When sediments are first deposited, they are unconsolidated and are not considered a rock . Lithification is the process of converting unconsolidated sediments into sedimentary rock. Lithification involves primarily the processes of compaction and cementation. Recrystallization is also an important process for some sediments.
Compaction is the rearrangement of sedimentary particles to reduce pore space and squeeze out pore water . Unlithified sediments generally contain some excess space between grains. This is especially true in the case of very fine-grained sediments such as clay and mud. Coarser grained sediments such as sand and gravels are heavy enough to settle with a minimum of pore space. Over time, as sediments accumulate in basins, the thickness and weight of the overlying sediments increases. The pressure on the buried sediments causes all the grains to compress together as tightly as possible. Excess interstitial water is also forced out and the sediments are now compacted.
The next step is cementation. Only after cementation has occurred can the sediments be considered a rock. Although compaction has reduced the pore space within sediments, some space remains. In addition, even though the sediments are compacted to a high degree and have been de-watered, the great pressures and higher temperatures in a sedimentary basin can force hot circulating waters, or basinal brines, to permeate through the sediments. These fluids have the ability to carry dissolved minerals and deposit them within the available pore space of sediments, binding them into rock.
The cement may be derived either from outside sediments or from within the sediments themselves. Externally sourced cementing material is dissolved from some other rock formation or sediments that the fluids permeated prior to entering the sediments to be cemented. Alternatively, a permeating fluid may dissolve the cementing mineral from within the sediments themselves and then redeposit them before exiting.
The most common cementing materials are calcium carbonate, silica, and iron oxides. Calcium carbonate cement is usually in the form of calcite. Silica cement is dominantly quartz but also can be chert or chalcedony. Iron oxide cements occur in the form of hematite or limonite.
Some sediments may become lithified by the recrystallization of mineral constituents rather than by cementation. This is an important process in the formation of limestone and some shales. As the name implies, it involves the in situ recrystallization of the sedimentary grains. In this process, minerals will recrystallize as a response to a change in their chemical environment, such as a rise in the pH . This is because some minerals are more stable than others are in a given set of conditions. Entire grains may reform, or just the rims of minerals. As mineral grains recrystallize, they grow together, forming new interlocking grain boundaries. These interlocking crystals bind the sediments into rock.
See also Sedimentary rocks; Sedimentation