Chemical Sedimentary Rocks
Limestones. The most common
chemical sedimentary rock is limestone. Composed mostly of the mineral calcite (CaCO3),
limestones are usually formed by biochemical processes in shallow seawater. Coral and algae are especially important limestone builders.
Oolitic limestones form in ocean shallows from the accumulation of
oolites, sand‐sized spheres of chemically precipitated calcite that develop in the tidal zone.
Other variations of limestone result from the deposition and cementation of calcium‐rich shells, shell fragments, corals, algae, and the remains of tiny marine organisms. Coquina is formed from the cementation of large pieces of broken shells. Bioclastic and skeletal limestones are fine‐ to coarse‐grained accumulations of a wider variety of shell fragments and fossils. Chalk is a very fine‐grained bioclastic limestone composed of accumulations of skeletal debris from tiny marine organisms that drifted down to the ocean floor. All of these “redeposited” limestones could be considered clastic sedimentary rocks, as well as organic sedimentary rocks.
Dolomites. Limestones are frequently converted into dolomites, or dolostones, during the early stages of compaction, dewatering, and lithification of the limestone sediment. The process of dolomitization involves the removal of calcium from the limestone by magnesium‐rich solutions and its replacement in the rock by magnesium. Dolomite's chemical formula is CaMg(CO3)2.
Cherts. Chert (varieties of which are flint, agate, and jasper) is a hard, glassy sedimentary rock composed of silica that precipitated from water. Chert nodules, also known as geodes, are commonly found in limestones and less so in clastic sedimentary rocks. They form in pockets or voids that might have once been occupied by gas or organic material that has since been removed or decomposed. Cherts can also occur as continuous layers in sedimentary rocks. Chert usually composes at least half of a spectacular layered rock called iron formation, which crystallized in shallow seas around the world and is an important source of iron.
Evaporites. Evaporites are rocks that are composed of minerals that precipitated from evaporating seawater or saline lakes. Common evaporites are halite (rock salt), gypsum, borates, potassium salts, and magnesium salts.