Metamorphic Rocks

 


This plasma membrane is in turn associated with a layer of carbohydrate-containing macromolecules known as the glycocalyx , that varies in thickness from one species to another. The fine structure of tegument is essentially the same in both the cestodes and trematodes. Defined by the occurrence of prehnite and pumpellyite in metabasites. Isograds represent mineral reactions not rock chemical composition. Certain animal immune-derived cells may form aggregate cells also, for example the osteoclast cells responsible for bone resorption.

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More recently various authors have recognised distinctions in the assemblages containing prehnite and pumpellyite, and erected three facies or subfacies based on the assemblages prehnite-pumpellyite, prehnite-actinolite and pumpellyite-actinolite. These facies or subfacies, involving prehnite and pumpellyite, may be collectively referred to as the subgreenschist facies.

Facies boundaries are defined by the appearance or disappearance of a mineral or group of minerals, and not a specific P and T. Such variations in turn affect the P-T location of the boundary reactions. Defined by the occurrence of zeolites in mafic rocks, but not identifiable in metapelites. Defined by the occurrence of prehnite and pumpellyite in metabasites. Defined by the occurrence of illite, chlorite and smectite in metapelites.

Characterized by the lack of biotite in metasediments and metabasites phengite instead of Bt. No albite epidote or grossular garnet are the stable Ca Al silicates. Eclogites can be subdivided into three groups: Eclogites that form in the mantle and are brought up to the surface with kimberlites in diatremes. Garnets in these eclogites are rich in pyrope. Eclogites that form in the lower crust and are associated with gneiss terrains granulites and high grade amphibolites.

Garnets in this group are rich in grossular and almandine. Eclogites that form at relatively low temperatures in what are now known as subduction zones. These eclogites contain almandine rich garnet and are associated with blueschists. Facies, Protolith and Mineral assemblage for different protolith. Protolith refers to the original rock, prior to metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith.

As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to determine the protolith: These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks. Because of their high concentrations of alumina they are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.

Rocks that originally contained mostly quartz and feldspar like granitic rocks and arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since these minerals are stable of a wide range of temperature and pressure. Those that exhibit mostly quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic.

Calcareous rocks are calcium rich. They are usually derivatives of carbonate rocks, although they contain other minerals that result from reaction of the carbonates with associated siliceous detrital minerals that were present in the rock. At low grades of metamorphism calcareous rocks are recognized by their abundance of carbonate minerals like calcite and dolomite.

With increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite Mg-rich biotite , chlorite, and tremolite. At even higher grades anhydrous minerals like diopside, forsterite, wollastonite, grossularite, and calcic plagioclase. Basic metamorphic rocks are generally derivatives of basic igneous rocks like basalts and gabbros. They have an abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like plagioclase and epidote.

Rocks that are rich in Fe with little Mg are termed ferriginous. Most fungi of Basidiomycota exist as a dikaryon in which thread-like cells of the mycelium are partially partitioned into segments each containing two differing nuclei, called a heterokaryon. A classic example of a syncytium is the formation of skeletal muscle. Large skeletal muscle fibers form by the fusion of thousands of individual muscle cells. The multinucleated symplastic arrangement is important in pathologic states such as myopathy , where focal necrosis death of a portion of a skeletal muscle fibers does not result in necrosis of the adjacent sections of that same skeletal muscle fiber, because those adjacent sections have their own nuclear material.

Thus, myopathy is usually associated with such "segmental necrosis", but with some of the surviving segments being functionally cut off from their nerve supply via loss of continuity with the neuromuscular junction.

The syncytium of cardiac muscle is important because it allows rapid coordinated contraction of muscles along their entire length. Action potentials propagate along the surface of the muscle fiber from the point of synaptic contact, through intercalated discs. Although a syncytium, cardiac muscle differs because the cells are not long and multinucleated.

Cardiac tissue is therefore described as a functional syncytium, as opposed to the true syncytium of skeletal muscle. Certain animal immune-derived cells may form aggregate cells also, for example the osteoclast cells responsible for bone resorption. Another important vertebrate syncytium is in the placenta of placental mammals. Embryo-derived cells that form the interface with the maternal blood stream fuse together to form a multinucleated barrier - the syncytiotrophoblast.

This is probably important to limit the exchange of migratory cells between the developing embryo and the body of the mother, as some blood cells are specialized to be able to insert themselves between adjacent epithelial cells. The syncytial epithelium of the placenta does not provide such an access path from the maternal circulation into the embryo.

Much of the body of Hexactinellid sponges is composed of syncitial tissue. This allows them to form their large siliceous spicules exclusively inside their cells. The fine structure of tegument is essentially the same in both the cestodes and trematodes.

It is a syncytium consisting of multinucleated tissues with no distinct cell boundaries. The outer zone of the syncytium, called the "distal cytoplasm," is lined with a plasma membrane. This plasma membrane is in turn associated with a layer of carbohydrate-containing macromolecules known as the glycocalyx , that varies in thickness from one species to another. The distal cytoplasm is connected to the inner layer called the "proximal cytoplasm", which is the "cellular region or cyton or perikarya" through cytoplasmic tubes that are composed of microtubules.

The proximal cytoplasm contains nuclei , endoplasmic reticulum , Golgi complex , mitochondria , ribosomes , glycogen deposits , and numerous vesicles. The basal lamina is followed by a thick layer of muscle. These syncytial formations create distinctive cytopathic effects when seen in permissive cells.

Because many cells fuse together, syncytium are also known as multinucleated giant cells , or polykaryocytes. Typically, the viral families that can cause syncytium are enveloped because viral envelope proteins on the surface of the host cell are needed to fuse with other cells.