GLY 421: Igneous & Metamorphic Petrology

El-Shazly, 2004

Classification of Igneous Rocks

The classification of igneous rocks is not easy. This is simply because igneous rocks

represent a continuum in nature (whether mineralogically or chemically), making their

classification rather artificial. Nevertheless, classifications are needed to improve

communication between scientists, and are generally of five different types:

Mineralogical classifications: Are based on the mineralogical composition of the

igneous rock, and the percentage by volume of essential minerals (minerals influencing

its classification and constituting more than 5% of the rock). The volume % of any

mineral is known as its mode. Obviously, in order to understand mineralogical

classifications, we must know what kind of minerals occur in igneous rocks. Table 1

summarizes the most common igneous minerals and their chemical composition. Only

rocks in which minerals can be identified in hand specimen and/or thin section can be

classified mineralogically.

Textural classifications: Textures are used as a secondary criterion for the classification

of igneous rocks. For example, in the most commonly used mineralogical classifications

of igneous rocks, textures are used to tell plutonic and hypabyssal rocks from volcanic

ones. Each mineral assemblage will therefore correspond to two rock names: a plutonic

one and its volcanic equivalent (see Streckeisen's classification below). On the other

hand, textures are used as the only criterion for the classification of tuffs, ... but then

again, these are sedimentary (pyroclastic) rocks!

Chemical classifications: Are based on the chemical composition of the igneous rocks,

and therefore require the availability of bulk rock chemical analyses (obtained by XRF,

ICP or wet chemical techniques). These classifications are needed for many volcanic

rocks that are either too fine-grained to allow for proper identification of their

mineralogies, or which contain considerable amounts of glass.

Genetic classifications: Although the ultimate aim of the petrologist is to understand

how the rocks formed, genetic classifications are not very useful for the field geologist or

petrographer who is just starting a study!

Tectonic classifications: These are useful for understanding the relationship between

plate tectonics and igneous activity, and rely heavily on studying present day igneous

activity at different types of plate boundaries/ interiors. Making use of these

classifications to understand the tectonic setting of "older" igneous rocks still requires

chemical, mineralogical and textural data.

Among these five different types, it is clear that the easiest are the mineralogical and

textural classifications (as they require the minimum amount of information and usually

only a petrographic microscope), followed by the chemical classifications which are

necessary for many volcanic rocks.

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El-Shazly, 2004 1

Classification of Igneous Rocks

The classification of igneous rocks is not easy. This is simply because igneous rocks represent a continuum in nature (whether mineralogically or chemically), making their classification rather artificial. Nevertheless, classifications are needed to improve communication between scientists, and are generally of five different types: Mineralogical classifications: Are based on the mineralogical composition of the igneous rock, and the percentage by volume of essential minerals (minerals influencing its classification and constituting more than 5% of the rock). The volume % of any mineral is known as its mode. Obviously, in order to understand mineralogical classifications, we must know what kind of minerals occur in igneous rocks. Table 1 summarizes the most common igneous minerals and their chemical composition. Only rocks in which minerals can be identified in hand specimen and/or thin section can be classified mineralogically. Textural classifications: Textures are used as a secondary criterion for the classification of igneous rocks. For example, in the most commonly used mineralogical classifications of igneous rocks, textures are used to tell plutonic and hypabyssal rocks from volcanic ones. Each mineral assemblage will therefore correspond to two rock names: a plutonic one and its volcanic equivalent (see Streckeisen's classification below). On the other hand, textures are used as the only criterion for the classification of tuffs, ... but then again, these are sedimentary (pyroclastic) rocks! Chemical classifications: Are based on the chemical composition of the igneous rocks, and therefore require the availability of bulk rock chemical analyses (obtained by XRF, ICP or wet chemical techniques). These classifications are needed for many volcanic rocks that are either too fine-grained to allow for proper identification of their mineralogies, or which contain considerable amounts of glass. Genetic classifications: Although the ultimate aim of the petrologist is to understand how the rocks formed, genetic classifications are not very useful for the field geologist or petrographer who is just starting a study! Tectonic classifications: These are useful for understanding the relationship between plate tectonics and igneous activity, and rely heavily on studying present day igneous activity at different types of plate boundaries/ interiors. Making use of these classifications to understand the tectonic setting of "older" igneous rocks still requires chemical, mineralogical and textural data. Among these five different types, it is clear that the easiest are the mineralogical and textural classifications (as they require the minimum amount of information and usually only a petrographic microscope), followed by the chemical classifications which are necessary for many volcanic rocks.

El-Shazly, 2004 2 Mineralogical (± Textural?) classifications 1- Streckeisen's classification of plutonic igneous rocks: (Fig. 1): This is based on the relative modal abundance of the minerals: quartz (Q) - alkali feldspar (A; sum of K-spar

albite) - plagioclase feldspar (P, plagioclase feldspars with Ab < 95) – feldspathoids (F). Note that feldspathoids cannot coexist with quartz, so you are really always recalculating the modes of only 3 minerals (either Qz - A - P, or F - A - P) to a sum of 100% and plotting the recalculated modes on one of the two ternary diagrams (either the top or the bottom one). 2- Volcanic equivalents of the Streckeisen classification: (Fig. 2). Figure 3 is a simplified version of Streckeisen's classification, where the rock is assigned to a "family", and is quite useful for field geologists. Another simplified (but inaccurate) summary of these two classifications is given in Table 2. 3- The use of the Colour Index (C.I.) to classify igneous rocks: This is a very simple classification used by field geologists, and depends on estimating the volume % of the dark (mafic) minerals. Igneous rocks are classified into 4 groups based on C.I.:  leucocratic rocks: < 30% light minerals (C.I. < 30)  mesocratic: C.I. = 30 - 60  melanocratic: C.I. = 60 - 90  hypermelanic: C.I. > 90. Textural Classifications (Pyroclastic rocks) Pyroclastic rocks consist of ejecta or tephra (particles that were thown into the air by the forceful eruption of a volcano). These solid particles may represent (i) essential ejecta or tephra: solidified lavas resulting from the latest eruption, (ii) accessory ejecta: fragments of volcanic rocks produced by earlier eruptions, or (iii) accidental ejecta: fragments of any crustal rocks torn off from the vicinity of the volcano. Based on their size, ejecta are classified into: (a) Volcanic blocks : large angular blocks broken off from the sides of the volcano (accessory ejecta). Could reach several meters in size. (b) Bombs : When the lava is thrown up in the air, it solidifies rapidly into irregularly shaped but elongated objects that range in size from a few centimeters to tens of centimeters. The shapes of bombs are often described as "streamlined" or "spindle", and are commonly cracked. (c) Lapilli to cinder : walnut to sand sized ejecta. (d) Ash : very fine-grained (silt-sized) particles that are blown off into the air. (e) Dust : Volcanic dust has the ability to travel long distances before being deposited. Rocks that form from the deposition of volcanic lapilli, ash and dust have many features in common with sedimentary rocks, but the particles forming them are clearly of volcanic origin.

El-Shazly, 2004 4 This concept is based on comparing the mole proportions (mole proportion = weight% of oxide/molecular weight of this oxide) of the alkalis to alumina in a rock. Igneous rocks are subdivided into three groups according to this classification: Group Chemical characteristics Modal minerals Normative minerals^1 Peraluminous Al 2 O 3 > Na 2 O + K 2 O + CaO Corundum, Andalusite, Spessartine, almandine, Topaz, Muscovite, Tourmaline Corundum Metaluminous K 2 O + Na 2 O + CaO > Al 2 O 3

Na 2 O + K 2 O Feldspars + most Cpx An + Di Peralkaline K 2 O + Na 2 O > Al 2 O 3 Aegirine, Riebeckite, Arfvedsonite, Aenigmatite Aegirine (Ac), Na-metasilicate Most igneous rocks are metaluminous. Peraluminous rocks are almost always plutonic, whereas volcanic peralkaline rocks are more common than plutonic ones. 3- Total alkali - silica diagram: A simple plot of total alkalis (Na 2 O + K 2 O) vs. SiO 2 has been used to classify volcanic rocks. Figure 4 shows this classification. 4- CIPW norm calculations and classification: In 1902, Cross, Iddings, Pirsson and Washington, four American petrologists, devised a system for converting the chemical analysis of an igneous rock into a group of minerals predicted to crystallize from a magma of this composition. These predicted minerals (listed in table 3), are known as "normative minerals" and are represented in weight % to give the "norm of the rock" which should add up to ~ 100%. Normative calculations are based on the observed sequence of crystallization of minerals from a melt, and take into account which minerals can and cannot coexist in equilibrium. However, these calculations are based on the following assumptions (which are rarely true): 1- No hydroxyl bearing minerals exist in the rock (i.e. are calculated) 2- The norm is expressed in the form of end-members, and does not take into account many types of solid solution (e.g. Ti in pyroxenes, ... etc). 3- Pyroxenes and olivines are assumed to have the same Fe/Mg ratio. 4- Normative minerals are always formed (calculated) in the same order, regardless of the composition of the rock (or magma). (^1) See C.I.P.W classification below.

El-Shazly, 2004 5 Because of these assumptions, the normative mineralogy will almost always be different from the modal mineralogy of a rock. Nevertheless, the norm and mode of many basalts are often quite close, and normative calculations are generally useful for basic volcanic rocks. This is simply because basic magmas have less H 2 O compared to acidic ones, and these volatiles are themselves lost upon eruption, leaving behind mostly anhydrous minerals to crystallize from such lavas. Based on these norm calculations, Cross, Iddings, Pirsson and Washington proposed a classification of the different igneous rocks. However, this classification quickly became unpopular, and is seldom used. Nevertheless, normative calculations are still being widely used to quickly compare different rocks chemically. They are particularly useful for comparing different rock types that may have the same chemical composition (e.g. basalt, gabbro and eclogite). Norm calculations are also useful for classifying basalts (as we shall see later). Another example for the use of norms, if it is stated that a rock is olivine - normative, one quickly knows that it is not an oversaturated rock. An acmite-normative rock indicates that it probably crystallized from a peralkaline magma, ... and so on.

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Classification of Igneous Rocks