Cement (geology)

Cement is in petrography of sedimentary rocks of the interstitial space filled with diagenetic precipitates between the particles or grains or even recognizable several generations of such precipitates. The cement of a sedimentary rock composed mostly of calcium carbonate or silica, more rarely, from other minerals. The sequence of cements, the shape and structure of the cements and their mineralogy allow conclusions on the diagenesis, ie to the change of the rock since its creation to removal. Cement and / or matrix form the base composition of a sedimentary rock. In contrast to the cement matrix consists of a very fine-grained material in which coarser particles "float" or support each other.

Formation of cements

Cements of sedimentary rocks formed during solidification or Lithifizierung (Greek fossilization ) of the original loosely deposited material. Mineral pore waters ( usually silica or calcium carbonate) enter the pore spaces of unconsolidated sediment and cases out there. The process is also referred to as cementation. The porosity of the starting material is thereby reduced, while at the same time, the grains of the sediment to be cemented together. The cementation may be multiphasic, and the resulting cements can fill the pore space between the particles completely or only partially. Early diagenetic cements, which are often composed of aragonite, can Recrystallization again during later stages of diagenesis. Such Diageneseprodukte but are still referred to as cement. The interpretation of the cement and its sequence in the interstitial spaces between the particles provide important information for the diagenesis of the rock. It is therefore also called Zementstratigraphie. Neomorphose, that is, the complete in situ recrystallization of calcareous components, can produce structures that are very similar and the cements that can be easily 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
  • Penetrate the cement is generally not in the grains, type or cut the grains
  • Often, two or more generations of cement
  • Often with straight edges and triple points at which the crystal edges at an angle of 180 ° meet (English " enfacial junction" )
  • The longitudinal axes of the crystals in the cement are generally perpendicular to the surface of the grains
  • The crystal sizes take frequent with increasing distance from the particle surface.

Frequently, the combination of light microscopy and cathodoluminescence microscopy is necessary in order to distinguish cements of newly formed products ( Mikritisierung crystallization and collection ).

Cement types

Especially in the Karbonatpetrographie a number of different cement types are distinguished (regardless of their mineralogical composition ) according to their shape:

  • Acicular cement; needle-like crystals, which grow perpendicularly to the particle surface. They are long and arranged parallel to each other, the ends are often worsened. The length is about 100 microns to about 10 microns in diameter (L / B ratio of 10:1). The cement crust around the particles is usually the same thickness ( isopach ). This type of cement is mainly composed of aragonite, but also from high -magnesium calcite. It is formed under marine conditions.
  • Fibrous cement; the crystals are needle-like or columnar manner with an L / B ratio of > 6:1, the thickness of which is always > 10 microns. Thick crusts are often confused formed around the particles. There usually are aragonitic or high -magnesium calcite, which is formed under marine conditions, rare even in marine- vadose or meteoric - vadose conditions.
  • Botryoidaler cement ( cement sphärulitischer ): pore-filling cement, which grows in single or coalescing hemispheres; The half-spheres are composed of individual, elongated, radially disposed crystals. This cement type is always aragonitisch and is often made in small Riffhöhlen and in the steep Vorriffbereichen.
  • Radiaxialer cement: large, often contaminated with calcite inclusions. The crystals are z.T. very long L / B ratio of 3:1 to 10:1. The crystals often have a subkristallines structure. Different extinction of Subkristalle used to distinguish three types of radiaxialen cement type. This type of cement is formed exclusively in the marine environment.
  • "Dog tooth cement ": Tapered, relatively large (some 10 microns to 100 microns some ) crystals of elongated skalenohedraler to rhombohedraler form are called " dog tooth cement " means. The name alludes to the shape of the crystals. They usually grow more or less perpendicular to the surface ( grain or older cements) and consist of calcite. They frequently form in the freshwater area, but also in the marine environment and under hydrothermal conditions.
  • "Bladed " Cement: The crystals are not the same size and not fibrous. The Längen-/Breiten-Verhältnis is therefore much smaller than in the fibrous cement (L / W from 1.5:1 to 6:1). The crystal peaks are broad and flattened or pyramidal. The crystal sizes are about 10 microns in width and about 20 to more than 100 microns in height. Usually the crystals are towards the top a little further. This type of cement is usually the same thickness margins on grains. Mineralogical consists of cement often from high -magnesium calcite or aragonite.
  • Geopetalzement, gravity cement ( " dripstone " ): It is hanging cements or cement to the thickening of the crust beneath a grain, or hanging cements on the ceiling of intergranular voids or solution cavities. The cement forms in water droplets among the grains after the pore water has drained away, or in water droplets that hang from the ceiling of cavities ( " Mikrostalaktit "). This cement is usually kalzitisch and forms in the meteoric - vadose and meteoric phreatic - range, rare in marine- vadose zone ( intertidal splash zone ). In the marine sector, the cement is then usually aragonitisch.
  • Meniscus cement: This cement type describes cement bridges between grains. They occur in water bridges between the grains due to the surface tension of the water after the pore water is largely drained. This type of cement is mainly engaged in the meteoric - vadose zone, but also in the marine- vadose zone as a so -called " Beach Rock". He is almost always kalzitisch.
  • Drusiger cement: There is a cement that fills cavities or pores. This growth is mostly from the surface of the components. The crystals are approximately the same length and width, or they are only slightly ausgelängt. The crystal form can be described as anhedral or subhedral. They usually measure more than 10 microns in diameter, and they are usually related to the pore center towards larger. This calcite cement occurs in the freshwater area, but also in the diagenesis in greater depth.
  • Granular cement: It is a calcite cement that forms in the pores between the particles. The crystals are small ( under 10 microns ) and length and width are approximately equal. They usually grow without being influenced by the surfaces of the components. This cement is produced in the meteoric - vadose and meteoric phreatic - range as well as diagenesis in greater depth.
  • Block cement: This cement type describes medium to large calcite crystals ( several tens of microns to a few millimeters in size ) with no preferred orientation. The crystals vary in size. This cement is typically formed under freshwater conditions ( meteoric - phreatic and meteoric - vados ) as well as diagenesis in greater depth. Also in marine hard substrates and reefs, this type of cement occur.
  • Syntaxialer Kalzitzement This Kalzitzement grows or a component consisting of a single crystal (usually a fragment of echinoderm skeleton ). The crystallographic axis of the component also determines the crystallographic axis of the cement. This type of cement is found in marine- vadose and very shallow marine- phreatic areas as well as in the meteoric - phreatic area.
  • Peloidaler microcrystalline cement: It is constructed from a peloidalen structure consisting of small peloids ( diameter less than 100 microns ) in a microcrystalline Kalzitmatrix. The peloids are composed of micrite - sized crystals that are arranged radially indistinct. This type of cement is formed under shallow marine conditions, mostly in reef areas
  • Mikritzement ( microcrystalline cement): This is called a cement consisting of micron - sized crystals which build up as a thin layer to components or lining pores. He can also form bridges between components, and is similar in this respect to the meniscus cement. Mikritzement almost always consists of high -magnesium calcite. He is very similar to the mikritisierten barks of components, but caused by microbial activity or by drillings.

Groups within the cement types

These cements can be summarized in Groups:

  • Cements that of a substrate in the pore space to grow ( and dog - tooth "bladed " acicular, fibrous, botryoidal, radiaxial - fibrous, cement)
  • "Hanging" or linking cements ( " dripstone " or gravity cement, meniscus cement)
  • Pore-filling cements which form a mosaic ( drusiger, granular or blocky cement)
  • Cements, which grow on a component and in the same orientation as the optical component to continue to grow ( syntaxialer cement)
  • Mikritzemente ( Mikritzement and peloidaler microcrystalline cement)

Cement structure

The cement can be divided even after the cement structure

  • Isopachen cements ( equally thick borders around components)
  • Zirkumgranulare cements
  • Gravity
  • Crusts
  • Subjects ( Splays )
  • Botryoide
  • Drusiges mosaic
  • Equant mosaic
  • Granular mosaic
  • Syntaxiales growing

Mega cements

A very special and rare case of calcite precipitates in larger cavities are the so- called mega cements. It is very long, usually single crystals, which can reach a length up to 25 cm and a diameter of about 4 mm. Such cements are formed on the seabed, in Neptunian Dikes of Beckenkarbonaten and larger submarine cavities. A special type of this mega cements (so-called " raggioni " ) is interpreted as pseudomorphs of Aragonitzements, which has formed under meteoric - vadose or marine- vadose conditions.

" Kristallsilt " - sediment or cement?

The term Kristallsilt describes a matrix in solution cavities, window structures, intraparticle pores and small Riffhöhlen. The matrix typically consists of small, often and generally rhombic angular calcite crystals ( about 5 to 40 microns ). This matrix has been interpreted as a solution residue from the partial solution of limestones, eg have arisen in the meteoric - vadose Supratidalbereich. Other authors explain this matrix as an internal marine sediment, as they occasionally contain marine microfossils, or reconditioned microcrystalline cements.