As shown in the triangular diagram (Figure 2.18), bonding in most minerals is neither 100% ionic, 100% covalent, nor 100% metallic, but some come close. Most minerals contain combinations of ionic and covalent bonding. Metallic ore minerals such as pyrite (FeS2), stibnite (Sb2S3), and copper (Cu) generally have little ionic character. Most of them, especially those in the sulfide and sulfosalt groups, contain combinations of covalent and metallic bonding. Metallic and ionic bonds do not often combine, although galena (PbS) may be one example in which they do to a limited extent.
2.18 Different kinds of bonds in different minerals
The degree to which a bond is ionic depends on both elements involved. For example, because alkali elements (group 1) have a very strong tendency to become cations, and halogens (group 17) have an equal tendency to become anions, halite (NaCl) and other alkali halides form crystals in which bonds are nearly 100% ionic. Fluorite (CaF2), too, is nearly completely ionic. Alkaline earth oxides such as periclase (MgO) or lime (CaO), involving cations from group 2 and oxygen from group 16, are about 75% ionic. Many other oxides too, are mostly ionic. Silicates, the most common kind of mineral in Earth’s crust, are generally about 50% ionic and 50% covalent. And, bonds in diamond (C) are entirely covalent.