This document provides an overview of igneous rocks and their classification. It discusses that igneous rocks crystallize from magmas, which can be felsic, intermediate, or mafic depending on their silica content. Igneous rocks are classified based on mineral content using diagrams like the QAPF diagram. The ordering of mineral crystallization from cooling magma is explained by Bowen's reaction series. Common igneous minerals like feldspars, pyroxenes, amphiboles, and micas are described in terms of their composition, properties, and where they occur in the crystallization sequence. Phase diagrams provide additional insight into crystallization beyond Bowen's series.

1. Dr. Helen Lang
Dept. of Geology & Geographyp gy g p y
West Virginia University
SPRING 2009
GEOLOGY 585:
Optical Mineralogy & PetrologyOptical Mineralogy & Petrology

2. Overview of Igneous
Rocks

3. Igneous rocks crystallize from magmas
d iliand 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 FeOSiO2 and Al2O3 and low in MgO and FeO
• Intermediate magmas are between Felsic
and Mafic magmasand Mafic magmas
• Mafic magmas contain less than 50 wt.%
SiO d hi h M O F O d F OSiO2 and higher MgO, FeO and Fe2O3
• Ultramafic magmas are even more SiO2
poor and MgO/FeO rich

4. Classification and Naming ofClassification and Naming of
Igneous Rocks
• Based on Felsic Mineral Content
QAPF or Streckeisen Diagram– QAPF or Streckeisen Diagram
• For fine-grained volcanic rocks, naming is
b d h i l iti ( l tbased on chemical composition (plot
Na2O+K2O vs. SiO2)

5. Streckeisen
(QAPF)
Diagram withDiagram with
Plutonic
Igneous Rock
namesnames

6. IUGS Volcanic Classification is Graphical
Alkaline
SubalkalineSubalkaline

7. How to plot on the Streckeisen Diagram
Note that several fields
contain 2 or 3 names
Q+A+P
contain 2 or 3 names,
which do you use?
Di it h C I < 35 dQ/Q Diorite has C.I. < 35 and
Na-rich plagioclase with
An<50
F/F
Gabbro has C.I. >35 and
Ca-rich plagioclase with
An>50
+A+P
An>50
Anorthosite has >90%
plagioclase (<10%
fi i l )mafic minerals)

8. Bowen’s Reaction SeriesBowen s Reaction Series
• Idealized model for crystallization of magmasIdealized model for crystallization of magmas
• Shows order in which minerals crystallize
from a typical mafic or basaltic magmafrom 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

9. Bowen’s Reaction Series
hi T
olivine
Ca plagioclase
hi T
orthopyroxene
clinopyroxene NaCa plagioclase
amphibole (Hb)
perature
biotite Na plagioclase
alkali feldspar
cooling
temp
alkali feldspar
muscovite
quartz
residual
phases
quartz
low T

10. We’ll start with OlivineWe ll start with Olivine
• Formula: (Mg Fe)2SiO4Formula: (Mg, Fe)2SiO4
• An Isolated Tetrahedral Silicate
• complete solid solution with two end-
bmembers:
Mg SiO Forsterite (Fo) and–Mg2SiO4 Forsterite (Fo) and
–Fe2SiO4 Fayalite (Fa)2 4 y ( )

11. Olivine PropertiesOlivine Properties
• Vitreous luster
• Olive green color• Olive-green color
• No good cleavagesg g
• H=6.5 G=3.2
• Optical: high relief, moderately
high birefringencehigh birefringence

12. Olivine Xenoliths from the MantleOlivine Xenoliths from the Mantle

13. Olivine in thin section

14. Pyroxenes
• Common in mafic and intermediate igneous rocks,
typically crystallize from basalt after Olivine (BRS)
• Single Chain Silicates
• General Formula: (Ca2+,Mg2+,Fe2+)2Si2O6( , g , )2 2 6
• Two kinds:
– Orthopyroxenes (Opx) (Mg,Fe)2Si2O6 are orthorhombicOrthopyroxenes (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

15. Pyroxene
Relief, Color
and Cleavage inand Cleavage in
PPL

16. Orthopyroxene Properties
• Orthopyroxenes (enstatite, hypersthene,
bronzite)bronzite)
– Color gray, bronze, brown
– Colorless to tan in thin section
– High relief
– Low birefringence
– Parallel extinctionParallel extinction

17. Clinopyroxene Properties
• Diopside (more likely in metamorphic rocks)
Ca Mg Si O– Ca~1Mg~1Si2O6
– Color light green
– Colorless in thin section
– Moderate birefringence, inclined extinctiong ,
• Augite Ca~1(Mg,Fe)~1Si2O6 (typical igneous Cpx)
Dark green to black– Dark green to black
– Tan, light brown or green in thin section
– Moderate birefringence, inclined extinction

18. Augite (moderate birefringence, twinning
and cleavage)

19. Hornblende (Hb)
• is the main amphibole in igneous rocks
• crystallizes after olivine and pyroxenes• crystallizes after olivine and pyroxenes
from mafic magmas, and at lower
temperature
• is most common in intermediate
composition igneous rocks
d bl h i ili t• double-chain silicate

20. HornblendeHornblende
• General Amphibole Formula:
– (Na,K)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
– large medium small tetrahedral cationsg
• Simple Amphibole (Tremolite):
Ca Mg Si O (OH)– _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

21. Hornblende PropertiesHornblende Properties
• Black or dark green
• Vitreous luster, translucent
• H=5-6 G=3 0-3 5H 5 6 G 3.0 3.5
• two perfect prismatic cleavages at about 60o and
120o to eachother sometimes “splintery” cleavage120o to eachother, sometimes splintery cleavage
• commonly in prisms with flattened hexagonal
icross-sections

22. Hornblende’s Optical PropertiesHornblende s Optical Properties
• Various shades of brown, red-brown, green,
bl i hi iblue-green or tan in thin section
• 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
i ill l t f lviews you will see only one set of cleavages

23. Igneous Amphiboles
Bi H bl d C l i hi fi i dBig Hornblende Crystals in this fine-grained
volcanic rock are called Phenocrysts

24. Hb cleavage, color & pleochroism

25. Kaersutite (Na,Ti-rich igneous amphibole)Kaersutite (Na,Ti rich igneous amphibole)
i h d b l h iwith strong red-brown pleochroism

26. Biotite is the lowest Mafic Mineral on
Di ti Sid f B ’ S iDiscontinuous Side of Bowen’s Series
I lli l f f i d fi• It crystallizes very late from fractionated mafic
magmas and is only common in intermediate to
f l i H O b f Bi ifelsic magmas, H2O must be present for Biotite
to crystallize
• Biotite is a trioctahedral, t-o-t (or 2:1) sheet
silicate
• Biotite’s properties: it’s strongly pleochroic,
has cleavage, parallel extinction, bird’s-eyeg , p , y
extinction and pleochroic haloes around zircon

27. Muscovite is a Residual Phase on BRSMuscovite is a Residual Phase on BRS
• Crystallizes only from felsic magmas withCrystallizes only from felsic magmas with
H2O in the liquid
• Structure and properties are similar to• Structure and properties are similar to
biotite, except for color and pleochroism
C i G i d P i• Common in Granites and Pegmatites

28. Felsic Minerals on BRS
hi T
olivine
Ca plagioclase
hi T
orthopyroxene
clinopyroxene NaCa plagioclase
amphibole (Hb)
perature
biotite Na plagioclase
alkali feldspar
cooling
temp
alkali feldspar
muscovite
quartz
residual
phases
quartz
low T

29. Quartz PropertiesQuartz Properties
• H=7 G=2 65H 7, G 2.65
• Generally clear and glassy, may have a
variety of colors (clear smoky brown rose;variety of colors (clear, smoky, brown, rose;
it’s allochromatic)
C h id l f l• Conchoidal fracture, no cleavage
• Habit: hexagonal (6-sided prisms) or
massive
• Optical: low relief and low birefringencep g

30. Quartz in Granite Thin Section
PPL XPLPPL XPL
From Atlas of Rocks & Minerals in Thin Section

31. The Feldspar Ternary
C Al Si OCaAl2Si2O8
Anorthite
solid solutions
All natural
f ld
solid solutions feldspars
No feldspars
Miscibility Gap
Albite O th l
Miscibility Gap
NaAlSi3O8 KAlSi3O8
Albite Orthoclase
alkali feldspars

32. Alkali Feldspar (esp Orthoclase)Alkali Feldspar (esp. Orthoclase)
Properties
• H=6, G=2.56
• Generally turbid (cloudy); color white pinkGenerally turbid (cloudy); color white, pink
or flesh-colored
• 2 Perfect to good perpendicular cleavages• 2 Perfect to good perpendicular cleavages
• Habit: stubby prisms, simple twins common
• Optical: low relief and low birefringence
• Commonly Perthitic (micro and macro)y ( )

33. Alkali Feldspars have Perthitesp
What do Perthites look like?
Thin
sectionsection
in XPL

34. Alkali Feldspars have Perthites
What do Perthites look like?
1mm

35. What causes Perthites?
• Caused by un-mixing, exsolution or
separation of Na+ (diameter~1.1Å) and K+p ( )
(diameter~1.6Å) as the feldspar cools
• At low temperatures, there is a miscibility
gap between NaAlSi O and KAlSi Ogap between NaAlSi3O8 and KAlSi3O8

36. Microcline hasMicrocline has
plaid twinning
1mm

37. Plagioclase PropertiesPlagioclase Properties
• H=6-6 5 G=2 62-2 76 (Or H=6 G=2 56)H 6-6.5, G 2.62-2.76 (Or H=6, G=2.56)
• Luster pearly, vitreous/translucent
C l hi• Color white to gray
• One perfect, one good cleavage
• Optical: low relief and low birefringence
• Polysynthetic albite twinning usually presentPolysynthetic albite twinning usually present
• Not Perthitic!
C l d• Commonly zoned

38. Plagioclase Feldspars have
( l th ti l ll ) Albit T i(polysynthetic, lamellar) Albite Twins
d-
blein
inhand
lyvisi
PL)
isiblei
usuall
on(XP
ybevi
cimen,
sectio
May
spec
thin

39. Minerals of PegmatitesMinerals of Pegmatites
Large crystals of Quartz,
Alkali Feldspar andAlkali Feldspar and
Muscovite
(which are low on Bowen’s
Reaction Series)Reaction Series)

40. Pegmatites are rich in IncompatiblePegmatites are rich in Incompatible
Elements and Volatiles
• Incompatible Elements – those elements
that don’t fit well in common igneousthat don t fit well in common igneous
minerals (Li, B, Be)
V l il h l h h• Volatiles – those elements that have a
strong preference for the gaseous phase
(H O CO Cl F S)(H2O, CO2, Cl, F, S)

41. Phase Diagrams tell MUCH more
about Crystallization and Melting
than Bowen’s Reaction Seriesthan Bowen s Reaction Series
• What follows is just a quick preview of
some of the things phase diagrams can beg p g
used for
• We’ll look at two really simple examplesWe ll look at two really simple examples

42. How do we use this diagram to tell
about crystallization of basalt?about crystallization of basalt?
Diopside-rich
li id (X) Yliquid (X)
80% CaMgSi2O6 (Di)
20% CaAl2Si2O8 (An)
X
Y
% 2 2 8 ( )
Draw isopleth=
line of constant
composition
What happens as
liquid cools?
Anorthite-rich liquid (Y)q ( )
65% CaAl2Si2O8 (An)
35% CaMgSi2O6 (Di)

43. Textures of rocks in Di-An System

44. 2-component system with
Complete Solid SolutionComplete Solid Solution
Crystallization of
Plagioclase in
Basalts (real
i l i ligneous plagioclase
is not pure An like
in the Di-Anin the Di An
system)

45. Pyroxene Compositions and the
P Q d il lPyroxene Quadrilateral
Diopside Hedenbergiteclinopyroxenes
CaFeSi2O6CaMgSi2O6
Diopside Hedenbergiteclinopyroxenes
Augite
Orthopyroxene
Mg2Si2O6 Fe2Si2O6
FerrosiliteEnstatite orthopyroxenes
py