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Classifications of Igneous Rocks Classifications of Igneous Rocks

Chapter 2

Chapter 2

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Classification of Igneous Rocks Classification of Igneous Rocks

Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams.

Figure 2-1a. from your text: An Introduction to Igneous and Metamorphic Petrology, John Winter.

X = 100%

X = 0%

Discussion:

Normalization

If total does not add to 100%, normalize by multiplying each term by

100/(X + Y + Z)

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Normalization Example Normalization Example

Normalization

If total does not add to 100%, normalize by multiplying each term by

100/(X + Y + Z)

Ex 1. A sample contains X = 9 g. Qtz, Y = 2.6 g Plag, Z= 1.3 g Microcline What are the percentages by weight?

Sol’n: Multiply each by

100/ (9 + 2.6 + 1.3) = 7.752

9 x 7.752 = 69.76, 2.6 x 7.752 = 20.15 1.3 x 7.752 = 10.1 percents

total 99.99% close enough to 70, 20,

10 percents respectively

(4)

IUGS IUGS

Classification of Classification of

Phaneritic Phaneritic

Igneous Rocks Igneous Rocks

Figure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. From your text: An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

The rock must contain a total of at least 10% of the minerals below.

Renormalize to 100%

(a)

Quartz-rich Granitoid

90 90

60 60

20 20

Alkali Fs.

Quartz Syenite Quartz Syenite

Quartz

Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing

Syenite

5

10 35 65

(Foid)-bearing

Monzonite (Foid)-bearing Monzodiorite

90

Alkali Fs.

Syenite

(Foid)-bearing Alkali Fs. Syenite

10

(Foid) Monzosyenite (Foid

) Sye nite

(Foid) Monzodiorite

(Foid) Gabbro

Qtz. Diorite/

Qtz. Gabbro

5

10

Diorite/Gabbro/

Anorthosite (Foid)-bearing Diorite/Gabbro

60

(Foid)olites

Quartzolite

Granite Grano- diorite

Tona lite

Alkali Feldspar Granite

Q

A P

F

60

Define Tonalite, Monzonite,

Syenite based on this.

International Union of Geological Sciences Don’t use “foid”

in a rock name.

Use the actual

Feldspathoid

mineral name

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(6)

Classification of Classification of Aphanitic Igneous Aphanitic Igneous

Rocks Rocks

Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. From your text:

An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

(foid)-bearing

Trachyte (foid)-bearing

Latite (foid)-bearing Andesite/Basalt

(Foid)ites

10

60 60

35 65

10

20 20

60 60

F

A P

Q

Rhyolite Dacite

Trachyte Latite Andesite/Basalt

Phonolite Tephrite

Define Dacite, Trachyte, Latite and Phonolite and Tephrite

based on this

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Classification of Aphanitic Igneous Rocks Classification of Aphanitic Igneous Rocks

Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al.

(1986) J. Petrol., 27, 745-750. Oxford University Press.

(8)

Classification of Pyroclastic Igneous Rocks Classification of Pyroclastic Igneous Rocks

Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298. From your text: An

Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

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Classification of Minerals Classification of Minerals

Common Silicate minerals Common Silicate minerals

Nesosilicates – Independent Tetrahedra Nesosilicates – Independent Tetrahedra

Olivine Olivine

High temperature Fe-Mg silicate (typical High temperature Fe-Mg silicate (typical mantle mineral - formed 100’s km in Earth mantle mineral - formed 100’s km in Earth

Individual tetrahedra linked together by iron Individual tetrahedra linked together by iron and magnesium ions

and magnesium ions

Forms small, rounded crystals with no Forms small, rounded crystals with no cleavage

cleavage

(Mg,Fe)

2

SiO

4

High interference colors

No consistent

cleavages

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Classification of Minerals Classification of Minerals

Common Silicate minerals Common Silicate minerals

Pyroxene Group Single Chain Inosilicates Pyroxene Group Single Chain Inosilicates

for example ( for example ( Mg,Fe)SiO Mg,Fe)SiO

33

Single chain structures involving iron and Single chain structures involving iron and magnesium, chains weakly paired

magnesium, chains weakly paired

Two distinctive cleavages at nearly 90 degrees Two distinctive cleavages at nearly 90 degrees

Augite Augite is the most common mineral in the is the most common mineral in the pyroxene group

pyroxene group

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Classification of Minerals Classification of Minerals

Common Silicate minerals Common Silicate minerals

Amphibole Group Double Chain Inosilicates Amphibole Group Double Chain Inosilicates

Ca Ca

22

(Fe,Mg) (Fe,Mg)

55

Si Si

88

O O

2222

(OH) (OH)

22

Double chain structures involving a variety of Double chain structures involving a variety of ions ions

Two perfect cleavages exhibiting angles of , e.g. Two perfect cleavages exhibiting angles of , e.g.

124 and 56 degrees in Hornblende.

124 and 56 degrees in Hornblende.

Hornblende Hornblende is the most common mineral in the is the most common mineral in the amphibole group

amphibole group

Pleochroic in Plane Polarized Light

Looks stringy

(12)

Hornblende Crystal Hornblende Crystal

56 and 124 degree 56 and 124 degree

Cleavages Cleavages

Distinguish Hornblende from Pyroxene Group by cleavage

Pyroxene Crystal

Two Cleavage Faces

at about 90 degrees

90

o

(13)

Cleavage in Pyroxenes Cleavage in Pyroxenes

It isn’t perfect in all slices

(14)

Cleavage in Amphiboles

Looking down the c-axis

(15)

Looking down the c-axis

Looking down the c-axis

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Amphiboles Amphiboles

Amphiboles such as Hornblende are pleochroic in Amphiboles such as Hornblende are pleochroic in Plane Polarized Light. Hornblende is monoclinic.

Plane Polarized Light. Hornblende is monoclinic.

With crossed polars, they have inclined extinction, With crossed polars, they have inclined extinction,

i.e. they go dark at an angle to ONE of their i.e. they go dark at an angle to ONE of their

cleavage planes

cleavage planes

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“ “ Clinopyroxenes” (monoclinic pyroxenes) Clinopyroxenes” (monoclinic pyroxenes) also have inclined extinction,

also have inclined extinction, but are not pleochroic in PPL but are not pleochroic in PPL

Any monoclinic mineral has one inclined Any monoclinic mineral has one inclined extinction when rotating with crossed polars extinction when rotating with crossed polars

http://www.youtube.com/watch?v=1DSqh5oEYOE

(18)

Classification of Minerals Classification of Minerals

Common Silicate minerals Common Silicate minerals

Mica Group Phyllosilicates Mica Group Phyllosilicates

Sheet structures that result in one direction of perfect Sheet structures that result in one direction of perfect cleavage

cleavage

Biotite Biotite is the common dark colored mica mineral. Has is the common dark colored mica mineral. Has wavy “bird’s eye extinction”

wavy “bird’s eye extinction”

Muscovite Muscovite is the common light colored mica mineral . is the common light colored mica mineral . Can have undulose extinction.

Can have undulose extinction.

https://www.youtube.com/watch?v=dvDankgGBIs https://www.youtube.com/watch?v=dvDankgGBIs

Muscovite

KAl

3

Si

3

O

10

(OH)

2

(19)

In plane polarized light, Biotite is seen as dark brown to grey against the surrounding mostly colorless minerals. Under crossed polars

"bird's eye " = “mottled” = “wavy”

extinction can easily be seen when the mineral is nearly extinct. Often, the mineral color masks the

interference colors when the mineral is not extinct.

http://www.youtube.com/watch?v=IjUdjGQyWtw http://www.youtube.com/watch?v=Bv3MVkyyxjk

Pleochroic in PPL

http://www.youtube.com/watch?v=-6LEW_H-ccQ

(20)

Orders of Interference colors

Orders of Interference colors

(21)

3-D (Framework) Tectosilicates 3-D (Framework) Tectosilicates

Quartz SiO2

(22)

Quartz Quartz

Undulose (aka “undulatory”) extinction Undulose (aka “undulatory”) extinction

1 1 o o gray in standard thin section 30 gray in standard thin section 30   m m

http://www.youtube.com/watch?v=O1I-_YdgaHg http://www.youtube.com/watch?v=O1I-_YdgaHg

Forms late in igneous, fills in gaps between earlier xtals Forms late in igneous, fills in gaps between earlier xtals

(23)

Identifying minerals with a Identifying minerals with a

Michel-Levy Chart Michel-Levy Chart

If you know the thickness of the thin section, you can narrow

down the possibilities by noting where the interference color of

an unknown crosses the thickness line

(24)

Thin section ~30 microns, mineral is dark second order blue, so birefringence about 0.020

Possibilities circled

(25)

Feldspars Feldspars

Common Silicate minerals Common Silicate minerals

Tectosilicates Tectosilicates

Feldspar Group Feldspar Group

Most common mineral group Most common mineral group

3-dimensional framework of tetrahedra exhibit two 3-dimensional framework of tetrahedra exhibit two directions of perfect cleavage at 90 degrees

directions of perfect cleavage at 90 degrees

K-spars (potassium feldspar) and K-spars (potassium feldspar) and Plagioclases (sodium Plagioclases (sodium to calcium feldspar solutions) are the two most

to calcium feldspar solutions) are the two most common groups

common groups

Pearly to vitreous Luster Pearly to vitreous Luster

(26)

Potassium feldspar Potassium feldspar

Note Pearly Luster

KAlSi

3

O

8

Perthitic Texture, Microcline plus exsolved Albite Tartan twins in Microcline.

Microcline is the low TP version of K-spars KAlSi

3

O

8

http://www.youtube.com/watch?v=7-KZREqrh44

Microcline is Triclinic, Orthoclase is Monoclinic

(27)

Plagioclase feldspar Plagioclase feldspar

Note the Twinning, seems to have ‘stripes’

(Ca,Na)AlSi

3

O

8

Labradorite Albite NaAlSi

3

O

8

http://www.youtube.com/watch?v=gLcVT_6y-MA

(28)

Glass

Glass (magma cooled to fast for crystals to form) (magma cooled to fast for crystals to form)

Plagioclase (Anorthite) xtals in basaltic glass. Crossed Polars

The glass is isotropic and so stays extinct under crossed polars,

i.e. it is black in all orientations.

(29)

Garnet Garnet

Garnet is also Isotropic, and has a very high refractive index, so cracks stand out strongly. Under crossed polars it stays dark.

Pink garnet (PPL) Garnet inclusion (crossed polars) Indistinct cleavage

Looks like a squashed

pink tomato

(30)

Next week: Chapter 3 Textures of Igneous Rocks

References

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