The Bowen reaction series, developed by geologist Norman L. Bowen, shows the sequence of mineral formation from magma based on temperature, pressure, and cooling rate. The series has two branches, the continuous and discontinuous series, which describe the formation of plagioclases and mafic minerals respectively. The sequence ranges from rocks high in calcium, magnesium, and iron to rocks low in these elements and high in sodium and silica. The series is used in geology, petrology, and volcanology.
The rocks that make up the earth’s crust are made up of a variety of minerals with different chemical compositions and physical properties. Minerals originate in igneous rocks, which solidified from magma, and the types of minerals present depend not only on the chemical composition of the original magma, but also on its temperature, pressure and rate at which it cooled. In the early 20th century, geologist Norman L. Bowen of the Geophysical Laboratory at the Carnegie Institution in Washington, DC carried out a series of experiments aimed at determining the crystallization sequence of different minerals from magma. He melted powdered samples of igneous rock, then allowed them to cool to predetermined temperatures so he could observe the formation of mineral crystals and the sequence in which they appeared. From these results, he compiled what became known as the Bowen reaction series, a mineral formation sequence widely used in geology, petrology, and volcanology.
When molten rock cools very quickly, there isn’t enough time for the minerals to form crystals; the result is instead an amorphous glass. The experimental procedure used by Bowen was designed to exploit this phenomenon to “freeze” the crystallization process in the different phases. The rock samples were placed in an extremely sturdy container known as a “bomb” and heated to approximately 2,912°F (1,600°C), ensuring that all material melted. The sample was allowed to cool to a certain temperature and held at that temperature long enough for some minerals to crystallize, then suddenly cooled with water to provide a “snapshot” of the process at that particular stage. The minerals that had already crystallized were preserved, while the rest of the material, which was still molten, solidified into glass.
By repeating this procedure for different temperatures, the Bowen reaction series was expanded, giving a picture of crystalline minerals produced at temperatures ranging from 2,552°F (1,400°C) up to 1472°F (800°C). Bowen identified two distinct branches of the series, distinct in mineral chemistry, which came together at lower temperatures. One, which he called the continuous series, described the crystallization sequence of minerals rich in sodium, calcium, aluminum and silica, collectively known as plagioclases. The other, called the discontinuous series, described the sequence of minerals rich in iron and magnesium, known as mafic minerals.
The continuous series is so called because it shows a smooth transition in the composition of the minerals formed with decreasing temperature. This is best illustrated by the relative proportions of calcium and sodium. When crystallization occurs at very high temperatures, the crystalline material is very rich in calcium and very low in sodium. As temperatures drop, the ratio of sodium to calcium steadily increases, until these proportions reverse. The proportion of silica in minerals also increases with decreasing temperature.
In the batch branch of the Bowen reaction series, the processes are more complex. As with the continuous series, the proportion of silica increases with decreasing temperature; however, instead of a steady increase in silica content there is a sequence of distinct minerals: olivine, pyroxene, amphibole and biotite. Olivine is the first to crystallize – at around 2,552°F (1,400°C), but as the temperature drops it reacts with the still molten material, forming the next mineral in the series, pyroxene. Similar processes convert pyroxene to amphibole and amphibole to biotite; however, each transition from one mineral to another will only occur if there is still enough silica in the magma. The sequence can also break down at any time if the magma cools very rapidly as it reaches the surface, leaving minerals such as olivine, pyroxene and amphibole still present in the solidified rock, just as in Bowen’s experiments.
Where the two branches join, the sequence continues. The remaining minerals, in increasing order of silica content, are orthoclase, also known as potassium feldspar, muscovite and quartz. Overall, the Bowen series of reactions ranges from rocks high in calcium, magnesium, and iron and low in sodium and silica — such as basalt — to rocks low in calcium, magnesium, and iron, and high in sodium. and silica – like granite. In a large underground magma chamber that cools very slowly, the high-calcium olivine and plagioclase will crystallize first and sink through the liquid magma to the bottom of the chamber, followed by other minerals in the sequence, leaving granite and rocks similar to the top when the whole mass has solidified. Good examples of this sequence, ranging from granite at the top to gabbro – a coarse crystalline rock with the same composition as basalt – at the bottom can be found in several locations around the world.
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