Minerals

In molecules, elements are not merely mixed together, but are joined by chemical bonds. Chemical bonds in minerals are of four types: covalent, ionic, metallic, or Van der Waals, with covalent and ionic bonds most common. Two or more of these bond types can and do coexist in most minerals.

Covalent bonds are very strong bonds formed when atoms share electrons with neighboring atoms. Sulfur, and both of carbon's natural forms, graphite and diamond, are covalently-bonded minerals. So is quartz, which contains only silicon and oxygen.

Ionic bonds are strong bonds formed when electrons are transferred from one element to another. Since electrons carry a negative electrostatic charge, the element that acquires extra electrons becomes a negatively charged ion, an anion. The element that gave off the electron becomes a positively charged ion, a cation. The attraction between opposite charges binds anions and cations together in ionic compounds. Most metals (the

elements iron, nickel, lead, aluminum, etc.) exist in nature as cations, rather than as electrically neutral atoms. Their mineral compounds are, therefore, usually ionic. This is true whether they are joined with one non-metal, as in oxides (oxygen), or with two, as in sulfates (sulfur and oxygen) and carbonates (carbon and oxygen).

Metallic bonds are generally weaker than either covalent or ionic bonds, which explains why metallically bonded minerals (true metals), like silver, gold, and copper, can be worked—beaten into flat sheets, or drawn into thin wires. In metallic bonds, electrons move about the crystal constantly flowing between adjacent atoms, redistributing their charge. Because of this flow of electrons, true metals are also good electrical conductors.

Van der Waals bonds are very weak bonds formed by residual charges from the other types of chemical bonds. Graphite is probably the best example of the nature of Van der Waals bonds. The atoms in graphite's carbon layers are covalently bonded, but a weak residual charge attracts the layers to one another. Van der Waals bonds make graphite a very soft mineral, excellent for use in pencil lead.