The 4,000 known minerals can be separated into a small number of groups, or mineral classes. You may think, “Why bother?” Classiﬁcation schemes are useful because they help organize information and streamline discussion. Biologists, for example, classify animals into groups based on how they feed their young and on the architecture of their skeletons, and botanists classify plants according to the way they reproduce and by the shape of their leaves. In the case of minerals, a good means of classiﬁcation eluded researchers until it became possible to determine the chemical makeup of minerals. A Swedish chemist, Baron Jöns Jacob Berzelius (1779–1848), analyzed minerals and noted chemical similarities among many of them. Berzelius, along with his students, established that most minerals can be classiﬁed by specifying the principal anion (negative ion) or anionic group (negative molecule) within the mineral. We now take a look at principal mineral classes, focusing especially on silicates, the class that constitutes most of the rock in the Earth.
The Mineral Classes
Mineralogists distinguish several principal classes of minerals. Here are some of the major ones.
|Physical characteristics of minerals.|
- Silicates: The fundamental component of most silicates in the Earth’s crust is the SiO44– anionic group. A well-known example, quartz (figure above a), has the formula SiO2.
- Oxides: Oxides consist of metal cations bonded to oxygen anions. Typical oxide minerals include hematite (Fe2O3; figure above b) and magnetite (Fe3O4; figure above g).
- Sulﬁdes: Sulfides consist of a metal cation bonded to a sulfide anion (S2–). Examples include galena (PbS) and pyrite (FeS2; figure above c).
- Sulfates: Sulfates consist of a metal cation bonded to the SO42– anionic group. Many sulfates form by precipitation out of water at or near the Earth’s surface. An example is gypsum (CaSO4s (2O).
- Halides: The anion in a halide is a halogen ion (such as chloride [Cl–] or fluoride [F –]), an element from the second column from the right in the periodic table (see Appendix). Halite, or rock salt (NaCl; Fig. 3.8d), and fluorite (CaF2), a source of fluoride, are common examples.
- Carbonates: In carbonates, the molecule CO32– serves as the anionic group. Elements such as calcium or magnesium bond to this group. The two most common carbonates are calcite (CaCO3; Fig. 3.8e) and dolomite (CaMg[CO3]2).
- Native metals: Native metals consist of pure masses of a single metal. The metal atoms are bonded by metallic bonds. Copper and gold, for example, may occur as native metals.
|The structure of silicate minerals.|
Silicate minerals, or silicates, make up over 95% of the continental crust and almost 100% of the oceanic crust and of the Earth’s mantle consist almost entirely of silicates. Thus, silicates are the most common minerals on Earth. As we've noted, silicates in the Earth’s crust and upper mantle contain the SiO44– anionic group. In this group, four oxygen atoms surround a single silicon atom, thereby deﬁning the corners of a tetrahedron, a pyramid-like shape with four triangular faces (figure above a). We refer to this anionic group as the silicon oxygen tetrahedron (or, informally, as the silica tetrahedron), and it acts, in effect, as the building block of silicate minerals.
Mineralogists distinguish among several groups of silicate minerals based on the way in which silica tetrahedra are arranged (figure above b). The arrangement, in turn, determines the degree to which tetrahedra share oxygen atoms. Note that the number of shared oxygen determines the ratio of silicon (Si) to oxygen (O) in the mineral. Here are the groups, in order from fewer shared oxygen to more shared oxygen:
- Independent tetrahedra: In this group, the tetrahedra are independent and do not share any oxygen atoms. The attraction between the tetrahedra and positive ions holds such minerals together. This group includes olivine, a glassy green mineral, and garnet ( 2nd figure above f).
- Single chains: In a single-chain silicate, the tetrahedra link to form a chain by sharing two oxygen atoms. The most common of the many different types of single-chain silicates are pyroxenes (2nd figure above b).
- Double chains: In a double-chain silicate, the tetrahedra link to form a double chain by sharing two or three oxygen atoms. Amphiboles are the most common type (2nd figure above c).
- Sheet silicates: The tetrahedra in this group share three oxygen atoms and therefore link to form two- dimensional sheets. Other ions and, in some cases, water molecules fit between the sheets in some sheet silicates. Because of their structure, sheet silicates have cleavage in one direction, and they occur in books of very thin sheets. In this group, we find micas (2nd figure above a) and clays. Clays occur only in extremely tiny flakes.
- Framework silicates: In a framework silicate, each tetrahedron shares all four oxygen atoms with its neighbours, forming a three-dimensional structure. Examples include feldspar and quartz. The two most common feldspars are plagioclase, which tends to be white, gray, or blue; and orthoclase (also called potassium feldspar, or K-feldspar), which tends to be pink (1st figure above d).