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8.3: Igneous Rock Composition

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    5597
  • Overview

    Often added to the Bowen’s reaction series diagram are the igneous rock classifications as well as example igneous rock names that are entirely dependent on the minerals that are found in them. For example, you can expect to find abundant olivine, and maybe a little pyroxene and a little Ca-rich plagioclase, in an ultramafic rock called peridotite or komatiite, or that pyroxene, plagioclase, and possibly some olivine or amphibole may be present in a mafic rock such as gabbro or basalt. You can also expect to see quartz, muscovite, potassium feldspar, and maybe a little biotite and Na-rich plagioclase in a felsic (or silicic) rock such as granite or rhyolite. Figure 8.1 demonstrates nicely that the classification of an igneous rock depends partly on the minerals that may be present in the rock, and since the minerals have certain colors due to their chemical makeup, then the rocks must have certain colors. For example, a rock composed of mostly olivine will be green in color due to olivine’s green color; such a rock would be called ultramafic. A rock that has a large amount of ferromagnesian minerals in it will be a dark-colored rock because the ferromagnesian minerals (other than olivine) tend to be dark-colored; an igneous rock that is dark in color is called a mafic rock (“ma-” comes from magnesium, and “fic” from ferric iron). An igneous rock with a large amount of nonferromagnesian minerals will be light in color, such as the silicic or felsic rocks (“fel” from feldspar, and “sic” from silica-rich quartz). So, based on color alone, we’ve been able to start classifying the igneous rocks.

     

    In Figure 8.2 are examples of igneous rocks that represent the mafic and felsic rock compositions (Figures 8.2A and 8.2C, respectively), as well as an intermediate rock type (Figure 8.2B). Notice that the felsic rocks can have a small amount of dark-colored ferromagnesian minerals, but is predominately composed of light-colored minerals, whereas the mafic rock has a higher percentage of dark-colored ferromagnesian minerals, which results in a darker-colored rock. A rock that is considered intermediate between the mafic and felsic rocks is truly an intermediate in terms of the color and mineral composition; such a rock would have less ferromagnesian minerals than the mafic rocks, yet more ferromagnesian minerals than the felsic rocks.

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    As previously mentioned, classifying rocks into one of the igneous rock compositions (ultramafic, mafic, intermediate, and felsic) depends on the minerals that each rock contains. Identification of the minerals can be difficult in rocks such as in Figure 8.2A, as the majority of minerals are dark in color and it can be difficult to distinguish each mineral. An easy method of determining the igneous rock composition is by determining the percentage of dark-colored minerals in the rock, without trying to identify the actual minerals present; this method of classification relies on a mafic color index (MCI), where the term mafic refers to any dark gray, black, or green-colored mineral (Figure 8.3). Igneous rocks with 0-15% dark-colored minerals (or 0-15% MCI) are the felsic rocks (Figure 8.3A), igneous rocks with 46-85% MCI are the mafic rocks (Figure 8.3C), and igneous rocks with over 85% MCI are considered ultramafic (Figure 8.3D). This means that any rock with an intermediate composition or with a 16-45% MCI is an intermediate igneous rock (Figure 8.3B).

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    Estimating the percentage of dark-colored minerals is only possible if the minerals are large enough to see. In that case, a person can still recognize a mafic rock by its dark-colored appearance and a felsic rock by its light-colored appearance. An intermediate rock will be somewhat lighter than a mafic rock, yet darker than a felsic rock. Finally, an ultramafic rock is typically green in color, due to the large amount of green-colored olivine in the rock. Such rocks that contain minerals that are too small to see are shown in Figure 8.4; note that you can still distinguish between mafic (Figure 8.4A), intermediate (Figure 8.4B) and felsic (Figure 8.4C) by the overall color of the rock. The intermediate igneous rock in Figure 8.4B does have a few visible phenocrysts; this odd texture will be covered later in this chapter.

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