Silicate Minerals and Bowen’s Reaction Series

Essentially all of the solid Earth, except for the slimy biological parts, is composed of minerals. Minerals on Earth may be divided into several categories, depending upon their composition and structure. Carbonate and phosphate minerals are important for life, in that they form the skeletons of many different groups of organisms. Native elements, like gold, are economically important. The vast majority of minerals that exist on Earth are in a class called the silicates.

Quartz is probably the best known of the silicate minerals. It’s found all over the place on the Earth’s surface, and why that is should be clear by the end of this article.

Silicate minerals get their name because they all include silica tetrahedra in their crystal structure. A silica tetrahedron is composed of a single silicon atom surrounded by four oxygen atoms forming a four-sided solid.

The Silica Tetrahedron
Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)Terms of UseAttribution

These tetrahedra may sit in the crystal matrix surrounded by other atoms, such as iron, magnesium or calcium, or they may bond together at the corners resulting in chains, sheets, or three-dimensional frameworks of connected tetrahedra. How the tetrahedra relate to each other is the basis of the subdivisons of the silicate minerals.

Nesosilicates – indepedent tetrahedra

Where tetrahedra are independent from each other, the mineral is formed by bonds between the tetrahedra and other atoms in the crystal matrix. The most common mineral in this group is olivine. This is the mineral that forms the green sand beaches in Hawai’i. When olivine crystals are of gem quality, they are called peridot.

This mineral is called olivine (actually this is a rock, made up of olivine crystals). It's easy to recognise because of its pimento olive color.
This mineral is called olivine (actually this is a rock, made up of olivine crystals). It’s easy to recognise because of its pimento olive color.

Olivine is relatively rare on the Earth’s surface, but highly abundant in the Earth’s mantle, where high temperatures and pressures keep it stable.

Inosilicates – tetrahedra in chains

In some silicates, the tetrahedra bond together to form chains. These can be linked into single chains or double chains.

Single-chain silicates include the pyroxene minerals, like augite.

Silicate-single-chain-plan-view-2D.png
Single-chain silicates structure. “Silicate-single-chain-plan-view-2D”. Licensed under Public domain via Wikimedia Commons.
This mineral is part of the pyroxene group of minerals. It is called augite and looks a lot like hornblende.
This mineral is part of the pyroxene group of minerals. It is called augite and looks a lot like hornblende.

Double-chain silicates include the amphibole minerals, like hornblende.

Silicate-double-chain-plan-view-2D.png
Double-chain silicate structure. “Silicate-double-chain-plan-view-2D”. Licensed under Public domain via Wikimedia Commons.
This mineral is part of the group of minearls called amphiboles. Specifically, this is hornblende.
This mineral is part of the group of minearls called amphiboles. Specifically, this is hornblende.

Phyllosilicates – the sheet silicates

Sheet silicates include the more recognizable minerals called micas. Clay minerals are also sheet silicates, as well as the vermiculite that you might use in your flower pots.

In sheet silicates, the tetrahedra are bonded to others at three of the four oxygen atoms, forming sheets. These minerals tend to peel apart easily along these layers.

Silicate-sheet-3D-polyhedra.png
Sheet silicate structure “Silicate-sheet-3D-polyhedra”. Licensed under Public domain via Wikimedia Commons.
This is biotite, a type of mica that is dark in color.
This is biotite, a type of mica that is dark in color.

Tectosilicates – the framework silicates

In tectosilicates, all four of each silica tetrahedron’s oxygens is shared with another tetrahedron, creating a three-dimensional framework.

Beta-quartz-CM-2D-balls.png
Framework silicate structure “Beta-quartz-CM-2D-balls” by Ben Mills – Own work. Licensed under Public domain via Wikimedia Commons.

For quartz, which is nothing but bonded silica tetrahedra, the result is a mineral that is very stable on the Earth’s surface, which is why it is so common.

Quartz. This is one mineral almost everyone has heard of!
Quartz. This is one mineral almost everyone has heard of!

Other tectosilicates, like the feldspars, tend to be far less stable on the surface of the Earth, perhaps due to the inclusion of other atoms, like calcium, sodium, potassium, and aluminum into the crystal matrix.

The mineral orthoclase. Also called potassium feldspar or K-spar.
The mineral orthoclase. Also called potassium feldspar or K-spar.

All right. So what does this have to do with Bowen’s Reaction Series?

Why, everything, of course!

Take a look at Bowen’s Reaction Series below. Especially look at the discontinuous series of minerals on the left. Does that order of minerals look familiar?

Bowen's Reaction Series.png
Bowen’s Reaction Series. “Bowen’s Reaction Series” by Colivine – Own work. Licensed under CC0 via Wikimedia Commons.

Notice that from the top of the discontinuous series going down, the silicates are increasing in the complexity of the bonds between tetrahedra. Olivine starts with independent tetrahedra, then (going down) there’s single chains (pyroxenes), double chains (amphiboles), sheets (biotite), and finally frameworks (Quartz and K-spar).

The minerals on the bottom of Bowen’s Reaction Series are ones you might recognize (the micas biotite and muscovite, and the omnipresent quartz). The other minerals are largely unfamiliar (though, they’re out there, I promise).

S.S. Goldich noted back in 1938 that minerals at the top of Bowen’s Reaction Series tended also to be the first to weather (or fall apart chemically) at the Earth’s surface, often degrading to clay minerals. This then explains why quartz is one of the most common minerals on the Earth’s surface.

Goldich Stability Series shows which minerals are most susceptible to weathering. Notice it’s like an upside down Bowen’s Reaction Series.

The order of minerals in Bowen’s Reaction Series, as well as the stability of minerals as shown in Goldich’s Dissolution Series, is related to the silicate structure of the minerals. The different structures provide the basis for the classification system for silicates, according to how the silica tetrahedra relate to one another.

It’s just another example of how all the answers are in Bowen’s Reaction Series.

Published by paleololigo

Scientist (Paleontology, Geochemistry, Geology); Writer (Speculative and Science Fiction, plus technical and non-technical Science); Mom to great boy on the Autism spectrum; possessor of too many hobbies.

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