Cement (geology)
In the petrography of sedimentary rocks, cement is the interstitial space between the particles or grains, which is filled with diagenetic excretions, or different generations of such excretions. The cement of a sedimentary rock usually consists of calcium carbonate or silicon dioxide , more rarely also of other minerals. The sequence of the cements, the shape and structure of the cements and their mineralogy allow conclusions to be drawn about the diagenesis , i.e. H. on the change in the rock from its formation to its erosion. Cement and / or matrix form the basic mass of sedimentary rock. In contrast to cement, the matrix consists of a very fine-grained material in which coarser particles “swim” or support each other.
Formation of cements
Cements from sedimentary rocks result from the solidification or lithification (Greek for petrification ) of the original loosely deposited material. Mineral pore water (mostly silicon dioxide or calcium carbonate ) get into the pore spaces of the loose sediment and precipitate there. The process is also known as cementation . The porosity of the starting material is thereby reduced, while at the same time the grains of the sediment are cemented together. The cementation can be multi-phase, and the cements produced can completely or only partially fill the pore space between the particles. Early diagenetic cements, which are often made of aragonite , can recrystallize again during later phases of diagenesis. Such diagenesis products are still referred to as cement. The interpretation of the cement and its sequence in the interstitial spaces between the particles provide important information on the diagenesis of the rock. One therefore also speaks of cement stratigraphy . Neomorphosis , d. H. the complete in situ recrystallization of calcareous components can produce structures that are very similar to cements and which can easily be confused with cements.
Cements are characterized by the following properties:
- often the crystals are clear and the crystal boundaries are usually well defined
- sharp contact between cement and grains
- the cements usually do not penetrate the grain or cut the grain
- often two or more generations of cement
- often with straight edges and triple points where the crystal edges meet at an angle of 180 ° ( English "enfacial junction")
- the longitudinal axes of the crystals in the cement are usually perpendicular to the surface of the grains
- the crystal sizes often increase with increasing distance from the particle surface.
A combination of light microscopic examinations and cathodoluminescence microscopy is often necessary in order to distinguish cements from neomorphic products ( micritization and bulk crystallization).
Cement types
In carbonate petrography in particular, a number of different types of cement (regardless of their mineralogical composition) are distinguished according to their shape:
- acicular cement; needle-shaped crystals that grow perpendicularly on the particle surface. They are long and parallel to one another, the ends often pointed. The length is up to about 100 µm with a diameter of about 10 µm (L / W ratio 10: 1). The cement crust around the particles is usually of the same thickness (isopach). This form of cement consists mainly of aragonite, but also of high-magnesium calcite. It forms under marine conditions.
- fibrous cement; the crystals are needle-like or columnar with an L / B ratio of> 6: 1, the thickness is always> 10 µm. Crusts of the same thickness are often formed around the particles. They are mostly aragonitic or high-magnesium calcites that are formed under marine conditions, and more rarely under marine vadose or meteoric-vadose conditions.
- botryoidal cement (spherulitic cement): pore-filling cement that grows in single or converging hemispheres; the hemispheres consist of individual, elongated, radially arranged crystals. This type of cement is always aragonitic and is often formed in small reef caves and in the steep fore reef areas.
- Radiaxial cement: large, often contaminated calcite crystals with inclusions. The crystals are e.g. Sometimes very long, L / B ratio from 3: 1 to 10: 1. The crystals often have a sub-crystalline structure. The different extinction of the sub-crystals is used to distinguish three sub-types of the radial-axial cement type. This type of cement forms exclusively in the marine area.
- “Dog tooth cement”: Pointed, relatively large (a few tens of µm to a few 100 µm) crystals of elongated, scalenohedral to rhombohedral shape are called “dog tooth cement”. The name alludes to the shape of the crystals. They usually grow more or less vertically on the surface (grains or older cements) and consist of calcite. They often form in freshwater areas, but also in marine areas and under hydrothermal conditions.
- "Bladed" cement: the crystals are not the same size and are not fibrous. The length / width ratio is therefore significantly smaller than with the fibrous cements (L / W 1.5: 1 to 6: 1). The crystal tips are broad and flattened or pyramidal in shape. The crystal sizes are from about 10 µm in width and from about 20 to more than 100 µm in height. Usually the crystals get a little wider towards the tip. This type of cement usually forms edges of the same thickness on grains. Mineralogically, this cement often consists of high-magnesium calcite or aragonite.
- Geopetal cement, gravity cement (“dripstone”): These are hanging cements or the thickening of a cement crust below a grain, or hanging cements on the ceiling of intergranular cavities or solution cavities. The cement forms in water droplets under the grains after the pore water has drained, or in water droplets that hang from the ceiling of cavities (“microstalactite”). This cement is mostly calcitic and forms in the meteoric-vadose and meteoric-phreatic area, more rarely also in the marine-vadose area (intertidal splash zone). In the marine area, the cement is then mostly aragonitic.
- Meniscus cement: This type of cement describes cement bridges between grains. They arise in water bridges between the grains, due to the surface tension of the water after the pore water has largely drained off. This type of cement forms mainly in the meteoric-vadose zone, but also in the marine-vadose zone as so-called “beach rock”. It is almost always calcitic.
- drusiger cement: It is a cement that fills cavities or pores. The growth usually starts from the surface of the components. The length and width of the crystals are approximately the same, or they are only slightly elongated. The crystal shape can be described as anhedral or subhedral . They usually measure more than 10 µm in diameter, and they usually get larger towards the center of the pore. This calcitic cement is created in fresh water, but also during diagenesis at greater depths.
- granular cement: it is a calcite cement that forms in the pores between the particles. The crystals are small (less than 10 µm) and length and width are approximately the same. They usually grow without being influenced by the surfaces of the components. This cement is created in the meteoric-vadose and meteoric-phreatic area as well as during diagenesis at greater depths.
- Block cement: This type of cement describes medium-sized to large calcite crystals (a few tens of µm to a few millimeters in size) without any preferred orientation. The crystals are of different sizes. This cement is typically formed under freshwater conditions (meteoric-phreatic and meteoric-vados) and during diagenesis at greater depths. This type of cement can also occur in hard marine grounds and reefs.
- Syntaxial calcite cement: This calcite cement grows on or around a component that consists of a single crystal (usually a fragment of an echinoderm skeleton). The crystallographic axis of the component also determines the crystallographic axis of the cement. This form of cement occurs in marine-vadose and very near-surface marine-phreatic areas as well as in meteoric-phreatic areas.
- Peloidal microcrystalline cement: It is made up of a peloidal structure consisting of small peloids (diameter smaller than 100 µm) in a microcrystalline calcite matrix. The peloids consist of micrite-sized crystals that are indistinctly arranged radially. This type of cement forms under shallow marine conditions, mostly in reef areas
- Micritzement (microcrystalline cement): This describes a cement that consists of micron-sized crystals that are placed around components as a thin layer or line pores. It can also form bridges between components and in this respect is similar to meniscal cement. Micrite cement almost always consists of high-magnesium calcite. It is very similar to the micritized rinds of components, but they are created by microbacterial activity or drilling.
Groups within the cement types
These cements can also be grouped together:
- Cements that grow from a substrate into the pore space (acicular, fibrous, botryoidal, radial-fibrous, dog-tooth and "bladed" cement)
- "Hanging" or connecting cements ("dripstone" or gravity cement, meniscus cement)
- Pore-filling cements that form a mosaic (drusy, granular or blocky cement)
- Cements that grow on a component and continue to grow in the same optical orientation as the component (syntax cement)
- Microcrystalline cement (microcrystalline cement and peloidal microcrystalline cement)
Cement structure
The cements can also be broken down according to the cement structure
- Isopache cements (edges of equal thickness around components)
- Circumgranular cements
- Gravity
- Crusts
- Fan (Splays)
- Botryoids
- Druzy mosaic
- Equant mosaic
- Granular mosaic
- Syntax growth
Mega-cement
A very special and rare case of calcitic precipitations in larger cavities are the so-called megacements. They are very long, mostly single crystals that can reach a length of 25 cm and a diameter of approx. 4 mm. Such cements form on the ocean floor, in Neptunian dikes of basin carbonates and larger, submarine cavities. A special type of these mega-cement (so-called “raggioni”) is interpreted as a pseudomorphosis of an aragonite cement that has formed under meteoric-vadose or marine-vadose conditions.
“Crystal silt” - sediment or cement?
The term crystal silt describes a matrix in solution cavities, window structures, intraparticle pores and small reef caves. This matrix typically consists of small, often rhombic and mostly angular calcite crystals (about 5 to 40 µm). This matrix was interpreted as a solution residue that occurs in the partial dissolution of limes, e.g. B. in the meteoric-vadose supratidal area. Other authors explain this matrix as internal marine sediment, as it occasionally contains marine microfossils, or as reclaimed microcrystalline cements.
literature
- Erik Flügel: Microfacies of Carbonate Rocks. Springer Verlag, Berlin, Heidelberg & New York 2004, ISBN 3-540-22016-X
- Hans Füchtbauer and Detlev K. Richter: Carbonate rocks. In: Hans Füchtbauer (Ed.), Sediment-Petrology, Part 2, Sediments and Sedimentary Rocks. 4th edition, 233-434 pp., E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 .
