Download mineralogy - West Virginia University

Dr. Helen Lang
Dept. of Geology & Geography
West Virginia University
FALL 2015
GEOLOGY 284:
MINERALOGY
Igneous Rocks and their Minerals:
Perkins, Chapter 5
What are IGNEOUS rocks?
• Rocks that are formed by solidification of a
MAGMA
• Magma is “a naturally occurring molten
(melted, liquid) rock material” (see glossary
in textbook)
Some General Terms about
Igneous Rocks
• intrusive
• plutonic
• magma
• extrusive
• volcanic
• lava
• usually phaneritic
• usually aphanitic
or porphyritic
Most Magmas are silicate magmas
(containing 40 to 75 wt.% SiO2)
• Felsic (silicic or sialic) magmas are high in
SiO2 and Al2O3 and low in MgO and FeO
• Intermediate magmas are between Felsic
and Mafic magmas
• Mafic magmas contain less than 50 wt.%
SiO2 and higher MgO, FeO and Fe2O3
• Ultramafic magmas are even more SiO2
poor and MgO/FeO rich
Sample
Chemical
Analyses
Wt. %
Oxide
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
H2O
P2O5
Granite
(felsic)
72.08
0.37
13.86
0.86
1.67
0.06
0.52
1.33
3.08
5.46
0.53
0.18
Gabbro
(mafic)
50.78
1.13
15.68
2.26
7.41
0.18
8.35
10.85
2.14
0.56
0.48
0.18
Magmas contain Volatiles
• Elements or compounds that “prefer” to be in
gaseous form
• Mostly H2O and CO2 in magmas
• Also S, Cl and F
• Contribute to formation of hydrous minerals
– like Biotite
K(Fe,Mg)3(AlSi3O10)(OH)2
• May separate and form bubbles that are
preserved as vesicles
Mount St. Helens 2004
Escape of gases from the magma causes explosive eruptions
like this October 1, 2004, eruption of Mount St. Helens
Magmas may crystallize in stages with
Crystals separating from the remaining Liquid
• Minerals are generally more dense than
liquid, fall to bottom of magma chamber
• Called fractional crystallization
• The last liquid to crystallize will contain
more volatiles and incompatible elements
(K, Rb, Li, Be, B, and REEs)
• Pegmatites form from these residual liquids,
large crystals because H2O acts as a flux
Bowen’s Reaction Series
• Idealized model for crystallization of magmas
• Shows order in which minerals crystallize
from a typical mafic or basaltic magma
• We will use it to organize igneous minerals
• Left side is called Discontinuous Side
– Mafic minerals change abruptly (discontinuously)
• Right side is called Continuous Side
– Plagioclase changes composition gradually
Bowen’s Reaction Series
high T
olivine
Ca plagioclase
orthopyroxene
temperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
Most of the Minerals in Igneous
Rocks are Silicate Minerals
• Felsic (high in SiO2 and alkalis) Silicate
Minerals
– Quartz and Feldspars
(framework silicates)
• Mafic (high in Mg and Fe) Silicate Minerals
– Pyroxenes, Amphiboles and Micas
(chain and sheet silicates)
We’ll start with Felsic Minerals
(framework silicates)
• Quartz SiO2
• Alkali Feldspars
(K,Na)AlSi3O8
• Plagioclase Feldspars
(Ca,Na)(Al,Si)4O8
• Right-hand side of Bowen’s Reaction Series
• Important in all igneous rocks
• Especially in Granites
Quartz and
Feldspars
in Polished
Granite
Quartz and Feldspar are
• Framework silicates (tectosilicates)
– #corners shared 4/tetrahedron
– Si,O formula
(SiO2)0 or
((Al,Si)4O8)1-to2– Si (+Al):O ratio 1:2
– Example
quartz, K-feldspar
– Formula
SiO2, KAlSi3O8
SiO2 like many other compounds
comes in several different structures
• Called Polymorphs (many forms)
– Minerals with the same formula, but different
structures
• Quartz - stable form of SiO2 at most conditions
found on Earth
• High Temperature (low P) forms: tridymite,
cristobalite – occur in hot volcanic rocks
• Very high Pressure forms: coesite, stishovite –
occur in meteorite impacts and deep subduction
SiO2 P-T Phase Diagram
Each Polymorph has a completely
different 3D framework of
linked SiO4 tetrahedra
3d Spirals
Quartz Structure
Tridymite Structure
Low Qtz
Low and
High Quartz
Structures
Same name
Change from one to
the other is
displacive
High Qtz
Same framework
Tridymite
High Quartz
and Tridymite
Structures
Different names
Change from one to
the other is
reconstructive
High Qtz
Different frameworks
View Crystal Structure Movies
http://socrates.berkeley.edu/~eps2/wisc/geo360
low-Quartz
http://socrates.berkeley.edu/~eps2/wisc/geo360/Quartz.mov
Cristobalite http://socrates.berkeley.edu/~eps2/wisc/geo360/cristobalite.mov
Tridymite
Coesite
http://socrates.berkeley.edu/~eps2/wisc/geo360/tridymite.mov
http://socrates.berkeley.edu/~eps2/wisc/geo360/Coesite.mov
SiO2 P-T Phase Diagram
G=4.30
G=2.93
G=2.65
G=2.65
G=2.33
G=2.28
Higher Density Minerals are Stable at Higher Pressure
Coesite has been found in
Crustal Rocks!
• Before 1984 Coesite and Stishovite had been found
only in impact craters (very high pressures for very
short times)
• In 1984 Coesite was found for the first time in rocks
that were once at the surface
• This means that in continent-continent collision zones
(like the Himalayas and Alps), rocks somehow got
from the surface down to >100km (>60mi) and back
fast enough to preserve coesite
Quartz Properties
• H=7, G=2.65
• Generally clear and glassy, may have a
variety of colors (clear, smoky, brown, rose;
it’s allochromatic)
• Conchoidal fracture, no cleavage
• Habit: hexagonal (6-sided prisms) or
massive
• Optical: low relief and low birefringence
Quartz Crystals
Quartz Crystals
Amethyst
Rutilated Quartz
Quartz and Feldspars in Granite
Quartz in Granite Thin Section
PPL
XPL
From Atlas of Rocks & Minerals in Thin Section
Feldspars
• Also framework silicates
• Most abundant minerals in the Earth’s
crust
• Also common in igneous rocks
• Almost all igneous rocks have
feldspars (not true for quartz)
How do we get framework silicates
with formulas different from SiO2?
• When all SiO44- tetrahedra share all corners
with other tetrahedra, formula is (SiO2)0, no
need for other cations to balance charge
• If Al3+ substitutes for Si4+ in some of the
tetrahedra, there is a net negative charge on
the framework and other cations are needed
to balance charge
• That’s how we get Feldspars!
Feldspars
• If Al3+ substitutes for 1/4 of the Si4+ in the
framework
• Formula changes from (Si4O8)0 to (AlSi3O8)1• Alkali Feldspars
(K+, Na+)AlSi3O8
• Orthoclase, KAlSi3O8, and Albite, NaAlSi3O8
• If Al3+ substitutes for 1/2 of the Si4+ in the
framework
• Formula changes from (Si4O8)0 to (Al2Si2O8)2• Anorthite
Ca(2+)Al2Si2O8
Feldspars all have similar 3D
Frameworks that contain
linked Double Crankshafts
View Crystal Structure Movies
K-feldspar (Sanidine)
http://socrates.berkeley.edu/~eps2/wisc/geo360/Sanidine.mov
Na-plagioclase (Albite)
http://socrates.berkeley.edu/~eps2/wisc/geo360/Albitem.mov
first frame shows Feldspar structure best
Ca-plagioclase (Anorthite)
http://socrates.berkeley.edu/~eps2/wisc/geo360/Anorthite.mov
Note the big open spaces available for the cations,
Na+, K+ and Ca2+, that balance the charge
Three Feldspar End-members
• Albite (Ab)
NaAlSi3O8
• Anorthite (An)
CaAl2Si2O8
• Orthoclase (Or)
KAlSi3O8
Relationships shown on Triangular (Ternary)
Diagram, one end-member at each corner
The Feldspar Ternary
CaAl2Si2O8
An
Anorthite
Ab
Or
Albite
Orthoclase
NaAlSi3O8
alkali feldspars
KAlSi3O8
Suppose we have a Feldspar with the
formula: Ca.05Na.25K.70Al1.05Si2.95O8
• To plot that feldspar on a triangular diagram, we
need % of each of the three feldspar end members
• Easiest way is to calculate the mole % of Ca, Na,
and K:
– Ca/(Ca+Na+K)*100 = (.05/1.00)*100 = 5 % An
– Na/(Ca+Na+K) * 100 = (.25/1.00)*100 = 25 % Ab
– K/(Ca+Na+K) * 100 = (.70/1.00)*100 = 70 % Or
How do Triangular Diagrams Work?
CaAl2Si2O8 100% CaAl Si O
2 2 8
Box 5.3 Shows CaO,
Al2O3, SiO2; we’ll
use feldspars
An
Count up from the
opposite side
Plot a Feldspar:
5% CaAl2Si2O8
25% NaAlSi3O8
70% KAlSi3O8
100% Ab
NaAlSi3O8
5% CaAl2Si2O8
NaAlSi3O8
0% CaAl2Si2O8
100%
KAlSi3O8
Or
KAlSi3O8
The Feldspar Ternary
CaAl2Si2O8
Anorthite
solid solutions
All natural
feldspars
No feldspars
Miscibility Gap
Albite
NaAlSi3O8
alkali feldspars
Orthoclase
KAlSi3O8
Alkali Feldspar (esp. Orthoclase)
Properties
• H=6, G=2.56
• Generally turbid (cloudy); color white, pink
or flesh-colored
• 2 Perfect to good perpendicular cleavages
• Habit: stubby prisms, simple twins common
• Optical: low relief and low birefringence
• Commonly Perthitic (micro and macro)
Typical Orthoclase Crystals
(alkali feldspar)
Orthoclase – Carlsbad Twin
Alkali Feldspars
have Perthites
What do Perthites
look like?
Alkali Feldspars have Perthites
What do Perthites look like?
Thin
section
in XPL
Alkali Feldspars have Perthites
What do Perthites look like?
1mm
What causes Perthites?
• Caused by un-mixing, exsolution or
separation of Na+ (diameter~1.1Å) and K+
(diameter~1.6Å) as the feldspar cools
• At low temperatures, there is a miscibility
gap between NaAlSi3O8 and KAlSi3O8
Miscibility gap in Alkali Fsp. causes Perthites
1000o
600o
400o
Albite
Temperature
800o
X’
albite
NaAlSi3O8
K-feldspar
One Homogeneous X
Alkali Feldspar
Two Feldspars
Perthite
alkali feldspars
X’’
orthoclase
KAlSi3O8
Why Alkali Feldspars CAN have perthites?
• Remember Al3+ substitutes for 1/4 of Si4+ in
framework (AlSi3O8)1• Al3+ is locked tightly in the feldspar framework
and can’t move
• K+ (1.46Å) and Na+ (1.08Å) with the same
charge can trade places freely in the structure and
still balance Al3+ charge
• Allows unmixing or exsolution (separation of
Na+ and K+) in solid feldspar
• Which allows Perthites to form as the rock cools
Microcline (a
polymorph of
KAlSi3O8
different from
orthoclase) is
sometimes
bluish green
Microcline has
plaid twinning
1mm
The Feldspar Ternary
CaAl2Si2O8
Anorthite
No feldspars,
Miscibility Gap
Albite
NaAlSi3O8
alkali feldspars
Orthoclase
KAlSi3O8
Plagioclase Properties
•
•
•
•
•
•
•
•
H=6-6.5, G=2.62-2.76
Luster pearly, vitreous/translucent
Color white to gray
One perfect, one good cleavage
Optical: low relief and low birefringence
Polysynthetic albite twinning usually present
Not Perthitic!
Commonly zoned
Plagioclase hand specimen - note
polysynthetic twinning (striations)
May be visible in handspecimen, usually visible
in thin section (in XPL)
Plagioclase Feldspars have
(polysynthetic, lamellar) Albite Twins
Zoning shows up with
crossed polars under the
microscope (diff. biref.)
Plagioclase Feldspars are
commonly zoned
Zoning in Plagioclase
Especially in volcanic
rocks, conditions may
change around a
growing plagioclase,
causing changes in
plagioclase composition
(variable Na and Ca)
This is zoning
Review Feldspars
CaAl2Si2O8
Anorthite
(An)
No feldspars,
Miscibility Gap
Albite
(Ab) NaAlSi3O8
alkali feldspars
Orthoclase
KAlSi3O8 (Or)
Plagioclase Feldspars CAN NOT
have perthites. Why?
• In some parts of the plagioclase, Al3+
substitutes for half of the Si4+; formula
(Al2Si2O8)2- Ca2+ or other divalent cation
must balance charge!
• In some parts of the plagioclase structure,
Al3+ substitutes for 1/4 of the Si4+; formula
(AlSi3O8)1Na1+ or other monovalent
cation must balance charge!
Why do Plagioclase Feldspars
NOT have perthites?
• Al3+ is locked tightly in feldspar framework
• Therefore, Al3+ can’t move
• Na+ and Ca2+ can’t move without Al3+ (That
would destroy the charge balance!)
• Therefore, exsolution or Perthites can’t
happen in Plagioclase Feldspar
• And alkali feldspars (except microcline)
don’t have albite twins!
Review Feldspars
CaAl2Si2O8
Anorthite
(An)
No feldspars,
Miscibility Gap
Albite
(Ab) NaAlSi3O8
alkali feldspars
Orthoclase
KAlSi3O8 (Or)
Bowen’s Reaction Series
high T
olivine
Ca plagioclase
orthopyroxene
temperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
SiO2 P-T Phase Diagram
Low Quartz, High
Quartz and Tridymite
Structures
Low Qtz
Tridymite
High Qtz
How do Triangular Diagrams Work?
CaAl2Si2O8 100% CaAl Si O
2 2 8
Box 5.3 Shows CaO,
Al2O3, SiO2; we’ll use
feldspars
An
Plot a Feldspar:
5% CaAl2Si2O8
25% NaAlSi3O8
70% KAlSi3O8
Count up from the
opposite side
100% Ab
NaAlSi3O8
5% CaAl2Si2O8
NaAlSi3O8
0% CaAl2Si2O8
100%
KAlSi3O8
Or
KAlSi3O8
CaAl2Si2O8
Anorthite
(An)
No feldspars,
Miscibility Gap
Albite
(Ab) NaAlSi3O8
alkali feldspars
Orthoclase
KAlSi3O8 (Or)
Miscibility gap in Alkali Fsp. causes Perthites
1000o
600o
400o
Albite
Temperature
800o
X’
K-feldspar
One Homogeneous X
Alkali Feldspar
Two Feldspars
Perthite
X’’
albite
orthoclase
NaAlSi3O8
KAlSi3O8