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In all of my time studying engineering and science, I have never come upon something as useful and nearly perfect as Bowen’s Reaction Series.

For those who have yet to be made aware of it, Bowen’s Reaction Series shows the relation between various minerals of igneous (molten rock) origin.  There are three parts to Bowen’s Reaction Series, the continuous reaction series, the discontinuous reaction series and the residual phase.  I’ll talk about each part in more detail and move on to a few interesting applications of Bowen’s Reaction Series in mineralogy.

The layout of Bowen’s Reaction Series is such that as you move up the series (towards olivine and calcium plagioclase), the melting temperature increases.  Calcium plagioclase and olivine do not have the same melting temperatures but are usually placed at the same height for aesthetic reasons.  This means that quartz, muscovite and potassium feldspar melt at the lowest temperatures.

Related to the melting temperature is thermodynamic stability.  Minerals which crystallize at a high temperature tend to be stable at such temperatures, and are less stable at atmospheric conditions.  Olivine, therefore, weathers very easily under standard conditions.  This helps to explain why quartz is the leftover in beach sand.  Quartz is resistant to chemical weathering and is commonly left behind.  The mechanical weathering of waves may wash away other materials, but quartz will remain behind.  Olivine and pyroxene, the two  mineral types on the discontinuous series with the highest melting points, weather extremely easily and are thus unlikely to form such deposits.

Anyone not interested in a more detailed discussion of the chemistry of Bowen’s Reaction Series should skip ahead to the final section on interesting applications of Bowen’s Reaction Series.

The Discontinuous Series

The discontinuous series is the most interesting part of Bowen’s Reaction Series.  It is here where the most interesting chemistry takes place.  As mentioned already, olivine is the first mineral to crystallize out of a melt.  Olivines have the general formula (Ca,Mg,Fe)(Mg,Fe)SiO4.  Calcic olivines are metamorphic minerals and will not be discussed further.  Magnesium rich olivine is known as Forsterite, and melts at a temperature much higher than the iron-rich end member, Fayalite.

If an olivine, (Ca,Mg,Fe)(Mg,Fe)SiO4, reacts with quartz, SiO2, it will form a pyroxene, (Ca,Mg,Fe)(Mg,Fe)Si2O3.  Unlike olivine, calcium pyroxene (known as clinopyroxene) are found in igneous formations.  Orthopyroxenes do not contain calcium.  Ortho and clinopyroxenes have different crystallographic structures since calcium ions are enough larger than iron and magnesium to force a change in the structure.

Adding another SiO2 and water to a pyroxene will produce an amphibole, (Ca,Mg,Fe)2(Mg,Fe)5Si8O22(OH)2.  Amphiboles are minerals that we begin to see more regularly in light coloured volcanic rocks, such as granites.  Amphiboles show a lot more chemical variability than either pyroxenes or olivines.  Sodium ions are capable of substituting for any of the (Ca,Mg,Fe) ions and minerals with up to three sodium ions exist.  To maintain charge balance, however, if a sodium is substituted for one of these atoms, an aluminum (3+) must substitute for a silicon (4+).  Al-O bonds are slightly weaker than Si-O bonds and thus only up to half of the silicon in a structure is available for substitution.

As all of the above minerals crystallize from the melt, thee melt becomes rich in potassium and silica (SiO2).  The last mineral in the  discontinuous series has plenty of silica in its structure and contains potassium, a very large ion.  Biotite’s chemical formula is K(Mg, Fe)3AlSi3O10(OH)2.  Like all other mineral classes on the discontinuous series, it has a high temperature magnesium and a low temperature iron end member.

Each mineral class in the discontinuous series has an increasing degree of polymerization than the last.  Olivine is a nesosilicate, meaning that the silicon-oxygen tetrahedra (which comes from the relative size of the two ions) are physically isolated from each other and no polymerization has taken place. Pyroxenes are single chained inosilicates, meaning that they have a linear chain of tetrahedrea linked together.  Amphiboles are also inosilicates, only two of these chains have joined to form a wider sheet.  Lastly, biotite is a phylosilicate, meaning that many such chains have joined such that the resulting polymer structure is infinite (compared to the size of the atoms) in two directions, forming a sheet.

The Continuous Series

Unlike the discontinuous series, where earlier minerals react with the melt to form  later minerals, once a mineral is formed it remains unless remelted.  The continuous series features only one mineral, plageoclase feldspars, varying from calcium-rich (anorthite) to sodium-rich (albite) end-members.  All feldspars are tectosilicates, meaning they are polymerized in all three directions forming a complex network of interconnected silica tetrahedra.  The way these minerals form tend to produce a zoning effect, where within a crystal calcium content decreases as you move from the centre to the edge.

As you move from albite to anorthite, an increasing number of silicon atoms must be replaced with aluminum atoms to ensure charge balance.

The Residual Phase

The residual phase is whatever is left over after both reaction series have run to completion.  As it cools, orthoclase (potassium) feldspar forms, as well as muscovite mica and quartz.  Quartz and orthoclase are both tectosilicates (3D network), while muscovite is a phylosilicate (sheet).

Interesting Uses

Now that I’ve walked you through the basic chemistry, its time to examine a few reasons why Bowen’s Reaction Series is such an amazing tool, aside from chemical and temperature relationships and weathering.

The first is something I’ve already referenced, as you move down the series, crystal structures generally become more complex.  From olivine with no polymerization to quartz with perfect three dimensional polymerization, all in one easy to see chart.  I say generally since it is common to group all of the feldspars together (as above), which separates biotite and muscovite mica.

Crystal structure determines many physical properties.  For example, micas break off into thin, flexible sheets because all of the silica tetrahedrons point in the same direction, all of the upward facing oxygen are coordinated (bonded) to potassium, which is so large that it needs 12 oxygen to bond to.  This means that the potassium-oxygen bond is extremely weak (1/12 of the silicon-oxygen bond).  Since the structure forces these to form linear sheets, its no wonder micas cleave the way they do.

The third thing is incompatibility.  You will never find an olivine and a quartz in the same rock.  Why?  If there was silica in the melt, it would have reacted with the olivine to form pyroxene, leaving either less olivine and no silica or no olivine and less silica.

Finally, Bowen’s Reaction Series allows us to explain what we see in rocks.  We know that a granite, rich in quartz, feldspar and biotite, is the result of a relatively iron-magnesium free magma which crystallized at lower temperatures.  A dunite, on the other hand, is mostly olivine and so had to crystallize at high temperatures and become physically separated from the rest of the melt and its silica.  Combine this information with the size of crystals observed in the rock, which determine the rate of cooling, and even an amateur like myself can tell a rock’s story with ease and accuracy.

That is why I find Bowen’s Reaction Series to be so fascinating, with only two other pieces of information (bonding rules and crystal size in relation to cooling rate) you can explain most common minerals and the rocks they make up.  It is a very powerful tool in earth history and mineralogy.