Engineering geology

Igneous Compositions

Compare and contrast the four basic igneous compositions: basaltic (mafic), granitic (felsic), andesitic (intermediate), and ultramafic.

Igneous rocks are composed mainly of silicate minerals. Chemical analyses show that silicon and oxygen are by far the most abundant constituents of igneous rocks.

These two elements, plus ions of aluminum (Al), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), and iron (Fe), make up roughly 98 percent, by weight, of most magmas.

In addition, magma contains small amounts of many other elements, including titanium and manganese, and trace amounts of much rarer elements, such as gold, silver, and uranium.

As magma cools and solidifies, these elements combine to form two major groups of silicate minerals (Figure 1).
The dark (or ferromagnesian) silicates are rich in iron and/or magnesium and comparatively low in silica. Olivine, pyroxene, amphibole, and biotite mica are the common dark silicate minerals of Earth’s crust.

Figure 1 – Mineralogy of common igneous rocks

By contrast, the light (or nonferromagnesian) silicates contain greater amounts of potassium, sodium, and calcium rather than iron and magnesium. The light silicate minerals, including quartz, muscovite mica, and the most abundant mineral group, the feldspars, are richer in silica than the dark silicates.

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