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Diagenesis is a mineralogical process that takes place in deposited rock formations. Diagenesis occurs at low temperatures and pressure. The rocks formations are often buried a few meters inside the Earth’s crust. If rock formations change as a result of being exposed to high temperature and pressure, we call that process: metamorphosis . Whether diagenesis takes place, as well as to what extent, strongly depends on the composition of the rock and the chemical composition of the ground water. If soluble minerals are already present in a rock formation, as is the case in limestones or evaporites, then diagenesis there will take place much easier.
An example of diagenesis is called cementation. Dissolved minerals inside the groundwater flow, precipitate inside bedrock. This form of diagenesis can also occur undersea. The density of the rock increases, while the porosity decreases. The rock will form a more solid entity.
Another form of diagenesis is called compaction. Due to the weight of overlying packages of sediment the underlying layers become compressed. In this situation, pressure melt can also occur. This is the result of individual grains, who have the biggest pressure on them, dissolving easily into the groundwater. The dissolved minerals then precipitate into the pores, thus increasing density.
A third form of diagenesis is called leaching . Depending on the chemical composition of the groundwater, dissolved minerals inside the groundwater flow of a rock formation, can leave the rock formation, Withdrawing lots of minerals from the deposited formation.
In calciferous fossils, recrystallization can occur due to diagenetic processes. The fossil can be transformed into more stable forms of calcium. For example, the conversion of aragonite into calcite. Fossils (containing aragonite) can dissolve completely. If the cavity is filled by another mineral as a result of diagenesis, typical stone cores are formed. Limestone for example is sometimes transformed by diagenetic processes into dolomite. Fossils often are lost or damaged.
Diagenesis also plays an important role in the transformation of organic substances in sediments. For example the formation of coal, oil and gas.
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