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.
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.
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.
Double-chain silicates include the amphibole minerals, like 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.
Tectosilicates – the framework silicates
In tectosilicates, all four of each silica tetrahedron’s oxygens is shared with another tetrahedron, creating a three-dimensional framework.
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.
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.
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?
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.
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.
Penny Higgins - Storyteller • Artist • Scientist 