An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, crustaceans such as crabs and lobsters; the shells of certain sponges and the various groups of shelled molluscs, including those of snails, tusk shells and nautilus, are exoskeletons. Some animals, such as the tortoise, have both an exoskeleton. Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, sensing, support and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defense from pests and predators, in providing an attachment framework for musculature. Exoskeletons contain chitin. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles.
These structures are composed of chitin, are six times as strong and twice as stiff as vertebrate tendons. Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts. Many different species produce exoskeletons. Bone, cartilage, or dentine turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters, in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria. One species of mollusc, the scaly-foot gastropod makes use of the iron sulfides greigite and pyrite; some organisms, such as some foraminifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton, as their test is always contained within a layer of living tissue. Exoskeletons have evolved independently many times.
Further, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals. This coating is constructed from bone in the armadillo, hair in the pangolin; the armor of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone. Since exoskeletons are rigid, they present some limits to growth. Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in snails and other molluscans. A true exoskeleton, like that found in arthropods, must be shed. A new exoskeleton is produced beneath the old one; as the old one is shed, the new skeleton is pliable. The animal will pump itself up to expand the new shell to maximal size let it harden; when the shell has set, the empty space inside the new skeleton can be filled up. Failure to shed the exoskeleton once outgrown can result in the animal being suffocated within its own shell, will stop subadults from reaching maturity, thus preventing them from reproducing.
This is the mechanism such as Azadirachtin. Exoskeletons, as hard parts of organisms, are useful in assisting preservation of organisms, whose soft parts rot before they can be fossilized. Mineralized exoskeletons can be preserved "as is", as shell fragments, for example; the possession of an exoskeleton permits a couple of other routes to fossilization. For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton, which may decay. Alternatively, exceptional preservation may result in chitin being mineralized, as in the Burgess Shale, or transformed to the resistant polymer keratin, which can resist decay and be recovered. However, our dependence on fossilized skeletons significantly limits our understanding of evolution. Only the parts of organisms that were mineralized are preserved, such as the shells of molluscs, it helps that exoskeletons contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone.
The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilized. Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago; the evolution of a mineralized exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian organisms produced tough outer shells while others, such as Cloudina, had a calcified exoskeleton; some Cloudina shells show evidence of predation, in the form of borings. On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they mineralised, this makes it difficult to comment on the early evolution of each lineage's exoskeleton.
It is known, that in a short course of time, just before the Cambrian period, exoskeletons
Orthoclase, or orthoclase feldspar, is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other, it is a type of potassium feldspar known as K-feldspar. The gem known as moonstone is composed of orthoclase. Orthoclase is a common constituent of most granites and other felsic igneous rocks and forms huge crystals and masses in pegmatite; the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember, of plagioclase. While cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium; the resulting intergrowth of the two feldspars is called perthite. The higher-temperature polymorph of KAlSi3O8 is sanidine. Sanidine is common in cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, is notably found in trachytes of the Drachenfels, Germany; the lower-temperature polymorph of KAlSi3O8 is microcline.
Adularia is a low temperature form of either microcline or orthoclase reported from the low temperature hydrothermal deposits in the Adula Alps of Switzerland. It was first described by Ermenegildo Pini in 1781; the optical effect of adularescence in moonstone is due to adularia. The largest documented single crystal of orthoclase was found in the Ural mountains in Russia, it weighed ~ 100 tons. Together with the other potassium feldspars, orthoclase is a common raw material for the manufacture of some glasses and some ceramics such as porcelain, as a constituent of scouring powder; some intergrowths of orthoclase and albite have an attractive pale luster and are called moonstone when used in jewellery. Most moonstones are translucent and white, although grey and peach-colored varieties occur. In gemology, their luster is called adularescence and is described as creamy or silvery white with a "billowy" quality, it is the state gem of Florida. The gemstone called rainbow moonstone is more properly a colorless form of labradorite and can be distinguished from "true" moonstone by its greater transparency and play of color, although their value and durability do not differ.
Orthoclase is one of the ten defining minerals of the Mohs scale of mineral hardness, on which it is listed as having a hardness of 6. NASA's Curiosity Rover discovery of high levels of orthoclase in Martian sandstones suggested that some Martian rocks may have experienced complex geological processing, such as repeated melting. Minerals portal List of minerals
Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current. The individual organisms constituting plankton are called plankters, they provide a crucial source of food to many large aquatic organisms, such as fish and whales. These organisms include bacteria, algae and drifting or floating animals that inhabit—for example—the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification. Though many planktonic species are microscopic in size, plankton includes organisms over a wide range of sizes, including large organisms such as jellyfish. Technically the term does not include organisms on the surface of the water, which are called pleuston—or those that swim in the water, which are called nekton; the name plankton is derived from the Greek adjective πλαγκτός, meaning errant, by extension, wanderer or drifter, was coined by Victor Hensen in 1887.
While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day, their horizontal position is determined by the surrounding water movement, plankton flow with ocean currents. This is in contrast to nekton organisms, such as fish and marine mammals, which can swim against the ambient flow and control their position in the environment. Within the plankton, holoplankton spend their entire life cycle as plankton. By contrast, meroplankton are only planktic for part of their lives, graduate to either a nektic or benthic existence. Examples of meroplankton include the larvae of sea urchins, crustaceans, marine worms, most fish; the amount and distribution of plankton depends on available nutrients, the state of water and a large amount of other plankton. The study of plankton is termed planktology and a planktonic individual is referred to as a plankter; the adjective planktonic is used in both the scientific and popular literature, is a accepted term.
However, from the standpoint of prescriptive grammar, the less-commonly used planktic is more the correct adjective. When deriving English words from their Greek or Latin roots, the gender-specific ending is dropped, using only the root of the word in the derivation. Plankton are divided into broad functional groups: Phytoplankton, autotrophic prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms, cyanobacteria and coccolithophores. Zooplankton, small protozoans or metazoans that feed on other plankton; some of the eggs and larvae of larger nektonic animals, such as fish and annelids, are included here. Bacterioplankton and archaea, which play an important role in remineralising organic material down the water column. Mycoplankton and fungus-like organisms, like bacterioplankton, are significant in remineralisation and nutrient cycling; this scheme divides the plankton community into broad producer and recycler groups.
However, determining the trophic level of many plankton is not always straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, many species perform both roles. In this mixed trophic strategy — known as mixotrophy — organisms act as both producers and consumers, either at the same time or switching between modes of nutrition in response to ambient conditions. For instance, relying on photosynthesis for growth when nutrients and light are abundant, but switching to predation when growing conditions are poor. Recognition of the importance of mixotrophy as an ecological strategy is increasing, as well as the wider role this may play in marine biogeochemistry. Plankton are often described in terms of size; the following divisions are used: However, some of these terms may be used with different boundaries on the larger end. The existence and importance of nano- and smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity.
The microplankton and smaller groups are microorganisms and operate at low Reynolds numbers, where the viscosity of water is much more important than its mass or inertia. Plankton inhabit oceans, lakes, ponds. Local abundance varies horizontally and seasonally; the primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy, confining primary production to surface waters, to geographical regions and seasons having abundant light. A secondary variable is nutrient availability. Although large areas of the tropical and sub-tropical oceans have abundant light, they experience low primary production because they offer limited nutrients such as nitrate and silicate; this results from large-scale ocean water column stratification. In such regions, primary production occurs at greater depth, although at a reduced level. Despite significant macronutrient concentrations, some ocean regions are unproductive; the micronutrient iron is deficient in these reg
Zeolites are microporous, aluminosilicate minerals used as commercial adsorbents and catalysts. The term zeolite was coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that heating the material, believed to have been stilbite, produced large amounts of steam from water, adsorbed by the material. Based on this, he called the material zeolite, from the Greek ζέω, meaning "to boil" and λίθος, meaning "stone"; the classic reference for the field has been Breck's book Zeolite Molecular Sieves: Structure, And Use. Zeolites occur but are produced industrially on a large scale; as of December 2018, 245 unique zeolite frameworks have been identified, over 40 occurring zeolite frameworks are known. Every new zeolite structure, obtained is examined by the International Zeolite Association Structure Commission and receives a three letter designation. Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others; these positive ions are rather loosely held and can be exchanged for others in a contact solution.
Some of the more common mineral zeolites are analcime, clinoptilolite, natrolite and stilbite. An example of the mineral formula of a zeolite is: Na2Al2Si3O10 the formula for natrolite; these cation exchanged. Natural zeolites form. Zeolites crystallize in post-depositional environments over periods ranging from thousands to millions of years in shallow marine basins. Occurring zeolites are pure and are contaminated to varying degrees by other minerals, quartz, or other zeolites. For this reason occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential. Zeolites are the aluminosilicate members of the family of microporous solids known as "molecular sieves", consist of Si, Al, O, metals including Ti, Sn, Zn, so on; the term molecular sieve refers to a particular property of these materials, i.e. the ability to selectively sort molecules based on a size exclusion process. This is due to a regular pore structure of molecular dimensions; the maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels.
These are conventionally defined by the ring size of the aperture, for example, the term "8-ring" refers to a closed loop, built from eight tetrahedrally coordinated silicon atoms and 8 oxygen atoms. These rings are not always symmetrical due to a variety of causes, including strain induced by the bonding between units that are needed to produce the overall structure, or coordination of some of the oxygen atoms of the rings to cations within the structure. Therefore, the pores in many zeolites are not cylindrical. Zeolites transform to other minerals under weathering, hydrothermal alteration or metamorphic conditions; some examples: The sequence of silica-rich volcanic rocks progresses from: Clay → quartz → mordenite–heulandite → epistilbite → stilbite → thomsonite–mesolite-scolecite → chabazite → calcite. The sequence of silica-poor volcanic rocks progresses from: Cowlesite → levyne–offretite → analcime → thomsonite–mesolite-scolecite → chabazite → calcite. Industrially important zeolites are produced synthetically.
Typical procedures entail heating aqueous solutions of silica with sodium hydroxide. Equivalent reagents include sodium silicate. Further variations include changes in the cations to include quaternary ammonium cations. Synthetic zeolites hold some key advantages over their natural analogues; the synthetic materials are manufactured in a phase-pure state. It is possible to produce zeolite structures that do not appear in nature. Zeolite A is a well-known example. Since the principal raw materials used to manufacture zeolites are silica and alumina, which are among the most abundant mineral components on earth, the potential to supply zeolites is unlimited. Conventional open-pit mining techniques are used to mine natural zeolites; the overburden is removed to allow access to the ore. The ore may be blasted or stripped for processing by using tractors equipped with ripper blades and front-end loaders. In processing, the ore is crushed and milled; the milled ore may be shipped in bags or bulk. The crushed product may be screened to remove fine material when a granular product is required, some pelletized products are produced from fine material.
As of 2016 the world's annual production of natural zeolite approximates 3 million tonnes. Major producers in 2010 included China, South Korea, Jordan, Turkey Slovakia and the United States; the ready availability of zeolite-rich rock at low cost and the shortage of competing minerals and rocks are the most important factors for its large-scale use. According to the United States Geological Survey, it is that a significant percentage of the material sold as zeolites in some countries is ground or sawn volcanic tuff that contains only a small amount of zeolites; some examples of such usage include dimension stone, lightweight aggregate, pozzolanic cement, soil conditioners. There are over 200 synthetic zeolites that have been synthesized by a process of slow crystallization of a silica-alumina gel in the presence of alkalis and organic templates. Many
Polymorphism (materials science)
In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can be found in any crystalline material including polymers and metals, is related to allotropy, which refers to chemical elements; the complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. Polymorphism is relevant to the fields of pharmaceuticals, pigments, dyestuffs and explosives; when polymorphism exists as a result of a difference in crystal packing, it is called packing polymorphism. Polymorphism can result from the existence of different conformers of the same molecule in conformational polymorphism. In pseudopolymorphism the different crystal types are the result of solvation; this is more referred to as solvomorphism as different solvates have different chemical formulae. An example of an organic polymorph is glycine, able to form monoclinic and hexagonal crystals.
Silica is known to form many polymorphs. A classical example is the pair of minerals and aragonite, both forms of calcium carbonate. An analogous phenomenon for amorphous materials is polyamorphism, when a substance can take on several different amorphous modifications. In terms of thermodynamics, there are two types of polymorphic behaviour. For a monotropic system, a plot of the free energy of the various polymorphs against temperature do not cross before all polymorphs melt—in other words, any transition from one polymorph to another below melting point will be irreversible. For an enantiotropic system, a plot of the free energy against temperature shows a crossing point threshold before the various melting points, it may be possible to revert interchangeably between the two polymorphs by heating or cooling, or through physical contact with a lower energy polymorph. The first observation of polymorphism in organic materials is attributed to Friedrich Wöhler and Justus von Liebig when in 1832 they examined a boiling solution of benzamide: upon cooling, the benzamide crystallised as silky needles, but when standing these were replaced by rhombic crystals.
Present-day analysis identifies three polymorphs for benzamide: the least stable one, formed by flash cooling is the orthorhombic form II. This type is followed by the monoclinic form III; the most stable form is monoclinic form I. The hydrogen bonding mechanisms are the same for all three phases. Polymorphs have different stabilities and may spontaneously convert from a metastable form to the stable form at a particular temperature. Most polymorphs of organic molecules only differ by a few kJ/mol in lattice energy. 50% of known polymorph pairs differ by less than 2 kJ/mol and stability differences of more than 10 kJ/mol are rare. They exhibit different melting points, solubilities, X-ray crystal and diffraction patterns. Various conditions in the crystallisation process is the main reason responsible for the development of different polymorphic forms; these conditions include: Solvent effects Certain impurities inhibiting growth pattern and favour the growth of a metastable polymorphs The level of supersaturation from which material is crystallised Temperature at which crystallisation is carried out Geometry of covalent bonds Change in stirring conditionsDespite the potential implications, polymorphism is not always well understood.
In 2006 a new crystal form of maleic acid was discovered 124 years after the first crystal form was studied. Maleic acid is a chemical manufactured on a large scale in the chemical industry and is a salt forming component in medicine; the new crystal type is produced when a co-crystal of caffeine and maleic acid is dissolved in chloroform and when the solvent is allowed to evaporate slowly. Whereas form I has monoclinic space group P21/c, the new form has space group Pc. Both polymorphs consist of sheets of molecules connected through hydrogen bonding of the carboxylic acid groups. 1,3,5-Trinitrobenzene is more than 125 years old and was used as an explosive before the arrival of the safer 2,4,6-trinitrotoluene. Only one crystal form of 1,3,5-trinitrobenzene was known in the space group Pbca. In 2004, a second polymorph was obtained in the space group Pca21 when the compound was crystallised in the presence of an additive, trisindane; this experiment shows. Walter McCrone has stated that "every compound has different polymorphic forms, that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound."
Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, states that in general it is not the most stable but the least stable polymorph that crystallises first. See for examples the aforementioned benzamide, dolomite or phosphorus, which on sublimation first forms the less stable white and the more stable red allotrope. Ostwald suggested that the solid first formed on crystallisation of a solution or a melt would be the least s
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY2O6 where X represents calcium, iron or magnesium and more zinc, manganese or lithium and Y represents ions of smaller size, such as chromium, iron, cobalt, scandium, vanadium or iron. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes, they share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that cystallize in the orthorhombic system are known as orthopyroxenes; the name pyroxene is derived from the Ancient Greek words for stranger. Pyroxenes were so named because of their presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic glass. However, they are early-forming minerals that crystallized before the lava erupted.
The upper mantle of Earth is composed of olivine and pyroxene. Pyroxene and feldspar are the major minerals in gabbro; the chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X site, the Y site, the tetrahedral T site. Cations in Y site are bound to 6 oxygens in octahedral coordination. Cations in the X site can be coordinated depending on the cation size. Twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 used names have been discarded. A typical pyroxene has silicon in the tetrahedral site and predominately ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT2O6; the names of the common calcium–iron–magnesium pyroxenes are defined in the'pyroxene quadrilateral' shown in Figure 2.
The enstatite-ferrosilite series contain up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite. Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite only crystallises in the monoclinic system. There is not complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. This leads to a miscibility gap between augite compositions. There is an arbitrary separation between the diopside-hedenbergite solid solution; the divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineral wollastonite has the formula of the hypothetical calcium end member but important structural differences mean that it is not grouped with the pyroxenes. Magnesium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure.
A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to nomenclature shown in Figure 3. The inclusion of sodium, which has a charge of +1, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In jadeite and aegirine this is added by the inclusion of a +3 cation on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron components are known as omphacite and aegirine-augite, with 80% or more of these components the pyroxene falls in the quadrilateral shown in Figure 2. Table 1 shows the wide range of other cations that can be accommodated in the pyroxene structure, indicates the sites that they occupy. In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and filling the site with the remaining aluminium and iron. Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, there are several alternative schemes: Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively.
For example, Na and Al give the jadeite composition. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site; this leads to e.g. NaFe2+0.5Ti4+0.5Si2O6. The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g. CaAlAlSiO6. In nature, more than one substitution may be found in the same mineral. Clinopyroxenes Aegirine, NaFe3+Si2O6 Augite, 2O6 Clinoenstatite, MgSiO3 Diopside, CaMgSi2O6 Esseneite, CaFe3+ Hedenbergite, CaFe2+Si2O6 Jadeite, NaSi2O6 Jervisite, Si2O6 Johannsenite, CaMn2+Si2O6 Kanoite, Mn2+Si2O6 Kosmochlor, NaCrSi2O6 Namansilite, NaMn3+Si2O6 Natalyite, NaV3+Si2O6 Omphacite, Si2O6 Petedunnite, CaSi2O6 Pigeonite, Si2O6 Spodumene, LiAl2 Orthopyroxenes Hypersthene, SiO3 Donpeacorite, MgSi2O6 Enstatite, Mg2Si2O6 Ferrosilite, Fe2Si2O6 Nchwaningite, Mn2+2SiO32•(H
Epidote is a calcium aluminium iron sorosilicate mineral. Well developed crystals of epidote, Ca2Al2O, crystallizing in the monoclinic system, are of frequent occurrence: they are prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry; the faces are deeply striated and crystals are twinned. Many of the characters of the mineral vary with the amount of iron present for instance, the color, the optical constants, the specific gravity; the color is green, brown or nearly black, but a characteristic shade of yellowish-green or pistachio-green. It displays strong pleochroism, the pleochroic colors being green and brown. Clinozoisite is green, white or pale rose-red group species containing little iron, thus having the same chemical composition as the orthorhombic mineral zoisite; the name, due to Haüy, is derived from the Greek word "epidosis" which means "addition" in allusion to one side of the ideal prism being longer than the other. Epidote is one of secondary origin.
It occurs in schistose rocks of metamorphic origin. It is a product of hydrothermal alteration of various minerals composing igneous rocks. A rock composed of quartz and epidote is known as epidosite. Well-developed crystals are found at many localities: Knappenwand, near the Großvenediger in the Untersulzbachthal in Salzburg, as magnificent, dark green crystals of long prismatic habit in cavities in epidote schist, with asbestos, adularia and apatite; the transparent, dark green crystals from the Knappenwand and from Brazil have been cut as gemstones. Belonging to the same isomorphous group with epidote are the REE-rich allanite, the manganese-rich piemontite. Piemontite occurs as small, reddish-black, monoclinic crystals in the manganese mines at San Marcel, near Ivrea in Piedmont, in crystalline schists at several places in Japan; the purple color of the Egyptian porfido rosso antico is due to the presence of this mineral. Allanite and dollaseite- have the same general epidote formula and contain metals of the cerium group.
In external appearance allanite differs from epidote, being black or dark brown in color, pitchy in lustre, opaque in the mass. The crystallographic and optical characters are similar to those of epidote. Although not a common mineral, allanite is of wide distribution as a primary accessory constituent of many crystalline rocks, granite, rhyolite and others, it was first found in the granite of east Greenland and described by Thomas Allan in 1808, after whom the species was named. Allanite is a mineral altered by hydration, becoming optically isotropic and amorphous: for this reason several varieties have been distinguished, many different names applied. Orthite was the name given by Jöns Berzelius in 1818 to a hydrated form found as slender prismatic crystals, sometimes a foot in length, at Finbo, near Falun in Sweden. Dollaseite is famous from the Ostanmossa mine in the Norberg district of Sweden; this article incorporates text from a publication now in the public domain: Leonard James. "Epidote".
In Chisholm, Hugh. Encyclopædia Britannica. 9. Cambridge University Press. P. 689. The mineral Epidote Mineral Galleries