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 Visual Estimation of Modal Abundance 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 granite granodiorite Q Classification of Phaneritic Igneous Rocks Quartzolite 90 90 Quartz rich Granitoid 60 Gra n Alka li Fel dsp ar Alkali Fs Syenite 20 20 Quartz Monzonite Quartz Syenite 5 Syenite 10 A Granodiorite Granite lite na To Alkali Fs Quartz Syenite Foid bearing Syenite 35 Monzonite Foid bearing Monzonite Quartz Monzodiorite 65 Monzodiorite Foid bearing Monzodiorite Figure 2 2 A classification of the phaneritic igneous rocks a Phaneritic rocks with more than 10 quartz feldspar feldspathoids After IUGS Foid Monzosyenite Foid Monzodiorite 60 Fo id G Foid bearing Alkali Fs Syenite ab bro 10 ite en Sy id Fo Q quartz A Alkali fledspars An0 An5 P Plagioclase feldspars An5An100 F Feldspathoid ite The rock must contain a total of at least 10 of the minerals below Renormalize to 100 60 60 Foid olites F Qtz Diorite Qtz Gabbro 5 Diorite Gabbro 90 Anorthosite P 10 Foid bearing Diorite Gabbro 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 Aphanitic rocks basalt rhyolite Q Classification of aphanitic Igneous Rocks 60 60 Rhyolite Dacite 20 20 Trachyte Latite 35 A 10 foid bearing Trachyte Andesite Basalt 65 foid bearing Latite Phonolite foid bearing Andesite Basalt 10 Tephrite Figure 2 3 A classification and nomenclature of volcanic rocks After IUGS 60 60 Foid ites F P 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 Classification of Igneous Rocks Plagioclase Feldspar Plagioclase Anorthosite Figure 2 2 A classification of the phaneritic igneous rocks b Gabbroic rocks c Ultramafic rocks After IUGS 90 Olivine Dunite Olivine gabbro Plagioclase bearing ultramafic rocks Pyroxene Pyroxene b Olivine Olivine Peridotites lite hr We Ha rzb urg ite 90 Lherzolite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 10 Orthopyroxene Websterite Clinopyroxenite Clinopyroxene reticulite Pele s tears Pele s Hair the really thin strands among the rocks 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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