Download GEOLOGY 585 - West Virginia University

Dr. Helen Lang
Dept.
p of Geology
gy & Geography
g p y
West Virginia University
SPRING 2009
GEOLOGY 585:
Optical Mineralogy & Petrology
Overview of Igneous
Rocks
Igneous rocks crystallize from magmas
andd most magmas are silicate
ili
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
d higher
hi h MgO,
M O F
FeO
O and
dF
Fe2O3
• Ultramafic magmas are even more SiO2
poor and MgO/FeO rich
Classification and Naming of
Igneous Rocks
• Based on Felsic Mineral Content
– QAPF or Streckeisen Diagram
• For fine-grained volcanic rocks, naming is
b d on chemical
based
h i l composition
iti (plot
( l t
Na2O+K2O vs. SiO2)
Streckeisen
(QAPF)
Diagram with
Plutonic
Igneous Rock
names
IUGS Volcanic Classification is Graphical
Alkaline
Subalkaline
Q/Q
Q+A+P
How to plot on the Streckeisen Diagram
Note that several fields
contain 2 or 3 names,
names
which do you use?
F/F+A+P
Diorite
Di
it has
h C.I.
C I < 35 andd
Na-rich plagioclase with
An<50
Gabbro has C.I. >35 and
Ca-rich plagioclase with
An>50
Anorthosite has >90%
plagioclase (<10%
mafic
fi minerals)
i
l)
Bowen’ss Reaction Series
Bowen
• Idealized model for crystallization of magmas
• Shows order in which minerals crystallize
from a typical mafic or basaltic magma
• Left side is called Discontinuous Side
– Mafic minerals change abruptly
• Right side is called Continuous Side
– Plagioclase changes composition gradually
Bowen’s Reaction Series
hi T
olivine
Ca plagioclase
orthopyroxene
tempperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
We’ll
We
ll start with Olivine
• Formula: (Mg
(Mg, Fe)2SiO4
• An Isolated Tetrahedral Silicate
• complete solid solution with two endmembers:
b
– Mg2SiO4 Forsterite (Fo) and
– Fe2SiO4
Fayalite
y
((Fa))
Olivine Properties
•
•
•
•
•
Vitreous luster
Olive green color
Olive-green
No ggood cleavages
g
H=6.5 G=3.2
Optical: high relief, moderately
high birefringence
Olivine Xenoliths from the Mantle
Olivine in thin section
Pyroxenes
• Common in mafic and intermediate igneous rocks,
typically crystallize from basalt after Olivine (BRS)
• Single Chain Silicates
• General Formula: ((Ca2+,,Mgg2+,,Fe2+)2Si2O6
• Two kinds:
– Orthopyroxenes (Opx) (Mg,Fe)2Si2O6 are orthorhombic
and have perpendicular axes, low birefringence, // ext.
• Enstatite
– Clinopyroxenes (Cpx) Ca(Mg,Fe)Si2O6 are monoclinic
and have one inclined axis, higher biref., inclined ext.
• Augite and Diopside
Pyroxene
Relief, Color
and Cleavage in
PPL
Orthopyroxene Properties
• Orthopyroxenes (enstatite, hypersthene,
bronzite)
– Color gray, bronze, brown
– Colorless to tan in thin section
– High relief
– Low birefringence
– Parallel extinction
Clinopyroxene Properties
• Diopside (more likely in metamorphic rocks)
– Ca~1Mg~1Si2O6
– Color light green
– Colorless in thin section
– Moderate birefringence,
g
, inclined extinction
• Augite Ca~1(Mg,Fe)~1Si2O6 (typical igneous Cpx)
– Dark green to black
– Tan, light brown or green in thin section
– Moderate birefringence, inclined extinction
Augite (moderate birefringence, twinning
and cleavage)
Hornblende (Hb)
• is the main amphibole in igneous rocks
• crystallizes after olivine and pyroxenes
from mafic magmas, and at lower
temperature
• is most common in intermediate
composition igneous rocks
• double-chain
d bl h i silicate
ili t
Hornblende
• General Amphibole Formula:
– (Na,K)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
– large
g
medium
small
tetrahedral cations
• Simple Amphibole (Tremolite):
– _Ca
Ca2Mg5Si8O22(OH)2
• Hornblende Formula:
– (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2
– Substitution of (Alvi, Aliv) for (Mgvi, Siiv) of tremolite
Hornblende Properties
•
•
•
•
Black or dark green
Vitreous luster, translucent
H=5-6
H
56
G=3
G
3.00-3
3.55
two perfect prismatic cleavages at about 60o and
120o to eachother,
eachother sometimes “splintery”
splintery cleavage
• commonly in prisms with flattened hexagonal
cross-sections
i
Hornblende’ss Optical Properties
Hornblende
• Various shades of brown, red-brown, green,
bl
blue-green
or tan in
i thin
hi section
i
• Moderate to strong pleochroism is typical
• Moderate birefringence, commonly
obscured by its color
• Two good cleavages at ~60o and 120o to
eachother
• You have to be looking down the prism to
see 2 crossing sets of cleavages, in many
views
i
you will
ill see only
l one sett off cleavages
l
Igneous Amphiboles
Big Hornblende
Bi
H bl d Crystals
C
l in
i this
hi fine-grained
fi
i d
volcanic rock are called Phenocrysts
Hb cleavage, color & pleochroism
Kaersutite (Na,Ti-rich
(Na,Ti rich igneous amphibole)
with
i h strong red-brown
db
pleochroism
l h i
Biotite is the lowest Mafic Mineral on
Di
Discontinuous
ti
Side
Sid off Bowen’s
B
’ Series
S i
• IIt crystallizes
lli
very late
l from
f
fractionated
f i
d mafic
fi
magmas and is only common in intermediate to
f l i magmas, H2O must bbe present ffor Bi
felsic
Biotite
i
to crystallize
• Biotite is a trioctahedral, t-o-t (or 2:1) sheet
silicate
• Biotite’s properties: it’s strongly pleochroic,
has cleavage,
g , pparallel extinction,, bird’s-eye
y
extinction and pleochroic haloes around zircon
Muscovite is a Residual Phase on BRS
• Crystallizes only from felsic magmas with
H2O in the liquid
• Structure and properties are similar to
biotite, except for color and pleochroism
• Common
C
iin Granites
G i andd Pegmatites
P
i
Felsic Minerals on BRS
hi T
olivine
Ca plagioclase
orthopyroxene
tempperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
Quartz Properties
• H=7
H 7, G=2
G 2.65
65
• Generally clear and glassy, may have a
variety of colors (clear,
(clear smoky,
smoky brown,
brown rose;
it’s allochromatic)
• Conchoidal
C h id l fracture,
f
no cleavage
l
• Habit: hexagonal (6-sided prisms) or
massive
• Optical:
p
low relief and low birefringence
g
Quartz in Granite Thin Section
PPL
XPL
From Atlas of Rocks & Minerals in Thin Section
The Feldspar Ternary
C Al2Si2O8
CaAl
Anorthite
solid solutions
All natural
f ld
feldspars
No feldspars
Miscibility Gap
Albite
NaAlSi3O8
alkali feldspars
Orthoclase
O
th l
KAlSi3O8
Alkali Feldspar (esp
(esp. Orthoclase)
Properties
• H=6, G=2.56
• Generally turbid (cloudy); color white,
white pink
or flesh-colored
• 2 Perfect to good perpendicular cleavages
• Habit: stubby prisms, simple twins common
• Optical: low relief and low birefringence
y Perthitic ((micro and macro))
• Commonly
Alkali Feldspars
p 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
p
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
Microcline has
plaid twinning
1mm
Plagioclase Properties
•
•
•
•
•
•
•
•
H 6-6.5,
H=6-6
5 G
G=2
2.62-2.76
62-2 76
(Or H=6,
H=6 G=2
G=2.56)
56)
Luster pearly, vitreous/translucent
C l white
Color
hi to gray
One perfect, one good cleavage
Optical: low relief and low birefringence
Polysynthetic albite twinning usually present
Not Perthitic!
C
Commonly
l zoned
d
May
y be viisible iin hand
dspeccimen, usuallly visible in
thin sectio
on (XP
PL)
Plagioclase Feldspars have
( l
(polysynthetic,
th ti lamellar)
l
ll ) Albite
Albit Twins
T i
Minerals of Pegmatites
Large crystals of Quartz,
Alkali Feldspar and
Muscovite
(which are low on Bowen’s
Reaction Series)
Pegmatites are rich in Incompatible
Elements and Volatiles
• Incompatible Elements – those elements
that don
don’tt fit well in common igneous
minerals (Li, B, Be)
• Volatiles
V l il – those
h
elements
l
that
h have
h
a
strong preference for the gaseous phase
(H2O,
O CO2, Cl,
Cl F,
F S)
Phase Diagrams tell MUCH more
about Crystallization and Melting
than Bowen’s
Bowen s Reaction Series
• What follows is just a quick preview of
g pphase diagrams
g
can be
some of the things
used for
• We
We’ll
ll look at two really simple examples
How do we use this diagram to tell
about crystallization of basalt?
Diopside-rich
li id (X)
liquid
80% CaMgSi2O6 (Di)
20%
% CaAl2Si2O8 ((An))
Draw isopleth=
line of constant
composition
What happens as
liquid cools?
Anorthite-rich liquid
q
((Y))
65% CaAl2Si2O8 (An)
35% CaMgSi2O6 (Di)
Y
X
Textures of rocks in Di-An System
2-component system with
Complete Solid Solution
Crystallization of
Plagioclase in
Basalts (real
i
igneous
plagioclase
l i l
is not pure An like
in the Di
Di-An
An
system)
Pyroxene Compositions and the
P
Pyroxene
Q
Quadrilateral
d il
l
Diopside
CaMgSi2O6
Enstatite
Mg2Si2O6
clinopyroxenes
Augite
Orthopyroxene
py
orthopyroxenes
Hedenbergite
CaFeSi2O6
Ferrosilite
Fe2Si2O6