So far, we have focused on ions and ionic bonding. Yet, minerals often contain other kinds of bonds. For example, fluorite has mostly ionic bonds. Bonds in other halides may only be 70% ionic. In contrast, the bonds between metals and oxygen in many minerals are up to 50% covalent. Overall, atoms in covalent bonds are significantly larger than their cations in ionic bonds. These difference can occasionally affect coordination number. Sulfides and native metals have bonds that range from nearly 100% covalent to 100% metallic, but covalent and metallic radii are nearly the same, and the variations in radii may not be significant. Combinations of ionic and metallic bonds are rare and usually minor.
Although bonds in minerals crystals, such as quartz crystals, may be up to 50% covalent, for most purposes consideration of ions and ionic bonding explains mineral properties. An ionic model, although incorrect in detail, works for many sulfides too, even though bonding in them has little ionic character. No matter the kind of bonding, (wholly or partially) positively charged ions alternate with (wholly or partially) negatively charged ions. Because ionic bonds are easier to analyze, and because they explain most mineral properties, we will stick with them for the rest of this chapter.