Lithification (greek : lithos = rock) is a process whereby freshly unconsolidated deposits of sediments are converted into rock. The main processes of lithification are Compaction, Cementation and Crystallization. The physical, chemical and biologic changes that occur under conditions of pressure (up to 1 kb) and temperature (maximum range of 100C to 300C) that are normal to the outer part of the Earth's crust to the deposited sediments from the time of their initial deposition to their lithification and after it are known as diagenesis. So the definition of diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification. In very thick sedimentary sequences, diagenesis may grade into metamorphism, and the sediment will change from a sedimentary rock to a meta-sedimentary rock to a metamorphic rock.
Some geologists restrict the term to the initial phase of post-sedimentary changes, occurring in the zone where the sediment is still unconsolidated, the process being complete when the sediment has been converted to a more or less compact sedimentary rock. In this usage, the term is equivalent to early diagenesis as used in the U.S. The main processes of diagenesis are:
Compaction
Definition: Compaction is the process by which the volume of sediment is reduced as the grains are squeezed together. The reduction in volume as the pore space is reduced helps to hold the rock together by binding the grains closer together and increasing the friction between grains. Intergranular fluid is expelled as volume is reduced. The degree of compaction is controlled by such factors as grain shape, sorting, original porosity, and the amount of pore fluid present.
Recrystallization
Definition: Recrystallization is a process in which physical or chemical conditions induce a reorientation of the crystal lattices of mineral grains. These textural changes cause the sediment to become lithified. It occurs in response to such factors as pressure, temperature, and fluid phase changes. It also occurs because of solution and reprecipitation of mineral phases already present in the rock.
Solution
Solution refers to the process in which a mineral is dissolved. As fluids pass through the sediment, the unstable constituents will dissolve and are transported away or are reprecipitated in nearby pores where conditions are different. The dissolution of soluble minerals by a solution under saturated with respect to that mineral depends on pH, Eh, temperature, pressure, PCO2, ion strength etc. Common minerals to dissolve are evaporates such as halite, sylvite and anhydrite. This process is important because it commonly leads to secondary porosity. Pressure solution is a process that occurs as pressure is concentrated at the point of contact between two grains in the sediment. This causes solution and subsequent migration of ions or molecules away from the point of contact, towards an area of lower pressure where the dissolved phase can be reprecipitated.
Cementation
Cementation is the process in which chemical precipitates (in the form of new crystals) form in the pores of a sediment or rock, binding the grains together. Some common cements are quartz, calcite and hematite, but a wide variety of cements are known, such as aragonite, gypsum, and dolomite. Pressure solution produces locally derived cement, but many cements consist of new minerals previously in solution in the fluid phase. Cementation reduces porosity by filling in the pore spaces between the grains.
Higher pH and higher temperatures favor carbonate cements. Lower pH and low temperatures favor quartz or chert cements. Syntaxial overgrowths are formed when cement growth occurs as and an extension of existing detrital "crystal".
The reverse process, called decementation, also is thought to occur. There is evidence that decementation has occurred in calcareous sandstones, in which case the calcareous cement or grains are dissolved in the same manner as the solution of limestones. The frosted and etched surfaces of quartz grains in some friable and loosely cemented sandstones seem to indicate the former presence of a carbonate cement that has been leached.
Authigenesis Or neocrystallization
Authigenesis (neocrystallization) is the process in which new mineral phases are crystallized in the sediment or rock during diagenesis. These new minerals may be produced:
• By reactions involving phases already present in the sediment (or rock)
• Through precipitation of materials introduced in the fluid phase, or
• From a combination of primary sedimentary and introduced components.
This process overlaps with weathering and cementation. It usually involves recrystallization and may result in replacement. Authigenic phases include silicates such as quartz, alkali feldspar, clays and zeolites; carbonates such as calcite and dolomite; evaporite minerals such as halite, sylvite and gypsum, as well as many others.
Replacement
Replacement involves the essentially simultaneous dissolution of existing minerals and the precipitation of a new mineral in situ.
Some geologists restrict the term to the initial phase of post-sedimentary changes, occurring in the zone where the sediment is still unconsolidated, the process being complete when the sediment has been converted to a more or less compact sedimentary rock. In this usage, the term is equivalent to early diagenesis as used in the U.S. The main processes of diagenesis are:
Compaction
Definition: Compaction is the process by which the volume of sediment is reduced as the grains are squeezed together. The reduction in volume as the pore space is reduced helps to hold the rock together by binding the grains closer together and increasing the friction between grains. Intergranular fluid is expelled as volume is reduced. The degree of compaction is controlled by such factors as grain shape, sorting, original porosity, and the amount of pore fluid present.
Recrystallization
Definition: Recrystallization is a process in which physical or chemical conditions induce a reorientation of the crystal lattices of mineral grains. These textural changes cause the sediment to become lithified. It occurs in response to such factors as pressure, temperature, and fluid phase changes. It also occurs because of solution and reprecipitation of mineral phases already present in the rock.
Solution
Solution refers to the process in which a mineral is dissolved. As fluids pass through the sediment, the unstable constituents will dissolve and are transported away or are reprecipitated in nearby pores where conditions are different. The dissolution of soluble minerals by a solution under saturated with respect to that mineral depends on pH, Eh, temperature, pressure, PCO2, ion strength etc. Common minerals to dissolve are evaporates such as halite, sylvite and anhydrite. This process is important because it commonly leads to secondary porosity. Pressure solution is a process that occurs as pressure is concentrated at the point of contact between two grains in the sediment. This causes solution and subsequent migration of ions or molecules away from the point of contact, towards an area of lower pressure where the dissolved phase can be reprecipitated.
Cementation
Cementation is the process in which chemical precipitates (in the form of new crystals) form in the pores of a sediment or rock, binding the grains together. Some common cements are quartz, calcite and hematite, but a wide variety of cements are known, such as aragonite, gypsum, and dolomite. Pressure solution produces locally derived cement, but many cements consist of new minerals previously in solution in the fluid phase. Cementation reduces porosity by filling in the pore spaces between the grains.
Higher pH and higher temperatures favor carbonate cements. Lower pH and low temperatures favor quartz or chert cements. Syntaxial overgrowths are formed when cement growth occurs as and an extension of existing detrital "crystal".
The reverse process, called decementation, also is thought to occur. There is evidence that decementation has occurred in calcareous sandstones, in which case the calcareous cement or grains are dissolved in the same manner as the solution of limestones. The frosted and etched surfaces of quartz grains in some friable and loosely cemented sandstones seem to indicate the former presence of a carbonate cement that has been leached.
Authigenesis Or neocrystallization
Authigenesis (neocrystallization) is the process in which new mineral phases are crystallized in the sediment or rock during diagenesis. These new minerals may be produced:
• By reactions involving phases already present in the sediment (or rock)
• Through precipitation of materials introduced in the fluid phase, or
• From a combination of primary sedimentary and introduced components.
This process overlaps with weathering and cementation. It usually involves recrystallization and may result in replacement. Authigenic phases include silicates such as quartz, alkali feldspar, clays and zeolites; carbonates such as calcite and dolomite; evaporite minerals such as halite, sylvite and gypsum, as well as many others.
Replacement
Replacement involves the essentially simultaneous dissolution of existing minerals and the precipitation of a new mineral in situ.
