Bowen's Reaction Series

Igneous rocks (e.g., granites, lava flows) are those created by the cooling of molten material called magma. As the magma cools, volatiles are lost, early formed minerals crystallize out and react with the remaining magma, and changes (either in a continuous fashion or a discontinuous one) in composition take place. This is called fractional crystallization. There is a sequence as to which minerals form first, second, third, and so on. This process is known as the Bowen's Reaction Series.

Note: The poster shown below depicts this process. I made it many years ago to help my students visualize the crystallization sequence. It was not made with the intention of showing it online. This explains why the the writing on the poster is a little hard to read, because I had to stand way back with my camera in order to photograph all the writing. Although the dark minerals do not show up very well, this poster is, nevertheless, useful because it conveys the main concept of the progressive sequence of crystallization of an ideal magma as it cools. It is especially helpful because I used actual minerals instead of just their names. 

The Bowen's Reaction Series is the result of research in the 1920's and 1930's by Norman Bowen, who was able to explain why certain types of minerals tend to found together (e.g., olivine and labradorite--see one of my earlier posts on labradorite) whereas others are almost never associated with one another (e.g., olivine and quartz). He experimented with powdered rock material that was heated until it melted and then allowed to cool to a target temperature, whereupon he observed the types of minerals that formed in the rocks produced. He repeated the process with progressively cooler temperatures, and the results allowed him to formulate his reaction series. His work showed that a geologist can infer, from the minerals present in a rock, the relative conditions under which the magma had formed.

The Bowen's Reaction Series can be combined also with crystal size and texture of the rock to determine whether the rock formed under the Earth's surface (plutonic igneous rocks with large crystals visible to the eye) or on the surface (volcanic igneous rocks with microscopic crystals). The rock names (e.g., peridotite, gabbro, granite) are indicated on the left side of the poster.

The following comments are intended only for individuals who are really interested in the details of the Bowen's Reaction Series.

The Bowen's Reaction Series consists of two main branches, the discontinuous reaction series (depicted on the left side of the poster: olivine, augite, hornblende, and biotite) and the continuous reaction series (depicted on the right side: anorthite, bytownite, labradorite, andesine, oligoclase, and albite). The last minerals (especially quartz) are generally considered to be the residual phases. Temperature cools as crystallization proceeds, with the high-temperature minerals forming first, and the low-temperature forming last. The word "continuous" refers to the continuous, series of solid solutions that are produced during the cooling. The word "discontinuous" refers to the ferro-magnesium minerals (e.g., augite) that form during the cooling process. They react with the melt and abruptly produce a new mineral, with different crystal structure and considerably different composition than the mineral that preceded them.

For the Bowen's Reaction Series to work in the perfect "flow chart" fashion (as shown on the poster) requires a magma with an ideal chemical composition and a long time for cooling. In reality, which minerals form are dictated largely by the chemical composition of the original molten material. Also, the Bowen's Reaction Series only goes to completion if there is enough time for the reactions to take place and enough Na, Al, and Si in the remaining melt to form each successive mineral. In the ideal situation, eventually only SiO2 (mineral quartz) is all that remains.

As the temperature (and pressure) decrease, a progression of feldspar minerals takes place. The plagioclase feldspars (anorthite, bytownite, labradorite, andesine, oligoclase, and albite), which have the formula (Ca, Na)(Al, Si)3O8, represent a single mineral (plagioclase) consisting of a solid-solution series with varying amounts of calcium (Ca) and sodium (Na). The highest temperature plagioclase (anorthite) has the highest calcium content. The lowest temperature plagioclase (albite) has the highest sodium content. In between, these ions mix in a continuous series with the calcium/sodium ratio varying from 100% Ca and 0% Na, to just the opposite.  This variable chemical composition creates the different colors of plagioclase. 

The cooler temperature alkali feldspars have varying amounts of potassium (K) and sodium (Na), and the ratio of these two elements also changes as the crystallization process proceeds. There are several of these minerals (anorthosite, microcline, sanidine, and orthoclase). Only orthoclase (KAlSi3 O8)is shown on the poster. 

The high-temperature minerals (e.g. anorthite, bytownite, labradorite) are the most unstable at the Earth's surface and quickest to weather because the Earth's surface is most different from the conditions which they were created. On the other hand, the low-temperature minerals (e.g., muscovite and quartz) are much more stable because the conditions at the surface are much more similar to the conditions under which they formed.

Common rock-forming minerals

The following is a very basic discussion of minerals, which is critical for the understanding the classification of igneous rocks, the topic of the next post.

Atoms (electrons, protons, and neutrons) make up elements (92 natural ones; (examples: oxygen, silicon, potassium).

Elements make up minerals (3,500 kinds).

     Minerals consisting of pure (native) single elements are rare     (e.g., gold, diamond, copper).

     Nearly all minerals consist of combinations of elements.

Only 20 minerals are common, and they are the rock-forming minerals. Seven of these are shown below.

white ("milky") quartz, 3.3 cm in height

Quartz (made up of the elements silicon + oxygen) is the most common mineral in the continental crust of Earth. The presence or absence of quartz in a rock is fundamental in the classification of igneous rocks. For more information, see my previous two posts, which review the subject of quartz.

potassium feldspar (K-spar), 3.7 cm in height

One of the largest groups of minerals is referred to as the "feldspar group," which consists of two subgroups. One of these subgroups is the "alkali feldspars,"which contain varying amounts of the elements potassium and sodium in their composition. The mineral pictured above is one of these alkali feldspars, which are generally called "potassium feldspar" or "K-spar" until x-ray and petrographic microscope studies are made to determine the exact mineral. 

plagioclase, 3.3 cm in height

The second subgroup of the "feldspar group" is called the "plagioclase feldspars," which contain varying amounts of the elements sodium and calcium in their composition. Plagioclase is characterized by the presence of striations, which can be seen as closely spaced lineations in the upper half of the specimen shown above.

flakes of muscovite, largest flake 4 cm height

Muscovite and biotite (see below) belong to the "mica group" of minerals. Both have one-dimensional cleavage, which splits into sheets or flakes. For a discussion of what cleavage is, see one of my more recent posts. Because of its clarity, large sheets of muscovite could have served as "windows" for pioneers living in wood cabins. Muscovite is common in quartz-rich igneous rocks (granite), metamorphic rocks like (schists and gneisses), and sedimentary rocks (siltstone and claystone).

large sheet of biotite, 6.5 cm height

Biotite has a more varied composition than does muscovite. For example, biotite has iron in its structure, which causes a dark coloration. Biotite occurs in igneous rocks, as well as a wide variety of metamorphic rocks. 

hornblende, 7 cm height

Hornblende has a very complicated composition and much chemical variability, depending on the chemicals available and the temperature of formation. Hornblende has two directions of cleavage (approximately at 60 degrees and 120 degrees). This mineral is found in a wide variety of igneous and metamorphic rocks. 

olivine, 3 cm height

The "olivine" group minerals contain varying amounts of iron and magnesium in their composition. "Olivine" is most common in quartz-free rocks and commonly has a green color.

In the minerals shown above, three of them occur as solid-solution series, which form when two elements can substitute for one another in a mineral. The three solid-solution series are the
alkali feldspars, plagioclase feldspars, and the "olivine" group.

Most of the minerals in these solid-solution series are not compositionally pure end members; rather they have an in-between composition.

Minerals make up rocks (3 main types: igneous, sedimentary, and metamorphic).

Rocks make up the Earth's crust (two kinds: continental and oceanic).

In the next post, igneous rocks will be discussed and illustrated, and the chart below is presented here to show how quartz is used to help define certain kinds of igneous rocks. For example, granite (think expensive kitchen-counter tops) is rich is quartz but basalt (think lava flows in Hawaii) is low in quartz.