Classifications of Igneous Rocks

1. Classifications of Igneous Rocks Chapter 2

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.

3. 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. (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

6. 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.

7. 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 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.

9. 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

10. 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

11. 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

12. Distinguish Hornblende from Pyroxene Group by cleavage Hornblende Crystal56 and 124 degreeCleavages Pyroxene CrystalTwo Cleavage Faces at about 90 degrees 90o

13. 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

16. 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

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

18. 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. • https://www.youtube.com/watch?v=dvDankgGBIs KAl3Si3O10(OH)2 Muscovite

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

21. 3-D (Framework) Tectosilicates Quartz SiO2

22. Quartz • Undulose (aka “undulatory”) extinction • 1o gray in standard thin section 30mm • http://www.youtube.com/watch?v=O1I-_YdgaHg • Forms late in igneous, fills in gaps between earlier xtals

23. 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

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

25. 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

26. Potassium feldspar KAlSi3O8 Note Pearly Luster http://www.youtube.com/watch?v=7-KZREqrh44 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

27. Plagioclase feldspar (Ca,Na)AlSi3O8 Note the Twinning, seems to have ‘stripes’ http://www.youtube.com/watch?v=gLcVT_6y-MA Labradorite Albite NaAlSi3O8

28. 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.

29. 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

30. Next week: Chapter 3 Textures of Igneous Rocks