3.1 Silicate Mineral Groups

The vast majority of the minerals that make up the rocks of Earth’s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a variety of clay minerals. The building block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom that form a four-sided pyramid shape with O at each corner and Si in the middle (Figure 3.1.1). The bonds in a silica tetrahedron have some of the properties of covalent bonds and some of the properties of ionic bonds. As a result of the ionic character, silicon becomes a cation (with a charge of +4) and oxygen becomes an anion (with a charge of –2). The net charge of a silica tetrahedron (SiO₄) is: 4 + 4(−2) = 4 − 8 = −4. As we will see later, silica tetrahedra (plural of tetrahedron) link together in a variety of ways to form most of the common minerals of the crust.

In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks (Table 3.2). The simplest silicate structure, that of the mineral olivine, 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 divalent (i.e., +2) iron or magnesium cations. Olivine can be either Mg₂SiO₄ or Fe₂SiO₄, or some combination of the two (Mg,Fe)₂SiO₄. The divalent cations of magnesium and iron are quite close in radius (0.73 versus 0.62 angstroms^[1]). Because of this size similarity, and because they are both divalent cations (both can have a charge of +2), iron and magnesium can readily substitute for each other in olivine and in many other minerals.

Recall that for non-silicate minerals, we classified minerals into groups according to their anion or anionic group. For silicate minerals, we group minerals based on their silicate structure into groups called: isolated,  pair, ring, single chain, double chain, sheet, and framework silicates. In this course, we will focus on just the isolated, single chain, double chain, sheet, and framework silicates.

Table 3.2 Silicate mineral configurations. The triangles represent silica tetrahedra.

Tetrahedron Configuration Picture	Tetrahedron Configuration Name	Example Minerals
	Isolated (nesosilicates)	Olivine, garnet, zircon, kyanite
	Pairs (sorosilicates)	Epidote, zoisite
	Rings (cyclosilicates)	Tourmaline
	Single chains (inosilicates)	Pyroxenes, wollastonite
	Double chains (inosilicates)	Amphiboles
	Sheets (phyllosilicates)	Micas, clay minerals, serpentine, chlorite
3-dimensional structure	Framework (tectosilicates)	Feldspars, quartz, zeolite

In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. Instead they are bonded to the iron and/or magnesium ions, in the configuration shown on Figure 3.1.2.

As already noted, the 2 ions of iron and magnesium are similar in size (although not quite the same). This allows them to substitute for each other in some silicate minerals. In fact, the ions that are common in silicate minerals have a wide range of sizes, as depicted in Figure 3.1.3. All of the ions shown are cations, except for oxygen. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). Fe²⁺  is known as ferrous iron. Fe³⁺  is known as ferric iron. Ionic radii are critical to the composition of silicate minerals, so we’ll be referring to this diagram again.

The structure of the single-chain silicate pyroxene is shown on Figures 3.1.4 and 3.1.5. In pyroxene, silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure. The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (−2 instead of −4).  Therefore, fewer cations are necessary to balance that charge. The structure of pyroxene is more “permissive” than that of olivine—meaning that cations with a wider range of ionic radii can fit into it. That’s why pyroxenes can have iron (radius 0.63 Å) or magnesium (radius 0.72 Å) or calcium (radius 1.00 Å) cations (see Figure 3.1.3 above). Pyroxene compositions are of the type MgSiO₃, FeSiO₃, and CaSiO₃, or some combination of these, written as (Mg,Fe,Ca)SiO₃, where the elements in the brackets can be present in any proportion.

In amphibole structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene and its compositions can be very complex. Hornblende, for example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and the hydroxyl ion (OH−).

In mica minerals, the silica tetrahedra are arranged in continuous sheets. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer cations are needed to balance the charge of the silica-tetrahedra structure in sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage in micas. Biotite mica can have iron and/or magnesium in it and that makes it a ferromagnesian silicate mineral (like olivine, pyroxene, and amphibole). Chlorite is another similar mineral that commonly includes magnesium. In muscovite mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral.

Apart from muscovite, biotite, and chlorite, there are many other sheet silicates (a.k.a. phyllosilicates), many of which exist as clay-sized fragments (i.e., less than 0.004 millimetres). These include the clay minerals kaolinite, illite, and smectite, and although they are difficult to study because of their very small size, they are extremely important components of rocks and especially of soils.

Silica tetrahedra are bonded in three-dimensional frameworks in both the feldspars and quartz. These are non-ferromagnesian minerals—they don’t contain any iron or magnesium. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Quartz contains only silica tetrahedra.

The three main feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and two types of plagioclase feldspar: albite (sodium only) and anorthite (calcium only). As is the case for iron and magnesium in olivine, there is a continuous range of compositions (solid solution series) between albite and anorthite in plagioclase. Because the calcium and sodium ions are almost identical in size (1.00 Å versus 0.99 Å) any intermediate compositions between CaAl₂Si₃O₈ and NaAlSi₃O₈ can exist (Figure 3.1.6).

The intermediate-composition plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% to 90% Ca). Potassium feldspar(KAlSi₃O₈) has a slightly different structure than that of plagioclase, owing to the larger size of the potassium ion (1.37 Å) and because of this large size, potassium and sodium do not readily substitute for each other, except at high temperatures. These high-temperature feldspars are likely to be found only in volcanic rocks because intrusive igneous rocks cool slowly enough to low temperatures for the feldspars to change into one of the lower-temperature forms.

In quartz (SiO₂), the silica tetrahedra are bonded in a “perfect” three-dimensional framework. Since in every silica tetrahedron one silicon cation has a +4 charge and the two oxygen anions each have a −2 charge, the charge is balanced. There is no need for aluminum or any of the other cations such as sodium or potassium. The hardness and lack of cleavage in quartz result from the strong bonds characteristic of the silica tetrahedron.

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• Figures 3.1.1, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6: © Steven Earle. CC BY.