CHEMICAL SEDIMENTARY ROCKS -- ORTHOCHEMICAL ROCKS
Ions transported away from a weathering environment in solution can be incorporated into new mineral structures in a depositional environment if chemical conditions are right. Rocks composed of minerals that form in this fashion are called CHEMICAL SEDIMENTARY ROCKS. When the mineral formation is accomplished by organisms growing shells or other skeletal structures for themselves the rocks are said to be BIOCHEMICAL ROCKS. When it is accomplished by inorganic, purely chemical processes the rocks are called ORTHOCHEMICAL ROCKS.
The easiest way to get a natural solution of ions to form minerals is to concentrate the ions in a smaller amount of water. This is usually accomplished in nature by evaporating some of the water away. Orthochemical rocks formed in this way are called EVAPORITES, and most commonly form in desert lakes and in enclosed ocean bay in hot, dry climates.
Two of the more common evaporite minerals are gypsum and halite. A rock made of gypsum is called ROCK GYPSUM. The first photograph shows an example.
An orthochemical rock made of halite is called ROCK SALT. The second photograph shows an example.
Other orthochemical rocks form when a solution of dissolved ions comes into contact with rocks of an appropriate composition to exchange ions with the solution. Dissolved silica in groundwater will dissolve calcite and replace it with finely crystalline quartz when it contacts a limestone. The result is an orthochemical rock called CHERT. The final photograph shows an example. Notice the conchoidal fracture of this rock.
