|Chemical Sedimentary Rocks
Chemical rocks are deposited from aqueous solutions and precipitation may be caused by chemical or biochemical processes. The formation of chemical rocks is not dependent on currents or energy, and particle size is not as important in classification as in clastic rocks. Instead, these rocks are classified on the chemistry of the dominant minerals. The three common groups of chemical rocks are Carbonates, Evaporites and Chert.
Carbonate rocks are those composed of the carbonate minerals, Calcite (CaC03) and Dolomite ((Mg,Ca)C03). Any rock dominated by Calcite is called a Limestone, although there are many different types. Limestones are almost all formed under the influence of the biota (organisms living in the water). Some examples are clams, corals and snails, which extract the constituent chemicals from the water to form their exoskeletons. When these animals die, they leave their skeletons behind and these particles can become incorporated into the rocks. More commonly, Calcite is precipitated by the actions of single-celled organisms (algaes and zooplankton). These microscopic Calcite particles make up most of Limestone. Limestone can be identified in the same way that Calcite is identified, by an obvious reaction to dilute hydrochloric acid. If a Limestone has no other distinguishing feature other, it is called either Limestone, or a Micrite. Some Limestones contain obvious fossils, these are called Fossiliferous Limestone. If a Limestone is composed entirely of obvious (and often large) shell fragments, it is called a Coquina. A Limestone are composed of the tiny exoskeletons of algaes is called Chalk and can be distinguished from a Micrite because the Chalk will feel somewhat gritty. Finally, some Limestones are made up of small (sand sized) spheres of Calcite. These spheres are called ooliths and the rock is called an Oolitic Limestone.
If a rock is dominated by the mineral Dolomite, it is called a Dolostone. Essentially all Dolostones started out as Limestone, but have altered over time. Therefore it is possible to have most of the same features seen in Limestones. However, in most cases, when Dolostones are formed the original textures of the Limestone are destroyed by recrystallization, so most Dolostones look like Micrites or fossiliferous Limestone. The key to distinguishing Dolostones is to judge the reaction to dilute hydrochloric acid.
Evaporite rocks, in contrast to Carbonate rocks, are deposited by chemical precipitation (not biochemical). They are classified by the dominant mineral, Gypsum (Rock Gypsum or Alabaster) and Halite (Rock Salt). These rocks form when waters become highly saline (such as Great Salt Lake in Utah) and supersatured for these minerals. Normal ocean waters contain about 34,500 ppm TDS (Total Dissolved Solids). If the concentration reaches about 100,000 ppm TDS, then gypsum will begin to precipitate from the water. At a salinity of 350,000 ppm, Halite will begin to precipitate. These rocks are identified by being able to identify the constituent mineral.
Chert is a form of the mineral quartz which forms under the temperature and pressure conditions seen at the Earth's surface (quartz also forms under metamorphic and igneous conditions). Chert is a cryptocrystalline form of quartz with the same general properties, but with a different mineralogic structure. Chert is often formed by biochemical processes. Certain types of algae, called diatoms, are capable of extracting silica from water (both freshwater and marine) and using it to form an exoskeleton. When the algaes die, these skeletons can accumulate into a siliceous ooze layer, which when lithified forms a chert layer, or a chert nodule (a nodule is a lump). In marine environments, another silica producing organism is an ameoba type cell called a radiolarian. In marine environments, radiolarians together with diatoms contribute to formation of chert layers