Geology And Geophysics

Chemical Sedimentary Rock Facts

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"Chemical Sedimentary Rock Facts"
Caption: Halite
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Chemical, or crystalline, sedimentary rocks contain mineral crystals that are made from dissolved water within them. These rocks are classified by the chemistry of the minerals. Their formation does not depend on any chemical or biochemical process. There are three common groups within chemical sedimentary rocks: carbonates, evaporites and chert.

Carbonate rocks are made of the minerals calcite and dolomite. When a rock contains calcite it is one of the types of limestone. This type of stone is usually formed by organisms that live in water, or biota, like clams or corals. This water life create their exoskeletons by taking the chemicals needed from the water. Their skeletal remains become part of the rocks. Algaes and zooplankton also become part of these rocks.

Geologists test for limestone by the way it reacts to dilute hydrochloric acid. When limestone contains large pieces of fossils they are called fossiliferous limes. If the shell fragments are even larger the rock is a coquina. Otherwise, the rock is called limestone or micrite.

Limestone made of very small exoskeletons of algae are chalk. This can be differentiated from micrite by its texture. Chalk has a gritty texture. Other limestones are made of spheres of calcite, or ooliths, and are called oolitic limestone.


A dolostone is a rock that is made up of dolomite. These stones begin as limestone, but they go through a change. Dolostones lose the limestone texture when they go through recrystallization. This makes them appears to be micrites or fossiliferous limestone.


Chemical sedimentary rocks that are created by precipitation are called evaporite. These stones are dominated by gypsum, or rock gypsum or alabaster, and halite, rock salt. When water becomes supersaturated with these minerals evaporite rocks form. To form gypsum the Total Dissolved Solids, or TDS, must reach 100,000 parts per million. Normally and ocean contains 34,500 parts per million. Halite will form when the salinity reaches 350,000 parts per million, according to Minot State EU.


When minerals are formed under the Earth's surface, the temperature and pressure produced the mineral quartz. Chert is what is called a cryptocrystalline type of quartz. Its mineral structure is different than quartz. When chert is formed, it is created by biochemical processes where silica is taken up from the water by diatoms to use in their exoskeleton. These skeletal remains turn into a siliceous layer which forms chert or a chert nodule. In marine environments, there is a radiolarian which works to form layers of chert.

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