Metal ions bond with only five major types of negatively charged ions, shown in the Figure. Consequently, metal–containing compounds are classified according to which type of negative ion they contain. Iron oxide is classified as an oxide and chalcopyrite is classified as a sulfide.
Halides, such as sodium chloride and magnesium chloride, are commonly referred to as salts. They have good solubility in water and so are readily washed away by the action of either surface water or groundwater. Most of these and other water–soluble metal–containing compounds therefore end up in the ocean. These compounds are recovered by evaporating seawater.
Alternatively, water–soluble compounds may end up in land basins, such as the Bonneville salt flats of Utah, where they are readily mined. In some regions, such as along the Gulf of Mexico, vast deposits of halides remain undissolved hundreds of meters below the surface, where groundwater cannot reach.
The compounds in these deposits tend to be very pure, which makes deep mining excavations like the one shown in the below Figure worthwhile.
In contrast to halides, compounds containing carbonate, phosphate, oxide or sulfide ions tend to have relatively low solubilities in water. Hence, their ores tend to stay put and are found in more diverse geologic locations. The form in which a metal is most likely to be found in nature is a function of its position in the periodic table.
Metals of group 1 tend to be found mostly as halides, group 2 metals as carbonates and group 3 metals and lanthanides mostly as phosphates. Most metals from groups IV to VIII along with aluminum Al, and tin Sn, tend to be found as oxides and most metals from groups IX to XV along with molybdenum Mo, tend to be found as sulfides.