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Names of Igneous Rocks

Names of Igneous Rocks. Texture + Composition = name Set up diagrams (many ternary ones again, you remember how these work?) to represent composition changes for rocks of a certain texture Composition can be related to specific minerals, or even physical characteristics of mineral grains

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Names of Igneous Rocks

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  1. Names of Igneous Rocks • Texture + Composition = name • Set up diagrams (many ternary ones again, you remember how these work?) to represent composition changes for rocks of a certain texture • Composition can be related to specific minerals, or even physical characteristics of mineral grains • Modal Composition - % of minerals comprising a rock

  2. Visual Estimation of Modal Abundance

  3. Classification based on Modal Mineralogy • Felsic rocks: mnemonic based on feldspar and silica. Also applies to rocks containing abundant feldspathoids, such as nepheline. GRANITE • Mafic rocks: mnemonic based on magnesium and ferrous/ferric. Synonymous with ferromagnesian, which refers to biotite, amphibole, pyroxene, olivine, and Fe-Ti oxides. BASALT • Ultramafic rocks: very rich in Mg and Fe. Generally have little feldspar. PERIDOTITE • Silicic rocks: dominated by quartz and alkali fsp. Sometimes refered to as sialic (Si + Al).

  4. granite granodiorite

  5. The rock must contain a total of at least 10% of the minerals below. Renormalize to 100% Q Classification of Phaneritic Igneous Rocks Quartzolite 90 90 Quartz-rich Granitoid 60 60 Grano- Tonalite Granite Alkali Feldspar Granite diorite Q=quartz A=Alkali fledspars (An0-An5) P=Plagioclase feldspars (An5-An100) F=Feldspathoid Alkali Fs. Qtz. Diorite/ 20 20 Quartz Syenite Qtz. Gabbro Quartz Quartz Quartz Monzonite Syenite Monzodiorite Alkali Fs. 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 Figure 2-2. A classification of the phaneritic igneous rocks. a.Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. 60 60 (Foid)olites F

  6. Classification of 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.

  7. Aphanitic rocks basalt rhyolite

  8. Classification of aphanitic Igneous Rocks Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS.

  9. Classification of 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.

  10. Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Classification of Igneous Rocks Plagioclase Feldspar Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. Pyroxene Olivine

  11. reticulite Pele’s tears Pele’s Hair (the really thin strands among the rocks…)

  12. Obsidian Volcanic glass formed from more felsic magmas (more silica), typically rhyolitic and usually black from small amount of nanocrystalline magnetite - Glass can get molecularly thin pretty easily, making it very sharp and using through antiquity for knives/spears, and still used today for scalpels (permits a much finer blade than does steel) Obsidian Cliffs, Yellowstone N.P.

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