Classifications of Igneous Rocks Chapter 2

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)

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/ ( ) = x = 69.76, 2.6 x = x = 10.1 percents total 99.99% close enough to 70, 20, 10 percents respectively

IUGS Classification of Phaneritic 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 Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing Syenite (Foid)-bearing Monzonite (Foid)-bearing Monzodiorite 90 Alkali Fs. Syenite (Foid)-bearing Alkali Fs.Syenite 10 (Foid) Monzosyenite (Foid) Syenite (Foid) Monzodiorite (Foid) Gabbro Qtz. Diorite/ Qtz. Gabbro 5 10 Diorite/Gabbro/ Anorthosite (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Quartzolite Granite Grano- diorite Tonalite 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

Classification of Aphanitic Igneous 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. Define Dacite, Trachyte, Latite and Phonolite and Tephrite based on this

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, 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, b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, From your text: An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

Classification of Minerals l Common Silicate minerals F Nesosilicates – Independent Tetrahedra F Olivine High temperature Fe-Mg silicate (typical mantle mineral - formed 100’s km in EarthHigh temperature Fe-Mg silicate (typical mantle mineral - formed 100’s km in Earth Individual tetrahedra linked together by iron and magnesium ionsIndividual tetrahedra linked together by iron and magnesium ions Forms small, rounded crystals with no cleavageForms small, rounded crystals with no cleavage (Mg,Fe) 2 SiO 4 High interference colors No consistent cleavages

Classification of Minerals Classification of Minerals l Common Silicate minerals F Pyroxene Group Single Chain Inosilicates F for example ( Mg,Fe)SiO 3 Single chain structures involving iron and magnesium, chains weakly pairedSingle chain structures involving iron and magnesium, chains weakly paired Two distinctive cleavages at nearly 90 degreesTwo distinctive cleavages at nearly 90 degrees Augite is the most common mineral in the pyroxene groupAugite is the most common mineral in the pyroxene group

Classification of Minerals Classification of Minerals l Common Silicate minerals F Amphibole Group Double Chain Inosilicates F Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2 Double chain structures involving a variety of ionsDouble chain structures involving a variety of ions Two perfect cleavages exhibiting angles of, e.g. 124 and 56 degrees in Hornblende.Two perfect cleavages exhibiting angles of, e.g. 124 and 56 degrees in Hornblende. Hornblende is the most common mineral in the amphibole groupHornblende is the most common mineral in the amphibole group Pleochroic in Plane Polarized Light Looks stringy

Hornblende Crystal 56 and 124 degree Cleavages Distinguish Hornblende from Pyroxene Group by cleavage Pyroxene Crystal Two Cleavage Faces at about 90 degrees 90 o

Cleavage in Pyroxenes It isn’t perfect in all slices

Cleavage in Amphiboles Looking down the c-axis

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

Classification of Minerals Classification of Minerals l Common Silicate minerals F Mica Group Phyllosilicates Sheet structures that result in one direction of perfect cleavageSheet structures that result in one direction of perfect cleavage Biotite is the common dark colored mica mineral. Has wavy “bird’s eye extinction”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.Muscovite is the common light colored mica mineral. Can have undulose extinction. Muscovite KAl 3 Si 3 O 10 (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 Pleochroic in PPL

Orders of Interference colors

3-D (Framework) Tectosilicates Quartz SiO2

Quartz l Undulose (aka “undulatory”) extinction 1 o gray in standard thin section 30  m 1 o gray in standard thin section 30  m l l 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 Possibilities circled

Feldspars Feldspars l Common Silicate minerals l Tectosilicates F Feldspar Group Most common mineral groupMost common mineral group 3-dimensional framework of tetrahedra exhibit two directions of perfect cleavage at 90 degrees3-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 groupsK-spars (potassium feldspar) and Plagioclases (sodium to calcium feldspar solutions) are the two most common groups Pearly to vitreous LusterPearly to vitreous Luster

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 Microcline is Triclinic, Orthoclase is Monoclinic

Plagioclase feldspar Plagioclase feldspar Note the Twinning, seems to have ‘stripes’ (Ca,Na)AlSi 3 O 8 Labradorite Albite NaAlSi 3 O 8

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. Pink garnet (PPL) Garnet inclusion (crossed polars) Indistinct cleavage Looks like a squashed pink tomato

Next week: Chapter 3 Textures of Igneous Rocks