There are three types of rocks, igneous, sedimentary and metamorphic. Igneous rocks form as a result of magma cooling and minerals within the magma crystallizing. There are a variety of minerals associated with igneous rocks and their abundance and crystal forms depend on the environment (pressure and temperature) under which the magma cooled. Igneous intrusive rocks crystallize very slowly as magma is pushed through the Earth's crust. Igneous extrusive rocks crystallize rapidly as magma is erupted through volcanic processes.
The Composition of Igneous Rocks
Eight elements make up about 98 percent, by weight, of most magmas from which igneous rocks are made. These elements are:
- Oxygen (O)
- Silicon (Si)
- Aluminum (Al)
- Iron (Fe)
- Calcium (C)
- Sodium (Na)
- Potassium (K)
- Magnesium (Mg)
The composition of igneous rocks is mostly oxygen and silica. This is in agreement with the composition of the Earth's crust, which is also mostly oxygen and silica. Other elements that make up the remainder of igneous rock compositions include manganese (Mn) and Titanium (Ti).
The Formation of Minerals in Igneous Rocks
The crystallization of magma fluids, either through extrusive or intrusive processes, creates the minerals that make up igneous rocks. The abundance of these minerals varies throughout the igneous rock family as the environment under which the magma cools controls the chemical and physical processes that lead to crystallization, otherwise known as the Bowen's Reaction Series.
The Bowen's Reaction Series describes the sequence in which igneous rock minerals crystallize within cooling magma. Rocks composed of minerals that crystallize at higher temperatures are known as mafic rocks and are relatively rich in magnesium and iron. Rocks composed of minerals that crystallize at lower temperatures are known as felsic rocks and are rich in silica, sodium and potassium. Rocks with a mineralogy in between these two extremes are known as intermediate rocks and contain a combination of mafic and felsic minerals.
Igneous Rock Minerals
The most abundant minerals in mafic rocks are olivine, augite and plagioclase feldspar. Olivine is a magnesium iron silicate which contains the elements Si, O and either Fe or Mg, or a combination of both. As a gemstone, it is known as peridote. Augite is a calcium magnesium iron silicate, similar to olivine, but can also contain the element Ca. Plagioclase feldspar is an aluminum silicate mineral which contains the elements Si, O, Al and either Ca or Na, or a combination of both.
The most abundant minerals in felsic rocks are biotite, amphibole, orthoclase and quartz. Biotite is a dark-colored member of a silicate mineral group called micas. Biotite contains the elements K, Mg, Fe, Si, Al and water. Amphibole (pronounced amfi-bowl) is another silicate mineral containing Fe, Mg, and Ca along with varying amounts of Si, O and water. Orthoclase, also known as potassium feldspar, is very similar to plagioclase feldspar except instead of containing Ca or Na, it contains K. Quartz, also known as silicon dioxide, is one of the most common minerals on the surface of the earth and contains the elements Si and O.
In the field, a mafic rock will generally be dark-colored, with an abundance of augite and olivine, and may have some light-colored specks of plagioclase feldspar. A felsic rock will generally be light-colored with an abundance of orthoclase and quartz, with some dark-colored specks of biotite and amphibole.
Sources:
"Igneous Rocks" - Cascades Volcano Observatory USGS
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Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
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