.5-4.3

White •Red, black, or brown; can be yellow, green, pink. Glassy. Translucent.

•Dodecahedrons (12-sided polygons)

•No cleavage. Brittle. Conchoidal fracture.

•Dodecahedron form, red, glassy, conchoidalfracture, H=7.

Olivine (Mg,Fe)2SiO4 7 3.3-3.4

White •Pale or dark olive green to yellow or brown. Glassy. Transparent.

•Short prisms (usually too small to see).

•Conchoidal fracture.•Brittle.

•Green, conchoidalfracture, glassy, H=7. Usually granular.

Quartz SiO2 7 2.7 White •Colorless, white, or gray; can occur in all colors. Glassy and/or greasy.

•Massive; or hexagonal prisms that end in a point.

•Conchoidal fracture.

•Glassy, conchoidalfracture, H=7. Hex. prism with point end.

Plagiociase Feldspar family: Anorthite and Labradorite CaAl2Si2O8 to Oligoclase and Albite NaAlSi3O8

6 2.6-2.8

White •Colorless, white, gray, or black; can have iridescent play of color from within. Translucent to opaque.

•Tabular crystals or thin needles

•2 good cleavage planes at nearly right angles.

•Twinning. 2 cleavages at 90°.

Potassium Feldspar family: Orthoclase and Microcline KAlSi3O8

6 2.5-2.6

White •Pink. Or white, orange, brown, gray, green. Translucent to opaque.

•Tabular crystals •2 good cleavage planes at nearly right angles.

•Subparallel exsolution lamellae. 2 cleavages at 90°. Pink color.

Pyroxene family: Augite Ca(Mg,Fe,Al)(Al,Si)O6

5.5-6

3.2-3.5

White, pale grey

•Green to black; opaque. •Short, 8-sided prisms (if visible).

•2 good cleavage planes at nearly right angles.

•H=5.5. Dark green or black. 2 cleavages at 90°. (Looks like HB.)

Amphibole family: Hornblende(Ca,Na)2-3(Fe,Mg,Al)5Si6(Si,Al)2O22(OH)2

5.5 3-3.3

Grey-green, white

•Dark green to black. Opaque.

•Long, perfect prisms.

•2 cleavages planes. Angles: 60°and 120°. Brittle. Splintery fracture.

•H=5.5. Dark green or black. 2 cleavages at 60° & 120°. Splintery fracture. Long prisms.

NONMETALLIC MINERALS (listed in decreasing hardness) Review mineral formula to connect to family! H=Hardness; SG = specific gravity

MineralH S

GStreak

•Color (and/or luster) •Form •Cleavage/Fracture •Distinctive properties

SerpentineMg6Si4O10(OH)8

2-5 2.2-2.6

White •Pale or dark green, yellow, grey. Opaque. Dull or silky.

•Smooth, rounded masses.

•No cleavage. •Mottled green color. Smooth, curved surfaces.

Fluorite CaF2 4 3-3.3

White •Colorless, purple, blue, grey, green, or yellow. Glassy. Opaque to transparent.

•Usually cubes or octahedrons.

4 excellent cleavage directions. Brittle.

•Cubic or octahedral form. 4 directions of cleavage.

Calcite CaCO3 3 2.7 White •Usually colorless, white, or yellow, can be green, brown, or pink. Glassy. Opaque to transparent.

•Rhombohedrons. •3 excellent cleavage planes. Angles: •< 90° and > 90°.

•Bubbles in HCL. Double refraction (2 images visible through clear sample). Rhombs, 3 cleavage planes (not 90°), H=3.

Mica family: Biotite K(Mg,Fe)3AlSi3O10(OH)2

2.5-3

2.7-3.1

Grey-brown

•Black, green-black, brown-black. Transparent to opaque.

•Short tablets. Like a tablet of paper.

•1 excellent cleavage – splits easily into thin, flexible sheets.

•1 flexible cleavage plane (sheet), dark colored; brown streak.

Mica family: Muscovite KAl3Si3O10(OH)2

2-2.5

2.7-3

White •Colorless, yellow, brown, or red-brown. Transparent to opaque.

•Short tablets. Like a tablet of paper.

•1 excellent cleavage – splits easily into thin, flexible sheets.

•1 flexible cleavage plane (sheet), light colored; white streak.

Halite NaCl 2.5 2.1-2.6

White •Colorless, white, yellow, blue, brown, or red. Glassy.

•Cubes. •Brittle. 3 excellent cleavage planes: cubes.

•Salty taste. H=2.5. Cubic form and cleavage.

GypsumCaSO4*2(H20)

2 2.3 White •Colorless, white, or grey. Translucent to transparent.

•Tabular, prisms, blades, or needles.

•1 good cleavage plane.

•H=2. 1 cleavage plane. Translucent.

TalcMg3Si4O10(OH)2

1 2.7-2.8

White •White, grey, pale green, or brown. Opaque. Greasy or silky luster.

Shapeless masses (if no cleavage visible) or tabular.

1 poor cleavage plane (may not be visible).

•Feels greasy or soapy. H=1. Opaque.

NONMETALLIC MINERALS (listed in decreasing hardness) Review mineral formula to connect to family! H=Hardness; SG = specific gravity

We have now covered the minerals section.Tips:

• Make and use flash cards.

• Form study groups.

• Frequently and repeatedly test yourself.

• Visit me in office hours.

Igneous Rocks

Devils Tower, Wyoming

Granite Basalt

Continental Crust

Buoyant (density 2.7g/cm3)Oceanic Crust

Density 3.0g/cm3

Rock Cycle

Melting

Heat and pressure(but not enough to melt)

Weathering and Erosion

Solids from Melts

Igneous rocks form from molten materials.

Magma: molten rock within the Earth. When

magma cools and crystallizes in the crust it form

plutonic/intrusive rocks.

Lava: molten rock on the Earth’s surface (magma

that has reached the surface). When lava

crystallizes it forms volcanic/extrusive rocks.

How are igneous rock is

classified?

• Texture: crystal size, frothy, vesicular, glassy,

and pyroclastic.

• Composition: chemical/mineralogical makeup.

Note--Volcanic/Extrusive or Plutonic/Intrusive:

a broad classification for where the igneous rock

crystalized. This directly relates to texture.

A First Look

Felsic Intermediate Mafic

Composition

Plutonic (Intrusive)

Volcanic (Extrusive)

�(Phaneretic: visible crystals)�

�(Aphanitic: crystals not visible to the unaided eye)�

Si% increasing, Mg and Fe% decreasing.

Magma

Lava

Volcanic and plutonic rock

emplacement

Melts occur in a variety of

geologic settings.Melts most often occur in the upper mantle or lower crust.

• Mid-Ocean Ridges (MOR): Rift zones/ Spreading Centers (including continental). These are places where tectonic plates move away from each other creating Oceanic Crust. The melt occurs due to decompression.

• Volcanic Arcs: Chains of volcanoes that form on the plate over-riding a subducting oceanic plate. Volcanic Arcs can be either Island Arcs or Continental Arcs. The melt occurs due to addition of water lowering the melting point.

• Hotspots: Locations where magma rises from deep within the Earth. Possibly as deep as the Core-Mantle Boundary.

The Upper Mantle is composed of a rock

called Peridotite which is mainly the

mineral olivine.

For a melt to occur there must be either :

an increase in temperature,

a decrease in pressure,

or the addition of volatiles like water.Peridotite

Decompression Melting

A

A’

If at a certain depth

the pressure on the

rocks is decreased,

then the rock can

begin to melt. As we

see in this diagram,

the rocks under a

certain pressure at

point A would be

solid for the

temperature

conditions. If the

temperature remains

the same but the

pressure is lower,

like at point A’, then

the rock begins to

melt.

A MOR can

develop

from Continental

Rifting.

Mid-Ocean Ridge(spreading centers)

Decompression Melting

Addition of Volatiles (Water)

Volcanic ArcContinental Crust has a lower density due to it’s overall chemical composition having less Fe and Mg, thus it is buoyant.

Oceanic Crust has a higher density than Continental Crust, so it is less buoyant especially when it cools and become more dense.

Addition of water into the mantle

causing rock to partially melt.

Partial Melting

Partial Melting

and magma

evolution.

As the magma rises it

can evolve. The material

it rises through can affect

the composition.

If the magma has to rise

through thick continental

crust then the magma is

likely to be more evolved.

Viscosity

high viscosity felsic lava

low viscosity mafic lava

Viscosity and Composition

• Viscosity is a measure of a fluids resistance to flowing.

• The higher temperature, the lowers viscosity.

• The Higher the silica content, the higher the viscosity.

• The higher the water content, the lower the viscosity.

• High viscosity can restrict atomic diffusion inhibiting crystal growth.

Factors controlling crystal size1) Rate of cooling--Intrusive vs. Extrusive.

2) Viscosity of magma—Temperature, Silica content,

and Water content.

Intrusive or plutonic: Cooled (solidified) at depth within the Earth’s

crust, thus slower cooling results in larger crystals.

Extrusive or Volcanic: Cooled (solidified) on the Earth’s surface, thus

rapid cooling results in small or even no crystal growth.

Silica Content: The more silica, the more viscous.

Temperature: Lower temperature means higher viscosity.

Water Content: More water means less viscous.

HotspotSometimes magma rises quickly and from greater depth and has less time to evolve.

Why does magma rise towards

the surface?

When a crystalline material is heated, the atoms that are bonded together

vibrate more and more vigorously with increasing in temperature. The space an

atom takes up increases as the vibration increases, so when a crystalline

material is heated, all the atoms in that material begin to take up more and more

space, thus the volume of that material increases but the amount of material

remains the same—this is a change (decrease) in density.

Density=Mass/Volume

Gold has a density of about 20 g/cc

Pure water at room temperature has a density of about 1 grams/cubic centimeter

Granite has a density of about 2.7 g/cc

As a melt (Magma or Lava)

cools, it begins to crystallize.

Unlike water, magma crystallizes over a broad

temperature range (several hundred degrees Celsius).

This is because magma is much more chemically

complicated than water (H2O crystallizes at one

temperature).

Magmas can contain a broad range of elements. These elements as we have

seen form a variety of compounds (we looked at minerals—remember the definition

of a mineral). Magma contains varying amount of volatiles which are dissolved

within the melt but can be released as a gas (just like a carbonated drink).

Magma is a mix of liquid and dissolved gases and a varying amount of crystals.

Classification of Igneous Rocks

• What is an igneous rock?

• Discuss with your neighbor how igneous

rocks are classified.

Classification of Igneous Rocks

• Texture: crystal size, frothy, vesicular, glassy,

and pyroclastic.

• Composition: chemical/mineralogical makeup.

Note--Volcanic/Extrusive or Plutonic/Intrusive:

a broad classification for where the igneous rock

crystalized. This directly relates to texture.

Factors controlling or effecting

magma composition.

• Origin/type: of melt (M.O.R., hot spot, volcanic

arcs).

• Location—what the melt travels through

(intrudes) or ponds in (the country rock type

and thickness—assimilation).

• Time and history (how quickly the magma rises

through the crust--assimilation, crystal

fractionation, and magma mixing events).

Basic Overview

Felsic

Feldspar and silica

Mafic

Magnesium and iron

Dark, dense rocks

“Basaltic composition”

Light color, lower density rocks

“Granitic composition”

Note: one must be careful with the term “granitic” because it also is a texture

(coarse grained or phaneritic) and an actual rock type.

Continents Oceans

Felsic Intermediate Mafic

Composition

Plutonic (Intrusive)

Volcanic (Extrusive)

�(Phaneritic: visible crystals)�

�(Aphanitic: crystals not visible to the unaided eye)�

Si% increasing, Mg and Fe% decreasing.Si richFe and Mg rich

Si poor

Granite Diorite Gabbro

Rhyolite Andesite Basalt

As a magma crystallizes the amount of crystals increases within the melt. Eventually it can

become a crystalline mush that has only a small liquid component remaining between the

crystals.

I have never

seen or heard

of it.

Dominant Accessory (could have been a mica) Dominant

How did a melt that likely

originated in the upper

mantle (peridotite), become

granite?

Partial Melting

and magma

evolution.

As the magma rises it

can evolve. The material

it rises through can affect

the composition.

Bowen’s Reaction Series

Differentiation in a magma body

leads to an evolution in composition.

Crystal Settling

Bowen’s Reaction Series

Gabbro

KomatiitePeridotite

Basalt

Scoria

DioriteAndesite

Tuff, Pumice

Granite Rhyolite

Obsidian

Tuff

Pumice

Granite Felsic: Silica>65%

Cooled slowly enough giving enough time for crystals to grow large.

RhyoliteFelsic: Silica>65%

Cooled too rapidly for crystals to grow large enough to see without a microscope.

This is a special case:

When there are large

crystals (called

phenocrysts) present in a

“groundmass” of

aphanitic crystals,

the texture of the

rock is called a

porphyritic. This is a

subcategory of aphanitic

texture.

What mineral is this?

What is the name of the rock?

Then large crystals (phenocrysts) are hornblende!

Thus this rock is called a Hornblende

Andesite Porphyry.

The magma was in the right conditions for hornblende to start to crystallize,

but was erupted and cooled rapidly enough that the remaining melt only grew

small crystals that form the groundmass.

Remember—hornblende is a mineral that crystallizes at a certain temperature

and is generally found in rocks of intermediate composition (think of Bowen’s

Reaction Series).

Hornblende phenocryst

Cooling rapid enough to freeze the lava before mineral crystals have time to form.

The formation of volcanic glass (obsidian) occurs in felsic lavas due to the high viscosity

restricting atomic diffusion. This is a mineraloid—does not have a crystal structure.

When crystals grow to very large

sizes, the texture of the rock is called

pegmatitic, thus this rock would be

called a pegmatite.

What minerals are these?

Quartz and Potassium Feldspar (K-spar)

make this a Granite Pegmatite. Pegmatitic

texture is a subcategory of Phaneritic.

Where does these minerals occur in

Bowen’s Reaction Series? Is this

Felsic, Intermediate, or Mafic?

Highly evolved magma

that is volatile and fluid rich,

thus readily allowing

ion diffusion and rapid

crystal growth.

Magma Gas Content and

Extrusive Rock Textures

Magma is full of volatile compounds to a varying degree. These volatiles are

released from the magma as gases when the pressure on the magma is

lowered, thus the gas could escape.

Gases escaping from lava while the lava is crystallizing can have an

effect on the texture of the rock which forms. This is how frothy and

vesicular textures are created.

Escaping gases from viscous felsic lava formed this pumice. Pumice can float on water!

Gases form bubbles in cooling lava which forms vesicular textures often seen in mafic

and intermediate lava rocks. Scoria will sink in water.

Vesicular

Basalt

Rocks From Ash and Debris

from Explosive Eruptions.• Rock debris and ash deposited from an explosive volcanic eruption can form rocks with a texture that is called pyroclastic (pyro=fire, clastic=pieces).

• Rocks with a pyroclastic texture are called tuff.

• Explosive volcanic eruptions can deposit very hot ash which is sometimes hot enough to fuse together after deposition forming a welded tuff which is harder or more durable than other tuffs.

Contact--

Note Crystal sizes

What does this tell us?

Porphyritic

Granite Density~2.7 g/cm^3

BasaltDensity~3.0 g/cm^3

We see plutonic rocks on the surface

when they are exposed by erosion.

magma chamberPluton

The Sierra Nevada Mts. is a batholith that is composed of many plutons.Composed of mostly granite and granodiorite (plutonic rocks), these mountains

were once magma chambers beneath a chain of volcanoes. These volcanoes

eroded away, greatly filling the valleys to the east and west with sediments. Rivers

Also carried much sediment all the way to the ocean.

As erosion removed the volcanoes the buoyant felsic rocks of the

Sierra Nevada Mts. had less weight above it so it began to rise. This

is called isostatic equilibrium. The Sierra Nevada continue to be

eroded and continue to rise.

Sill

Bedding

Figure 4.32

Basalt Dike

Figure 4.23