6.6: Mineral Modes

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6.101.jpg — Figure 6.101: Mineral modes of some common igneous rocks

K-feldspar, quartz, plagioclase, pyroxene, olivine, muscovite, biotite, and amphibole are the most common minerals in igneous rocks. Figure 6.101 depicts the typical amounts by volume – also called the mineral modes – of these eight minerals in some common plutonic rocks. Volcanic rock names are included in the table, but many volcanic rocks contain glass that replaces some or all of the minerals that could be present. The table below gives more explicit information about the minerals common to different kinds of rocks.

Figure 6.101, and table below, emphasize the affinities that some minerals have for others. Silicic to intermediate rocks always contain K-feldspar and quartz and are likely to contain hydrous minerals such as muscovite or biotite. They may contain Na-rich plagioclase. The most common mafic rocks contain pyroxenes and Ca-rich plagioclase. They may contain olivine, and almost never contain quartz or K-feldspar. Olivine, nepheline, and feldspathoid minerals can never coexist with quartz, because they would react to form other silica-rich minerals. Thus, certain minerals can be associated with each other in nature, while others cannot. The minerals present are primarily controlled by the composition of the magma, which in turn reflects the process and source that generated it. So, different igneous rocks, formed in different environments, have characteristic mineral assemblages.