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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