Sediment and Sedimentation

Unlike clastic and organic sediment, chemical sediment can not simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution, that is, it must be precipitated. A common way for precipitation to occur is by evaporation. As water evaporates from the surface, if it is not replaced by water from another source (rainfall or a stream) any dissolved minerals in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. This often occurs in the desert in what are known as salt pans or lakes. It may also occur along the sea coast in a salt marsh.

Another mechanism that triggers mineral precipitation is a change in water temperature. When ocean waters with different temperatures mix, the end result may be sea water in which the concentration of dissolved minerals is higher than can be held in solution at that water temperature, and minerals will precipitate. For most minerals, their tendency to precipitate increases with decreasing water temperature. However, for some minerals, calcite (calcium carbonate) for example, the reverse is true.

Minerals may also be forced to precipitate by the biological activity of certain organisms. For example, when algae remove carbon dioxide from water, this decreases the acidity of the water, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions, to promote mineral precipitation and use the minerals to form their skeletons. Clams, snails, hard corals, sea urchins, and a large variety of other marine organisms form their exoskeletons by manipulating water chemistry in this way.