Questions to be Considered in this Article:
- On what basis are igneous rocks classified ?
- What general terms are used to describe the basic textural and compositional parameters by which igneous rocks are classified ?
- What is the generally accepted classification scheme ?
- How do we deal with the plethora of igneous rock names that we might run across in the literature?
INTRODUCTION
The preferred method for classifying any rock type (igneous, sedimentary, or metamorphic) is based on texture and Composition (the latter usually in terms of mineral proportions). Textural criteria are commonly considered first, as textures provide the best evidence for rock origin and permit classification into the broadest genetic categories. The first step in igneous rock description should be to determine whether the rock falls into one of the following three categories:
Phaneritic :- The majority of crystals that compose the rock are readily visible with the naked eye (> ~0.1 mm). If a rock exhibits phaneritic texture, it typically crystallized slowly beneath the surface of the Earth and may be called plutonic, or intrusive.
Aphanitic :- Most of the crystals are too small to be seen readily with the naked eye (< ~ 0.1 mm). If a rock is aphanitic, it crystallized rapidly at the Earth’s surface and may be called volcanic, or extrusive.
Fragmental :- The rock is composed of pieces of disaggregated igneous material, deposited and later amalgamated. The fragments themselves may include pieces of preexisting (predominantly igneous) rock, crystal fragments, or glass. Fragmental rocks are typically the result of a volcanic explosion or collapse and are collectively called pyroclastic.
The grain size of phaneritic rocks may be further subdivided as follows :
| Fine Grained | < 1 mm diameter (< sugar granules) |
| Medium Grained | 1-5 mm diameter (sugar to pea sized) |
| Coarse Grained | 5-50 mm diameter |
| Very coarse Grained | > 50 mm diameter (the lower size limit is not really well defined) |
Pegmatitic is an alternative term for very coarse grain size but has compositional implications for many geologists because pegmatites have historically been limited to late-stage crystallization of granitic magmas. please notice the distinction between aphanitic (too fine to see individual grains) and fine grained (grains are visible without a hand lens but less than 1 mm in diameter).
Some rocks classified as phaneritic and aphanitic are relatively equigranular (of uniform grain size), whereas others exhibit a range of grain sizes because different minerals may experience somewhat different growth rates. The grain size usually varies over only a modest range, and it does so somewhat gradually. If, on the other hand, the texture displays two dominant grain sizes that vary by a significant amount, the texture is called porphyritic. The larger crystals are called phenocrysts, and the finer crystals are referred to as groundmass. Whether such rocks are considered plutonic or volcanic is based on the grain size of the groundmass. Because the grain size is generally determined by cooling rate, porphyritic rocks generally result when a magma experiences two distinct phases of cooling. This 1s most common in, although not limited to, volcanics, in which the phenocrysts form in the slow-cooling magma chamber, and the finer groundmass forms upon eruption.
COMPOSITIONAL TERMS
The composition of a rock may refer either to its chemical composition or the proportions of minerals in it. These different Compositional aspects are related but may lead to some confusion at times. Nearly all igneous rocks are composed principally of silicate minerals, which are most commonly those included in Bowen’s Series: quartz, plagioclase, alkali feldspar, muscovite, biotite, hornblende, pyroxene, and olivine. Of these, the first four (and any feldspathoids present) are felsic minerals (from feldspar + silica), and the latter four are mafic (from magnesium + ferric iron). Generally, felsic refers to the light-colored silicates (feldspars, quartz, feldspathoids), whereas mafic refers to the darker ones, but composition has precedence (e.g., smoky quartz and dark feldspars are felsic). In addition to these principal minerals, there may also be a number of accessory minerals, present in small quantities, usually consisting of apatite, zircon, titanite, epidote, an oxide or a sulfide, or a silicate alteration product such as chlorite.
Related Posts
Most geologists agree that the best way to form the compositional basis for a classification of igneous rocks is to use the exact principal mineral content, as will be described shortly. A number of general descriptive terms, however, are not meant to name specific rocks but to emphasize some compositional aspect of a rock. Unfortunately, many of these terms address similar, but not equivalent, compositional parameters, resulting at times in confusion. For example, the terms in the previous paragraph are commonly applied not only to minerals but also to the rocks that they compose. Felsic, then, describes a rock composed predominantly of felsic minerals, whereas mafic describes a rock with far more mafic minerals. The term ultramafic refers to a rock that consists of over 90% mafic minerals. Similar, but not equivalent, terms are leucocratic, indicating a lightcolored rock, and melanocratic, indicating a dark-colored rock. The first two terms are based on mineral content, whereas the latter ones are based on rock color, but the relationship between these two parameters is obvious: rocks composed principally of light-colored minerals (felsic) should be light-colored themselves (leucocratic). Color, however, is not a very reliable measure of the composition of a rock. Thus terms such as mafic, which are defined rather loosely by color and composition, can be confusing. For example, when plagioclase becomes more calcic than about Ansg, it is commonly dark gray or even black. Smoky quartz is also quite dark. Should these minerals be considered mafic? Most geologists would resist this, as the mnemonic roots of the felsic and mafic terms refer to their chemical composition, even though color may have replaced composition as the common distinguishing feature. A rock composed of 90% dark feldspar would thus be considered both felsic and melanocratic. The color of a rock has been quantified by a value known as the color index (M’), which is taken here to simply mean the volume percentage of dark minerals. (It is more specifically defined for advanced work by Le Maitre et al., 2002.)
Purely chemical terms such as silicic, magnesian, alkaline, and aluminous that refer, respectively, to the SiO2, MgO, (Na2O + K2O), and Al2O3 content of a rock may also be used, particularly when the content of some particular component is unusually high. Silica content is of prime importance, and the term acidic is synonymous with silicic. Although based on the outdated concept that silicic acid is the form of silica in solution, even in melts, the term is still in use. The opposite of acidic is basic, and the spectrum of silica content in igneous rocks has been subdivided as follows :
| Acidic | > 66 wt. % SiO2 |
| Intermediate | 52-66 wt. % SiO2 |
| Basic | 45-52 wt.% SiO2 |
| Ultrabasic | < 45 wt. % SiO2 |
Because the concept behind “acidic” and “basic” is not accurate, many petrologists consider these terms outdated as well, whereas others consider them useful. Naturally, basic rocks are also mafic, so we see some of the unnecessary complexity involved in simply describing the most general compositional properties of igneous rocks.
Yet a further problem arises when we attempt to refer to the composition of a melt. How, for example, do we refer to the magma that, when crystallized, becomes a basalt? Because there are few, if any, minerals in a melt, the mineralbased terms felsic and mafic are technically inappropriate. Color, as mentioned above, is an unreliable compositional measure, and it is notoriously poor for magmas and glasses, as the common occurrence of black obsidian, that is quite silicic, can testify. Basic would apply to melts, and it is here that the term may best be used. Many North American geologists consider the terms mafic and felsic appropriate compositional terms and prefer to call a dark magma mafic rather than basic, which they consider outmoded. British geologists prefer basic and acidic to refer to magmas because they are appropriately based on composition and not mineralogy. Others escape the problem altogether by naming the magma after the rock equivalent. The literature is thus full of descriptions of “basaltic magmas,” “mafic magmas,” “basic magmas,” etc.
THE IUGS CLASSIFICATION
Over time, a number of classification schemes have been applied to igneous rocks, resulting in a plethora of equivalent or overlapping rock names.
Young provided an excellent review of the tumultuous history of igneous rock classification. In the 1960s, largely at the initiative of Albert Streckeisen, the International Union of Geological Sciences (IUGS) formed the Subcommission on the Systematics of Igneous Rocks to attempt to develop a standardized and workable system of igneous rock nomenclature. The latest version of this classification was recently published by Le Maitre et al. (2002).
Modifying Terms
It is acceptable under the IUGS system to include mineralogical, chemical, or textural features in a rock name. The goal here is to impart some descriptive information that you consider important enough to put in the name. This is a matter of judgment and is flexible. If the rock is unusually light colored for its category, you may want to add the prefix leuco-, as in Ieucogranite. If it’s unusually dark, add the prefix mela-, as in melagranite. You may also use textural terms, such as porphyritic granite, rapakivi granite, graphic granite, etc. when they are very obvious. When naming a rock, always try to find the correct name from the proper IUGS diagram. Names such as pegmatite, aplite, and tuff are incomplete. Rather, use these textural terms to modify the rock name, as in pegmatitic orthoclase granite, aplitic granite, and rhyolite tuff. If you want to convey some important mineralogical information, you can add that to the name as well. Naturally, quartz, plagioclase, and alkali feldspar are already implicit in the name so are redundant if mentioned specifically. However, you may want to describe a rock as a niebeckite granite or a muscovite biotite granite. If more than one mineral is included, they are listed in the order of increasing modal concentration. In the previous example, then, there should be more biotite than muscovite in the rock. At times, it may also be desirable to add a chemical modifier, such as alkaline, calc-alkaline, aluminous, etc. A common example is the use of the prefix alkali-. High contents of alkalis can stabilize an alkali amphibole or an alkali pyroxene. We usually don’t think of pyroxene-bearing granites, but some “alkali granites” may indeed contain a sodium-rich pyroxene. Some chemical characteristics are manifested throughout a whole series of cogenetic magmas in some igneous provinces. The chemical terms are thus more commonly applied to “suites” of igneous rocks (i.e., groups of rocks that are genetically related).
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FIGURE 2 A classification of the phaneritic igneous rocks. (a) Phaneritic rocks with more than 10% (quartz + feldspar+ feldspathoids). (b) Gabbroic rocks. (c) Ultramafic rocks. After Le Maitre et al. (2002). |
Mafic and Ultramafic Rocks
Gabbroic rocks (plagioclase + mafics) and ultramafic rocks (with over 90% mafics) are classified using separate diagrams (Figures 2b and 2c, respectively). As with any classification, the IUGS subcommittee has had to find a delicate balance between the tendencies for splitting and lumping. The same is true for us. Whereas the IUGS must serve the professional community and guide terminology for professional communication, we must find a classification suitable for more common use in student petrology laboratories. Figure 2b for gabbroic rocks is simplified from the TUGS recommendations. When one can distinguish pyroxenes in a gabbro, there is more specific terminology in Le Maitre et al. (2002) (e.g, an orthopyroxene gabbro is called a norite), Figure 2c is more faithful to the IUGS recommendations. In hand specimen work, it may be difficult to distinguish orthofrom clinopyroxene in black igneous rocks. Hence the terms peridotite and pyroxenite are commonly used for ultramafics because they are independent of pyroxene type. When the distinction can be made, the more specific terms in Figure 2c are preferred. The presence of over 5% hornblende further complicates the nomenclature of both mafic and ultramafic rocks. I believe that the IUGS distinction between an olivine-pyroxene hornblendite, an olivine-hornblende pyroxenite, and a pyroxene hornblende peridotite adds more detail than necessary at this point. The student is referred to the complete IUGS classification (Le Maitre et al., 2002) for proper names if it becomes important to make more detailed distinctions in nomenclature.
APHANITIC ROCKS
Volcanic rocks for which a mineral mode can be determined are treated in the same way as plutonics in the original IUGS classification. One determines the mode, normalizes to find P, A, and Q or F, and plots the result in Figure 3, in a manner identical to that described for Figure 2. Because the mode is commonly difficult to determine accurately for volcanics, Figure 3 is a simplification, modified from the more
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FIGURE 3 A classification and nomenclature of volcanic rocks. After Le Maitre et al. (2002). |
detailed diagram published by the IUGS (Le Maitre et al., 2002). The matrix of many volcanics is composed of minerals of extremely fine grain size and may even consist of a considerable proportion of vitreous (glassy) or amorphous material. Thus it is commonly impossible, even in thin section, to determine a representative mineralogical mode. If it is impossible to recognize the mineralogy of the matrix, a mode must be based on phenocrysts. The IUGS recommends that rocks identified in such a manner be called phenotypes and have the prefix phenoinserted before the name (e.g., pheno-latite). As we shall soon see, minerals crystallize from a melt in a sequence (as indicated by, but certainly not restricted to, Bowen’s Series), so the first minerals to crystallize do not necessarily represent the mineralogy of the rock as a whole. If based on phenocrysts, the position of a rock on Figure 3 will be biased toward the early-forming phases and usually erroneous for the rock as a whole.
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FIGURE 4 A chemical classification of volcanics based on total alkalis versus silica (“TAS”). After Le Bas et al. (1986) and Le Maitre et al. (2002). The line between the (Foid)ite field and the Basanite-Tephrite field is dashed, indicating that further criteria are needed to distinguish these types: if normative ne > 20%, the rock should be called a nephelinite, and if ne < 20% and normative ab is present but < 5%, the rock is a melanephelinite. Abbreviations: o/ = normative olivine and
Q = normative 100 * q/(q + or + ab + an). See Appendix B for an explanation and calculation of norms. Reprinted by permission of Cambridge University Press. |
Again, rocks that plot near P in Figure 3 present a problem in the volcanic classification, just as in the plutonic one. One cannot distinguish andesite from basalt using Figure 3. The IUGS recommends a distinction based on color index or silica content (see below) and not on plagioclase composition. An andesite is defined as a plagioclase-rich rock either with a color index below 35% or with greater than 52% SiO2). Basalt has a color index greater than 35% and has less than 52% SiO2). Many andesites defined on color index or silica content have plagioclases of composition Angs or greater.
The most reliable way to avoid the matrix problem discussed above is to analyze the volcanic rock chemically and use a Classification scheme based on the analytical results (as is implicit using % SiO2 in the IUGS distinction between andesite and basalt discussed above). The IUGS has subsequently recommended a classification of volcanics based on a simple diagram comparing the total alkalis with silica, also called a “TAS” diagram (Le Bas et al., 1986). The diagram (Figure 4) requires a chemical analysis and is divided into 15 fields. To use it, we normalize a chemical analysis of a volcanic to a 100% nonvolatile basis, combine Na2O + K2O, and plot the total against SiO2 . Results are generally consistent with the QAPF diagram when a good mode ts available. The shaded fields in the TAS diagram can be further subdivided by considering the concentrations of Na2O and K2O independently, if desired, according to the lower box. Further refinements are presented for Mg-rich volcanics, and the IUGS (Le Maitre et al., 2002) recommends the name picrite for volcanics containing less than 3% alkalis and 12 to 18% MgO and komatiite (TiO2 < 1%) or meimechite (TiO2 > 1%) for volcanics with similar alkali content but richer in MgO. The term boninite is recommended for an andesite or basaltic andesite with more than 8% MgO and less than 0.5% TiO2.
The diagrams shown in Figures 2 to 4 provide you with the names of most common igneous rocks, but a number of important rock types classified by the IUGS are not included in the figures. For instance, the classification shown does not cover any hypabyssal (shallow intrusive) rocks such as diabase (or dolerite in Britain), nor does it cover the less common rock types such as carbonatites (igneous carbonates), lamproites/lamprophyres, (highly alkaline, volatile rich mafic flow/dike rocks), spilites (sodic basalts), or keratophyres (sodic intermediate volcanics), etc.
Highly alkaline rocks, particularly those of continental origin, are varied, both mineralogically and chemically. The composition of highly alkaline rocks ranges to high concentrations of several elements present in only trace amounts in more common igneous rocks. The great variety results in a similarly complex nomenclature. Although these alkaline rocks comprise less than 1% of igneous rocks, fully half of the formal igneous rock names apply to them. Such an intricate nomenclature is far beyond the intended scope of this article.
I have tried to avoid the cumbersome detail that a comprehensive classification requires and have attempted to provide a useful compromise between completeness and practicality. Table 1 (Above pdf of common rock names) lists a much broader spectrum of igneous rock names that can be found in the literature. Those in bold are recommended by the IUGS and can be found either in Figures 2 to 4 or in Le Maitre et al. (2002). The other terms are not recommended by the IUGS because they are too colloquial, too restrictive, inappropriate, or obsolete. I have attempted to provide a very brief definition of each term, including the IUGS approved term that most closely approximates it. It is impossible to do this with precision, however, as the chemical, mineralogical, and/or textural criteria seldom coincide perfectly. Table 1 is intended to provide you with a quick reference to rock terms that you may encounter in the literature and not a rigorous definition of each. For the latter, you are referred to the AG/ Glossary or Le Maitre et al. (2002). For the sake of brevity, I have included only the root IUGS terms, such as granite, andesite, or trachyte, and not compound terms such as alkali-feldspar granite, basaltic trachy-andesite, etc.
PYROCLASTIC ROCKS
The initial IUGS classification (Streckeisen, 1973) did not cover pyroclastic rocks, but they were addressed in a later installment (Schmid, 1981). As mentioned previously, if the chemical composition is available, these rocks could be classified compositionally in the same manner as any other volcanics, but they commonly contain significant impurities, and only those for which the foreign material is minimal can a meaningful compositional name be applied. Pyroclastics are thus typically classified on the basis of the type of fragmental material (collectively called pyroclasts) or on the size of the fragments (in addition to a chemical or modal name, if possible). Pyroclasts need not be of volcanic origin: some may be fragments of sedimentary or metamorphic country rock caught up in a violent eruption.
To name a pyroclastic rock, determine the percentage of the fragments that fall into each of the following categories:
| > 64 mm diameter | Bombs (if molten during fragmentation thus typically rounded/blobby, flattened, or stretched) Blocks (if not molten during fragmentation thus typically angular or broken) |
| 2-64 mm | Lapilli |
| < 2 mm | Ash |
The IUGS maintains that a true pyroclastic rock must contain at least 75% pyroclasts. The fragments may be individual crystals, glass, or rock fragments. Individual crystals in pyroclastic rocks are referred to as “crystal fragments,” not “phenocrysts,” because their origin is uncertain. Glass may occur as pumice, ash-sized fragments of shattered thin pumice vesicle-walls, or as dense angular or rounded droplet shaped pieces. The relative proportions of fragments in the size categories above are then plotted on Figure § to determine the rock name. Tuffs and ashes may be further qualified by the type of fragments they contain. Coarse (ash) tuffs contain particles predominantly in the 1/16 mm to 2 mm range, whereas the particles in fine (ash) tuffs or dust tuffs are generally < 1/16 mm. Lithic tuff would contain a predominance of rock fragments, vitric tuff a predominance of pumice and glass fragments, and crystal tuff a predominance of crystal fragments. Again, it is good practice to include a compositional name whenever possible, based on a chemical analysis, the color index, or crystal fragment mineralogy. A name such as rhyolitic lapilli tuff is thus a complete and descriptive name for a light pink, tan, or very light gray pyroclastic rock dominated by lapilli-sized fragments.
For rocks containing both pyroclasts and sedimentary clastic material (epiclasts), the IUGS subcommission (Le Maitre et al., 2002) suggests the general term tuffite, which may be subdivided further by adding the prefix tuffaceous to the normal sedimentary name, such as shale, siltstone, sandstone, conglomerate, or breccia. Epivolcaniclastics are secondary deposits, meaning that they are not deposited directly by eruptive activity. These may occur due to volcanic flank collapse or as marine aprons around volcanic islands, volcanic mudflows (lahars), or reworked epiclasts. An aquagene tuff is a waterborne accumulation of ash. It may result from a subaqueous eruption, or it may be an airborne accumulation that has been reworked by water. Hyalotuff or hyaloclastite is an aquagene tuff that is created when magma is shattered upon contact with water.
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FIGURE 5 A classification of the pyroclastic rocks. After Fisher (1966). |
Important “First Principle” Concepts
- Igneous rocks are dominated by silicate minerals and typically range in composition from ultramafic through mafic and intermediate to silicic varieties. More specific (and generally more unusual) varieties may be described chemically, using appropriate modifiers, such as alkalic, potassic, calcic, aluminous, etc.
- Texturally, igneous rocks are either phaneritic (intrusive rocks), aphanitic (extrusive or volcanic rocks), or fragmental (pyroclastic rocks).
- Igneous rocks may be classified and named on the basis of their mineral content (which is a reflection of their composition) and their texture.
Conclusion
Most common igneous rocks may be classified and named on the basis of their texture and mineral content. The International Union of Geological Sciences has developed a standardized igneous rock nomenclature for classifying and naming igneous rocks based on the modal (volume) percentages of the constituent minerals. To name a typical rock, one determines the modal percentages of quartz minerals (Q'), Al-kali feldspars (A'), Plagioclase (P'), Feldspathoids (F'), and Mafics (M'). If the rock has 90% or more Q' + A' +P' +F', the rock is named by plotting the relative proportions of these mineral constituents (normalized to 100%) on the appropriate phaneritic or aphanitic diagram. Volcanic rocks in which a mode is impossible to determine should be classified on the basis of chemical composition on a total alkali versus silica (TAS) diagram. Pyroclastic rocks are classified separately on the basis of the size and nature of the fragments (pyroclasts) that compose them, including a compositional name, whenever possible.