Silicate Minerals

Karla Panchuk

Silicon and oxygen bond covalently to create a silicate tetrahedron (SiO44-), which is a four-sided pyramid shape with oxygen at each corner and silicon in the middle (Figure below). This structure is the building block of many important minerals in the crust and mantle. Silicon has a charge of +4, and oxygen has a charge of -2, so the total charge of the silicate anion is -4.

three spheres labelled "oxygen" are arranged in a triangle pattern. A smaller sphere labeled "silicon" is nestled on top of them. A fourth oxygen sits on top of the silicon sphere.Label: SiO4 4- Silicate tetrahedron
Figure The silica tetrahedron is the building block of all silicate minerals. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modified after Helgi (2013), CC BY-SA 3.0. View source.

In silicate minerals, these tetrahedra are arranged and linked together in various ways, from single units to chains, rings, and more complex frameworks.  In the rest of this section, we will look at the structures of the most common silicate minerals in Earth’s crust and mantle.

Isolated Tetrahedra

The simplest silicate structure—that of the mineral olivine (Figure below)—is composed of isolated tetrahedra bonded to iron and/or magnesium ions. In olivine, the –4 charge of each silica tetrahedron is balanced by two iron or magnesium cations, each with a charge of +2.

Left: A pattern of triangles (representing tetrahedra) with circles (representing cations) between them. The triangles do not touch. Right: Yellowish green crystals labelled "Olivine (Mg, Fe)2SiO4"
Figure Olivine is a silicate mineral made of isolated silica tetrahedra bonded to Fe and Mg ions (left). Source: Karla Panchuk (2021), CC BY-SA 4.0. Click for more attributions.

Olivine can be pure Mg2SiO4 or pure Fe2SiO4, or a combination of the two, written as (Mg,Fe)2SiO4.

Chain Silicates

The mineral pyroxene is an example of a single-chain silicate (Figure 5.25), where one oxygen from each tetrahedron is shared with the next tetrahedron. Sharing means that fewer oxygens are needed to make the tetrahedra, so there’s less oxygen in this structure compared to olivine. This can be expressed as a silicon-to-oxygen ratio (Si:O).

In olivine, tetrahedra aren’t connected directly to each other, so each silicon atom must have four oxygen atoms all to itself to complete a tetrahedron. That’s a ratio of 1:4. In pyroxene, each silicon atom only needs three unique oxygen atoms because it can borrow one from a neighbor to have a tetrahedron. For pyroxene, the Si:O is 1:3. With one less oxygen in the mix per tetrahedron, the net charge per silicon atom that cations must balance is lower (-2 instead of -4).

Figure Pyroxene (dark mineral in the photo) is a silicate mineral in which tetrahedra are linked in strings, with adjacent tetrahedra sharing an oxygen atom. Source: Karla Panchuk (2021), CC BY-SA 4.0. End-on view modified after Klein & Hurlbut (1993). Click for more attributions.

The way tetrahedra share oxygens in single-chain silicates is why, even though pyroxene is built out of silicate tetrahedra (with the silicate anion being SiO44-), its formula has the tetrahedra represented as SiO3 (e.g., MgSiO3, FeSiO3, and CaSiO3.

 

The mineral amphibole is also a chain silicate, but the silica tetrahedra are linked in double chains (Figure below). Amphibole has a silicon-to-oxygen ratio higher than pyroxene; hence, fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene, and its compositions can be very complex.
Figure Amphibole (dark mineral in the photo) is a silicate mineral in which tetrahedra are linked in doubled-up strings. Tetrahedra share two oxygen atoms with adjacent tetrahedra. End-on view modified after Klein & Hurlbut (1993). Source: Karla Panchuk (2021), CC BY-SA 4.0. Click for more attributions.
Sheet Silicates

In mica structures, the silica tetrahedra are arranged in continuous sheets (Figure below), where each tetrahedron shares three oxygen anions with adjacent tetrahedra. Because even more oxygens are shared between adjacent tetrahedra, fewer charge-balancing cations are needed for sheet silicate minerals.

Bonding between sheets is relatively weak, which accounts for the tendency of mica minerals to split apart in sheets (Figure below, to the bottom right). Two common micas in silicate rocks are biotite (Figure below, to the bottom left), which contains iron and/or magnesium, making it a dark mineral, and muscovite (Figure below on the right), which contains aluminum and potassium and is light in color. All of the sheet silicate minerals have water in their structure, in the form of the hydroxyl (OH-) anion.

Figure Micas are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite mica (lower left) has Fe and Mg cations. Muscovite mica (lower right) has Al and K instead. The muscovite mica shows how thin layers can split away in a sheet silicate. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Top left- Modified after Steven Earle (2015), CC BY 4.0. Top right- Modified after Klein & Hurlbut (1993). Click for more attributions.

Some sheet silicates typically occur in clay-sized fragments (i.e., less than 0.004 mm). These include the clay minerals kaolinite, illite, and smectite, which are important components of rocks and especially of soils.

Framework Silicates

In framework silicates, tetrahedra are connected in three-dimensional structures rather than in two-dimensional chains and sheets.

Feldspar

Feldspars are a group of very abundant framework silicates in Earth’s crust. They include alumina tetrahedra as well as silicate tetrahedra. In alumina tetrahedra, an aluminum cation is at the center instead of a silicon cation.

Feldspars are classified using a ternary (3-fold) system with three end-members (“pure” feldspars). This system is illustrated with a triangular diagram with each end member at one corner (Figure below). The distance along a side of the diagram represents the relative abundance of the composition of each end member.

Figure Ternary diagram showing the feldspar group of framework silicate minerals. Alkali feldspars are those with compositions ranging between albite (with a Na cation) and orthoclase and its polymorphs (with a K cation. Plagioclase feldspars are those with compositions ranging between albite and anorthite (with a Ca cation). Source: Karla Panchuk (2018) CC BY-SA 4.0. Ternary diagram modified after Klein & Hurlbut (1993). Click for more attributions and a ternary diagram without mineral images.

One end-member is potassium feldspar (also referred to as K-feldspar), which has the composition KAlSi3O8. Another end member is albite, which has sodium instead of potassium (formula NaAlSi3O8). As is the case for iron and magnesium in olivine, there is a continuous range of compositions between albite and orthoclase. Feldspars in this series are referred to as alkali feldspars.

The third end-member is anorthite, which has calcium instead of potassium or sodium (formula CaAl2Si3O8). Feldspars with a mixture of sodium and calcium in their composition are called plagioclase feldspars.

Quartz

Quartz (SiO2; Figure below) contains only silica tetrahedra. In quartz, each silica tetrahedron is bonded to four other tetrahedra (with oxygen shared at every corner of each tetrahedron), making a three-dimensional framework. As a result, the ratio of silicon to oxygen is 1:2. The hardness of quartz and the fact that it breaks irregularly (notice the bottom of the crystal in the right of the figure below) and not along smooth planes result from the strong covalent/ionic bonds characteristic of the silica tetrahedron.

Figure Quartz is another silicate mineral with a three-dimensional framework of silica tetrahedra. Sometimes quartz occurs as well-developed crystals (left), but it also occurs in common rocks such as granite (right). In addition to quartz, the granite contains potassium feldspar, albite, and amphibole. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Click for more attributions.

References

Klein, C. & Hurlbut, C. S., Jr. (1993). Manual of Mineralogy (after J. D. Dana). New York, NY: John Wiley & Sons, Inc.

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