These precipitates are referred to as cements in that they form in the interstices between grains of sediment and tend to stick the grains togeher. The increrases in temperature and pressure also cause chemical reactions to occur between the mineral grains of the sediment and between the minerals and the fluids that are trapped between the grains. These reactions, called diagenetic reactions also tend to decrease the amount of interstitial volume between the grains of the rock, further cementing the grains together. As mentioned above, chemical sediments form by direct precipitation from solution. One common class of chemical sedimentary rocks is called evaporites .
Evaporites form by precipitation from sea water or brackish fresh water.
The scenario for the formation of an evaporite requires that a batch of sea water becomes isolated from input of additional sea water, for example in a lagoon. The isolated sea water then begins to evaporate, which concentrates the dissloved salts and other components of sea water. As evaporation proceeds, various minerals will be precipitated from the water. Minerals that are formed in this way and are found in evaporites are listed in the table below. An example of an evaporite is the salt deposits located at Sussex, New Brunswick.