Limpets are aquatic snails with a shell, broadly conical in shape and a strong, muscular foot. Although all limpets are members of the class Gastropoda, limpets are polyphyletic, meaning the various groups which are referred to as "limpets" have descended independently from different ancestral gastropods; this general category of conical shell is known as "patelliform". All members of the large and ancient marine clade. Within that clade, the members of the Patellidae family in particular are referred to as "true limpets". Other groups, not in the same family, are called limpets of one type or another, due to the similarity of their shells' shape. Examples include the Fissurellidae family, part of the Vetigastropoda clade and the Siphonariidae, which use a siphon to pump water over their gills; some species of limpet live in fresh water. The basic anatomy of a limpet consists of the usual molluscan organs and systems: A nervous system centered around the paired cerebral and pleural sets of ganglia.

These ganglia create a ring around the limpet's esophagus called a circumesophageal nerve ring or nerve collar. Other nerves in the head/ snout are the optic nerves which connect to the two eye spots located at the base of the cerebral tentacles, as well as the labial and buccal ganglia which are associated with feeding and controlling the animal's odontophore, the muscular cushion used to support the limpet's radula that scrapes algae off the surrounding rock for nutrition. Behind these ganglia lie the pedal nerve cords which control the movement of the foot, the visceral ganglion which in limpets has been torted during the course of evolution; this means, among other things, that the limpet's left osphradium and oshradial ganglion is controlled by its right pleural ganglion and vice versa. For most limpets, the circulatory system is based around a single triangular three-chambered heart consisting of an atrium, a ventricle, a bulbous aorta. Blood enters the atrium via the circumpallial vein and through a series of small vesicles that deliver more oxygenated blood from the nuchal cavity.

Many limpets still retain a ctenidium in this nuchal chamber instead of the circumpallial gills as a means for exchanging oxygen and carbon dioxide with the surrounding water or air. Blood moves from the atrium into the ventricle and into the aorta where it is pumped out to the various lacunar blood spaces / sinuses in the hemocoel; the odontophore may play a large role in assisting with blood circulation as well. The two kidneys are different in size and location; this is a result of torsion. The left kidney is diminutive and in most limpets is functional; the right kidney, has taken over the majority of blood filtration and extends over and around the entire mantle of the animal in a thin, almost-invisible layer. The digestive system takes up a large part of the animal's body. Food enters via the downward-facing mouth, it moves through the esophagus and into the numerous loops of the intestines. The large digestive gland helps break down the microscopic plant material, the long rectum helps compact used food, excreted through the anus located in the nuchal cavity.

The anus of most molluscs and indeed many animals is located far from the head. In limpets and most gastropods, the evolutionary torsion which took place and allowed the gastropods to have a shell into which they could withdraw has caused the anus to be located near the head. Used food would foul the nuchal cavity unless it was compacted prior to being excreted; the torted condition of the limpets remains though they no longer have a shell into which they can withdraw and though the evolutionary advantages of torsion appear to therefore be negligible. The gonad of a limpet is located beneath its digestive system just above its foot, it swells and bursts, sending gametes into the right kidney which releases them into the surrounding water on a regular schedule. Fertilized eggs hatch and the floating veliger larvae are free-swimming for a period before settling to the bottom and becoming an adult animal. True limpets in the family Patellidae live on hard surfaces in the intertidal zone. Unlike barnacles or mussels, limpets are capable of locomotion instead of being permanently attached to a single spot.

However, when they need to resist strong wave action or other disturbances, limpets cling firmly to the surfaces on which they live, using their muscular foot to apply suction combined with the effect of adhesive mucus. It is difficult to remove a true limpet from a rock without injuring or killing it. All "true" limpets are marine; the most primitive gro


Parisite is a rare mineral consisting of cerium and calcium fluoro-carbonate, Ca23F2. Parisite is parisite-, but when neodymium is present in the structure the mineral becomes parisite-, it is found only as crystals, which belong to the trigonal or monoclinic pseudo-hexagonal system and have the form of acute double pyramids terminated by the basal planes. The crystals are translucent; the hardness is 4.5 and the specific gravity is 4.36. Light which has traversed a crystal of parisite exhibits a characteristic absorption spectrum. At first, the only known occurrence of this mineral was in the famous emerald mine at Muzo in Colombia, South America, where it was found by J. J. Paris, who rediscovered and worked the mine in the early part of the 19th century. Allied to parisite, indeed first described as such, is a mineral from the nepheline-syenite district of Julianehaab in south Greenland. To this the name synchysite has been given; the crystals are rhombohedral (as distinct from hexagonal. At the same locality there is found a barium-parisite, which differs from the Colombian parisite in containing barium in place of calcium, the formula being 2Ba3: this is named cordylite on account of the club-shaped form of its hexagonal crystals.

Bastnasite is a cerium lanthanum and neodymium fluoro-carbonate CO3, from Bastnas, near Riddarhyttan, in Vestmanland and the Pikes Peak region in Colorado, United States. Anthony, John W.. "Parisite-". Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America; this article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed.. "Parisite". Encyclopædia Britannica. 20. Cambridge University Press. P. 825


Kerogen is solid, insoluble organic matter in sedimentary rocks. Consisting of an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter by 10,000 fold, it is insoluble in normal organic solvents and it does not have a specific chemical formula. Upon heating, kerogen converts in part to gaseous hydrocarbons. Petroleum and natural gas form from kerogen. Kerogen may be classified by its origin: lacustrine and terrestrial; the name "kerogen" was introduced by the Scottish organic chemist Alexander Crum Brown in 1906, derived from the Greek for "wax birth". Increasing production of hydrocarbons from shale has motivated a revival of research into the composition and properties of kerogen. Many studies have documented dramatic and systematic changes in kerogen composition across the range of thermal maturity relevant to the oil and gas industry. Analyses of kerogen are performed on samples prepared by acid demineralization with critical point drying, which isolates kerogen from the rock matrix without altering its chemical composition or microstructure.

Kerogen is formed during sedimentary diagenesis from the degradation of living matter. The original organic matter can comprise lacustrine and marine algae and plankton and terrestrial higher-order plants. During diagenesis, large biopolymers from, e.g. proteins and carbohydrates in the original organic matter decompose or completely. This breakdown process can be viewed as the reverse of photosynthesis.. These resulting units can polycondense to form geopolymers; the formation of geopolymers in this way accounts for the large molecular weights and diverse chemical compositions associated with kerogen. The smallest units are the fulvic acids, the medium units are the humic acids, the largest units are the humins; this polymerization happens alongside the formation and/or sedimentation of one or more mineral components resulting in a sedimentary rock like oil shale. When kerogen is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide elevated pressure and temperature owing to lithostatic and geothermal gradients in Earth's crust.

Resulting changes in the burial temperatures and pressures lead to further changes in kerogen composition including loss of hydrogen, nitrogen and their associated functional groups, subsequent isomerization and aromatization Such changes are indicative of the thermal maturity state of kerogen. Aromatization allows for molecular stacking in sheets, which in turn drives changes in physical characteristics of kerogen, such as increasing molecular density, vitrinite reflectance, spore coloration. During the process of thermal maturation, kerogen breaks down in high-temperature pyrolysis reactions to form lower-molecular-weight products including bitumen and gas; the extent of thermal maturation controls the nature of the product, with lower thermal maturities yielding bitumen/oil and higher thermal maturities yielding gas. These generated species are expelled from the kerogen-rich source rock and in some cases can charge into a reservoir rock. Kerogen takes on additional importance in unconventional resources shale.

In these formations and gas are produced directly from the kerogen-rich source rock. Much of the porosity in these shales is found to be hosted within the kerogen, rather than between mineral grains as occurs in conventional reservoir rocks. Thus, kerogen controls much of the transport of oil and gas in shale. Kerogen is a complex mixture of organic chemical compounds that make up the most abundant fraction of organic matter in sedimentary rocks; as kerogen is a mixture of organic materials, it is not defined by a single chemical formula. Its chemical composition varies between and within sedimentary formations. For example, kerogen from the Green River Formation oil shale deposit of western North America contains elements in the proportions carbon 215: hydrogen 330: oxygen 12: nitrogen 5: sulfur 1. Kerogen is insoluble in normal organic solvents in part because of its high molecular weight of its component compounds; the soluble portion is known as bitumen. When heated to the right temperatures in the earth's crust, some types of kerogen release crude oil or natural gas, collectively known as hydrocarbons.

When such kerogens are present in high concentration in rocks such as organic-rich mudrocks shale, they form possible source rocks. Shales that are rich in kerogen but have not been heated to required temperature to generate hydrocarbons instead may form oil shale deposits; the chemical composition of kerogen has been analyzed by several forms of solid state spectroscopy. These experiments measure the speciations of different types of atoms in kerogen. One technique is 13C NMR spectroscopy. NMR experiments have found that carbon in kerogen can range from entirely aliphatic to entirely aromatic, with kerogens of higher thermal maturity having higher abundance of aromatic carbon. Another technique is Raman spectroscopy. Raman scattering is characteristic of, can be used to identify, specific vibrational modes and symmetries of molecular bonds; the first-order Raman spectra of kerogen comprises two principal