The Formation of Non-Clastic Chemical Precipitates
Non-clastic, Chemical Sedimentary Rocks are not composed of weathered
rock fragments. Chemical Sediments form when a solid comes out
of solution either by evaporation or precipitation.
A precipitate is the solid resulting from a chemical reaction that occurs with
the mixing of 2 solutions. The original substances change into a slightly soluble
solid that settles in an aqueous medium.
These equations show chemical reactions that can occur in seawater, perhaps
over a deep sea shelf, and which result in the formation of 2 precipitates:
Ca2+ + CO32- = CaCO3
Forms the Mineral: Calcite - Rock Name: Limestone
H4SiO4 = SiO2 + H2O
Forms the Mineral: Silica or Quartz - Rock Name: Chert or Flint
These are the most common Chemical Sedimentary Rocks formed by precipitation:
Limestone - Composed of the Mineral Calcite - Chemical Formula: CaCO3
Dolostone - Composed of the Mineral Dolomite - Chemical Formula: CaMg(CO3)2
Chert - Composed of Cryptocrystalline Quartz - Chemical Formula: SiO2
Flint - Composed of Cryptocrystalline Quartz - Chemical Formula: SiO2
- Black variety of Chert.
Precipitates settle to the bottom of the ocean. Over time, the ooze covering
the seafloor hardens into chemical limestone rock.
The composition of chemical limestone is the mineral calcite (CaCO3)
though it is possible for limestone to contain some clay in its composition.
In the field a geologist might use the following names to denote a mixed composition:
limey shale or shaley limestone. The second word in each term indicates the
dominant composition of the rock.
More About Limestone: The most common chemical sedimentary rock is limestone.
In fact, limestone comprises about 10% of all sedimentary rocks including both
the clastic and non-clastic subgroups. The majority of limestone rocks are organic
meaning they were formed from once living organisms.
However, crystalline limestone can form directly from chemical reactions in
water, specifically from the precipitation of Calcium, Oxygen and Carbon Dioxide
ions. A limestone precipitate is classified as Chemical Limestone.
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Chemical limestone formation is not limited to only marine
environments. For example, water traveling through rocks dissolves minerals
in the rocks then redeposits them when the water evaporates or when minerals
precipitate out of a supersaturated solution. The stalactites and stalagmites
found in caves form in such a manner. 
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Chemical Limestone is not to be confused with a Bioclastic Fossiliferous form of Limestone, also composed of Calcite. This type of limestone was formed from the compaction and cementation of once living calcareous organisms, i.e. shells and corals. See the Bioclastic Rocks subgroup on the main menu.
