Silicate Minerals and the Silica Tetrahedron

Silicate minerals are by far the most abundant minerals on the surface of the Earth. All silicate minerals share a common building block: the silica tetrahedron. This four-sided molecule consists of four oxygen (O) atoms and one silicon (Si) atom. The oxygen atoms are anions with a minus-2 charge (O^2-) that are covalently bonded to a single silicon cation with a plus-4 charge (Si⁴⁺). Each face of the silica tetrahedron forms an equilateral triangle. As you will see below, different types of silicate minerals are characterized based on how the silica tetrahedra are bonded to each other, as well as to other elements. Because of the abundance of silicate minerals and their underlying silica tetrahedra building blocks, Si and O comprise 75% of the mass of the Earth’s crust.

The four oxygen anions have a combined charge of -8 and the silicone cation has a charge of +4, giving the silica tetrahedron building block a total negative charge of -4 and a chemical equation of [SiO₄]⁴⁻. This negative charge causes the tetrahedron to have a great affinity for atoms or molecules with positive charges.

There are two common solutions to this problem of excess negative charge. One is to add more cations. The mineral olivine is formed in this way, with two magnesium cations (Mg²⁺) attached to each silica tetrahedron, resulting in the chemical equation Mg₂SiO₄.

The second way for the silica tetrahedra to balance their charges is to link to one another. The mineral quartz provides an example of this solution, where all four corners of each tetrahedron are linked to others, producing the overall mineral chemical formula SiO₂.

Most silicate minerals are constructed through a combination of tetrahedral linking and the addition of cations, for example, pyroxene (MgSiO₃). The degree and manner of tetrahedral linking is important in controlling the behavior and properties of individual silicate minerals, and also provides a useful way to classify these minerals. There are five major groups of silicate minerals: isolated tetrahedra, single-chain silicates, double-chain silicates, sheet silicates, and framework silicates.

Before exploring these five groups, however, it is useful to be introduced to the close relationship between the atomic structure and properties of silicate minerals and the temperatures in which they crystallize.

Bowen's Reaction Series

Silicate minerals are formed from the crystallization of magma. In the early 1900s, a geologist named Norman L. Bowen (1887-1956) developed a model, later called "Bowen's reaction series," that characterized the order in which silicate minerals form as magma cools. This model is shown in the figure below.

Imagine that you could look inside a chamber deep inside the Earth that is full of magma. As that magma cools, certain minerals will form crystals in a predictive, orderly fashion, which is explained by Bowen's reaction series. Different minerals have different crystallization temperatures: some crystallize out of magma at incredibly high temperatures, while others crystallize at much lower temperatures. For example, the olivine mineral forsterite crystallizes out of magma at temperatures near 1800 ºC, while pyroxenes do not form until the magma temperature reaches 1557 ºC. Further, the temperatures at which some minerals crystalize depends upon the initial chemical composition of the magma itself.

Central to Bowen's model is the concept of fractional crystallization, which dictates that minerals are removed from the cooling magma as they crystalize, for example by sinking to the bottom of a magma chamber. As a consequence, the chemical composition of the magma body changes as it cools because some components of its initial composition become sequestered away in minerals as they crystallize and are "removed" from the liquid magma.

The types of minerals (and therefore rocks) formed from a body of cooling magma is ultimately a consequence of the chemistry of the magma itself. For example, magmas that have relatively low amounts of silica will produce minerals like olivines, pyroxenes, and amphiboles as they cool, but virtually no quartz. These minerals are dark in color and the resulting rocks are characterized as mafic. Alternatively, magmas that are rich in silica produce minerals such as quartz and potassium feldspar, but not minerals like olivine. Quartz and potassium feldspar are light in color and rocks that are rich in these minerals are characterized as felsic.

Bowen's model has two component series. The first is a continuous series describes the formation of plagioclase feldspar crystals that become increasingly enriched in sodium and depleted in calcium. The second is a discontinuous series that describes the successive formation of minerals belonging to the olivine, pyroxene, and amphibole groups, as well as biotite mica. Note that the crystallization of minerals in the continuous and discontinuous series happens simultaneously.

Continuous Reaction Series

The continuous reaction series in Bowen's model explains the formation of different types of plagioclase feldspar (geologists often refer to members of this mineral group as "plag," pronounced with a "g" as in George). In members of this mineral group, atoms of calcium and sodium can substitute for each other, with the relative percentages of Ca and Na defining individual minerals, without any important changes in the underlying crystal structure. As a body of magma begins to cool, the first plagioclase feldspar minerals to crystallize are enriched in calcium (Ca). The purest endmember is a mineral called anorthite (CaAl₂Si₂O₈), which has no sodium. As cooling proceeds, the magma becomes depleted in calcium (because it is "locked away" in minerals like anorthite) and the plagioclase feldspar minerals formed become enriched instead in sodium (Na). Some individual crystal grains of plagioclase feldspar record this history with centers that are rich in calcium and outer zones with greater amounts of sodium.The endmember mineral albite (NaAlSi₃O₈) has almost no calcium, just as anorthite has virtually no sodium. Intermediate member minerals of the plagioclase feldspar series include oligoclase and the lovely mineral labradorite.

Discontinuous Reaction Series

The discontinuous reaction series in Bowen's model describes the crystallization of distinct types of minerals as a body of magma cools. Members of the olivine group have the highest crystallization temperature (~1200–1800 °C), so are the first to crystallize out of a silica-poor magma and sink to the bottom of the magma chamber. (A different way to think about this is that olivines have the highest melting temperature.) As the magma continues to cool, pyroxenes are the next to crystallize, followed by amphiboles and then biotite mica.

Potassium Feldspar, Muscovite Mica, and Quartz

Members of the potassium feldspar group (commonly also called "K-feldspar," "K-spar," or "alkali feldspar"); orthoclase is an example),  muscovite mica, and quartz are three of the final minerals that might crystallize from a cooling body of magma once it reaches a temperature of around 600 ºC, but only if that magma is rich in silica. These minerals are indicative of granite, which also includes albite, which is sodium-rich potassium feldspar.

Trends Associated with Bowen's Reaction Series

The elegance of Bowen’s Reaction Series (BRS) lies in its ability to explain an array of seemingly disparate aspects of silicate minerals and the rocks that they comprise. As one moves from the top of Bowen's model to the bottom, a handful of trends become apparent, some of which have already been alluded to previously (compare with figure of Bowen's model above). Note that other than crystallization temperature, most of these trends apply to the discontinuous reaction series, rather than the continuous series.

Crystallization temperature: Minerals higher in BRS have higher crystallization temperatures than those that are lower in BRS. For example, olivine minerals crystallize at a much higher temperature than quartz. (Alternative, quartz melts at a much lower temperature than olivine).

Mineral color: In general, minerals higher in the discontinuous series of BRS are darker in color than those that are lower in BRS. For example, pyroxene is darker in color than potassium feldspar. Rocks that are enriched in dark-colored minerals like olivine and pyroxene are mafic, while those that are enriched in light-colored minerals like potassium feldspar and quartz are felsic.

Iron and magnesium content: Minerals that are higher in the discontinuous series of BRS have greater amounts of iron (Fe) and magnesium (Mg) than those that are lower in the series.

Silica content: Minerals that are lower in the discontinuous series of BRS have greater amounts of silica than those that are higher in the series.

Silicon-oxygen bonding (complexity): Minerals that are lower in the discontinuous series of BRS have greater amounts of silica tetrahedra bonding than those that are higher in the series (see more below). This makes minerals that are lower in the series more resistant to weathering than those that are higher in the series.

Resistance to weathering (stability): Minerals that are lower in the discontinuous series of BRS are more resistant to weathering than those that are higher in the series. This greater resistance corresponds to their increased degree of silica tetrahedral bonding.

Minerals with Isolated Tetrahedra (Nesosilicates)

In this class of silicate minerals, the tetrahedra anions ([SiO₄]⁴⁻) are connected to cations, rather than each other. Olivines, mentioned above, are a group of minerals that have this structure. One type of olivine is forsterite. Its chemical formula, Mg₂SiO₄, demonstrates that the silica tetrahedra are bonded to magnesium (Mg); in particular, each tetrahedron shares four Mg²⁺ cations. The atomic-level crystal structure of forsterite is shown in the interactive model below. Note in the model that the silica tetrahedra (shaded white) do not connect directly with each other; instead, they are isolated.

Fayalite is the other common mineral in the olivine group. In this mineral, the silica tetrahedra are bonded to iron cations (Fe²⁺) rather than magnesium, resulting in the chemical formula Fe₂SiO₄. One result of the insular tetrahedral structure of olivine minerals is the relatively low Si to O ratio of 1:4.