Turquoise

Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula . It is rare and valuable in finer grades and has been prized as a gemstone and ornamental stone for thousands of years owing to its unique hue. Like most other opaque gems, turquoise has been devalued by the introduction onto the market of treatments, imitations and synthetics.
The gemstone has been known by many names. Pliny the Elder referred to the mineral as "callais" (from Ancient Greek ) and the Aztecs knew it as "chalchihuitl". The word "turquoise" dates to the 17th century and is derived from the French "turquois" meaning "Turkish" because the mineral was first brought to Europe through Turkey from mines in the historical Khorasan of Iran (Persia)..
Properties.
The finest of turquoise reaches a maximum Mohs hardness of just under 6, or slightly more than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals, and all of its properties are highly variable. X-ray diffraction testing shows its crystal system to be triclinic. With lower hardness comes lower specific gravity (2.60–2.90) and greater porosity; these properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and its transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.
The refractive index of turquoise (as measured by sodium light, 589.3 nm) is approximately 1.61 or 1.62; this is a mean value seen as a single reading on a gemological refractometer, owing to the almost invariably polycrystalline nature of turquoise. A reading of 1.61–1.65 (birefringence 0.040, biaxial positive) has been taken from rare single crystals. An absorption spectrum may also be obtained with a hand-held spectroscope, revealing a line at 432 nm and a weak band at 460 nm (this is best seen with strong reflected light). Under longwave ultraviolet light, turquoise may occasionally fluoresce green, yellow or bright blue; it is inert under shortwave ultraviolet and X-rays.
Turquoise is insoluble in all but heated hydrochloric acid. Its streak is a pale bluish white, and its fracture is conchoidal, leaving a waxy lustre. Despite its low hardness relative to other gems, turquoise takes a good polish. Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining.
Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Crystals, even at the microscopic scale, are rare. Typically the form is a vein or fracture filling, nodular, or botryoidal in habit. Stalactite forms have been reported. Turquoise may also pseudomorphously replace feldspar, apatite, other minerals, or even fossils. Odontolite is fossil bone or ivory that has historically been thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphate vivianite. Intergrowth with other secondary copper minerals such as chrysocolla is also common.
Formation.
Turquoise deposits probably form in more than one way. However, a typical turquoise deposit begins with hydrothermal deposition of copper sulfides. This takes place when hydrothermal fluids leach copper from a host rock, which is typically an intrusion of calc-alkaline rock with a moderate to high silica content that is relatively oxidized. The copper is redeposited in more concentrated form as a copper porphyry, in which veins of copper sulfide fill joints and fractures in the rock. Deposition takes place mostly in the potassic alteration zone, which is characterized by conversion of existing feldspar to potassium feldspar and deposition of quartz and micas at a temperature of
Turquoise is a secondary or supergene mineral, not present in the original copper porphyry. It forms when meteoric water (rain or snow melt infiltrating the Earth's surface) percolates through the copper porphyry. Dissolved oxygen in the water oxidizes the copper sulfides to soluble sulfates, and the acidic, copper-laden solution then reacts with aluminum and potassium minerals in the host rock to precipitate turquoise. This typically fills veins in volcanic rock or phosphate-rich sediments. Deposition usually takes place at a relatively low temperature, , and seems to occur more readily in arid environments.
Turquoise in the Sinai Peninsula is found in lower Carboniferous sandstones overlain by basalt flows and upper Carboniferous limestone. The overlying beds were presumably the source of the copper, which precipitated as turquoise in nodules, horizontal seams, or vertical joints in the sandstone beds. The classical Iranian deposits are found in sandstones and limestones of Tertiary age were intruded by apatite-rich porphyritic trachytes and mafic rock. Supergene alteration fractured the rock and converted some of the minerals in the rock to alunite, which freed aluminum and phosphate to combine with copper from oxidized copper sulfides to form turquoise. This process took place at a relatively shallow depth, and by 1965 the mines had "bottomed" at a depth averaging just below the surface.
Turquoise deposits are widespread in North America. Some deposits, such as those of Saguache and Conejos Counties in Colorado or the Cerrillos Hills in New Mexico, are typical supergene deposits formed from copper porphyries. The deposits in Cochise County, Arizona, are found in Cambrian quartzites and geologically young granites and go down at least as deep as .
Occurrence.
Turquoise was among the first gems to be mined, and many historic sites have been depleted, though some are still worked to this day. These are all small-scale operations, often seasonal owing to the limited scope and remoteness of the deposits. Most are worked by hand with little or no mechanization. However, turquoise is often recovered as a byproduct of large-scale copper mining operations, especially in the United States.
Iran.
Iran has been an important source of turquoise for at least 2,000 years. It was initially named by Iranians "pērōzah" meaning "victory", and later the Arabs called it ""fayrūzah", which is pronounced in Modern Persian as "fīrūzeh"". In Iranian architecture, the blue turquoise was used to cover the domes of palaces because its intense blue colour was also a symbol of heaven on earth.
This deposit is blue naturally and turns green when heated due to dehydration. It is restricted to a mine-riddled region in Nishapur, the mountain peak of Ali-mersai near Mashhad, the capital of Khorasan Province, Iran. Weathered and broken trachyte is host to the turquoise, which is found both "in situ" between layers of limonite and sandstone and amongst the scree at the mountain's base. These workings are the oldest known, together with those of the Sinai Peninsula. Iran also has turquoise mines in Semnan and Kerman provinces.
Sinai.
Since at least the First Dynasty (3000 BCE) in ancient Egypt, and possibly before then, turquoise was used by the Egyptians and was mined by them in the Sinai Peninsula. This region was known as the "Country of Turquoise" by the native Monitu. There are six mines in the peninsula, all on its southwest coast, covering an area of some . The two most important of these mines, from a historical perspective, are Serabit el-Khadim and Wadi Maghareh, believed to be among the oldest of known mines. The former mine is situated about 4 kilometres from an ancient temple dedicated to the deity Hathor.
The turquoise is found in sandstone that is, or was originally, overlain by basalt. Copper and iron workings are present in the area. Large-scale turquoise mining is not profitable today, but the deposits are sporadically quarried by Bedouin peoples using homemade gunpowder. In the rainy winter months, miners face a risk from flash flooding; even in the dry season, death from the collapse of the haphazardly exploited sandstone mine walls may occur. The colour of Sinai material is typically greener than that of Iranian material but is thought to be stable and fairly durable. Often referred to as "Egyptian turquoise", Sinai material is typically the most translucent, and under magnification, its surface structure is revealed to be peppered with dark blue discs not seen in material from other localities.
United States.
The Southwest United States is a significant source of turquoise; Arizona, California (San Bernardino, Imperial, Inyo counties), Colorado (Conejos, El Paso, Lake, Saguache counties), New Mexico (Eddy, Grant, Otero, Santa Fe counties) and Nevada (Clark, Elko, Esmeralda County, Eureka, Lander, Mineral County and Nye counties) are (or were) especially rich. The deposits of California and New Mexico were mined by pre-Columbian Native Americans using stone tools, some local and some from as far away as central Mexico. Cerrillos, New Mexico is thought to be the location of the oldest mines; prior to the 1920s, the state was the country's largest producer; it is more or less exhausted today. Only one mine in California, located at Apache Canyon, operates at a commercial capacity today.
The turquoise occurs as vein or seam fillings, and as compact nuggets; these are mostly small in size. While quite fine material is sometimes found, rivalling Iranian material in both colour and durability, most American turquoise is of a low grade (called "chalk turquoise"); high iron levels mean greens and yellows predominate, and a typically friable consistency in the turquoise's untreated state precludes use in jewelry.
Arizona is currently the most important producer of turquoise by value. Several mines exist in the state, two of them famous for their unique colour and quality and considered the best in the industry: the Sleeping Beauty Mine in Globe ceased turquoise mining in August 2012. The mine chose to send all ore to the crusher and to concentrate on copper production due to the rising price of copper on the world market. The price of natural untreated Sleeping Beauty turquoise has risen dramatically since the mine's closing. The Kingman Mine as of 2015 still operates alongside a copper mine outside of the city. Other mines include the Blue Bird mine, Castle Dome, and Ithaca Peak, but they are mostly inactive due to the high cost of operations and federal regulations. The Phelps Dodge Lavender Pit mine at Bisbee ceased operations in 1974 and never had a turquoise contractor. All Bisbee turquoise was "lunch pail" mined. It came out of the copper ore mine in miners' lunch pails. Morenci and Turquoise Peak are either inactive or depleted.
Nevada is the country's other major producer, with more than 120 mines which have yielded significant quantities of turquoise. Unlike elsewhere in the US, most Nevada mines have been worked primarily for their gem turquoise and very little has been recovered as a byproduct of other mining operations. Nevada turquoise is found as nuggets, fracture fillings and in breccias as the cement filling interstices between fragments. Because of the geology of the Nevada deposits, a majority of the material produced is hard and dense, being of sufficient quality that no treatment or enhancement is required. While nearly every county in the state has yielded some turquoise, the chief producers are in Lander and Esmeralda counties. Most of the turquoise deposits in Nevada occur along a wide belt of tectonic activity that coincides with the state's zone of thrust faulting. It strikes at a bearing of about 15° and extends from the northern part of Elko County, southward down to the California border southwest of Tonopah. Nevada has produced a wide diversity of colours and mixes of different matrix patterns, with turquoise from Nevada coming in various shades of blue, blue-green, and green. Some of this unusually-coloured turquoise may contain significant zinc and iron, which is the cause of the beautiful bright green to yellow-green shades. Some of the green to green-yellow shades may actually be variscite or faustite, which are secondary phosphate minerals similar in appearance to turquoise. A significant portion of the Nevada material is also noted for its often attractive brown or black limonite veining, producing what is called "spiderweb matrix". While a number of the Nevada deposits were first worked by Native Americans, the total Nevada turquoise production since the 1870s has been estimated at more than , including nearly from the Carico Lake mine. In spite of increased costs, small scale mining operations continue at a number of turquoise properties in Nevada, including the Godber, Orvil Jack and Carico Lake mines in Lander County, the Pilot Mountain Mine in Mineral County, and several properties in the Royston and Candelaria areas of Esmerelda County.
In 1912, the first deposit of distinct, single-crystal turquoise was discovered at Lynch Station in Campbell County, Virginia. The crystals, forming a druse over the mother rock, are very small; is considered large. Until the 1980s Virginia was widely thought to be the only source of distinct crystals; there are now at least 27 other localities.
In an attempt to recoup profits and meet demand, some American turquoise is treated or "enhanced" to a certain degree. These treatments include innocuous waxing and more controversial procedures, such as dyeing and impregnation (see Treatments). There are some American mines which produce materials of high enough quality that no treatment or alterations are required. Any such treatments which have been performed should be disclosed to the buyer on sale of the material.
Other sources.
Turquoise prehistoric artifacts (beads) are known since the fifth millennium BCE from sites in the Eastern Rhodopes in Bulgaria – the source for the raw material is possibly related to the nearby Spahievo lead–zinc ore field.
China has been a minor source of turquoise for 3,000 years or more. Gem-quality material, in the form of compact nodules, is found in the fractured, silicified limestone of Yunxian and Zhushan, Hubei province. Additionally, Marco Polo reported turquoise found in present-day Sichuan. Most Chinese material is exported, but a few carvings worked in a manner similar to jade exist. In Tibet, gem-quality deposits purportedly exist in the mountains of Derge and Nagari-Khorsum in the east and west of the region respectively.
Other notable localities include: Afghanistan; Australia (Victoria and Queensland); north India; northern Chile (Chuquicamata); Cornwall; Saxony; Silesia; and Turkestan.
History of use.
The pastel shades of turquoise have endeared it to many great cultures of antiquity: it has adorned the rulers of Ancient Egypt, the Aztecs (and possibly other Pre-Columbian Mesoamericans), Persia, Mesopotamia, the Indus Valley, and to some extent in ancient China since at least the Shang Dynasty. Despite being one of the oldest gems, probably first introduced to Europe (through Turkey) with other Silk Road novelties, turquoise did not become important as an ornamental stone in the West until the 14th century, following a decline in the Roman Catholic Church's influence which allowed the use of turquoise in secular jewellery. It was apparently unknown in India until the Mughal period, and unknown in Japan until the 18th century. A common belief shared by many of these civilizations held that turquoise possessed certain prophylactic qualities; it was thought to change colour with the wearer's health and protect him or her from untoward forces.
The Aztecs inlaid turquoise, together with gold, quartz, malachite, jet, jade, coral, and shells, into provocative (and presumably ceremonial) mosaic objects such as masks (some with a human skull as their base), knives, and shields. Natural resins, bitumen and wax were used to bond the turquoise to the objects' base material; this was usually wood, but bone and shell were also used. Like the Aztecs, the Pueblo, Navajo and Apache tribes cherished turquoise for its amuletic use; the latter tribe believe the stone to afford the archer dead aim. Among these peoples turquoise was used in mosaic inlay, in sculptural works, and was fashioned into toroidal beads and freeform pendants. The Ancestral Puebloans (Anasazi) of the Chaco Canyon and surrounding region are believed to have prospered greatly from their production and trading of turquoise objects. The distinctive silver jewellery produced by the Navajo and other Southwestern Native American tribes today is a rather modern development, thought to date from around 1880 as a result of European influences.
In Persia, turquoise was the "de facto" national stone for millennia, extensively used to decorate objects (from turbans to bridles), mosques, and other important buildings both inside and out, such as the Medresseh-i Shah Husein Mosque of Isfahan. The Persian style and use of turquoise was later brought to India following the establishment of the Mughal Empire there, its influence seen in high purity gold jewellery (together with ruby and diamond) and in such buildings as the Taj Mahal. Persian turquoise was often engraved with devotional words in Arabic script which was then inlaid with gold.
Cabochons of imported turquoise, along with coral, was (and still is) used extensively in the silver and gold jewellery of Tibet and Mongolia, where a greener hue is said to be preferred. Most of the pieces made today, with turquoise usually roughly polished into irregular cabochons set simply in silver, are meant for inexpensive export to Western markets and are probably not accurate representations of the original style.
The Ancient Egyptian use of turquoise stretches back as far as the First Dynasty and possibly earlier; however, probably the most well-known pieces incorporating the gem are those recovered from Tutankhamun's tomb, most notably the Pharaoh's iconic burial mask which was liberally inlaid with the stone. It also adorned rings and great sweeping necklaces called "pectorals". Set in gold, the gem was fashioned into beads, used as inlay, and often carved in a scarab motif, accompanied by carnelian, lapis lazuli, and in later pieces, coloured glass. Turquoise, associated with the goddess Hathor, was so liked by the Ancient Egyptians that it became (arguably) the first gemstone to be imitated, the fair structure created by an artificial glazed ceramic product known as faience.
The French conducted archaeological excavations of Egypt from the mid-19th century through the early 20th. These excavations, including that of Tutankhamun's tomb, created great public interest in the western world, subsequently influencing jewellery, architecture, and art of the time. Turquoise, already favoured for its pastel shades since around 1810, was a staple of Egyptian Revival pieces. In contemporary Western use, turquoise is most often encountered cut "en cabochon" in silver rings, bracelets, often in the Native American style, or as tumbled or roughly hewn beads in chunky necklaces. Lesser material may be carved into fetishes, such as those crafted by the Zuni. While strong sky blues remain superior in value, mottled green and yellowish material is popular with artisans.
Cultural associations.
In many cultures of the Old and New Worlds, this gemstone has been esteemed for thousands of years as a holy stone, a bringer of good fortune or a talisman. The oldest evidence for this claim was found in Ancient Egypt, where grave furnishings with turquoise inlay were discovered, dating from approximately 3000 BCE. In the ancient Persian Empire, the sky-blue gemstones were earlier worn round the neck or wrist as protection against unnatural death. If they changed colour, the wearer was thought to have reason to fear the approach of doom. Meanwhile, it has been discovered that the turquoise certainly can change colour, but that this is not necessarily a sign of impending danger. The change can be caused by the light, or by a chemical reaction brought about by cosmetics, dust or the acidity of the skin.
The goddess Hathor was associated with turquoise, as she was the patroness of Serabit el-Khadim, where it was mined. Her titles included "Lady of Turquoise", "Mistress of Turquoise", and "Lady of Turquoise Country".
In Western culture, turquoise is also the traditional birthstone for those born in the month of December. The turquoise is also a stone in the Jewish High Priest's breastplate, described in Exodus chapter 28. The stone is also considered sacred to the indigenous Zuni and Pueblo peoples of the American Southwest. The pre-Columbian Aztec and Maya also considered it to be a valuable and culturally important stone.
Imitations.
The Egyptians were the first to produce an artificial imitation of turquoise, in the glazed earthenware product faience. Later glass and enamel were also used, and in modern times more sophisticated porcelain, plastics, and various assembled, pressed, bonded, and sintered products (composed of various copper and aluminium compounds) have been developed: examples of the latter include "Viennese turquoise", made from precipitated aluminium phosphate coloured by copper oleate; and "neolith", a mixture of bayerite and copper(II) phosphate. Most of these products differ markedly from natural turquoise in both physical and chemical properties, but in 1972 Pierre Gilson introduced one fairly close to a true synthetic (it does differ in chemical composition owing to a binder used, meaning it is best described as a simulant rather than a synthetic). Gilson turquoise is made in both a uniform colour and with black "spiderweb matrix" veining not unlike the natural Nevada material.
The most common imitation of turquoise encountered today is dyed howlite and magnesite, both white in their natural states, and the former also having natural (and convincing) black veining similar to that of turquoise. Dyed chalcedony, jasper, and marble is less common, and much less convincing. Other natural materials occasionally confused with or used in lieu of turquoise include: variscite and faustite; chrysocolla (especially when impregnating quartz); lazulite; smithsonite; hemimorphite; wardite; and a fossil bone or tooth called odontolite or "bone turquoise", coloured blue naturally by the mineral vivianite. While rarely encountered today, odontolite was once mined in large quantities—specifically for its use as a substitute for turquoise—in southern France.
These fakes are detected by gemologists using a number of tests, relying primarily on non-destructive, close examination of surface structure under magnification; a featureless, pale blue background peppered by flecks or spots of whitish material is the typical surface appearance of natural turquoise, while manufactured imitations will appear radically different in both colour (usually a uniform dark blue) and texture (usually granular or sugary). Glass and plastic will have a much greater translucency, with bubbles or flow lines often visible just below the surface. Staining between grain boundaries may be visible in dyed imitations.
Some destructive tests may be necessary; for example, the application of diluted hydrochloric acid will cause the carbonates odontolite and magnesite to effervesce and howlite to turn green, while a heated probe may give rise to the pungent smell so indicative of plastic. Differences in specific gravity, refractive index, light absorption (as evident in a material's absorption spectrum), and other physical and optical properties are also considered as means of separation.
Treatments.
Turquoise is treated to enhance both its colour and durability (increased hardness and decreased porosity). As is so often the case with any precious stones, full disclosure about treatment is frequently not given. Gemologists can detect these treatments using a variety of testing methods, some of which are destructive, such as the use of a heated probe applied to an inconspicuous spot, which will reveal oil, wax or plastic treatment.
Waxing and oiling.
Historically, light waxing and oiling were the first treatments used in ancient times, providing a wetting effect, thereby enhancing the colour and lustre. This treatment is more or less acceptable by tradition, especially because treated turquoise is usually of a higher grade to begin with. Oiled and waxed stones are prone to "sweating" under even gentle heat or if exposed to too much sun, and they may develop a white surface film or bloom over time. (With some skill, oil and wax treatments can be restored.)
Stabilization.
Material treated with plastic or water glass is termed "bonded" or "stabilized" turquoise. This process consists of pressure impregnation of otherwise unsaleable chalky American material by epoxy and plastics (such as polystyrene) and water glass (sodium silicate) to produce a wetting effect and improve durability. Plastic and water glass treatments are far more permanent and stable than waxing and oiling, and can be applied to material too chemically or physically unstable for oil or wax to provide sufficient improvement. Conversely, stabilization and bonding are rejected by some as too radical an alteration.
The epoxy binding technique was first developed in the 1950s and has been attributed to Colbaugh Processing of Arizona, a company that still operates today. The majority of American material is now treated in this manner although it is a costly process requiring many months to complete.
Dyeing.
The use of Prussian blue and other dyes (often in conjunction with bonding treatments) to "enhance” its appearance, make uniform or completely change the colour, is regarded as fraudulent by some purists, especially since some dyes may fade or rub off on the wearer. Dyes have also been used to darken the veins of turquoise.
Reconstitution.
Perhaps the most extreme of treatments is "reconstitution", wherein fragments of fine turquoise material, too small to be used individually, are powdered and then bonded with resin to form a solid mass. Very often the material sold as "reconstituted turquoise" is artificial, with little or no natural stone, made entirely from resins and dyes. In the trade reconstituted turquoise is often called "block turquoise" or simply "block".
Backing.
Since finer turquoise is often found as thin seams, it may be glued to a base of stronger foreign material for reinforcement. These stones are termed "backed", and it is standard practice that all thinly cut turquoise in the Southwestern United States is backed. Native indigenous peoples of this region, because of their considerable use and wearing of turquoise, have found that backing increases the durability of thinly cut slabs and cabochons of turquoise. They observe that if the stone is not backed it will often crack. Early backing materials included the casings of old model T batteries, old phonograph records, and more recently epoxy steel resins. Backing of turquoise is not widely known outside of the Native American and Southwestern United States jewellery trade. Backing does not diminish the value of high quality turquoise, and indeed the process is expected for most thinly cut American commercial gemstones.
Valuation and care.
Hardness and richness of colour are two of the major factors in determining the value of turquoise; while colour is a matter of individual taste, generally speaking, the most desirable is a strong sky to robin egg blue (in reference to the eggs of the American robin). Whatever the colour, for many applications, turquoise should not be soft or chalky; even if treated, such lesser material (to which most turquoise belongs) is liable to fade or discolour over time and will not hold up to normal use in jewellery.
The mother rock or "matrix" in which turquoise is found can often be seen as splotches or a network of brown or black veins running through the stone in a netted pattern; this veining may add value to the stone if the result is complementary, but such a result is uncommon. Such material is sometimes described as "spiderweb matrix"; it is most valued in the Southwest United States and Far East, but is not highly appreciated in the Near East where unblemished and vein-free material is ideal (regardless of how complementary the veining may be). Uniformity of colour is desired, and in finished pieces the quality of workmanship is also a factor; this includes the quality of the polish and the symmetry of the stone. Calibrated stones—that is, stones adhering to standard jewellery setting measurements—may also be more sought after. Like coral and other opaque gems, turquoise is commonly sold at a price according to its physical size in millimetres rather than weight.
Turquoise is treated in many different ways, some more permanent and radical than others. Controversy exists as to whether some of these treatments should be acceptable, but one can be more or less forgiven universally: This is the "light" waxing or oiling applied to most gem turquoise to improve its colour and lustre; if the material is of high quality to begin with, very little of the wax or oil is absorbed and the turquoise therefore does not rely on this impermanent treatment for its beauty. All other factors being equal, untreated turquoise will always command a higher price. Bonded and reconstituted material is worth considerably less.
Being a phosphate mineral, turquoise is inherently fragile and sensitive to solvents; perfume and other cosmetics will attack the finish and may alter the colour of turquoise gems, as will skin oils, as will most commercial jewellery cleaning fluids. Prolonged exposure to direct sunlight may also discolour or dehydrate turquoise. Care should therefore be taken when wearing such jewels: cosmetics, including sunscreen and hair spray, should be applied before putting on turquoise jewellery, and they should not be worn to a beach or other sun-bathed environment. After use, turquoise should be gently cleaned with a soft cloth to avoid a buildup of residue, and should be stored in its own container to avoid scratching by harder gems. Turquoise can also be adversely affected if stored in an airtight container.


Whiteite

Whiteite is a rare hydrated phosphate mineral, with hydroxyl
Whiteite subgroup.
The name whiteite refers to three minerals in the jahnsite-whiteite group, whiteite subgroup. Subgroup members (formulae from IMA):
In the whiteite formulae, the symbols in brackets indicate the dominant atom in three distinct structural positions, designated X, M(1), and M(2), in that order; for instance, magnesium Mg is always the dominant atom in the M(2) position for all the whiteite minerals.
Whiteite was named after John Sampson White Jr (born 1933), associate curator of minerals at the Smithsonian Institution, and founder, editor and publisher (1970-1982) of the Mineralogical Record.
Unit cell.
All members of the series belong to the monoclinic crystal system with point group 2/m. Most sources give the space group as P21/a for the Ca Fe rich member, which was the first of the series to be described, but Dana gives it as P2/a.
The other members are variously described in different sources as having space groups P21/a, P2/a or Pa.
There are two formula units per unit cell (Z = 2). The cell parameters vary slightly between the group members, but to the nearest angstrom they all have a = 15 Å, b = 7 Å and c = 10 Å, with β 112.5 to 113.4. Individual values are:
Appearance.
The whiteite minerals are generally brown, pink or yellow, and whiteite-(CaMnMg) may also be light lavender coloured. They are transparent to translucent, with a vitreous luster and a white to brownish white streak. They occur as aggregates of tabular crystals, or thick tabular canoe-shaped crystals. Whiteite from Rapid Creek in the Yukon, Canada, is often associated with deep blue lazulite crystals (33 out of 49 photos on Mindat.org).
Optical properties.
The optical class is thought to be biaxial (+), but whiteite-(CaFeMg) may be biaxial (-). The refractive indices are Nx = 1.580, Ny = 1.584 to 1.585 and Nz = 1.590 to 1.591, similar to those for quartz.
Physical properties.
Whiteite is invariably twinned, giving the crystals a pseudo-orthorhombic appearance, and the cleavage is good to perfect.
Whiteite is quite soft, with hardness 3 to 4, between calcite and fluorite. Its specific gravity is 2.58, similar to that of quartz. Whiteite is not radioactive.
Occurrence.
The type locality for whiteite-(CaFeMg)and whiteite-(MnFeMg) is the Ilha claim, Taquaral, Itinga, Jequitinhonha valley, Minas Gerais, Brazil, and for whiteite-(CaMnMg) it is the Tip Top Mine (Tip Top pegmatite), Fourmile, Custer District, Custer County, South Dakota, US. The type material is conserved at the National School of Mines, Paris, France, and at the National Museum of Natural History, Washington DC, US, reference 123013.
At the Lavra da Ilha pegmatite, Taquaral, Brazil, whiteite is found in a complex zoned granite pegmatite associated with eosphorite, zanazziite, wardite, albite and quartz.
At Blow River, the Yukon, Canada, it is found in iron-rich sedimentary rocks with siderite, lazulite, arrojadite and quartz.
At Ilha de Taquaral, Minas Gerais, Brazil, it occurs along joints and fractures in quartz and albite associated with other phosphates.


Technetium(IV) bromide

Technetium(IV) bromide is an inorganic compound with the formula TcBr4. A brown solid, it is moderately soluble in water.
Prepearation.
Technetium tetrabromide is produced by combining the elements at elevated temperatures:
Structure.
As verified by X-ray crystallography, the compound is an inorganic polymer consisting of interconnected TcBr6 octahedra. Platinum(IV) bromide and osmium(IV) bromide adopt similar structures.


Metatungstate

In chemistry, metatungstate refers to the anion with the formula [W12O40]8- and salts derived from this anion. The term also refers to protonated derivatives of this anion, including [H2W12O40]6-. The unprotonated anion [W12O40]12- has Td symmetry.


Crimean Titan

Crimea TITAN ( ; ) is the largest manufacturer of titanium dioxide pigment in Eastern Europe. It is located in Armyansk, Crimea.
Location and geography.
The plant is located in Crimea, in the northern part on the Perekop Isthmus near the coast of the Sivash. It covers an area of and is connected by railroad to Vadim station on the Odessa railway.
History.
The decision to build the Crimean State Production Association "TITAN" (KPO "TITAN") [Russian:] was made 28 Dec 1969. In 1971 a factory producing Ammonium Phosphate was commissioned; in 1973 aluminium sulfate, and water glass; in 1974 red iron oxide pigment; and by 1978 two titanium dioxide pigment plants were commissioned.
In Mar 1999 the KPO "TITAN" became part of the 'Syvash economic zone' [Russian:] - this free economic zone was an initiative of the Government of Ukraine as an experiment. On 10 Feb 2000 the KPO "TITAN" was converted into the state joint stock company SJSC "TITAN" [Russian: ]. On 31 Aug 2004 the closed joint stock company "Crimean TITAN" [Russian: ] was formed comprising 50% less one share held by Ostchem Germany GmbH (controlled by Dmitry Firtash), the remaining majority held by SJSC "TITAN".
After the Annexation of Crimea by the Russian Federation in 2014 a company was registered in Moscow as the private limited company "Titanium Investments" [Russian: ]; whilst the name of the plant itself was changed to "Ukrainian Chemical Products" [Russian: ], with the location of registration of the company changed from Armyansk to Kiev; and with the Moscow-based company leasing the "Ukrainian Chemical Products" plant, as well as supplying and exporting products. The plant was reliant on the production of two Ukrainian mining and enriching plants at Volnogoprsk and Irshanksy - previously these were leased by the plant but in 2014 they were transferred by the Ukrainian government to the Ministry of the Economy, and transferred to the state owned "United Mining and Chemical Company" [Russian: ]. The company remained registered as a Ukrainian enterprise, paying tax on profits in Ukraine, though local taxes were also paid in Crimea - as such the company attempted to continue to receive supplies from Ukraine, avoiding sanctions.
Post-2014 the plant suffered with revenue falling by nearly three quarters (from ~$300 million in 2013 to ~$86 million in 2015). Import substitution of Ilmentite from Sri Lanka was confirmed to have begun in 2016. Other issues relating to the Ukraine-Russia-Crimea situation included a lack of water for industrial production due to restrictions on water from Ukraine to supply the North Crimean Canal. Additional wages were reported as being paid at much reduced levels. In late 2017 it was reported that the business was still suffering shortages of Ilmenite, and operating at ~40-50% capacity, and had been received ore from Norway, Sri Lanka, and Brazil.
In 2017 Russian state bank VTB began bankruptcy proceedings against Titanium Investments, claiming $40 million in unpaid debts. In reaction Dmitry Firtash sought to transfer the business to a new company "TitanActive" [Russian: ], to avoid effects on the business due to the litigation.
Places close to the titanium plant, namely Armyansk and Perekop, were (first reportedly) on 23 August 2018 hit by a noxious sulphur dioxide gas allegedly coming from the water reservoir of the plant. 4,000 children were evacuated from Armyansk and Ukrainian authorities claimed that by 10 September (2018) dozens of people had sought medical assistance in mainland Ukraine. Crimean ecologist Margarita Litvinenko claimed that gas was caused because the water reservoir of the plant did not contain enough water due to the water shortage in Crimea caused by Ukraine's decision to stop the flow from the North Crimean Canal after the Russian annexation of Crimea.
Production.
The plant has two main units («Титан-1» and «Титан-2»), each with a (design) production capacity of 40,000 tons pa.
The production of Titanium dioxide at the plant uses Ilmenite as a feedstock, using concentrated Sulfuric acid to dissolve the ground ore. Iron based impurities are reduced to Iron (II) in solution; the liquid mixture is then filtered, and Iron (II) is removed by selective crystallization and centrifugal separation on cooling of the solution. Next the Titanyl sulfate solution is evaporatively concentrated.
The Titanyl sulfate solution is hydrolyzed producing amorphous flakes of Titanium dioxide, which are then filtered and washed. Filtered Titanium dioxide is then calcined to drive off water. Further surface treatment and grinding may be applied to the Titanium dioxide for specific uses or properties of the Titanium dioxide pigment.
In addition to its primary product (~90% of exports) the plant also generates red iron oxide pigment, mineral fertilizers, sulfuric acid, aluminum sulfate, water glass, iron sulfate, and copper sulfate, and alkaline sodium and lithium chemicals.


Gadonanotube

Gadonanotube are carbon nanotubes containing superparamagnetic clusters of Gd3+ ions. They are linear molecular magnets and efficient contrast agents for magnetic resonance imaging (MRI). The term gadonanotube was introduced in 2005.
Preparation.
Owing to their large magnetic moment (µ2 = 63µB2) Gd3+ ions are the most popular MRI contrast agents. They are toxic in aqueous solutions, but can be neutralized by chelation or by encapsulating them into carbon nanotubes or fullerenes, resulting in gadonanotubes and gadofullerenes. The as-prepared nanotubes have their ends capped by carbon that hinders their filling. They are cut open by a chemical or/and ultrasonic treatment, and their side walls are chemically modified (functionalized) to impart solubility in water and biological compatibility. Then the nanotubes are sonicated in an aqueous solution of GdCl3 to form gadonanotubes, where the Gd3+ forms clusters of 2–5 nm diameter which contain up to 10 ions. The small size of the clusters is responsible for the superparamagnetism and superior MRI contrast properties of gadonanotubes. The non-encapsulated Gd is removed by repeated washing with water.
Properties and applications.
Gadonanotubes are similar to gadofullerenes in structure and functionality, but they are significantly cheaper and easier to prepare, and have superior magnetic properties. For 20–100 MHz frequencies, their relaxation rate is 3–6 times that of gadofullerenes and about 40 times that of Magnevist, a common commercial MRI contrast agent. This difference is even much larger at lower frequencies. The dispersibility of gadonanotubes in water has also been significantly increased, without the need of a surfactant, by coating the nanotube surface with polyacrylic acid (PAA) polymer. PAA coated gadonanotubes have proven to be good MRI contrast agents when dispersed in water at physiological pH with a relaxivity of 150 mM−1 ·s −1 per Gd3+ ion at 1.5 T, and have also been used to safely label porcine bone-marrow-derived mesenchymal stem cells for MRI imaging.


Nitroxyl

Nitroxyl (common name) or azanone (IUPAC name) is the chemical compound HNO. It is well known in the gas phase. Nitroxyl can be formed as a short-lived intermediate in the solution phase. The conjugate base, NO−, nitroxide anion, is the reduced form of nitric oxide (NO) and is isoelectronic with dioxygen. The bond dissociation energy of H−NO is , which is unusually weak for a bond to the hydrogen atom.
Generation.
Nitroxyl is produced from the reagents Angeli's salt (Na2N2O3) and Piloty's acid (PhSO2NHOH). Other notable studies on the production of HNO exploit cycloadducts of acyl nitroso species, which are known to decompose via hydrolysis to HNO and acyl acid. Upon photolysis these compounds release the acyl nitroso species which then further decompose.
HNO is generated via organic oxidation of cyclohexanone oxime with lead tetraacetate to form 1-nitrosocyclohexyl acetate:
This compound can be hydrolyzed under basic conditions in a phosphate buffer to HNO, acetic acid, and cyclohexanone.
Dichloramine reacts with hydroxyl ion, which is always present in water, to yield nitroxyl and the chloride ion.
Reactions.
Nitroxyl is a weak acid, with p"K"a of about 11, the conjugate base being the triplet state of NO−, sometimes called nitroxide. Nitroxyl itself, however, is a singlet ground state. Thus, deprotonation of nitroxyl uniquely involves the forbidden spin crossing from the singlet state starting material to triplet state product:
Due to the spin-forbidden nature of deprotonation, proton abstraction is many orders of magnitude slower ("k" = for deprotonation by OH−) than what one would expect for a heteroatom proton-transfer process (processes that are so fast that they are sometimes diffusion-controlled).
The "K"a of starting from or ending with the electronic excited states has also been determined. When process of deprotonating singlet state HNO to obtain singlet state NO− has a p"K"a is about 23. On the other hand, when deprotonating triplet HNO to obtain triplet NO−, the p"K"a is about −1.8.
Nitroxyl rapidly decomposes by a bimolecular pathway to nitrous oxide ("k" at 298 K = ):
The reaction proceeds via dimerization to hyponitrous acid, H2N2O2, which subsequently undergoes dehydration. Therefore, HNO is generally prepared "in situ" as described above.
Nitroxyl is very reactive towards nucleophiles, including thiols. The initial adduct rearranges to a sulfinamide:
Detection.
In biological samples, nitroxyl can be detected using fluorescent sensors, many of which are based on the reduction of Cu(II) to Cu(I) with concomitant increase in fluorescence.
Medicinal chemistry.
Nitroxyl donors, known as nitroso compounds, show potential in the treatment of heart failure and ongoing research is focused on finding new molecules for this task.


Sodium monofluorophosphate

Sodium monofluorophosphate, commonly abbreviated SMFP, is an inorganic compound with the chemical formula Na2PO3F. Typical for a salt, MFP is odourless, colourless, and water-soluble. This salt is an ingredient in some toothpastes.
Uses.
MFP is best known as an ingredient in some toothpastes. It functions as a source of fluoride via the following hydrolysis reaction:
Fluoride protects tooth enamel from attack by bacteria that cause dental caries (cavities). Although developed by a chemist at Procter and Gamble, its use in toothpaste (Colgate toothpaste and Ultra Brite) was patented by Colgate-Palmolive, as Procter and Gamble was engaged in the marketing of Crest toothpaste (containing stannous fluoride, marketed as "Fluoristan"). In the early 1980s, Crest was reformulated to use MFP, under the trademark "Fluoristat"; today Crest toothpastes use sodium fluoride or stannous fluoride. Compared to straight fluorides, sodium monofluorophosphate has slightly less aftertaste.
MFP is also used in some medications for the treatment of osteoporosis.
In 1991, sodium monofluorophosphate was found by Calgon to inhibit the dissolution of lead in drinking water when used in concentrations between 0.1 mg/L and 500 mg/L.
Tooth decay.
Tooth decay is caused by bacteria naturally present in one's mouth. These bacteria form a sticky, colorless soft film on the teeth called plaque. When foods containing carbohydrates (starches and sugars) are eaten, the bacteria that form plaque use the sugar as a form of energy. They also turn it into a glue-like substance that helps them stick to the surface of the tooth. The plaque produces acid, which attacks the enamel.
Chemistry of decay.
Tooth enamel consists mostly of calcium hydroxyphosphate, Ca5(PO4)3OH, also known as the mineral hydroxyapatite. Apatite is a hard, insoluble compound. Acid (H+), produced especially after a high-sugar meal, attacks the apatite:
Chemistry of enamel fluoridation.
The degradation of apatite by loss of OH− causes the enamel to dissolve. The process is reversible as saliva supplies back OH− to reform apatite. If fluoride, F−, ions are present in saliva, fluorapatite, Ca5(PO4)3F, also forms.
Fluorapatite resists attacks by acids better than apatite itself, so the tooth enamel resists decay better than enamel containing no fluoride.
Preparation and structure.
Sodium monofluorophosphate is produced industrially by the reaction of sodium fluoride with sodium metaphosphate:
The process involves scission of a pyrophosphate bond, analogous to hydrolysis. NaMFP can also be prepared by treating tetrasodium pyrophosphate or disodium phosphate with hydrogen fluoride.
In the laboratory, MFP can be prepared by hydrolysis of difluorophosphate ions with dilute sodium hydroxide:
Structure.
The structure of the fluorophosphate anion consists of phosphorus at the center of a tetrahedron defined by three oxygen atoms and one fluorine. Formal representations depict a double bond between one oxygen atom and phosphorus, with single bonds for the other two oxygen atoms and the fluorine. In this very formal depiction, negative charge is localized on the O atoms of the single P-O bonds. MFP is similar to and isoelectronic with Na2SO4. The anion has C3v symmetry.
Discovery and development.
Sodium monofluorophosphate was first described in 1929 by the German chemist Willy Lange, who was then with the University of Berlin. His fruitless attempts to prepare the free monofluorophosphoric acid led him to check the stability of its esters. Together with Gerda von Krüger, one of his students, Lange thus synthesized diethyl fluorophosphate and some analogs, which proved to be quite toxic, being related to nerve agents. In the 1930s, Gerhard Schrader, working for the German company IG Farben, tried to develop synthetic insecticide. His work focused on esters of phosphoric acid and resulted in an accidental discovery of some other nerve agents such as DFP (diisopropyl fluorophosphate), Tabun, Soman, and Sarin. In the meantime, Lange, who was married to a Jewish woman, emigrated from Germany to the United States and started work for Procter and Gamble Company. In 1947, he and Ralph Livingston of Monsanto Company published the preparation of the free fluorophosphoric acids and mentioned the use of some toxic esters of monofluorophosphoric acid (like DFP) in the treatment of glaucoma and myasthenia gravis. The well known toxicity of these esters led to fears that the simple salts might also be toxic, and such fears precluded any large scale commercial use of the salts. In 1950, under sponsorship of the manufacturer of the compounds, Ozark Chemical Company, the toxicity of sodium monofluorophosphate was studied by Harold Hodge at the University of Rochester who included anti-cavity testing. In 1967 Colgate-Palmolive filed several patents on the use of sodium monofluorophosphate in toothpaste.
Safety.
The usual content of MFP in toothpaste is 0.76%. The compound is used in place of sodium fluoride, particularly in children's toothpastes, because it is less acutely toxic, although both have modest toxicities. The in rats is 0.9 g/kg.


Diiodoacetylene

Diiodoacetylene is the organoiodine compound with the formula C2I2. It is a white, volatile solid that dissolves in organic solvents. It is prepared by iodination of trimethylsilylacetylene. Although samples explodes above 80 °C, diiodoacetylene is the most readily handled of the dihaloacetylenes. Dichloroacetylene, for example, is more volatile and more explosive. As confirmed by X-ray crystallography, diiodoacetylene is linear. It is however a shock, heat and friction sensitive compound.


Organoselenium chemistry

Organoselenium compounds (or seleno-organic) are chemical compounds containing carbon-to-selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity. Selenium belongs with oxygen and sulfur to the group 16 elements or chalcogens, and similarities in chemistry are to be expected. Organoselenium compounds are found at trace levels in ambient waters, soils and sediments.
Selenium can exist with oxidation state −2, +2, +4, +6. Se(II) is the dominant form in organoselenium chemistry. Down the group 16 column, the bond strength becomes increasingly weaker (234 kJ/mol for the C−Se bond and 272 kJ/mol for the C−S bond) and the bond lengths longer (C−Se 198 pm, C−S 181 pm and C−O 141 pm). Selenium compounds are more nucleophilic than the corresponding sulfur compounds and also more acidic. The p"K"a values of XH2 are 16 for oxygen, 7 for sulfur and 3.8 for selenium. In contrast to sulfoxides, the corresponding selenoxides are unstable in the presence of β-protons and this property is utilized in many organic reactions of selenium, notably in selenoxide oxidations and in selenoxide eliminations.
The first organoselenium compound to be isolated was diethyl selenide in 1836.
Organoselenium compounds in nature.
Selenium, in the form of organoselenium compounds, is an essential micronutrient whose absence from the diet causes cardiac muscle and skeletal dysfunction. Organoselenium compounds are required for cellular defense against oxidative damage and for the correct functioning of the immune system. They may also play a role in prevention of premature aging and cancer. The source of Se used in biosynthesis is selenophosphate.
Glutathione oxidase is an enzyme with a selenol at its active site. Organoselenium compounds have been found in higher plants. For example, upon analysis of garlic using the technique of high-performance liquid chromatography combined with inductively coupled plasma mass spectrometry (HPLC-ICP-MS), it was found that γ-glutamyl-"Se"-methylselenocysteine was the major Se-containing component, along with lesser amounts of "Se"-methylselenocysteine. Trace quantities of dimethyl selenide and allyl methyl selenide are found in human breath after consuming raw garlic.
Selenocysteine and selenomethionine.
Selenocysteine, called the twenty-first amino acid, is essential for ribosome-directed protein synthesis in some organisms. More than 25 selenium-containing proteins (selenoproteins) are now known. Most selenium-dependent enzymes contain selenocysteine, which is related to cysteine analog but with selenium replacing sulfur. This amino acid is encoded in a special manner by DNA.
Selenomethionine is a selenide-containing amino acid that also occurs naturally, but is generated by post-transcriptional modification.
Organoselenium chemistry in organic synthesis.
Organoselenium compounds are specialized but useful collection of reagents useful in organic synthesis, although they are generally excluded from processes useful to pharmaceuticals owing to regulatory issues. Their usefulness hinges on certain attributes, including (i) the weakness of the C−Se bond and (ii) the easy oxidation of divalent selenium compounds.
Vinylic selenides.
Vinylic selenides are organoselenium compounds that play a role in organic synthesis, especially in the development of convenient stereoselective routes to functionalized alkenes. Although various methods are mentioned for the preparation of vinylic selenides, a more useful procedure has centered on the nucleophilic or electrophilic organoselenium addition to terminal or internal alkynes. For example, the nucleophilic addition of selenophenol to alkynes affords, preferentially, the Z-vinylic selenides after longer reaction times at room temperature.The reaction is faster at a high temperature; however, the mixture of Z- and E-vinylic selenides was obtained in an almost 1:1 ratio. On the other hand, the adducts depend on the nature of the substituents at the triple bond. Conversely, vinylic selenides can be prepared by palladium-catalyzed hydroselenation of alkynes to afford the Markovnikov adduct in good yields. There are some limitations associated with the methodologies to prepare vinylic selenides illustrated above; the procedures described employ diorganoyl diselenides or selenophenol as starting materials, which are volatile and unstable and have an unpleasant odor. Also, the preparation of these compounds is complex.
Selenoxide oxidations.
Selenium dioxide is useful in organic oxidation. Specifically, SeO2 will convert an allylic methylene group into the corresponding alcohol. A number of other reagents bring about this reaction.
In terms of reaction mechanism, SeO2 and the allylic substrate react via pericyclic process beginning with an ene reaction that activates the C−H bond. The second step is a [2,3] sigmatropic reaction. Oxidations involving selenium dioxide are often carried out with catalytic amounts of the selenium compound and in presence of a sacrificial catalyst or co-oxidant such as hydrogen peroxide.
SeO2-based oxidations sometimes afford carbonyl compounds such as ketones, β-Pinene and cyclohexanone oxidation to 1,2-cyclohexanedione. Oxidation of ketones having α-methylene groups affords diketones. This type of oxidation with selenium oxide is called Riley oxidation.
Selenoxide eliminations.
In presence of a β-hydrogen, a selenide will give an elimination reaction after oxidation, to leave behind an alkene and a "SeO"-selenoperoxol. The "SeO"-selenoperoxol is highly reactive and is not isolated as such. In the elimination reaction, all five participating reaction centers are coplanar and, therefore, the reaction stereochemistry is syn. Oxidizing agents used are hydrogen peroxide, ozone or MCPBA. This reaction type is often used with ketones leading to enones. An example is acetylcyclohexanone elimination with benzeneselenylchloride and sodium hydride.
The Grieco elimination is a similar selenoxide elimination using o-nitrophenylselenocyanate and tributylphosphine to cause the elimination of the elements of H2O.
Functionalization of olefins.
In the past two decades, the functionalization of carbon-carbon double bonds with electrophilic organoselenium compounds addition have been received particular attention. The selanylation reaction is initiated by the formation of seleniranium ion intermediates from alkenes and the selenium electrophile RSeX followed by backside nucleophilic attack, leading to the anti-addition product. The attack of the nucleophile occurs on the carbon atom that has the more stable positive charge, usually the most substituted carbon atom. Addition reactions of various selenium electrophiles with alkenes have been explored in details by using internal and external nucleophiles.


Alf Gregson

Alfred Gregson (2 March 1889 – March 1968) was an English professional football inside left who played in the Football League for Grimsby Town and Bury.
Personal life.
Gregson was married with a daughter born in 1915 and a son, who died in infancy in May 1917. In 1914, he was working as a tinsmith. In February 1915, six months after the outbreak of the First World War, Gregson enlisted as a private with the Football Battalion of the Middlesex Regiment. He suffered a gunshot wound to the left thigh near Bruay-la-Buissière in April 1916. Gregson finished the war as a corporal in the 4th (Service) Battalion and was discharged from the army in March 1920.


Zinc nitrate

Zinc nitrate is an inorganic chemical compound with the formula Zn(NO3)2 . This white, crystalline salt is highly deliquescent and is typically encountered as a hexahydrate Zn(NO3)2•6H2O. It is soluble in both water and alcohol.
Synthesis and reactions.
Zinc nitrate is usually prepared by dissolving zinc in nitric acid, this reaction is concentration dependent, with a reaction in concentrated acid also forming ammonium nitrate:
On heating, it undergoes thermal decomposition to form zinc oxide, nitrogen dioxide and oxygen.
Applications.
Zinc nitrate has no large scale application but is used on a laboratory scale for the synthesis of coordination polymers. Its controlled decomposition to zinc oxide has also been used for the generation of various ZnO based structures, including nanowires.
It can be used as a mordant in dyeing. An example reaction gives a precipitate of zinc carbonate:


Terbium(III) nitrate

Terbium(III) nitrate is an inorganic chemical compound with the formula Tb(NO3)3. The hexahydrate crystallizes as triclinic colorless crystals with the formula [Tb(NO3)3(H2O)4]·2H2O. It can be used to synthesize materials with green emission.
Preparation.
Terbium(III) nitrate can be prepared by dissolving terbium(III,IV) oxide in a mixture of aqueous HNO3 and H2O2 solution.
Properties.
It reacts with NH4HCO3 to produce Tb2(CO3)3 along with its basic carbonate. It forms Tb(NO3)52- in CH3CN with excess nitrate anions.


Unoctnilium


Organozirconium chemistry

Organozirconium compounds are organometallic compounds containing a carbon to zirconium chemical bond. Organozirconium chemistry is the corresponding science exploring properties, structure, and reactivity of these compounds. Organozirconium compounds have been widely studied, in part because they are useful catalysts in Ziegler-Natta polymerization.
Comparison with organotitanium chemistry.
Many organozirconium compounds have analogues on organotitanium chemistry. Zirconium(IV) is more resistant to reduction than titanium(IV) compounds, which often convert to Ti(III) derivatives. By the same token, Zr(II) is a particularly powerful reducing agent, forming robust dinitrogen complexes. Being a larger atom, zirconium forms complexes with higher coordination numbers, e.g. polymeric [CpZrCl3]n vs monomeric CpTiCl3 (Cp = C5H5).
History.
Zirconocene dibromide was prepared in 1953 by a reaction of the cyclopentadienyl magnesium bromide and zirconium(IV) chloride. In 1966, the dihydride Cp2ZrH2 was obtained by the reaction of Cp2Zr(BH4)2 with triethylamine. In 1970, the related hydrochloride (now called Schwartz's reagent) was obtained by reduction of zirconacene dichloride (Cp2ZrCl2) with lithium aluminium hydride (or the related LiAlH(t-BuO)3). The development of organozirconium reagents was recognized by a Nobel Prize in Chemistry to Ei-Ichi Negishi.
Zirconocene chemistry.
The foremost applications of zirconocenes involve their use as catalysts for olefin polymerization.
Schwartz's reagent ([Cp2ZrHCl]2) participates in hydrozirconation, which enjoys some use in organic synthesis. Substrates for hydrozirconation are alkenes and alkynes. Terminal alkynes give vinyl complexes. Secondary reactions are nucleophilic additions, transmetalations, conjugate additions, coupling reactions, carbonylation, and halogenation.
Extensive chemistry has also been demonstrated from decamethylzirconocene dichloride, Cp*2ZrCl2. Well-studied derivatives include Cp*2ZrH2, [Cp*2Zr]2(N2)3, Cp*2Zr(CO)2, and Cp*2Zr(CH3)2.
Zirconocene dichloride can be used to cyclise enynes and dienes to give cyclic or bicyclic aliphatic systems.
Alkyl and CO complexes.
The simplest organozirconium compounds are the homoleptic alkyls. Salts of [Zr(CH3)6]2- are known. Tetrabenzylzirconium is a precursor to many catalysts for olefin polymerization. It can be converted to mixed alkyl, alkoxy, and halide derivatives, Zn(CH2C6H5)3X (X = CH3, OC2H5, Cl).
In addition to mixed Cp2Zr(CO)2, zirconium forms the binary carbonyl [Zr(CO)6]2-.
Organohafnium chemistry.
Organohafnium compounds behave nearly identically to organozirconium compounds, as hafnium, which is just below zirconium on the periodic table. Many Hf analogues of Zr compounds are known, including bis(cyclopentadienyl)hafnium(IV) dichloride, bis(cyclopentadienyl)hafnium(IV) dihydride, and dimethylbis(cyclopentadienyl)hafnium(IV).
Cationic hafnocene complexes, post-metallocene catalysts, are used on an industrial scale for the polymerization of alkenes.


Organogold chemistry

Organogold chemistry is the study of compounds containing gold–carbon bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation states for organogold compounds are I with coordination number 2 and a linear molecular geometry and III with CN = 4 and a square planar molecular geometry. The first organogold compound discovered was gold(I) carbide Au2C2, which was first prepared in 1900.
Gold(I).
Gold(I) complexes are 2-coordinate, linear, diamagnetic, 14 electron species. They typically exist as adducts LAuR with as ligand L for instance a triphenylphosphine or an isocyanide. The ligand prevents reduction of Au(I) to metallic Au(0) with dimerization of the organic residue. Gold(I) can also exist as the aurate M[AuR2] (the ate complex) whereby the cation is usually fitted with a complexing agent to improve stability. The AuR2− anion is also linear just as other M(d10) species such as Hg(Me)2 and Pd(Me)22+. Gold is known to form acetylides (capable of forming polymeric structures), carbenes and carbynes. The classic method for the preparation of LAuR compounds is by reaction of a Grignard reagent with a gold(I) halide. A subsequent reaction with an organolithium R-Li forms the ate complex.
In a special group of compounds, an aryl carbon atom acts as a bridge between two gold atoms. One such compound, (MesAu)5, is formed in a reaction between Au(CO)Cl and the mesityl Grignard. Carbon can be coordinated with gold up to a value to 6. Compounds of the type C(AuL)4 are isolobal with methane and those of type C(AuL)5+ isolobal with the methanium ion. These hypercoordinated organogold clusters are often stabilized by aurophilic interactions between the formally closed-shell gold centers.
Gold cyanide compounds (MAu(CN)2) are of some importance to gold cyanidation, a process for the extraction of gold from low-grade ore. The carbon to metal bond in metal cyanides is usually ionic but evidence exists that the C-Au bonding in the gold cyanide ion is covalent.
Gold(III).
Gold(III) complexes are 4 coordinate, square planar, diamagnetic, toxic, 16 electron species. When the formal coordination number is less than 4, ligands such as chlorine can make up for it by forming a bridging ligand. Intramolecular chelation is another strategy. In general gold(III) compounds are toxic and therefore less studied than gold(I). Monoarylgold(III) complexes are one well-studied class of complexes. They are often prepared by direct electrophilic auration of arenes by AuCl3. Homoleptic tetraalkylaurate(III) complexes (e.g. Li[AuMe4]) are also well-characterized.
Gold catalysis.
General considerations.
Gold-catalyzed reactions fall into two major categories: heterogeneous catalysis including catalysts by gold nanoparticles (e.g., Au/TiO2) and thiol-monolayer gold surfaces, and catalysts on alumina support, including alumina supported Au/CeO2. These catalysts have been investigated for industrially important processes like the oxidation of alcohols, oxidation of carbon monoxide (CO), and various selective hydrogenation reactions (e.g. butadiene to butene). Though often efficient and exhibiting useful or unique selectivities, there is considerable uncertainty with respect to the mechanism of processes catalyzed by various heterogeneous gold catalysts, even compared to other heterogeneous transition metal catalysts.
In contrast, homogeneous catalysis with gold uses simple or ligand-bound gold(I) or gold(III) compounds that are soluble in organic solvents and is used for the synthesis of fine chemicals in organic chemistry. Binary gold halides and simple complexes, including gold(I) chloride, gold(III) chloride, and chloroauric acid, have been employed as complexes. These gold sources, however, quickly give rise to ill-defined and easily deactivated (via reduction to Au0) active catalysts in solution. The development of well-defined phosphine- or NHC-ligated gold(I) complexes was an important advance and led to significant increase in interest in the synthetic applications of gold catalysis. Ligated gold(I) complexes are typically prepared and stored as the bench-stable (but unreactive) chlorides, LAuCl, e.g., chloro(triphenylphosphine)gold(I), which are typically activated via halide abstraction with silver salts like AgOTf, AgBF4, or AgSbF6 to generate a cationic gold(I) species. Although the coordinatively unsaturated complex "LAu+" is notionally generated from a LAuCl/AgX mixture, the exact nature of the cationic gold species and the role of the silver salt remains somewhat contentious. The "para"-nitrobenzoate, bistriflimide, and certain nitrile complexes represent catalytically active yet isolable silver-free precatalysts.
Cationic gold(I) forms π-complexes with alkene or alkyne bonds, following the Dewar–Chatt–Duncanson model. Gold is certainly not the only metal showing this type of bonding and reactivity, several metal ions isolobal with the simple proton (i.e., an empty s-orbital) do as well: for example, mercury(II) and platinum(II). Electrophilic ions and complexes such as these with a strong propensity to form π-complexes are generally known as pi(π)-acids (see also: cation–pi interaction).
Gold(I)-alkene and -alkyne complexes are electrophilic and susceptible toward nucleophilic attack. In oxymercuration the resultant organomercurial species is generated stoichiometrically, and requires an additional step to liberate the product. In the case of gold, protonolysis of the Au-C bond closes the catalytic cycle, allowing the coordination of another substrate. Some practical advantages of gold(I) catalysis include: 1) air stability (due to the high oxidation potential of Au(I)), 2) tolerance towards adventitious moisture (due its low oxophilicity), and 3) relatively low toxicity compared to other pi-acids (e.g., Pt(II) and Hg(II)). Chemically, Au(I) complexes typically do not undergo oxidation to higher oxidation states, and Au(I)-alkyls and -vinyls are not susceptible to β hydride elimination.
Historical development.
In 1976, Thomas and coworkers reported conversion of phenylacetylene to acetophenone using tetrachloroauric acid in a 37% yield. In this reaction gold(III) was used as a homogeneous catalyst replacing mercury in oxymercuration. This same study lists a published yield >150%, indicating catalysis that perhaps was not acknowledged by the chemists.
In 1991, Utimoto reacted gold(III) (NaAuCl4) with alkynes and water. Teles identified a major drawback of this method as Au(III) was rapidly reduced to catalytically dead metallic gold and in 1998 returned to the theme of ligand supported Au(I) for the same transformation:
This particular reaction demonstrated fantastic catalytic efficiency and would trigger a flurry of research into the use of phosphinegold(I) complexes for the activation C-C multiple bonds in the years to come. In spite of the lower stability of gold(III) complexes under catalytic conditions, simple AuCl3 was also found to be an efficient catalyst in some cases. For instance, Hashmi reported an AuCl3-catalyzed alkyne / furan Diels–Alder reaction - a type of cycloaddition that does not ordinarily occur - for the synthesis of 2,3-disubstituted phenols:
Further mechanistic studies conclude that this is not a concerted transformation, but rather an initial alkyne hydroarylation followed by a series of non-obvious intramolecular rearrangements, concluding with a 6π electrocyclization and rearomatization.
Relativistic effects are significant in organogold chemistry due to the large nuclear charge of the metal ("Z" = 79). As a consequence of relativistically expanded 5"d" orbitals, the LAu fragment can stabilize a neighboring carbocation via electron donation into the empty "p"-type orbital. Thus, in addition to their expected carbocation-like reactivity, these cations also exhibit significant carbene character, a property that has been exploited in catalytic transformations such as cyclopropanation and C-H insertion. Propargyl esters can serve as precursors for cationic gold-vinylcarbene intermediates, which can react with alkenes in a concerted manner to afford the cyclopropanation product. The use of a chiral ligand (("R")-DTBM-SEGPHOS) resulted in good to excellent levels of enantioselectivity.
Although Echavarren first reported the preparation of chiral bisphosphinedigold(I) complexes for enantioselective gold catalysis proceeding via the typical pi-activation mechanism, an early, atypical example of enantioselective catalysis by gold was described by Hayashi and Ito in 1986. In this process, benzaldehyde and methyl isocyanoacetate undergo cyclization in the presence of a chiral ferrocenylphosphine ligand and a bis(isocyanide)gold(I) complex to form a chiral oxazoline. Since oxazolines can be hydrolyzed to provide a 1,2-aminoalcohol, this reaction constitutes the first example of a catalytic, asymmetric aldol reaction.
In contrast to the other reactions described above, this reaction does not involve activation of a C-C double or triple bond by gold. In a simple mechanistic picture, gold(I) simultaneously coordinates to two phosphine ligands and the carbon isocyanate group which is then attacked by the carbonyl group. Further studies on the bonding mode of Au(I) indicate that this simple picture may have to be revised.
Heterogeneous gold catalysis is an older science. Gold is an attractive metal to use because of its stability against oxidation and its variety in morphology for instance gold cluster materials. Gold has been shown to be effective in low-temperature CO oxidation and acetylene hydrochlorination to vinyl chlorides. The exact nature of the catalytic site in this type of process is debated. The notion that gold can catalyse a reaction does not imply it is the only way. However, other metals can do the same job inexpensively, notably in recent years iron (see organoiron chemistry).
Gold catalyzed reactions.
Although of no commercial importance, gold catalyzes many organic transformations, usually carbon-carbon bond formation from Au(I), and C-X (X = O, N) bond formation from the Au(III) state, due to that ion's harder Lewis acidity. During the past decade, several studies have demonstrated that gold can efficiently catalyze C-C and C-heteroatom cross-coupling reactions that proceed through an Au(I)/Au(III) cycle. Hong C. Shen summarized homogeneous reactions forming cyclic compounds into 4 main categories:
Other reactions are the use of gold in C–H bond activation and aldol reactions. Gold also catalyses coupling reactions.
Limitations.
While gold-catalyzed hydrofunctionalization of alkynes, allenes, and allylic alcohols occurs readily under comparatively mild conditions, unactivated alkene remain poor substrates in most cases, in large part due to the resistance of the intermediate alkylgold(I) complexes to protodeauration. The development of intermolecular gold-catalyzed transformations has also lagged behind the development of intramolecular ones.


Ammonium paratungstate

Ammonium paratungstate (or APT) is a white crystalline salt with the chemical formula (NH4)10(H2W12O42)·4H2O. It is described as "the most important raw material for all
other tungsten products."
Production.
From tungsten ores.
Tungsten ores, which are typically oxides, are digested in base to give solutions of tungstate together with many contaminating species. This crude extract is acidified and treated with sulfide to separate molybdenum trisulfide. Upon further acidification APT eventually crystallizes.
Laboratory methods.
If a calcined WO3 is used, refluxing the ammonia solution is advisable to accelerate its dissolution.
Conversion to tungsten metal.
Heating ammonium paratungstate to its decomposition temperature of 600 °C yields tungsten(VI) oxide, as described in this idealized equation:
From there, the trioxide is heated in an atmosphere of hydrogen, yielding elemental tungsten:
Structure.
The anion in (NH4)10(W12O41)·5H2O has been shown to be [H2W12O42]10−, containing two hydrogen atoms, keeping two hydrogen atoms inside the cage. The correct formula notation for ammonium paratungstate is therefore (NH4)10[H2W12O42]·4H2O. The [H2W12O42]10− ion is known as the paratungstate B ion, as opposed to the paratungstate A ion, that has the formula [W7O24]6−, similar to the paramolybdate ion. The existence of the paratungstate A ion, could not be confirmed by NMR spectroscopy, however.
Before about 1930, there has been some dispute about the exact composition of the salt, and both (NH4)10W12O41 and (NH4)6W7O24 were proposed. O.W. Gibbs remarked about this:
Other hydrates.
When concentrating an ammoniacal solution of tungstic acid (i.e. hydrous WO3), the product obtained is ammonium paratungstate. Below 50 °C, the hexahydrate is formed, whereas when the temperature of the solution is above 50 °C, the pentahydrate or heptahydrate is formed. The former crystallizes as triclinic plates or prisms, whereas the latter as pseudorhombic needles. The tetrahydrate is most significant in a commercial sense. Also known:


Deuterated solvent

Deuterated solvents are a group of compounds where one or more hydrogen atoms are substituted by deuterium atoms.
These compounds are often used in Nuclear magnetic resonance spectroscopy.


Titanium(II) chloride

Titanium(II) chloride is the chemical compound with the formula TiCl2. The black solid has been studied only moderately, probably because of its high reactivity. Ti(II) is a strong reducing agent: it has a high affinity for oxygen and reacts irreversibly with water to produce H2. The usual preparation is the thermal disproportionation of TiCl3 at 500 °C. The reaction is driven by the loss of volatile TiCl4:
The method is similar to that for the conversion of VCl3 into VCl2 and VCl4.
TiCl2 crystallizes as the layered CdI2 structure. Thus, the Ti(II) centers are octahedrally coordinated to six chloride ligands.
Derivatives.
Molecular complexes are known such as TiCl2(chel)2, where chel is DMPE (CH3)2PCH2CH2P(CH3)2 and TMEDA ((CH3)2NCH2CH2N(CH3)2). Such species are prepared by reduction of related Ti(III) and Ti(IV) complexes.
Unusual electronic effects have been observed in these species: TiCl2[(CH3)2PCH2CH2P(CH3)2]2 is paramagnetic with a triplet ground state, but Ti(CH3)2[(CH3)2PCH2CH2P(CH3)2]2 is diamagnetic.
A solid-state derivative of TiCl2 is Na2TiCl4, which has been prepared by the reaction of Ti metal with TiCl3 in a NaCl flux. This species adopts a linear chain structure wherein again the Ti(II) centers are octahedral with terminal, axial halides.


Diamond enhancement

Diamond enhancements are specific treatments, performed on natural diamonds (usually those already cut and polished into gems), which are designed to improve the visual gemological characteristics of the diamond in one or more ways. These include clarity treatments such as laser drilling to remove black carbon inclusions, fracture filling to make small internal cracks less visible, color irradiation and annealing treatments to make yellow and brown diamonds a vibrant fancy color such as vivid yellow, blue, or pink.
The CIBJO and government agencies such as the United States Federal Trade Commission explicitly require the disclosure of all diamond treatments at the time of sale. Some treatments, particularly those applied to clarity, remain highly controversial within the industry—this arises from the traditional notion that diamonds hold a unique or "sacred" place among the gemstones, and should not be treated too radically, if for no other reason than a fear of damaging consumer confidence.
Clarity and color enhanced diamonds sell at lower price points when compared to similar, untreated diamonds. This is because enhanced diamonds are originally lower quality before the enhancement is performed, and therefore are priced at a substandard level. After enhancement, the diamonds may visually appear as good as their non-enhanced counterparts.
Clarity enhancements.
The clarity, or "purity", of a diamond refers to internal inclusions of the diamond, and is one of the 4-C's in determining a diamonds' value. Common "inclusions" that appear inside diamonds are black carbon spots and small cracks, commonly referred to as fractures or "feathers", due to their feathery whitish appearance when viewed from above or through the side. Diamonds may also have other inclusions such as air bubbles and mineral deposits such as iron or garnet. The size, color, and position of the inclusions are factors in determining the value of a diamond, especially when the other gemological characteristics are of a higher standard.
Laser drilling.
The development of laser drilling techniques have increased the ability to selectively target, remove and significantly reduce the visibility of black carbon inclusions on a microscopic scale. Diamonds containing hematite inclusions have been laser-drilled since the late 1960s, a technique credited to Louis Perlman that did a successful test a year after General Electric had made a similar one with a diamond for industrial use in 1962.
The laser drilling process involves the use of an infrared laser (of surgical grade at a wavelength about 1064 nm) to bore very fine holes (less than 0.2 millimeters or 0.005 inches in diameter) into a diamond to create a route of access to a black carbon crystal inclusion. Because diamond is transparent to the wavelength of the laser beam, a coating of amorphous carbon or other energy-absorbent substance is applied to the surface of the diamond to initiate the drilling process. The laser then burns a narrow tube or channel to the inclusion. Once the location of included black carbon crystal has been reached by the drill channel, the diamond is soaked in sulfuric acid. After soaking in sulfuric acid the black carbon crystal will dissolve and become transparent (colorless) and sometimes slightly whitish opaque. Under microscopic inspection the fine drill or bore holes can be seen, but are not distracting and do not affect sparkle or brilliance of the diamond. While the channels are usually straight in direction, from an entry point on the surface, some drilling techniques are drilled from within, using naturally occurring fractures inside the stone to reach the inclusion in a way that mimics organic "feathers" (This method is sometimes referred to as KM drilling which stands for special drilling in Hebrew). The channels are microscopic so that dirt or debris cannot travel down the channel. The surface-reaching holes can only be seen by reflecting light off of the surface of the diamond during microscopic viewing such as a jeweler's 10x magnifying lens or loupe and are invisible to the naked eye.
Fracture filling.
While fracture filling as a method to enhance gems has been found in gems over 2,500 years old, the diamond's unique refractive index required a more advanced filler than simple wax and oil treatments. This technology became available roughly 20 years after the time the laser drilling technique was developed. Simply put, "fracture filling" makes tiny natural fractures inside diamonds less visible to the naked eye or even under magnification.
Fractures are very common inside diamonds and are created during the diamond's creation in the earth's crust. As the rough diamond travels up from the earth's crust through volcanic pipes it comes under extreme stresses and pressures, and during this travel tiny fractures can form inside the diamond. If these fractures are visible and damaging to the beauty of the diamond, it will have much lower demand and won't be as salable to jewelers and the general public, making them candidates for fracture filling and thus visually improve the appearance of the diamond.
The fracture filling of diamond is often the last step in the process of diamond enhancement, following laser drilling and acid-etching of inclusions, though if the fractures are surface-reaching, no drilling may be required. The process involves the use of specially-formulated fillers with a refractive index approximating that of diamond. It was pioneered by Zvi Yehuda of Ramat Gan, Israel, and "Yehuda" is now used as a brand name applied to diamonds treated in this manner. Koss & Schechter, another Israel-based firm, attempted to modify Yehuda's process in the 1990s by using halogen-based glasses, but this was unsuccessful. The details behind the Yehuda process have been kept secret, but the filler used is reported to be lead oxychloride glass, which has a fairly low melting point. The New York-based Dialase also treats diamonds via a Yehuda-based process, which is believed to use lead-bismuth oxychloride glass, but research into creating better, more durable, less traceable fillers is still being done, creating more silicone-based fillers for the fracture filling process.
The filler present in fracture-filled diamonds can usually be detected by a trained gemologist under the microscope: While each diamond gets a treatment that fits its unique shape, state and fracture status, there may be traces of surface-reaching bore holes and fractures associated with drilled diamonds, air bubbles and flow lines within the glass, which are features never seen in untreated diamond.
More dramatic is the so-called "flash effect", which refers to the bright flashes of color seen when a fracture-filled diamond is rotated; the color of these flashes ranges from an electric blue or purple to an orange or yellow, depending on lighting conditions (light field and dark field, respectively). The flashes are best seen with the field of view nearly parallel to the filled fracture's plane (although specific fractures in untreated diamonds may cause similar "flash effect"). In strongly colored diamonds the flash effect may be missed if examination is less than thorough, as the stone's body color will conceal one or more of the flash colors. For example, in brown-tinted "champagne" diamonds, the orange-yellow flashes are concealed, leaving only the blue-purple flashes to be seen.
One last but important feature of fracture-filled diamonds is the color of the filler itself: It is sometimes a yellowish to brownish, and along with being visible in transmitted light, it can affect the overall color of the diamond, making the diamond fall an entire color grade after fracture-filling. For this reason fracture-filling is normally only applied to stones whose size is large enough to justify the treatment, although stones as small as 0.02 carats (4 mg) have been fracture-filled.
The fracture-filling of diamond is a controversial treatment within the industry—and increasingly among the public as well—because some companies do not disclose this process when selling these stones. It is important to note that while fracture filling is a durable process, some fillers are damaged and may even melt at certain temperatures (), causing the diamond to "sweat" the filler under the heat of a jeweler's torch; thus routine jewelry repair can lead to degradation of clarity caused by the loss of the filler used to fill the cracks, especially if the jeweler is not aware of the treatment.
Positions on certification of enhanced diamonds are polarized. On one hand some gemological laboratories, including that of the influential Gemological Institute of America, refuse to issue certificates for fracture-filled diamonds. Conversely others including European Gemological Laboratories (EGL) and Global Gem Labs (GGL) will certify such diamonds to their achieved clarity level while also stating on the certificate that the diamond is clarity enhanced.
A third type of labs may certify these diamonds to the original clarity level. This rends any treatment benefit moot by disregarding apparent clarity, and instead assigning the diamond a grade reflecting its original, pre-treatment clarity. This has raised quite a commotion, as this puts fracture-filled diamonds outside of the traditional realm of diamond certification, damaging their legitimacy as 'mostly natural' diamonds. This demand for clarity-enhanced diamond grading has caused the creation of new labs, or an update to existing lab procedures, to include remarks regarding any clarity enhancements procedures (drilling, fracture filling) into their regular reports, boosting the validity of this trade.
Color enhancements.
Generally there are three major methods to artificially alter the color of a diamond: irradiation with high-energy subatomic particles; the application of thin films or coatings; and the combined application of high pressure and high temperature (HPHT). However, there is recent evidence that fracture filling is not only used to improve clarity, but that it can be used for the sole purpose to change the color into a more desirable color as well.
The first two methods can only "modify" color, usually to turn an off-color Cape series stone (see Material properties of diamond: Composition and color) into a more desirable fancy-colored stone. Because some irradiation methods produce only a thin "skin" of color, they are applied to diamonds that are already cut and polished. Conversely, HPHT treatment is used to modify and "remove" color from either rough or cut diamonds—but only certain diamonds are treatable in this manner. Irradiation and HPHT treatments are usually permanent insofar as they will not be reversed under normal conditions of jewelry use, whereas thin films are impermanent.
Irradiation.
Sir William Crookes, a gem fancier as well as a chemist and physicist, was the first to discover radiation's effects on diamond color when in 1904 he conducted a series of experiments using radium salts. Diamonds enveloped in radium salt slowly turned a dark green; this color was found to be localized in blotchy patches, and it did not penetrate past the surface of the stone. The emission of alpha particles by the radium was responsible. Unfortunately radium treatment also left the diamond strongly radioactive, to the point of being unwearable. A diamond octahedron so treated was donated by Crookes to the British Museum in 1914, where it remains today: it has lost neither its color nor radioactivity.
Presently diamonds are safely irradiated in four ways: Proton and deuteron bombardment via cyclotrons; gamma ray bombardment via exposure to cobalt-60; neutron bombardment via the piles of nuclear reactors; and electron bombardment via van de Graaff generators. These high-energy particles physically alter the diamond's crystal lattice, knocking carbon atoms out of place and producing color centers. Irradiated diamonds are all some shade of green, black, or blue after treatment, but most are annealed to further modify their color into bright shades of yellow, orange, brown, or pink. The annealing process increases the mobility of individual carbon atoms, allowing some of the lattice defects created during irradiation to be corrected. The final color is dependent on the diamond's composition, and the temperature and length of annealing.
Cyclotroned diamonds have a green to blue-green color confined to the surface layer: they are later annealed to 800 °C to produce a yellow or orange color. They remain radioactive for only a few hours after treatment, and due to the directional nature of the treatment and the cut of the stones, the color is imparted in discrete zones. If the stone was cyclotroned through the pavilion (back), a characteristic "umbrella" of darker color will be seen through the crown (top) of the stone. If the stone was cyclotroned through the crown, a dark ring is seen around the girdle (rim). Stones treated from the side will have one half colored deeper than the other. Cyclotron treatment is now uncommon.
Gamma ray treatment is also uncommon, because although it is the safest and cheapest irradiation method, successful treatment can take several months. The color produced is a blue to blue-green which penetrates the whole stone. Such diamonds are not annealed. The blue color can sometimes approach that of natural Type IIb diamonds, but the two are distinguished by the latter's semiconductive properties. As with most irradiated diamonds, most gamma ray-treated diamonds were originally tinted yellow; the blue is usually modified by this tint, resulting in a perceptible greenish cast.
The two most common irradiation methods are neutron and electron bombardment. The former treatment produces a green to black color that penetrates the whole stone, while the latter treatment produces a blue, blue-green, or green color that only penetrates about 1 mm deep. Annealing of these stones (from 500–900 °C for neutron-bombarded stones and from 500–1200 °C for electron-bombarded stones) produces orange, yellow, brown, or pink. Blue to blue-green stones that are not annealed are separated from natural stones in the same manner as gamma ray-treated stones.
Prior to annealing, nearly all irradiated diamonds possess a characteristic absorption spectrum consisting of a fine line in the far red, at 741 nm – this is known as the GR1 line and is usually considered a strong indication of treatment. Subsequent annealing usually destroys this line, but creates several new ones; the most persistent of these is at 595 nm.
Some irradiated diamonds are completely natural. One famous example is the Dresden Green Diamond. In these natural stones the color is imparted by "radiation burns" in the form of small patches, usually only skin deep, as is the case in radium-treated diamonds. Naturally irradiated diamonds also possess the GR1 line. The largest known irradiated diamond is the Deepdene.
Coatings.
The application of colored tinfoil to the pavilion (back) surfaces of gemstones was common practice during the Georgian and Victorian era; this was the first treatment—aside from cutting and polishing—applied to diamond. Foiled diamonds are mounted in closed-back jewelry settings, which may make their detection problematic. Under magnification, areas where the foil has flaked or lifted away are often seen; moisture that has entered between the stone and foil will also cause degradation and uneven color. Because of its antique status, the presence of foiled diamonds in "older" jewelry will not detract from its value.
In modern times, more sophisticated surface coatings have been developed; these include violet-blue dyes and vacuum-sputtered films resembling the magnesium fluoride coating on camera lenses. These coatings effectively whiten the apparent color of a yellow-tinted diamond, because the two colors are complementary and act to cancel each other out. Usually only applied to the pavilion or girdle region of a diamond, these coatings are among the hardest treatments to detect—while the dyes may be removed in hot water or alcohol with ease, the vacuum-sputtered films require a dip in sulfuric acid to remove. The films can be detected under high magnification by the presence of raised areas where air bubbles are trapped, and by worn areas where the coating has been scratched off. These treatments are considered fraudulent unless disclosed.
Another coating treatment applies a thin film of synthetic diamond to the surface of a diamond simulant. This gives the simulated diamond certain characteristics of real diamond, including higher resistance to wear and scratching, higher thermal conductivity, and lower electrical conductivity. While resistance to wear is a legitimate goal of this technique, some employ it in order to make diamond simulants more difficult to detect through conventional means, which may be fraudulent if they are attempting to represent a simulated diamond as real.
High-pressure high-temperature treatment.
A small number of otherwise gem-quality stones that possess a brown body color can have their color significantly lightened or altogether removed by HPHT treatment, or, depending on the type of diamond, improve existing color to a more desirable saturation. The process was introduced by General Electric in 1999. Diamonds treated to become colorless are all Type IIa and owe their marring color to structural defects that arose during crystal growth, known as "plastic deformations", rather than to interstitial nitrogen impurities as is the case in most diamonds with brown color. HPHT treatment is believed to repair these deformations, and thus whiten the stone. (This is probably an incorrect conclusion, the whitening due to destruction of stable vacancy clusters according to one of the researchers). Type Ia diamonds, which have nitrogen impurities present in clusters that do not normally affect body color, can also have their color altered by HPHT. Some synthetic diamonds have also been given HPHT treatment to alter their optical properties and thus make them harder to differentiate from natural diamonds. Pressures of up to 70,000 atmospheres and temperatures of up to 2,000 °C (3,632 °F) are used in HPHT procedure.
Also in 1999, Novatek, a Provo, UT manufacturer of industrial diamonds known for its advancements in diamond synthesis, accidentally discovered that the color of diamonds could be changed by the HPHT process. The company formed NovaDiamond, Inc. to market the process. By applying heat and pressure to natural stones, NovaDiamond could turn brown Type I diamonds light yellow, greenish yellow, or yellowish green; improve Type IIa diamonds by several color grades, even to white; intensify the color of yellow Type I diamonds; and make some bluish gray Type I and Type IIb colorless (although in some cases natural bluish gray diamonds are more valuable left alone, as blue is a highly desired hue). In 2001, however, NovaDiamond quit the HPHT gem business because of what the company's leader, David Hall, characterized as the underhanded practices of dealers. Apparently, dealers were passing off NovaDiamond enhanced gems as naturally colored, and the company refused to be party to this deception.
Definitive identification of HPHT stones is left to well-equipped gemological laboratories, where Fourier transform spectroscopy (FTIR) and Raman spectroscopy are used to analyze the visible and infrared absorption of suspect diamonds to detect characteristic absorption lines, such as those indicative of exposure to high temperatures. Indicative features seen under the microscope include: internal graining (Type IIa); partially healed feathers; a hazy appearance; black cracks surrounding inclusions; and a beaded or frosted girdle. Diamonds treated to remove their color by General Electric are given laser inscriptions on their girdles: these inscriptions read "GE POL", with "POL" standing for Pegasus Overseas Ltd, a partnered firm. It is possible to polish this inscription away, so its absence cannot be a trusted sign of natural color. Although it is permanent, HPHT treatment should be disclosed to the buyer at the time of sale.