3.1 Silicate Mineral Groups
The vast majority of the minerals that make up the rocks of Earth’s crust are 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 , 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.
The element (Si) is one of the most important geological elements and is the second-most abundant element in Earth’s crust (after oxygen). Silicon bonds readily with oxygen to form a tetrahedron (Figure 3.1.1). Pure silicon crystals (created in a lab) are used to make semi-conductive media for electronic devices. A mineral is one in which silicon and oxygen are present as silica tetrahedra. Silica also refers to a chemical component of a rock and is expressed as % SiO2. The mineral quartz is made up entirely of silica tetrahedra, and some forms of quartz are also known as “silica”. is a synthetic product (e.g., silicone rubber, resin, or caulking) made from silicon-oxygen chains and various organic molecules. To help you keep the “sili” names straight, here is a summary table:
|Silicon||The 14th element on the periodic table (Si)|
|Silicon wafer||A crystal of pure silicon sliced very thinly and used for electronics|
|Silica tetrahedron||A combination of one silicon atom and four oxygen atoms that form a tetrahedron|
|% silica||The proportion of a rock that is composed of the component SiO2|
|Silica||A solid made out of SiO2 (but not necessarily a mineral – e.g., opal)|
|Silicate||A mineral that contains silica tetrahedra (e.g., quartz, feldspar, mica, olivine)|
|Silicone||A flexible synthetic material made up of Si–O chains with attached organic molecules|
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 , 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 (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). 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.
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 iron. Fe3+ is known as 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 , 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 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 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. mica can have iron and/or magnesium in it and that makes it a silicate mineral (like olivine, pyroxene, and amphibole). is another similar mineral that commonly includes magnesium. In 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 (a.k.a. ), many of which exist as clay-sized fragments (i.e., less than 0.004 millimetres). These include the clay minerals , , and , 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 and . These are —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 minerals are , (a.k.a. K-feldspar or K-spar) and two types of plagioclase feldspar: (sodium only) and (calcium only). As is the case for iron and magnesium in olivine, there is a continuous range of compositions ( 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). (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.
The names “pyroxene”, “amphibole”, “mica”, and “feldspar” can be confusing at first, as these are technically names of mineral “families” and not names of a specific mineral. Minerals within the same family tend to share common structures, but each individual mineral is distinguished by its chemical formula. In the examples below the mineral names are bolded.
- One type of pyroxene mineral that you will see in this course is called augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6). Augite is one of many minerals within the pyroxene family.
- One of the most common amphibole minerals is called hornblende ((Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2), which is just one of many minerals within the amphibole family.
- Two common minerals from the mica family that you will see in this course are biotite (K(Mg,Fe)3AlSi3O10(OH)2) and muscovite ( KAl2(AlSi3O10(F,OH)2).
- Three feldspar minerals you will encounter in this course are potassium feldspar (KAlSi3O8), albite (NaAlSi3O8), and labradorite ((Ca, Na)(Al, Si)4O8).
In (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.
Silicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending on whether or not they have iron (Fe) and/or magnesium (Mg) in their formula. A number of minerals and their formulas are listed below. For each one, indicate whether or not it is a ferromagnesian silicate.
See Appendix 2 for Practice Exercise 3.1 answers.*Some of the formulas, especially the more complicated ones, have been simplified.
- Figures 3.1.1, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6: © Steven Earle. CC BY.
- An angstrom is the unit commonly used for the expression of atomic-scale dimensions. One angstrom is 10−10 metres or 0.0000000001 metres. The symbol for an angstrom is Å. ↵
A mineral that includes silica tetrahedra.
A combination of 1 silicon atom and 4 oxygen atoms that form a tetrahedron.
The 14th element.
A form of the mineral quartz (SiO2).
A flexible synthetic material made up of Si–O chains with attached organic molecules
A silicate mineral made up of isolated silica tetrahedra and with either iron or magnesium (or both) as the cations.
An ion with a charge or +2 or −2.
the reduced (non-oxidized) form of an ion of iron (Fe2+)
The oxidized form of an ion of iron (Fe3+).
A single chain silicate mineral.
A double-chain ferromagnesian silicate mineral (e.g., hornblende).
A sheet silicate mineral (e.g., biotite).
A sheet silicate mineral (mica) that includes iron and or magnesium, and is therefore a ferromagnesian silicate.
Referring to a silicate mineral that contains iron and or magnesium.
A ferromagnesian sheet silicate mineral, typically present as fine crystals and forming from the low-temperature metamorphism of mafic rock.
A potassium-bearing non-ferromagnesian mica.
A silicate mineral in which the silica tetrahedra are combined within sheets.
A silicate mineral in which the silica tetrahedra are made up of sheets.
A clay mineral that does not have cations other than Al and Si.
A clay mineral with a composition similar to that of muscovite mica.
A fine-grained sheet silicate mineral that can accept water molecules into interlayer spaces, resulting is swelling.
A very common family of framework silicate minerals.
A silicate mineral with the formula SiO2.
A silicate mineral that does not contain iron or magnesium (e.g., feldsspar).
Feldspar with the formula KAlSi3O8.
Sodium-rich plagioclase feldspar.
Calcium-rich plagioclase feldspar.
The substitution of one element for another in a mineral (e.g., iron can be substituted for magnesium in the mineral olivine).