A common member of the pyroxene family is augite, itself containing several solid solution series with a complex chemical formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 that gives rise to a number of individual mineral names. Amphibole is even more permissive than pyroxene and its compositions can be very complex. To avoid these complications, the following figure presents a simplified version of igneous rock nomenclature focusing on the four main groups, which is adequate for an introductory student. Exercise: Classifying Igneous Rocks by the Proportion of Dark Minerals The four igneous rocks shown below have differing proportions of ferromagnesian silicates (dark minerals). Mineral Group: non-ferromangnesian silicate Luster/Color: non-metallic, glassy/colorless Cleavage: 1 perfect direction Hardness: 2 to 3 Other Characteristics: splits into thin elastic sheets, transparent to translucent BIOTITE Chemical Formula: K (Mg,Fe) 3 (Al,Fe)Si 3 O 10 (OH) 2 Mineral Group: ferromagnesian silicate There is no need for aluminum or any of the other cations such as sodium or potassium. Sulfides are well known for being important ore minerals. Feldspars, micas, and ferromagnesian minerals (pyroxenes, amphiboles, olivines, etc.) These combinations and others create the chemical structure in which positively charged ions can be inserted for unique chemical compositions forming silicate mineral groups. in, Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher. The term is used to cover such minerals as the olivines, pyroxenes, amphiboles, and the micas, biotite and phlogopite. As a result of the ionic character, silicon becomes a cation (with a charge of +4) and oxygen becomes an anion (with a charge of 2). Halite, Calcite ________ exhibit a sheet-like silicate structure. Peacock, M. A. Therefore, fewer cations are necessary to balance that charge. 1. The solid parts, called tephra, settle back to earth and cool into rocks with pyroclastic textures. In muscovite mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral. A silicate mineral made up of isolated silica tetrahedra and with either iron or magnesium (or both) as the cations. Quartz and orthoclase feldspar are felsic minerals. It can be found in the Andes Mountains and in some island arcs (see. Biotite mica has more iron and magnesium and is considered a ferromagnesian silicate mineral. The diagram below represents a single chain in a silicate mineral. As previously described, the comma between iron (Fe) and magnesium (Mg) indicates these two elements occur in a solid solution. Laccoliths bulge upwards; a similar downward-bulging intrusion is called a lopolith. A sill is a concordant intrusion that runs parallel to the sedimentary layers in the country rock. Amphibole is even more permissive than pyroxene and its compositions can be very complex. Classification of Igneous Rock Series. The chemical formula is (Fe,Mg)2SiO4. Olivine has a pure iron end-member (called fayalite) and a pure magnesium end-member (called forsterite). Mafic materials can also be described as ferromagnesian. Since in every silica tetrahedron one silicon cation has a +4 charge and the two oxygen anions each have a 2 charge, the charge is balanced. 2.4 Silicate Minerals - Physical Geology - 2nd Edition Chemically, sheet silicates usually contain silicon and oxygen in a 2:5 ratio (Si4O10). Silica tetrahedra are bonded in three-dimensional frameworks in both the feldspars and quartz. Dikes are therefore discordant intrusions, not following any layering that was present. Micas contain mostly silica, aluminum, and potassium. Andesite is a fine crystalline intermediate extrusive rock. A potassium-bearing non-ferromagnesian mica. Diamond and graphite are also native element minerals, both composed entirely of carbon. These are generally called the rock-forming minerals. 3.2 Magmass press Magma Formation. In olivine, it takes two divalent cations to balance the 4 charge of an isolated tetrahedron. In quartz (SiO2), the silica tetrahedra are bonded in a perfect three-dimensional framework. Another is the native rock is melted and consumed into the rising magma or broken into pieces that settle into the magma, a process known as stoping. For igneous rock, the composition is divided into four groups: felsic, intermediate, mafic, and ultramafic. Count the number of tetrahedra versus the number of oxygen ions (yellow spheres). The structure of pyroxene is more permissive than that of olivine meaning that cations with a wider range of ionic radii can fit into it. Physical Geology Lab Samples - Georgia Southwestern State University Thats why pyroxenes can have iron (radius 0.63 ) or magnesium (radius 0.72 ) or calcium (radius 1.00 ) cations (see Figure 3.1.3 above). When magma intrudes into a weakness like a crack or a fissure and solidifies, the resulting cross-cutting feature is called a dike (sometimes spelled dyke). Since the one silicon cation has a +4 charge and the two oxygen anions each have a 2 charge, the charge is balanced. Examples of rhyolite include several lava flows in Yellowstone National Park and the altered rhyolite that makes up the Grand Canyon of the Yellowstone. An Introduction to Geology (Johnson, Affolter, Inkenbrandt, and Mosher), { "4.01:_Classification_of_Igneous_Rocks" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "4.02:_Bowens_Reaction_Series" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "4.03:_Magma_Generation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "4.04:_Partial_Melting_and_Crystallization" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "4.05:_Volcanism" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { "00:_Front_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "01:_Understanding_Science" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02:_Plate_Tectonics" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03:_Minerals" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04:_Igneous_Processes_and_Volcanoes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05:_Weathering_Erosion_and_Sedimentary_Rocks" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06:_Metamorphic_Rocks" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07:_Geologic_Time" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "08:_Earth_History" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "09:_Crustal_Deformation_and_Earthquakes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10:_Mass_Wasting" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "11:_Water" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "12:__Coastlines" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "13:_Deserts" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "14:_Glaciers" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "15:_Global_Climate_Change" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "16:_Energy_and_Mineral_Resources" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "zz:_Back_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic", "showtoc:no", "license:ccbyncsa", "authorname:johnsonaffolterinkenbmosher" ], https://geo.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fgeo.libretexts.org%2FBookshelves%2FGeology%2FBook%253A_An_Introduction_to_Geology_(Johnson_Affolter_Inkenbrandt_and_Mosher)%2F04%253A_Igneous_Processes_and_Volcanoes%2F4.01%253A_Classification_of_Igneous_Rocks, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), Aphanitic/Phaneritic Rock Types with Images. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Common mafic rocks include basalt, diabase and gabbro. 2. Impurities consisting of atoms within this framework give rise to many varieties of quartz among which are gemstones like amethyst, rose quartz, and citrine. Since the silicon ion has a charge of 4 and each of the four oxygen ions has a charge of 2, the silica tetrahedron has a net charge of 4. Biotite mica can have iron and/or magnesium in it and that makes it a ferromagnesian silicate mineral (like olivine, pyroxene, and amphibole). Plus, they are, by definition, exposed to the elements of erosion immediately. Because potassium ions are so much larger than sodium and calcium ions, which are very similar in size, the inability of the crystal lattice to accommodate both potassium and sodium/calcium gives rise to the two families of feldspar: orthoclase and plagioclase respectively. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Non-ferromagnesian Silicates are silicate minerals without substantial Fe and Mg in their crystalline structure. If we focus on the non-ferromagnesian silicates, it is evident that felsic rocks can have from 0% to 35% K-feldspar, from 25% to 35% quartz (the vertical thickness of the quartz field varies from 25% to 35%), and from 25% to 50% plagioclase (and that plagioclase will be sodium-rich, or albitic). In fact, the ions that are common in silicate minerals have a wide range of sizes, as depicted in Figure 3.1.3. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). Another feldspar is plagioclase with the formula (Ca,Na)AlSi3O8, the solid solution (Ca,Na) indicating a series of minerals, one end of the series with calcium CaAl2Si2O8, called anorthite, and the other end with sodium NaAlSi3O8, called albite. Both are sheet silicates and split easily into thin layers along planes parallel to the sheets. Micas contain mostly silica, aluminum, and potassium. The building block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom that form a four-sided pyramid shape with O at each corner and Si in the middle (Figure 3.1.1). 3. The intermediate-composition plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% to 90% Ca).
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