The nature of chemical bonding in minerals controls many properties. The table below makes some comparisons. Covalent bonds are stronger than ionic bonds, which in turn are stronger than metallic bonds. Consequently, covalent minerals are hard and tenacious, while metallic ones are usually not, and ionic minerals fall between. Melting temperatures follow a similar pattern (covalent compounds generally melt at the highest temperatures due to their stronger bonds), while solubility in water is greatest for ionic crystals (because of their weak bonds and easy ionization).
Bond type also affects crystal symmetry. Both ionic and metallic bonds are nondirectional so bonding can occur equally in all directions. In contrast, covalent bonds involve pairs of atoms and are linear. Consequently, metallic and ionic minerals generally have high symmetry compared with covalent ones.
|property||ionic bonds||covalent bonds||metallic bonds|
|common elements involved||from opposite sides of the
|close together in the
|solubility in water||high||low||very low|
|melting temperature||moderate to high||high||variable|
|hardness||medium to hard||very hard||often malleable|
|ability to break||brittle, good cleavage||brittle, common fractures||variable|
|crystal symmetry||high symmetry||low symmetry||very high symmetry|
|transparency||fully to partially transparent||partially transparent or opaque||opaque|
|how common?||most nonmetallic minerals||some minerals||most metallic minerals|
|examples||halite (NaCl); calcite (CaCO3)||diamond (C); sphalerite (ZnS)||copper (Cu); silver (Ag)|