Chemical Sediments are formed by precipitation of chemicals out of a water solution. The most common chemical sedimentary rock is limestone , composed of carbonate minerals. An impure limestone containing calcium-magnesium carbonates is termed dolomite or dolostone. Gypsum, anhydrite, and borax are calcium sulfates. Under some conditions, SiO 2 may form a noncrystalline rock called chert .
Biological sediments play a role.
Coral reefs form carbonate rocks. Silica from sponge spicules, diatoms, and radiolarians can form chert layers. And we should not forget coal, or its relatives peat and lignite. Lithification usually involves compaction by overburden pressure, followed by cementation by some other mineral that fills in the gaps, transported by water. Most fossils, say, a dinoaur bone, have had their organic molecules replaced over the eons by other minerals.