Crystallize from molten material:
Igneous rocks are classified on their texture and their composition.
Granite - polished
Vesicular basalt with olivine phenocrysts, building stone at Hawaiian Volcano Observatory, Big Island of Hawaii
Igneous rocks can be placed into four groups based on their chemical compositions:
Other types of igneous rock:
A polished syenite called larvikite with centimeter- to inch-scale gray to blue plagioclase crystals. The industrial name for the rocks is "blue pearl". Photographed in an above-ground cemetery in New Orleans, LA
Bowen's Reaction Series also lists the minerals in the order in which they melt with increasing temperatures.
Minerals higher on the reaction series crystallize before minerals lower on the reaction series. The earlier-formed minerals react with the magma to form minerals lower on the series. The process ends when the magma has completely crystallized. The composition of the resulting igneous rock depends on the composition of the magma.
Visit this interactive web site to see the minerals of Bowen's Reaction Series (in order) http://www.cat.cc.md.us/courses/eas101/unit1/brs.html.
Bowen's Reaction Series has two branches. They are:
The discontinuous reaction series involves the dark-colored ferromagnesian minerals:
As a magma cools, olivine crystallizes first. The olivine crystals react with the remaining magma to form pyroxene. Pyroxene reacts with the magma to form amphibole. Amphibole reacts with the magma to form biotite. Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure. As crystallization proceeds, the crystal structures become more complex (olivine has an isolated tetrahedral structure, pyroxene has a single chain structure, amphibole has a double chain structure, and biotite has a sheet structure). The series of minerals is called discontinuous because a series of different minerals is formed, each with a different crystal structure.
The continuous reaction series involves the plagioclase feldspars. Plagioclase feldspars are an example of a "solid solution series", exhibiting gradations in chemical and physical properties. Chemically, this series consists of two "end members":
There is a continuous chemical and physical gradation between the two end members. (Various plagioclase mineral names are given, based on the percentages of calcium and sodium present, including anorthite, bytownite, labradorite, andesine, oligoclase, and albite).
Ca-plagioclase is the first to crystallize. It reacts with the melt to become more sodium rich. (If reaction is not complete, a zoned plagioclase crystal results which has a calcium-rich center and sodium-rich edges).
This series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure. The minerals differ primarily in the proportions of calcium and sodium present.
During the last stages of crystallization, potassium feldspar (KAlSi308) crystallizes. Muscovite may also form. If the remaining melt contains excess silica, quartz will crystallize.
Bowen's Reaction Series helps us to understand why certain minerals tend to occur together in igneous rocks. For example, the mafic rocks, basalt and gabbro tend to contain olivine, pyroxene, and calcium-rich plagioclase feldspar. These are all minerals which crystallize at high temperatures. As another example, felsic or sialic rocks such as granite and rhyolite tend to contain quartz, potassium feldspar, sodium-rich plagioclase feldspar, and sometimes muscovite. These are minerals which crystallize at lower temperatures. The minerals that ultimately form are controlled by the initial composition of the magma.
Bowen's Reaction Series also helps us to understand why certain minerals do NOT occur together in igneous rocks. For example, olivine and quartz are unlikely to occur in the same igneous rock, because olivine is a high temperature mineral, and quartz is a low temperature mineral.
Bowen's Reaction Series also shows us that the range of igneous rocks, from ultramafic to sialic (or felsic), can be produced by the same original mafic magma. The magma changes as it cools. As a magma cools, the early-formed crystals may settle to the bottom of the magma chamber. This would produce a rock type at the bottom of the magma chamber that is dominated by early-formed minerals such as olivine, pyroxene, and calcic plagioclase (a mafic or ultramafic rock). The remaining melt would be enriched in silica (relative to its original composition), and may continue moving upward toward the earth's surface. Crystal settling may occur again, producing an intermediate rock. As the last remaining melt moves toward the Earth's surface, it will crystallize to produce a sialic or felsic rock. The removal of crystals from the magma by settling (or other processes) is known as fractional crystallization.
The formation of several different rock types from one initial magma, through separation of earlier-formed crystals, causing the magma to evolve to become more silica-rich, is known as magmatic differentiation. Magmatic differentiation can produce a variety of types of igneous rocks through evolution of the original parent magma.
You need to memorize Bowen's Reaction Series. It helps to use a mnemonic device (or phrase using the initial letters of each mineral) to help remember the names of the minerals in order. Remember the Y-like shape. For the discontinuous reaction series and the minerals in the "trunk" of the Y, you would have the following first letters of mineral names: O P A B K M Q. (K is for potassium feldspar, which is abbreviated with the chemical symbol, K.) Try to make up your own mnemonic device or phrase, but here is one that one of my students made up. I don't like it, but I remember it. Old People Are Bad; Keep Mother Quiet. Hopefully you can make up a better one.
You have to know the series AND understand the concepts of how Bowen's Reaction Series relates to melting and to crystallization, and to the origin of igneous rocks of various composition.
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Copyright 2000 Pamela Gore
This page created by
Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, Clarkston, GA
October 2, 1995
Modified January 24, 1997
Modified September 21, 1998
Modified July 17, 1999
Last modified August 21, 2000