How Does Bowen's Reaction Series Relate to the Classification of Igneous Rock?
The Bowen reaction series is a description of how magma's minerals change as they cool. It’s a means of ranking common igneous silicate minerals by the temperature at which they crystallise. Bowen's Reaction Series describes the temperatures at which different common silicate minerals change from the liquid to solid phase (or from the solid to liquid).
The Principles that Bowen realized are as follows:
- A) As a melt cools minerals crystallize that are in thermodynamic equilibrium with the melt (dissolution equals crystallization; if no equilibrium either crystallization will dominate [supersaturation], or dissolution [undersaturated]).
- B) As the melt keeps cooling and minerals keep crystallizing, the melt will change its composition.
- C) 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.
- D) 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)
- E) This reaction series implies that from a single "parental magma" all the various kinds of igneous rocks can be derived by Magmatic Differentiation.
In general terms, the higher temperature minerals have a higher proportion of iron and magnesium, and are therefore considered to be mafic. The lower temperature minerals are related to the opposite end of the compositional spectrum (lower in iron and magnesium, higher in silicon and oxygen), and are considered to be felsic. Some minerals are clearly mafic, some are clearly felsic, and some fall in between these two extremes.
The other factor which contributes to the differences in igneous rocks relates to the length of time it took for the magma to crystallize. In general terms, the faster the rate of cooling (common to extrusive, volcanic rocks), the smaller the resulting mineral grains. Slower cooling histories (commonly intrusive) result in a coarser-grained rock.
Mafic igneous rocks (olivine, pyroxene, and the plagioclase feldspars) include basalt (extrusive) and gabbro (intrusive), while felsic igneous rocks (quartz, amphibole, mica, and the orthoclase feldspars) include granite (intrusive) and rhyolite (extrusive).
Magmas of intermediate composition result in the crystallization of intermediate minerals (actually a mix of the mid-range minerals: amphibole and both types of feldspar), with the common igneous rocks being andesite (extrusive) and diorite (intrusive).
Simply put, the high temperature minerals, the first ones to crystallize in a mass of magma, are most unstable at the Earth's surface and quickest to weather because the surface is most different from the conditions under which they were created. On the other hand, the low temperature minerals are much more stable because the conditions at the surface are much more similar to the conditions under which they formed.
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