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.
What’s with all of these “sili” names?
The element silicon (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 silica tetrahedron (Figure 3.1.1). Pure silicon crystals (created in a lab) are used to make semi-conductive media for electronic devices. A silicate 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”. Silicone 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:
| “Sili” name | Definition |
|---|---|
| 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 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 clo