Page 1 :
12/3/2016, , General Classification of Igneous Rocks, , EENS 2120, Prof. Stephen A. Nelson, , Petrology, Tulane University, , General Classification of Igneous Rocks, This document last updated on 11Jan2011, , Classification of igneous rocks is one of the most confusing aspects of geology. This is partly, due to historical reasons, partly due to the nature of magmas, and partly due to the various, criteria that could potentially be used to classify rocks., Early in the days of geology there were few rocks described and classified. In those days, each new rock described by a geologist could have shown characteristics different than, the rocks that had already been described, so there was a tendency to give the new and, different rock a new name. Because such factors as cooling conditions, chemical, composition of the original magma, and weathering effects, there is a potential to see an, infinite variety of igneous rocks, and thus a classification scheme based solely on the, description of the rock would eventually lead to a plethora of rock names. Still, because, of the history of the science, many of these rock names are firmly entrenched in the, literature, so the student must be aware of all of these names, or at least know where to, look to find out what the various rocks names mean., Magmas, from which all igneous rocks are derived, are complex liquid solutions., Because they are solutions, their chemical composition can vary continuously within a, range of compositions. Because of the continuous variation in chemical composition, there is no easy way to set limits within a classification scheme., There are various criteria that could be used to classify igneous rocks. Among them are:, 1. Minerals Present in the Rock (the mode). The minerals present in a rock and, their relative proportions in the rock depend largely on the chemical composition, of the magma. This works well as a classification scheme if all of the minerals that, could potentially crystallize from the magma have done so usually the case for, slowly cooled plutonic igneous rocks. But, volcanic rocks usually have their, crystallization interrupted by eruption and rapid cooling on the surface. In such, rocks, there is often glass or the minerals are too small to be readily identified., Thus a system of classification based solely on the minerals present can only be, used., We can easily the inadequacy of a mineralogical classification based on minerals, present if you look at the classification schemes for volcanic rocks given in, introductory geology textbooks. For example, most such schemes show that a, dacite is a rock that contains small amounts of quartz, somewhat larger amounts of, sanidine or alkali feldspar, plagioclase, biotite, and hornblende, In all the years I, have been looking at igneous rocks (since about the midcretaceous) I have yet to, see a dacite that contains alkali feldspar. Does this mean that the intro geology, textbooks lie? Not really, these are the minerals that should crystallize from a, dacite magma, but don't because the crystallization history is interrupted by rapid, cooling on the surface., 2. Texture of the Rock. Rock texture depends to a large extent on cooling history of, the magma. Thus rocks with the same chemical composition and same minerals, http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm, , 1/7
Page 2 :
12/3/2016, , General Classification of Igneous Rocks, , present could have widely different textures. In fact we generally use textural, criteria to subdivide igneous rocks in to plutonic (usually medium to coarse, grained) and volcanic (usually fine grained, glassy, or porphyritic.) varieties., 3. Color. Color of a rock depends on the minerals present and on their grain size., Generally, rocks that contain lots of feldspar and quartz are light colored, and rocks, that contain lots of pyroxenes, olivines, and amphiboles (ferromagnesium, minerals) are dark colored. But color can be misleading when applied to rocks of, the same composition but different grain size. For example a granite consists of, lots of quartz and feldspar and is generally light colored. But a rapidly cooled, volcanic rock with the same composition as the granite could be entirely glassy, and black colored (i.e. an obsidian). Still we can divide rocks in general into felsic, rocks (those with lots of feldspar and quartz) and mafic rocks (those with lots of, ferromagnesium minerals). But, this does not allow for a very detailed, classification scheme., 4. Chemical Composition. Chemical composition of igneous rocks is the most, distinguishing feature., The composition usually reflects the composition of the magma, and thus provides, information on the source of the rock., The chemical composition of the magma determines the minerals that will, crystallize and their proportions., A set of hypothetical minerals that could crystallize from a magma with the same, chemical composition as the rock (called the Norm), can facilitate comparison, between rocks., Still, because chemical composition can vary continuously, there are few natural, breaks to facilitate divisions between different rocks., Chemical composition cannot be easily determined in the field, making, classification based on chemistry impractical., , Because of the limitations of the various criteria that can used to classify igneous rocks,, geologists use an approach based on the information obtainable at various stages of examining, the rocks., 1. In the field, a simple field based classification must be used. This is usually based on, mineralogical content and texture. For plutonic rocks, the IUGS system of classification, can be used. For volcanic rocks, the following table can be used., , Simple Field Classification of Volcanic Rocks, (For use in EENS 212), Rock, http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm, , 2/7
Page 4 :
12/3/2016, , General Classification of Igneous Rocks, , SiO2 (Silica) Content, > 66 wt. % Acid, 5266 wt% Intermediate, 4552 wt% Basic, < 45 wt % Ultrabasic, This terminology is based on the onetime idea that rocks with a high % SiO2 were precipitated, from waters with a high concentration of hyrdosilicic acid H4SiO4. Although we now know, this is not true, the acid/base terminology is well entrenched in the literature., Silica Saturation, If a magma is oversaturated with respect to Silica then a silica mineral, such as quartz,, cristobalite, tridymite, or coesite, should precipitate from the magma, and be present in the, rock. On the other hand, if a magma is undersaturated with respect to silica, then a silica, mineral should not precipitate from the magma, and thus should not be present in the rock. The, silica saturation concept can thus be used to divide rocks in silica undersaturated, silica, saturated, and silica oversaturated rocks. The first and last of these terms are most easily seen., Silica Undersaturated Rocks In these rocks we should find minerals that, in general, do, not occur with quartz. Such minerals are:, Nepheline NaAlSiO4, , Leucite KAlSi2O6, , Forsteritic Olivine Mg2SiO4, , Sodalite 3NaAlSiO4.NaCl, , Nosean 6NaAlSiO4.Na2SO4, , Haüyne 6NaAlSiO4.(Na2,Ca)SO4, , Perovskite CaTiO3, , Melanite Ca2Fe+3Si3O12, , Melilite (Ca,Na)2(Mg,Fe+2,Al,Si)3O7, Thus, if we find any of these minerals in a rock, with an exception that we'll see in a, moment, then we can expect the rock to be silica undersaturated., If we calculate a CIPW Norm (we'll see how to do this in lab) the normative minerals, that occur in silica undersaturated rocks are nepheline and/or leucite., Silica Oversaturated Rocks. These rocks can be identified as possibly any rock that does, not contain one of the minerals in the above list., If we calculate a CIPW Norm, silica oversaturated rocks will contain normative quartz., Silica Saturated Rocks. These are rocks that contain just enough silica that quartz does, not appear, and just enough silica that one of the silica undersaturated minerals does not, appear. In the CIPW norm, these rocks contain olivine, or hypersthene + olivine, but no, quartz, no nepheline, and no leucite., To get an idea about what silica saturation means, let's look at a simple silicate system the, system Mg2SiO4 SiO2, http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm, , 4/7
Page 5 :
12/3/2016, , General Classification of Igneous Rocks, , Note how compositions, between Fo and En will end, their crystallization with only, Fo olivine and enstatite., These are SiO2, undersaturated., compositions. All, compositions between En and, SiO2 will end their, crystallization with quartz and, enstatite. These are SiO2 , oversaturated compositions., Note also that this can cause, some confusion in volcanic, rocks that do not complete, their crystallization due to, rapid cooling on the surface., Let's imagine first a, composition in the silica, undersaturated field. Cooling, to anywhere on the liquidus, will result in the, crystallization of Forich, olivine. If this liquid, containing olivine is erupted, and the rest of the liquid, quenches to a glass, then this, will produce a rock with, phenocrysts of olivine in a, glassy groundmass., Applying the criteria above for identifying silica undersaturated rocks would tell us that this is, a silicaundersaturated rock, which we know to be correct. Next, let's look at a silica, oversaturated composition, such as one just to the left of the point labeled 'P' in the diagram. If, this liquid is cooled to the liquidus and olivine is allowed to crystallize, and is then quenched, on the surface, it will contain phenocrysts of Forich olivine in a glassy groundmass. Applying, the criteria above would suggest that this rock is also silica undersaturated, but we know it is, not. This illustrates one of the difficulties of applying any criteria of classification to volcanic, rocks where incomplete crystallization/reaction has not allowed all minerals to form., Alumina (Al2O3) Saturation, After silica, alumina is the second most abundant oxide constituent in igneous rocks. Feldspars, are, in general, the most abundant minerals that occur in igneous rocks. Thus, the concept of, alumina saturation is based on whether or not there is an excess or lack of Al to make up the, feldspars. Note that Al2O3 occurs in feldspars in a ratio of 1 Al to 1 Na, 1K, or 1 Ca:, KAlSi3O8 1/2K2O : 1/2Al2O3, NaAlSi3O8 1/2Na2O : 1/2Al2O3, CaAl2Si2O8 1CaO : 1Al2O3, http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm, , 5/7
