In the early 1900's, N. L. Bowen and others at the Geophysical Laboratories in Washington D.C. began experimental studies into the order of crystallization of the common silicate minerals from a magma. The idealized progression which they determined is still accepted as the general model for
the evolution of magmas during the cooling process.
The Principles that Bowen realized are as follows:
- As a melt (the liquid portion of the magma) cools minerals crystallize that are in thermodynamic equilibrium with the melt (dissolution equals crystallization; if no equilibrium exists either crystallization will dominate [supersaturation], or dissolution will dominate [undersaturated]).
- As the melt keeps cooling and minerals keep crystallizing, the melt will change its composition.
- The earlier formed crystals will not be in equilibrium with this melt any more and will be dissolved again to form new minerals. In other words: these crystals react with the melt to form new crystals, therefore the name, reaction series.
- The common minerals of igneous rocks can be arranged into two series, a continuous reaction series of the feldspars, and a discontinuous reaction series of the ferromagnesian minerals (olivine, pyroxene, hornblende, biotite)
- This reaction series implies that from a single "parental magma" all the various kinds of igneous rocks can be derived by Magmatic Differentiation (see below)
To summarize: Bowen determined that specific minerals form at specific temperatures as a magma cools. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches (see below). The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich. Plagioclase feldspar crystals have a core that is calcium-rich and a rim that is sodium-rich. The average composition of the calcium and sodium content in plagioclase feldspars will approximate the calcium and sodium composition of the magma.
The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. The remarkable thing that Bowen found concerned the discontinuous branch. At a certain temperature a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma (the melt), and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to hornblende, and then to biotite. The reason for this "stepped" evolution of minerals is that with dropping temperature we we have decreasing thermal vibration of molecules, and that allows silica to form more complex structures. Thus, olivine with its isolated silica tetrahedra forms at the highest temperatures, and as temperatures drop silica tetrahedra first manage to join together in single chains (pyroxenes), then in double chains (hornblende and other amphiboles), and then sheets (micas). Finally, at the lowest temperatures the two branches merge and we get the minerals that are common to felsic rocks - muscovite mica (sheet), and potassium feldspar (K-spar which is a 3D framework) and quartz (3D framework). This reaction series is illustrated in the table below. (PDF version)