Darker rocks (containing a predominance of dark minerals) tend to be:

Triangular shaped diagram shows where rock composition falls between end members (at each corner): quartz, alkali feldspar, and plagioclase feldspar. Because this diagram doesn’t include mafic minerals (olivine and pyroxene), it can’t be used to differentiate the intermediate diorite from the mafic gabbro.ĭiagram Showing the Classification of Intrusive Igneous Rocks Rock types are plotted to show the relative amounts of quartz, alkali feldspar, and plagioclase felspar they contain. The diagram below shows the classification of intrusive igneous rocks in Blue Ridge Parkway and the some of the different types of intrusive rocks beyond the three main types. Gabbro consists mostly of pyroxene, olivine, and plagioclase feldspar.Diorite consists mostly of plagioclase feldspar, amphibole, and pyroxene.

Granite consists mostly of quartz and alkali-feldspar, with relatively minor plagioclase feldspar and mafic minerals (biotite, muscovite and/or amphibole ).The minerals found in the three main types of plutonic rocks are: The different types of mafic intrusive rocks are classified on their relative amounts of olivine, pyroxene, and plagioclase. Mafic and intermediate intrusive rocks also have a significant proportion of minerals that contain iron and magnesium such as olivine, pyroxene, amphibole, and biotite. Intrusive rocks are classified on the proportion of major minerals in them, particularly the relative amounts of quartz, alkali-feldspar, and plagioclase. ► Intrusive (Plutonic) Rock Classification Silicic - high in silica (and low in magnesium and iron) Mafic - high in magnesium and iron (and low in silica) The two major divisions of igneous rocks based on composition are: In turn, high silica rocks have less iron and magnesium, and more sodium and potassium. Low silica rocks have higher concentrations of iron and magnesium. Composition of igneous rocks is expressed as weight percent silica, which ranges from about 45 to 78 (wt % SiO2). In descending order, they are:Īlmost all igneous rocks are silicates meaning that silicon and oxygen are the two most common elements in them. Igneous rocks are mostly made of the most common elements found in the Earth’s crust. Texture and structure refer to the relationships between crystals, groundmass, vesicles, and other components of a rock. Igneous rocks are classified both by their chemical composition, mineral composition, texture, and structure.

Darker rocks (containing a predominance of dark minerals) tend to be: