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| Propriedade / Aquisição: UniCEUB / Prof.ª Odette Rezende Roncador | ||||
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Usos: Usa-se a fluorita, principalmente,
como fluxo na fabricação do aço, na manufatura
de vidros opalescentes, na esmaltação de utensílios
de cozinha, para a preparação de acido fluorídrico
e, ocasionalmente, como material de ornamentação de
vasos e pratos. Usam-se pequenas quantidades de fluorita ótica
para lentes e prismas, em sistemas óticos diversos.
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Ocorrência: A fluorita é
um mineral comum e amplamente distribuído. Encontra-se, comumente,
seja como um mineral de ganga, associado a minérios de metais,
espacialmente os de chumbo e de prata. É comum em dolomitos
de calcários, encontra-se, também, como um mineral
acidental, de menor importância, em varias rochas magmáticas
e pegmatitos.associada a muitos minerais diferentes, a saber:calcita,
dolomita, gipso, celestina, barita, quartzo,esfalerita, cassiterita,
topázio, turmalina e apatita.
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No Brasil: A fluorita vem sendo produzida
no Brasil para o uso principalmente na industria siderúrgica,
para a fabricação de ferro-ligas. Encontra-se nos
estados do Ceará, Rio Grande do Norte, Paraíba, Bahia,
Espírito Santo, Minas Gerais Paraná e Santa Catarina.
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No Mundo:A fluorita encontra-se, em quantidade,
na Inglaterra. Encontra-se, comumente, nas minas da Alemanha, suíça,
do Tirol, da Tchecoslováquia e da Noruega. Os grandes produtores
de flourita comercial (espato de flúor) alem dos EUA, são
o México, Canadá e Alemanha.
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Gemas do Brasil: Tudo sobre pedras preciosas, garimpo (ouro, diamante, esmeralda, opala), feiras e cursos de Gemologia online Hotmart. Aprenda a ganhar dinheiro com gemas no país mais rico do mundo.
domingo, 21 de julho de 2013
FLUORITA
Fluorite
Fluorite
| Fluorite | |
|---|---|
Deep green isolated fluorite crystal showing cubic and octahedral
faces, set upon a micaceous matrix, from Erongo Mountain, Erongo Region,
Namibia (overall size: 50 mm x 27 mm, crystal size: 19 mm wide, 30 g)
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|
| General | |
| Category | Halide mineral |
| Formula (repeating unit) |
CaF2 |
| Strunz classification | 03.AB.25 |
| Crystal symmetry | Isometric H–M Symbol 4/m 3 2/m |
| Unit cell | a = 5.4626 Å; Z=4 |
| Identification | |
| Color | Colorless, white, purple, blue, green, yellow, orange, red, pink, brown, bluish black; commonly zoned; can be any color of the spectrum. |
| Crystal habit | Occurs as well-formed coarse sized crystals also nodular, botryoidal, rarely columnar or fibrous; granular, massive |
| Crystal system | Isometric, cF12, SpaceGroup Fm3m, No. 225 |
| Twinning | Common on {111}, interpenetrant, flattened |
| Cleavage | Octahedral, perfect on {111}, parting on {011} |
| Fracture | Subconchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 4 (defining mineral) |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.175–3.184; to 3.56 if high in rare-earth elements |
| Optical properties | Isotropic; weak anomalous anisotropism |
| Refractive index | 1.433–1.448 |
| Fusibility | 3 |
| Other characteristics | sometimes phosphoresces when heated or scratched. Other varieties fluoresce |
| References | [1][2][3][4] |
Fluorite is a colorful mineral, both in visible and ultraviolet light, and the stone has ornamental and lapidary uses. Industrially, fluorite is used as a flux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride for hydrofluoric acid manufacture, which is the intermediate source of most fluorine-containing fine chemicals. Optically clear transparent fluorite lenses have low dispersion, so lenses made from it exhibit less chromatic aberration, making them valuable in microscopes and telescopes. Fluorite optics are also usable in the far-ultraviolet range where conventional glasses are too absorbent for use.
Contents |
History and etymology
Fluorite derives from the Latin noun fluo, meaning a stream or flow of water. In verb form this was fluor or fluere, meaning to flow. The mineral is used as a flux in iron smelting to decrease the viscosity of slags. The melting point of calcium fluoride is 1676 K. The term flux comes from the Latin noun fluxus, a wash or current of water. Agricola, a German scientist with expertise in philology, mining, and metallurgy, named fluorspar as a Neo Latinization of the German Flussespar from Flusse (stream, river) and "Spar" (meaning a nonmetallic mineral akin to gypsum, spærstān, spear stone, referring to its crystalline projections).In 1852 fluorite gave its name to the phenomenon of fluorescence, which is prominent in fluorites from certain locations, due to certain impurities in the crystal. Fluorite also gave the name to its constitutive element fluorine.[2]
The mineral fluorite was originally termed fluorospar and was first discussed in print in a 1530 work Bermannus, sive de re metallica dialogus [Bermannus; or a dialogue about the nature of metals], by Georgius Agricola, as a mineral noted for its usefulness as a flux.[5][6] Presently, the word "fluorspar" is most commonly used for fluorite as the industrial and chemical commodity, while "fluorite" is used mineralogically and in most other senses.
Structure
Main article: calcium fluoride
Fluorite crystallises in a cubic motif. Crystal twinning is common and adds complexity to the observed crystal habits.Occurrence and mining
Fluorite is a widely occurring mineral which is found in large deposits in many areas. Notable deposits occur in China, Germany, Austria, Switzerland, England, Norway, Mexico, and both the Province of Ontario and Newfoundland and Labrador in Canada. Large deposits also occur in Kenya in the Kerio Valley area within the Great Rift Valley. In the United States, deposits are found in Missouri, Oklahoma, Illinois, Kentucky, Colorado, New Mexico, Arizona, Ohio, New Hampshire, New York, Alaska, and Texas. Fluorite has been the state mineral of Illinois since 1965. At that time, Illinois was the largest producer of fluorite in the United States, but the last fluorite mine in Illinois was closed in 1995.[7]
The world reserves of fluorite are estimated at 230 million tonnes (Mt) with the largest deposits being in South Africa (about 41 Mt), Mexico (32 Mt) and China (24 Mt). China is leading the world production with about 3 Mt annually (in 2010), followed by Mexico (1.0 Mt), Mongolia (0.45 Mt), Russia (0.22 Mt), South Africa (0.13 Mt), Spain (0.12 Mt) and Namibia (0.11 Mt).[8]
One of the largest deposits of fluorspar in North America is located in the Burin Peninsula, Newfoundland, Canada. The first official recognition of fluorspar in the area was recorded by geologist J.B. Jukes in 1843. He noted an occurrence of "galena" or lead ore and fluorite of lime on the west side of St. Lawrence harbour. It is recorded that interest in the commercial mining of fluorspar began in 1928 with the first ore being extracted in 1933. Eventually at Iron Springs Mine, the shafts reached depths of 970 feet (300 m). In the St. Lawrence area, the veins are persistent for great lengths and several of them have wide lenses. The area with veins of known workable size comprises about 60 square miles (160 km2).[9][10][11]
Cubic crystals up to 20 cm across have been found at Dalnegorsk, Russia.[12] The largest documented single crystal of fluorite was a cube 2.12 m in size and weighing ~16 tonnes.[13]
"Blue John"
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| Small specimen of blue fluorite from China |
Main article: Derbyshire Blue John
One of the most famous of the older-known localities of fluorite is Castleton in Derbyshire, England, where, under the name of Derbyshire Blue John, purple-blue fluorite was extracted from several mines or caves, including the famous Blue John Cavern.
During the 19th century, this attractive fluorite was mined for its
ornamental value. The mineral Blue John is now scarce, and only a few
hundred kilograms are mined each year for ornamental and lapidary use. Mining still takes place in both the Blue John Cavern and the nearby Treak Cliff Cavern.[14]Recently discovered deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone.[15]
Fluorescence
Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite.[18] Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal.[19]
The color of visible light emitted when a sample of fluorite is fluorescing is dependent on where the original specimen was collected; different impurities having been included in the crystal lattice in different places. Neither does all fluorite fluoresce equally brightly, even from the same locality. Therefore, ultraviolet light is not a reliable tool for the identification of specimens, nor for quantifying the mineral in mixtures. For example, among British fluorites, those from Northumberland, County Durham, and eastern Cumbria are the most consistently fluorescent, whereas fluorite from Yorkshire, Derbyshire, and Cornwall, if they fluoresce at all, are generally only feebly fluorescent.
Fluorite also exhibits the property of thermoluminescence.[20]
Color
Uses
Source of fluorine and fluoride
Fluorite is a major source of hydrogen fluoride, a commodity chemical used to produce a wide range of materials. Hydrogen fluoride is liberated from the mineral by the action of concentrated sulfuric acid:The resulting HF is converted into fluorine, fluorocarbons, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.[21]
There are three principal types of industrial use for natural fluorite, commonly referred to as "fluorspar" in these industries, corresponding to different grades of purity. Metallurgical grade fluorite (60–85% CaF2), the lowest of the three grades, has traditionally been used as a flux to lower the melting point of raw materials in steel production to aid the removal of impurities, and later in the production of aluminium. Ceramic grade fluorite (85–95% CaF2) is used in the manufacture of opalescent glass, enamels and cooking utensils. The highest grade, "acid grade fluorite" (97% or more CaF2), accounts for about 95% of fluorite consumption in the US where it is used to make hydrogen fluoride and hydrofluoric acid by reacting the fluorite with sulfuric acid.[22]
Internationally, acid-grade fluorite is also used in the production of AlF3 and cryolite (Na3AlF6), which are the main fluorine compounds used in aluminium smelting. Alumina is dissolved in a bath that consists primarily of molten Na3AlF6, AlF3, and fluorite (CaF2) to allow electrolytic recovery of aluminium. Fluorine losses are replaced entirely by the addition of AlF3, the majority of which will react with excess sodium from the alumina to form Na3AlF6.[22]
Niche uses
Lapidary uses
Natural fluorite mineral has ornamental and lapidary uses. Fluorite may be drilled into beads and used in jewelry, although due to its relative softness it is not widely used as a semiprecious stone.Laboratory applications
In the laboratory, calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits extremely low change in refractive index with wavelength. Furthermore the material is attacked by few reagents. At wavelengths as short as 157 nm, a common wavelength used for semiconductor stepper manufacture for integrated circuit lithography, the refractive index of calcium fluoride shows some non-linearity at high power densities which has inhibited its use for this purpose. In the early years of the 21st century the stepper market for calcium fluoride collapsed and many large manufacturing facilities have been closed. Canon and other manufacturers have used synthetically grown crystals of calcium fluoride components in lenses to aid apochromatic design, and to reduce light dispersion. This use has largely been superseded by newer glasses and computer aided design. As an infrared optical material, calcium fluoride is widely available and was sometimes known by the Eastman Kodak trademarked name "Irtran-3," although this designation is obsolete.Optics
Fluorite has a very low dispersion, so lenses made from it exhibit less chromatic aberration than those made of ordinary glass.[23] However, naturally-occurring fluorite crystals without optical defects were only large enough to produce microscope elements.With the advent of synthetically-grown fluorite (calcium fluoride crystal), it could be used instead of glass in some high-performance telescopes and camera lens elements. Its use for prisms and lenses was studied and promoted by Victor Schumann near the end of the 19th century.[24]
In telescopes, fluorite elements allow high-resolution images of astronomical objects at high magnifications. Canon Inc. produces synthetic fluorite crystals that are used in their more expensive telephoto lenses.
Exposure tools for the semiconductor industry make use of fluorite optical elements for ultraviolet light at wavelengths of about 157 nanometers. Fluorite has a uniquely high transparency at this wavelength. Fluorite objective lenses are manufactured by the larger microscope firms (Nikon, Olympus, Carl Zeiss and Leica). Their transparence to ultraviolet light enables them to be used for fluorescence microscopy.[25] The fluorite also serves to correct optical aberrations in these lenses. Nikon has previously manufactured at least one all-fluorite element camera lens (105 mm f/4.5 UV) for the production of ultraviolet images.[26] Konica Produced a flourite lens for their SLR cameras - the Hexanon 300mm f6.3.
Source of fluorine gas in nature
In 2012, the first source of naturally occurring fluorine gas was found in fluorite mines in Bavaria, Germany. It was previously thought that fluorine gas did not occur naturally because it is so reactive and would rapidly react with other chemicals.[27] Fluorite is normally colourless but some varied forms found nearby look black and are known as 'fetid fluorite' or antozonite. The minerals, containing small amounts of uranium and its daughter products, release radiation sufficiently energetic to induce oxidation of fluoride anions within the structure to fluorine that becomes trapped inside the mineral. The colour of fetid fluorite is predominantly due to the calcium atoms remaining. Solid state fluorine-19 NMR was carried out on the gas escaping the antozonite revealed a peak at 425 ppm, which is consistent with F2.[28]See also
- List of minerals
- Magnesium fluoride – also used in UV optics
Brazilianite
Brazilianite
| Brazilianite | |
|---|---|
 Brazilianite from type locality, Conselheiro Pena, Minas Gerais, Brazil |
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| General | |
| Category | Phosphate minerals |
| Formula (repeating unit) |
NaAl3(PO4)2(OH)4 sodium aluminum phosphate hydroxide |
| Strunz classification | 08.BK.05 |
| Identification | |
| Color | Yellow, green, colorless |
| Crystal habit | Prismatic; Spherical druses |
| Crystal system | Monoclinic |
| Cleavage | [010] Good |
| Fracture | Conchoidal |
| Mohs scale hardness | 5.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.98 |
| Refractive index | 1.60 - 1.62 |
| Birefringence | 0.021 |
It occurs in the form of perfect crystals grouped in druses, in pegmatites, and is often of precious-stone quality. The only noted deposit of brazilianite is in the surroundings of Conselheiro Pena, in the state of Minas Gerais in Brazil. During the past few years this deposit has yielded a great quantity of beautiful raw material, which has included crystals of surprisingly large dimensions and perfectly bounded crystal faces.
Some of these are found on leaves of muscovite with their strong silvery glitter, ingrown in their parent rock. Such specimens are not ground, but find their way into museums and private collections. The most exquisite crystals, dark greenish-yellow to olive-green, sometimes measure up to 12 cm in length and 8 cm in width. Crystals of similar shape and dimensions have discovered in another deposit in Minas Gerais, near Mantena, but they lack the perfection of the crystal bounding. Brazilianites have also been discovered in many large collections; they originated from the Palermo mine and the Charles Davis mine in Grafton County, New Hampshire, USA.
References
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Wikimedia Commons has media related to: Brazilianite |
Azurite
Azurite
| Azurite | |
|---|---|
Azurite from China with large crystals and light surface weathering.
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| General | |
| Category | Carbonate mineral |
| Formula (repeating unit) |
Cu3(CO3)2(OH)2 |
| Strunz classification | 05.BA.05 |
| Crystal symmetry | Monoclinic 2/m |
| Unit cell | a = 5.01 Å, b = 5.85 Å, c = 10.35 Å; β = 92.43°; Z=2 |
| Identification | |
| Formula mass | 344.67 g/mol |
| Color | Azure-blue, Berlin blue, very dark to pale blue; pale blue in transmitted light |
| Crystal habit | Massive, prismatic, stalactitic, tabular |
| Crystal system | Monoclinic Prismatic |
| Twinning | Rare, twin planes {101}, {102} or {001} |
| Cleavage | Perfect on {011}, fair on {100}, poor on {110} |
| Fracture | Conchoidal |
| Tenacity | brittle |
| Mohs scale hardness | 3.5 to 4 |
| Luster | Vitreous |
| Streak | Light Blue |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.773 (measured), 3.78 (calculated) |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.730 nβ = 1.758 nγ = 1.838 |
| Birefringence | δ = 0.108 |
| Pleochroism | Visible shades of blue |
| 2V angle | Measured: 68°, calculated: 64° |
| Dispersion | relatively weak |
| References | [1][2][3] |
Contents
Mineralogy
Azurite crystals are monoclinic, and when large enough to be seen they appear as dark blue prismatic crystals.[2][3][5] Azurite specimens are typically massive to nodular, and are often stalactitic in form. Specimens tend to lighten in color over time due to weathering of the specimen surface into malachite. Azurite is soft, with a Mohs hardness of only 3.5 to 4. The specific gravity of azurite is 3.77 to 3.89. Azurite is destroyed by heat, losing carbon dioxide and water to form black, powdery copper(II) oxide. Characteristic of a carbonate, specimens effervesce upon treatment with hydrochloric acid.
Color
The optical properties (color, intensity) of minerals such as azurite and malachite are explained in the context of conventional electronic spectroscopy of coordination complexes. Relatively detailed descriptions are provided by ligand field theory.Weathering
Azurite is unstable in open air with respect to malachite, and often is pseudomorphically replaced by malachite. This weathering process involves the replacement of some the carbon dioxide (CO2) units with water (H2O), changing the carbonate:hydroxide ratio of azurite from 1:1 to the 1:2 ratio of malachite:- 2 Cu3(CO3)2(OH)2 + H2O → 3 Cu2(CO3)(OH)2 + CO2
Uses
Pigments
Azurite was used as a blue pigment for centuries. Depending on the degree of fineness to which it was ground, and its basic content of copper carbonate, it gave a wide range of blues. It has been known as mountain blue or Armenian stone, in addition it was formerly known as Azurro Della Magna (from Italian). When mixed with oil it turns slightly green. When mixed with egg yolk it turns green-grey. It is also known by the names Blue Bice and Blue Verditer, though Verditer usually refers to a pigment made by chemical process. Older examples of azurite pigment may show a more greenish tint due to weathering into malachite. Much azurite was mislabeled lapis lazuli, a term applied to many blue pigments. As chemical analysis of paintings from the Middle Ages improves, azurite is being recognized as a major source of the blues used by medieval painters. True lapis lazuli was chiefly supplied from Afghanistan during the Middle Ages while azurite was a common mineral in Europe at the time. Sizable deposits were found near Lyons, France. It was mined since the 12th century in Saxony, in the silver mines located there.[6]Heating can be used to distinguish azurite from purified natural ultramarine blue, a similar but much more expensive pigment, as described by Cennino D'Andrea Cennini. Ultramarine withstands heat, but azurite turns to black copper oxide. However, gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.
Jewelry
Azurite is used occasionally as beads and as jewelry, and also as an ornamental stone. However, its softness and tendency to lose its deep blue color as it weathers limit such uses. Heating destroys azurite easily, so all mounting of azurite specimens must be done at room temperature.Collecting
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| Small specimen of Azurite from China. |
Prospecting
While not a major ore of copper itself, the presence of azurite is a good surface indicator of the presence of weathered copper sulfide ores. It is usually found in association with the chemically very similar malachite, producing a striking color combination of deep blue and bright green that is strongly indicative of the presence of copper ores.History
The use of azurite and malachite as copper ore indicators led indirectly to the name of the element nickel in the English language. Nickeline, a principal ore of nickel that is also known as niccolite, weathers at the surface into a green mineral (annabergite) that resembles malachite. This resemblance resulted in occasional attempts to smelt nickeline in the belief that it was copper ore, but such attempts always ended in failure due to high smelting temperatures needed to reduce nickel. In Germany this deceptive mineral came to be known as kupfernickel, literally "copper demon". The Swedish alchemist Baron Axel Fredrik Cronstedt (who had been trained by Georg Brandt, the discoverer of the nickel-like metal cobalt) realized that there was probably a new metal hiding within the kupfernickel ore, and in 1751 he succeeded in smelting kupfernickel to produce a previously unknown (except in certain meteorites) silvery white, iron-like metal. Logically, Cronstedt named his new metal after the nickel part of kupfernickel. An unintended later consequence of his choice is that both Canadian and American coins worth one-twentieth of a dollar are now named after a German term for "kobolds"—that is, they are called nickels.Kimberlite
Kimberlite
Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes as well as igneous dykes and sills. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), potentially from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.
Kimberlite has attracted more attention than its relative volume might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide information about the composition of the deep mantle and about melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.
Contents |
Morphology and volcanology
Many kimberlites are emplaced as carrot-shaped, vertical intrusions termed 'pipes'. This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma which inherits a large proportion of both CO2 and H2O in the system, which produces a deep explosive boiling stage that causes a significant amount of vertical flaring (Bergman, 1987). Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement and Skinner 1985, and Clement, 1982).The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle-derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within 1.5–2 km (0.93–1.2 mi) of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved, but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer (up to 0.6 mile).
Two Jurassic kimberlite dikes exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners.[1]
Petrology
Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source.Historically, kimberlites have been subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who renamed these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that Group II kimberlites actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion.
Group I kimberlites
Group-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic (0.5–10 mm or 0.020–0.39 in) to megacrystic (10–200 mm or 0.39–7.9 in) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite, in a fine to medium grained groundmass.The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.
Group II kimberlites
Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite); Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite); potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite.
Kimberlitic indicator minerals
Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (a pyroxene), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium, are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.These indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence may indicate the presence of a kimberlite within the erosional watershed which produced the alluvium.
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