Where to find chemical sedimentary rocks

Because most chemical sedimentary rocks are formed in marine environments, it is not unlikely to find fossils within chemically precipitated rocks. In shallow marine environments a specific type of precipitation, called an ooid, can occur. In these environments material is often under constant motion due to tidal or wave action. As these materials roll back and forth on the sea floor, precipitates will crystallize on only the exposed surface.

Eventually, thin spherical layers of precipitate will develop on the original particle or nucleus. Similar material precipitates within limestone caves to form stalactites , stalagmites , and a wide range of other speleothems. Tufa, travertine and speleothems make up only a tiny proportion of all limestone. Dolomite CaMg CO 3 2 is another carbonate mineral, but dolomite is also the name for a rock composed of the mineral dolomite although some geologists use the term dolostone to avoid confusion.

All of the dolomite found in ancient rocks has been formed through magnesium replacing some of the calcium in the calcite in carbonate muds and sands. This process is known as dolomitization , and it is thought to take place where magnesium-rich water percolates through the sediments in carbonate tidal flat environments.

In some cases, chert is deposited along with limestone in the moderately deep ocean, but the two tend to remain separate, so chert beds within limestone are quite common Figure 6. In other situations, and especially in very deep water, chert accumulates on its own, commonly in thin beds. Banded iron formation BIF is a deep sea-floor deposit of iron oxide that is a common ore of iron Figure 6.

BIF forms when iron dissolved in seawater is oxidized, becomes insoluble, and sinks to the bottom in the same way that silica tests do to form chert. BIF is prevalent in rocks dating from to Ma, a result off changes in the atmosphere and oceans that took place over that time period.

Photosynthetic bacteria i. These bacteria first evolved around Ma, and for the next billion years, almost all of that free oxygen was used up by chemical and biological processes, but by Ma free oxygen levels started to increase in the atmosphere and the oceans.

After Ma, little dissolved iron was left in the oceans and the formation of BIF essentially stopped. In arid regions many lakes and inland seas have no stream outlet and the water that flows into them is removed only by evaporation. Under these conditions, the water becomes increasingly concentrated with dissolved salts, and eventually some of these salts reach saturation levels and start to crystallize Figure 6.

Sylvite is mined at numerous locations across Saskatchewan Figure 6. This is an easy experiment that you can do at home. Stir until all or almost all of the salt has dissolved, then pour the salty water leaving any undissolved salt behind into a shallow wide dish or a small plate. Leave it to evaporate for a few days and observe the result.

This is a common way for chemical sedimentary rocks to form and the rocks are commonly called evaporites. They are typically made up of the minerals halite calcium chloride, or rock salt and gypsum calcium sulfate.

Another way to precipitate minerals out of water is to change the temperature or the acidity of the water rather than evaporate the water. This is how rocks such as limestones form. Limestones commonly form in oceans, which do not evaporate away. For example, when cold sea water heats up, it can no longer hold as much calcium carbonate calcite, the mineral that limestones are made out of in solution.