On the first day of my introductory geology class, I advise my students to memorize this thing called Bowen’s Reaction Series (BRS). I tell them that knowing it will help them out tremendously throughout the class and the rest of their lives.
A great deal of history of a rock can be told by merely recognizing the minerals that are in it. Bowen’s Reaction Series provides the basic relationships of some of the most common rock-forming minerals on Earth. Not only does BRS show which minerals will be found with which other minerals, it also provides a scheme of stability and chemical composition of those minerals.
In this post, I’ll introduce the important rock forming minerals of Bowen’s Reaction Series, and place this in the context of stability and composition that will help students of geology understand the history of a particular rock.
I’ll start by saying that BRS applies primarily to igneous rocks – those rocks that formed when molten rock cooled and crystals developed. When the once molten rock completely solidifies, the solid rock left behind is an igneous rock. The crystals that form might be any one of several different minerals. What the minerals are is dictated by the chemical composition of the original molten rock.
The most important chemical components are iron (Fe), magnesium (Mg), potassium (K), aluminum (Al), calcium (Ca), and sodium (Na). We’re also concerned with the relative amounts of silica (SiO4) present.
All right, so what minerals are we talking about? I’ve already introduced them once before in this blog post.
To list them, they are:
- Pyroxene (Augite)
- Amphibole (Hornblende)
- Potassium Feldspar (K-spar or Orthoclase)
- Plagioclase (actually a series of minerals)
If you can identify these eight minerals, you can identify most of the minerals found on the Earth’s surface.
Here’s how they sit on Bowen’s Reaction Series:
There are three parts to BRS: The Discontinuous series, that includes olivine, pyroxene, amphibole, and biotite; the continuous series, which includes the plagioclase minerals (anorthite to oligoclase); and the residual phases, including quartz, muscovite, and potassium feldspar.
Minerals toward the top of BRS have high abundances of iron and magnesium, and are termed mafic. These mafic minerals, though still having quite a lot of silica, have substantially less silica than do the minerals toward the bottom of BRS. This characteristic chemical composition tend to make mafic minerals less stable at the surface of the Earth. However, the vast majority of rock making up the Earth is mafic, with olivine and related minerals composing most of the Earth’s mantle.
At the bottom of BRS are the felsic minerals of quartz, muscovite, and potassium feldspar. Felsic minerals have high abundances of aluminum and potassium, and higher amounts of silica than mafic minerals. In fact, quartz is 100% silica.
The plagioclase series is interesting, because it spans from mafic compositions all the way to felsic – but it’s all one mineral. This is what we in the biz call a ‘solid solution series,’ which is a mouthful, to be sure. We know all the minerals on this series are plagioclase because they show the characteristic striations of plagioclase, but they differ in the relative amounts of certain elements, in particular calcium and sodium, which causes them to be different colors.
Thus, we have anorthite at the mafic end of the series, which is dark in color, and oligoclase at the felsic end which is light in color.
All right, so now we have all the minerals in place and understand something about their chemical composition. So how do we use this?
These minerals do not just form random associations. Those on the mafic end of the series only occur with other mafic minerals. Thus, you can expect olivine, pyroxene, and calcium plagioclase to occur together to form a mafic rock. This is a rock you’ve probably seen before. It’s called basalt, and is the typical rock that forms from the volcanoes on Hawaii.
Quartz, muscovite, and potassium feldspar also commonly occur together. This rock is known as granite. You might have a granite counter top in your home.
Now, I’ll point out here that there are a lot of things out there called ‘granite’ that aren’t granite at all. At least not by the strict geological definition. Granite has quartz, muscovite (maybe biotite), and potassium feldspar in big crystals that you can see. If you have a lovely countertop made of augite and calcium plagioclase, they might have sold it to you calling it granite, but it’s really a rock called ‘gabbro.’ I’ll get to the names of rocks here in a minute.
The point is that you’ll never see olivine and quartz together in a rock. Muscovite won’t occur with amphibole. It’s always felsic with felsic, mafic with mafic, and intermediate (between felsic and mafic) with other intermediate minerals. So if you can identify one mineral, you can start guessing at the others simply based on where you are on BRS.
But wait! There’s more!
Minerals toward the mafic end of BRS tend overall to be darker than those on the felsic end. Thus, if you have a dark colored igneous rock, you can guess that it’s mafic and from there know what minerals might be present even if you can’t actually see the crystals. Felsic rocks, like granite, tend to be very light, even almost white in color. Intermediate rocks are interesting because rather than being grey, they tend to have a mixture of light and dark minerals giving them a salt-and-pepper appearance.
So, you probably have two questions at this point.
- How do I know if I have an igneous rock?
- How do we name all these rocks?
How do I know if I have an igneous rock?
This is a question I get year after year, and I swear I have yet to come up with a good answer that works every time. You just have to think about the origin of the rock. It crystallized out of a liquid, much like you get ice cubes from water that was poured into a square container then allowed to freeze.
An igneous rock usually lacks any obvious layering (although, sometimes things like basalt show lines of flow that look like layers). If you can see crystals, they’ll have no obvious preferred direction. They’ll be floating every which way in the rock. You’ll also see that the crystals have sharp edges and interlock with other crystals as if they grew together (because they did). The crystals shouldn’t be rounded or smoothed as if they tumbled in a stream.
How do we name all these rocks?
The naming scheme for igneous rocks can be very simple or very complex. For most people, and for students in my class, I choose to go with the simple classification, based only on composition (mafic, intermediate, etc.) and the size of the crystals. The crystals are either too small to see with a naked eye, big enough to see and all of similar size, or some big and some small all in the same rock.
If the crystals are too small to see without magnification, the rock is called aphanitic (AY-fan-it-ic).
If the crystals can be seen with the naked eye, and are all about the same size, the rock is called phaneritic (FAN-er-it-ic).
If some crystals are much, much larger than others, with the small ones perhaps too small to be seen without help, the rock is called porphyritic (PORE-fur-it-ic), with the word porphyry (PORE-fur-ee or por-IF-fur-ee) simply attached to the aphanitic name to show that it’s porphyritic.
Aphanitic Phaneritic Porphyritic Mafic Basalt Gabbro Basalt porphyry Intermediate Andesite Diorite Andesite porphyry Felsic Rhyolite Granite Rhyolite porphyry
So far, we can now use BRS to say something about what minerals are present in an igneous rock and know something about the rock’s composition. Can Bowen’s tell us anything else?
Yes. Of course.
We know that mafic minerals are most stable at high temperatures and pressures. Thus, if they’re sitting on the Earth’s surface they tend to break down very quickly. So if we find a mafic rock, we know it hasn’t traveled far from its source.
Felsic minerals, on the other hand are quite stable at the Earth’s surface, which is why we have massive beaches comp0sed of glorious quartz sand.
You might ask, well, if the majority of the Earth (the mantle) is made up of mafic minerals like olivine, how come we have so much granite on the Earth’s surface.
The answer to that question might be found in this post. And if that doesn’t satisfy you, then ask me and I’ll write a longer post about it some time…