Classifications of Igneous RocksClassifications of Igneous Rocks

Chapter 2Chapter 2

Classification of Igneous RocksClassification 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:NormalizationIf total does not add to 100%, normalize by multiplying each term by 100/(X + Y + Z)

Normalization ExampleNormalization Example

NormalizationIf 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 MicroclineWhat are the percentages by weight?Sol’n: Multiply each by 100/ (9 + 2.6 + 1.3) = 7.7529 x 7.752 = 69.76, 2.6 x 7.752 = 20.151.3 x 7.752 = 10.1 percentstotal 99.99% close enough to 70, 20, 10 percents respectively

IUGS IUGS Classification of Classification of PhaneriticPhaneriticIgneous RocksIgneous 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 ofat least 10% of the minerals below.Renormalize to 100%

(a)

Quartz-richGranitoid

9090

6060

2020Alkali Fs.Quartz Syenite Quartz

SyeniteQuartz

MonzoniteQuartz

Monzodiorite

Syenite Monzonite Monzodiorite

(Foid)-bearingSyenite

5

10 35 65

(Foid)-bearingMonzonite

(Foid)-bearingMonzodiorite

90

Alkali Fs.Syenite

(Foid)-bearingAlkali Fs. Syenite

10

(Foid)Monzosyenite

(Foid) Syenite

(Foid)Monzodiorite

(Foi

d) G

abbr

o

Qtz. Diorite/Qtz. Gabbro

5

10

Diorite/Gabbro/Anorthosite

(Foid)-bearingDiorite/Gabbro

60

(Foid)olites

Quartzolite

Granite Grano-diorite

Tonalite

Alka

li Feld

spar

Gra

nite

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

Classification of Classification of Aphanitic Igneous Aphanitic Igneous

RocksRocks

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, Latiteand Phonolite and Tephritebased on this

Classification of Aphanitic Igneous RocksClassification 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 RocksClassification 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 MineralsClassification of Minerals Common Silicate mineralsCommon Silicate minerals

Nesosilicates – Independent TetrahedraNesosilicates – Independent Tetrahedra OlivineOlivine

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

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

• Forms small, rounded crystals with no Forms small, rounded crystals with no cleavagecleavage

(Mg,Fe)2SiO4

High interference colorsNo consistent cleavages

Classification of MineralsClassification of Minerals Common Silicate mineralsCommon Silicate minerals

Pyroxene Group Single Chain InosilicatesPyroxene Group Single Chain Inosilicates for example (for example (Mg,Fe)SiOMg,Fe)SiO33

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

• Two distinctive cleavages at nearly 90 degreesTwo distinctive cleavages at nearly 90 degrees

• AugiteAugite is the most common mineral in the is the most common mineral in the pyroxene grouppyroxene group

Classification of MineralsClassification of Minerals

Common Silicate mineralsCommon Silicate minerals Amphibole Group Double Chain InosilicatesAmphibole Group Double Chain Inosilicates CaCa22(Fe,Mg)(Fe,Mg)55SiSi88OO2222(OH)(OH)22

• Double chain structures involving a variety of Double chain structures involving a variety of ionsions

• 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.

• HornblendeHornblende is the most common mineral in the is the most common mineral in the amphibole groupamphibole group

Pleochroic in Plane Polarized Light

Looks stringy

Hornblende CrystalHornblende Crystal56 and 124 degree56 and 124 degreeCleavagesCleavages

Distinguish Hornblende fromPyroxene Group by cleavage

Pyroxene CrystalTwo Cleavage Faces at about 90 degrees

90o

Cleavage in PyroxenesCleavage in Pyroxenes

It isn’t perfect in all slices

Cleavage in Amphiboles

Looking down the c-axis

Looking down the c-axisLooking down the c-axis

AmphibolesAmphiboles 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 planescleavage planes

““Clinopyroxenes” (monoclinic pyroxenes) Clinopyroxenes” (monoclinic pyroxenes) also have inclined extinction,also have inclined extinction,but are not pleochroic in PPLbut are not pleochroic in PPL

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

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

Classification of MineralsClassification of Minerals Common Silicate mineralsCommon Silicate minerals

Mica Group PhyllosilicatesMica Group Phyllosilicates• Sheet structures that result in one direction of perfect Sheet structures that result in one direction of perfect

cleavagecleavage

• BiotiteBiotite 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”

• MuscoviteMuscovite 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

Muscovite

KAl3Si3O10(OH)2

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

Orders of Interference colorsOrders of Interference colors

3-D (Framework) Tectosilicates3-D (Framework) Tectosilicates

Quartz SiO2

QuartzQuartz Undulose (aka “undulatory”) extinctionUndulose (aka “undulatory”) extinction 11oo gray in standard thin section 30 gray in standard thin section 30m m http://www.youtube.com/watch?v=O1I-_YdgaHghttp://www.youtube.com/watch?v=O1I-_YdgaHg Forms late in igneous, fills in gaps between earlier xtalsForms late in igneous, fills in gaps between earlier xtals

Identifying minerals with a Identifying minerals with a Michel-Levy ChartMichel-Levy Chart

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

FeldsparsFeldspars

Common Silicate mineralsCommon Silicate minerals TectosilicatesTectosilicates

Feldspar GroupFeldspar Group• Most common mineral groupMost common mineral group

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

• K-sparsK-spars (potassium feldspar) and (potassium feldspar) and PlagioclasesPlagioclases (sodium (sodium to calcium feldspar solutions) are the two most to calcium feldspar solutions) are the two most common groupscommon groups

• Pearly to vitreous LusterPearly to vitreous Luster

Potassium feldsparPotassium feldsparNote Pearly Luster

KAlSi3O8

Perthitic Texture, Microcline plus exsolved Albite

Tartan twins in Microcline. Microcline is the low TP version of K-spars KAlSi3O8

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

Microcline is Triclinic, Orthoclase is Monoclinic

Plagioclase feldsparPlagioclase feldsparNote the Twinning, seems to have ‘stripes’

(Ca,Na)AlSi3O8

Labradorite Albite NaAlSi3O8

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

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 PolarsThe glass is isotropic and so stays extinct under crossed polars, i.e. it is black in all orientations.

GarnetGarnetGarnet 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

Next week: Chapter 3 Textures of Igneous Rocks