Classification of igneous rocks
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This document classifies igneous rocks based on their visible crystal size, composition, and mineralogy. It divides rocks into phaneritic, aphanitic, porphyritic, fragmental, pegmatitic, and glassy categories based on crystal size. It further classifies rocks as felsic, intermediate, mafic, or ultramafic based on their silica and iron/magnesium content. Diagrams show classifications of specific rock types like plutonic rocks, volcanic rocks, gabbroic rocks, and ultramafic rocks based on their mineralogy and compositions.
Classification of igneous rocks
- 1. Classification of igneous rocks Phaneritic: crystals visible with naked eye Plutonic or intrusive rocks Phaneritic Aphanitic: crystal too small for naked eye Volcanic or extrusive rocks Aphanitic
- 2. Porphyritic: two different, dominant grain sizes Large Crystals = phenocrysts; Small Crystals = groundmass Porphyritic Fragmental: composed of disagregated igneous material Pyroclastic rocks Pyroclastic
- 3. Pegmatitic Pegmatitic: very large xtals (cm to 10s of cm); i.e., slowly cooled Forms veins or layers within plutonic body Glassy Glassy: non-crystalline; cools very fast (e.g., obsidian) Volcanic rocks Vesicular: Vesicular vesicles (holes, pores, cavities) form as gases expand Volcanic rocks
- 4. Classification of Igneous Rocks Pyroclastic rocks 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.
- 5. Compositional terms for igneous rocks Felsic: feldspar + silica ~55-70% silica, K-feldspar > 1/3 of feldspars present light-colored silicate minerals — Continental crust Intermediate: between felsic and mafic ~55-65% silica, plag > 2/3 of feldspars present Na-rich plag predominates over Ca-rich plag Mafic: magnesium + ferric iron ~45-50% silica; Ca-rich plag dominant feldspar dark silicate minerals — Oceanic crust Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars Mantle-derived
- 6. Classification of common igneous rocks Composition Phaneritic Aphanitic Color index (% dark minerals) Felsic Granite Rhyolite 10 Syenite Trachyte 15 Monzonite Latite 20 Intermediate Granodiorite Dacite 20 Diorite Andesite 25 Mafic Gabbro Basalt 50 Ultramafic Peridotite 95
- 7. Mineralogical terms of Igneous Rocks
- 8. Mineralogical terms of Igneous Rocks
- 9. Classification of Igneous Rocks . 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.
- 10. (a) Q The rock must contain a total of at least 10% of the minerals below. Quartzolite Renormalize to 100% 90 90 Quartz-rich Classification of Granitoid 60 60 Phaneritic ite n Gra To Grano- na Granite ar diorite lite dsp Igneous Rocks Fel li Alka Alkali Fs. 20 20 Qtz. Diorite/ Quartz Syenite Quartz Qtz. Gabbro Quartz Quartz Plutonic rocks Alkali Fs. Syenite 5 Syenite Syenite Monzonite Monzodiorite Monzodiorite 5 Diorite/Gabbro/ 10 35 Monzonite 65 90 Anorthosite A (Foid)-bearing (Foid)-bearing (Foid)-bearing P Syenite Monzonite Monzodiorite 10 10 (Foid)-bearing Diorite/Gabbro (Foid)-bearing (Fo Alkali Fs. Syenite id) ro (Foid) (Foid) bb Sy Monzosyenite Monzodiorite Ga en ite id) (Fo 60 60 Figure. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + (Foid)olites feldspathoids). After IUGS. F
- 11. Q Classification of Aphanitic Igneous Rocks 60 60 Volcanic rocks Rhyolite Dacite 20 20 Trachyte Latite Andesite/Basalt 35 65 A (foid)-bearing (foid)-bearing (foid)-bearing P Trachyte Latite Andesite/Basalt 10 10 Phonolite Tephrite A classification and nomenclature of volcanic rocks. After IUGS. 60 60 (Foid)ites F
- 12. Classification of Igneous Rocks Plagioclase Gabbroic Anorthosite Figure. A classification of the phaneritic 90 rocks igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. ro T ro Olivine bb cto Ga Ultramafic lite Olivine Dunite gabbro 90 rocks Peridotites We te hr i urg Plagioclase-bearing ultramafic rocks lite Lherzolite rzb Pyroxene Olivine Ha (b) 40 Orthopyroxenite Olivine Websterite Pyroxenites 10 (c) 10 Websterite Clinopyroxenite Orthopyroxene Clinopyroxene