Classifications of Igneous Rocks

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Classifications of Igneous Rocks. Chapter 2. Classification of Igneous Rocks. X = 100%. Discussion: Normalization If total does not add to 100%, normalize by multiplying each term by 100/(X + Y + Z). X = 0%.

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### Classifications of Igneous Rocks

Chapter 2

Classification of Igneous Rocks

X = 100%

Discussion:

Normalization

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

100/(X + Y + Z)

X = 0%

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.

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

(a)

Q

The rock must contain a total of

at least 10% of the minerals below.

Quartzolite

Renormalize to 100%

90

90

IUGS Classification of PhaneriticIgneous Rocks

Quartz-rich

Granitoid

60

60

Grano-

Tonalite

Granite

Alkali Feldspar Granite

diorite

Alkali Fs.

Qtz. Diorite/

20

20

Quartz Syenite

Qtz. Gabbro

Quartz

Quartz

Quartz

Monzonite

Syenite

Monzodiorite

Alkali Fs.

Define Tonalite,

Monzonite,

Syenite based on this.

5

Diorite/Gabbro/

5

Syenite

Syenite

Monzodiorite

Monzonite

Anorthosite

90

35

10

65

A

P

(Foid)-bearing

(Foid)-bearing

(Foid)-bearing

Syenite

Monzonite

Monzodiorite

(Foid)-bearing

10

10

Diorite/Gabbro

(Foid)-bearing

Alkali Fs.

Syenite

(Foid) Syenite

(Foid)

(Foid)

Monzosyenite

Monzodiorite

(Foid) Gabbro

International Union of Geological Sciences

Don’t use “foid” in a rock name. Use the actual Feldspathoid mineral name

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.

60

60

(Foid)olites

F

Classification of Aphanitic Igneous Rocks

Define Dacite, Trachyte, Latite

and Phonolite and Tephrite

based on this

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.

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.

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.

Classification of Minerals
• Common Silicate minerals
• Nesosilicates – Independent Tetrahedra
• Olivine
• High temperature Fe-Mg silicate (typical mantle mineral - formed 100’s km in Earth
• Individual tetrahedra linked together by iron and magnesium ions
• Forms small, rounded crystals with no cleavage

(Mg,Fe)2SiO4

High interference colors

No consistent cleavages

Classification of Minerals
• Common Silicate minerals
• Pyroxene Group Single Chain Inosilicates
• for example (Mg,Fe)SiO3
• Single chain structures involving iron and magnesium, chains weakly paired
• Two distinctive cleavages at nearly 90 degrees
• Augite is the most common mineral in the pyroxene group
Classification of Minerals

Looks stringy

• Common Silicate minerals
• Amphibole Group Double Chain Inosilicates
• Ca2(Fe,Mg)5Si8O22(OH)2
• Double chain structures involving a variety of ions
• Two perfect cleavages exhibiting angles of , e.g. 124 and 56 degrees in Hornblende.
• Hornblende is the most common mineral in the amphibole group

Pleochroic in Plane Polarized Light

Distinguish Hornblende from

Pyroxene Group by cleavage

Hornblende Crystal56 and 124 degreeCleavages

Pyroxene CrystalTwo Cleavage Faces at about 90 degrees

90o

Cleavage in Pyroxenes

It isn’t perfect in all slices

Cleavage in Amphiboles

Looking down the c-axis

Amphiboles
• Amphiboles such as Hornblende are pleochroic in Plane Polarized Light. Hornblende is monoclinic. With crossed polars, they have inclined extinction, i.e. they go dark at an angle to ONE of their cleavage planes
“Clinopyroxenes” (monoclinic pyroxenes) also have inclined extinction,but are not pleochroic in PPL
• Any monoclinic mineral has one inclined extinction when rotating with crossed polars

Classification of Minerals
• Common Silicate minerals
• Mica Group Phyllosilicates
• Sheet structures that result in one direction of perfect cleavage
• Biotite is the common dark colored mica mineral. Has wavy “bird’s eye extinction”
• Muscovite is the common light colored mica mineral . Can have undulose extinction.

KAl3Si3O10(OH)2

Muscovite

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

Quartz
• Undulose (aka “undulatory”) extinction
• 1o gray in standard thin section 30mm
• Forms late in igneous, fills in gaps between earlier xtals
Identifying minerals with a 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

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

Possibilities circled

Feldspars
• Common Silicate minerals
• Tectosilicates
• Feldspar Group
• Most common mineral group
• 3-dimensional framework of tetrahedra exhibit two directions of perfect cleavage at 90 degrees
• K-spars (potassium feldspar) and Plagioclases (sodium to calcium feldspar solutions) are the two most common groups
• Pearly to vitreous Luster
Potassium feldspar

KAlSi3O8

Note Pearly Luster

Tartan twins in Microcline.

Microcline is the low TP version of K-spars KAlSi3O8

Microcline is Triclinic, Orthoclase is Monoclinic

Perthitic Texture, Microcline plus exsolved Albite

Plagioclase feldspar

(Ca,Na)AlSi3O8

Note the Twinning, seems to have ‘stripes’

Glass (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.

Garnet

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

Looks like a squashed pink tomato

Pink garnet (PPL) Garnet inclusion (crossed polars)

Indistinct cleavage