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Maret 25, 2011

Nitrogen [N]

Maret 25, 2011 0

Characteristics

An: 7 N: 7
Am: 14.0067 g/mol
Group No: 15
Group Name: Pnictogen
Block: p-block Period: 2
State: gas at 298 K
Colour: colourless Classification: Non-metallic
Boiling Point: 77.36K (-195.79oC)
Melting Point: 63.05K (-210.1oC)
Critical temperature: 126.2K (-146.9oC)
Density: 1.251g/l

Discovery Information

Who: Daniel Rutherford
When: 1772
Where: Scotland/Sweden

Name Origin

Latin nitrogenium, where nitrum (from Greek nitron) means "native soda", and genes means "forming". "Nitrogen" in different languages.

Sources

Nitrogen can be made by liquification and then fractional distillation of the air. It is very easily done commercially. It can also be made by heating NaN3 to 300 degrees C.
Around 44 million tons are produced annually.

Abundance

Universe: 1000 ppm (by weight)
Sun: 1000 ppm (by weight)
Carbonaceous meteorite: 1400 ppm
Earth’s Crust: 25 ppm
Seawater: Atlantic surface: 8 x 10-5 ppm; Atlantic deep: 2.7 x 10-1 ppm; Pacific surface: 8 x 10-5 ppm; Pacific deep: 5.4 x 10-1 ppm
Human: 2.6 x 107 ppb by weight; 1.2 x 107 ppb by atoms

Uses

Nitrogen has many industrial uses in the gaseous forms, but probably the most interesting is liquid nitrogen, which is extremely cold. Items that must be frozen to extremely low temperatures for preservation are frequently stored in liquid nitrogen. Fertility clinics store sperm, eggs and embryos used to help infertile couples become pregnant in ampoules in liquid nitrogen. Since nitrogen gas is very stable, at standard temperature and pressure, it is used as the air in inert welding atmospheres. Documents, foods and chemicals are sometimes stored in nitrogen to keep them from oxidizing or reacting with air or water.

Notes

Nitrogen is the largest single component of the Earth’s atmosphere (78.084% by volume, 75.5% by weight).
Nitrogen in the elemental form was considered to be inert and was even named ozote which refers to the fact that it is not reactive. Of course nitrogen does form compounds, but the gaseous form consists of diamers (2 nitrogens bonded together). The diamer is very stable.
Nitrogen is a major element in organic compounds, especially proteins. Some nitrogen compounds are highly reactive. Trinitrotoluene is TNT or dynamite. Ammonium Nitrate is a fertilizer, but was used as the major explosive ingredient in the Oklahoma City bombing. Anfo, or Ammonium Nitrate and fuel oil mixture is the primary explosive used in the mining industry because it is inexpensive, easy to manufacture and can be easily manufactured near the mine site thus reducing the risks and expenses related to the transportation of explosives. Nitrates, Nitrites and Azides (all nitrogen compounds are either oxidizers or reactives and will react violently under the right conditions.
The triple bond in molecular nitrogen (N2) is one of the strongest in nature. The resulting difficulty of converting (N2) into other compounds, and the ease (and associated high energy release) of converting nitrogen compounds into elemental N2, have dominated the role of nitrogen in both nature and human economic activities.

Hazards

Rapid release of nitrogen gas into an enclosed space can displace oxygen, and therefore represents an asphyxiation hazard. Nitrogen also dissolves in the bloodstream, and rapid decompression (particularly in the case of divers ascending too quickly) can lead to a potentially fatal condition called decompression sickness, when nitrogen bubbles form in the bloodstream. It can also cause nitrogen narcosis.


Nitrogen Compounds

Chloramine NH2Cl
Commonly used in low concentrations as a disinfectant in municipal water systems as an alternative to chlorination. Water treated with chloramine lacks the distinct chlorine odour of the gaseous treatment and so has improved taste.
Aquarium owners must remove the chloramine from their tap water because it is toxic to fish. Many animals are sensitive to chloramine and it must be removed from water given to many animals in zoos.
Dinitrogen tetroxide N2O4
Dinitrogen tetroxide is one of the most important rocket propellants ever developed and by the late 1950s it became the storable oxidizer of choice for rockets in both the USA and USSR.
Nitric oxide NO : Irritant :
It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known. It is also a toxic air pollutant produced by automobile engines and power plants.
The nitric oxide molecule is a free radical, which makes it very reactive and unstable. In air, it quickly reacts with oxygen to form the poisonous nitrogen dioxide (NO2).
Nitric dioxide NO2 : Highly Toxic :
It is one of several nitrogen oxides (Nox). This orange/brown gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants and a poison by inhalation.
Nitrogen trichloride NCl3 : Explosive :
A yellow, oily, pungent-smelling liquid, often found as a byproduct of chemical reactions between nitrogen-containing compounds and chlorine.
Nitrogen trichloride is a very strong explosive; an explosion involving it blinded Sir Humphry Davy temporarily, and forced him to take on Michael Faraday as a worker. This was in part because some of Davy’s junior lab workers had recently been fired for fighting. Nitrogen trichloride is incredibly sensitive; it will explode upon exposure to cold or hot temperatures, sunlight, or organic substances such as turpentine.
Nitrogen triiodide NI3
Also called nitrogen iodide, is a highly explosive compound of nitrogen and iodine. It is a contact explosive, and small quantities explode with a gunpowder-like snap when touched by even a feather, releasing a volatile cloud of iodine vapour.
Small amounts of nitrogen triiodide are sometimes synthesized as a demonstration to chemistry students. However, because the compound is so unstable, it has not been used in blasting caps or primers for explosives.
The reason for it’s instability is due to the size difference between the two different types of atoms. The three iodine atoms are much bigger than the nitrogen atom holding them together. Because of this, not only is the bond between nuclei under much stress and therefore weaker, but the outside electrons of the different iodine atoms are very close, which increases the overall instability of the molecule.
Nitrous oxide N2O
Under room conditions, it is a colourless non-flammable gas, with a pleasant, slightly-sweet odour. It is used in surgery and dentistry for its anaesthetic and analgesic effects, where it is commonly known as laughing gas due to the euphoric effects of inhaling it. It is also used as an oxidizer in internal combustion engines. Nitrous oxide is present in the atmosphere where it acts as a powerful greenhouse gas.
The gas was discovered by Joseph Priestley in 1772, who called it phlogisticated nitrous air (see phlogiston). Humphry Davy in the 1790s tested the gas on himself and some of his friends, including the poets Samuel Taylor Coleridge and Robert Southey. They soon realised that nitrous oxide considerably dulled the sensation of pain, even if the inhaler were still semi-conscious. And so it came into use as an anaesthetic, particularly by dentists, who do not typically have access to the services of an anesthesiologist and who may benefit from a patient who can respond to verbal commands.
Urea phosphate CO(NH2)2H3PO4
It is made by reacting urea with phosphoric acid (H3PO4). It is sometimes used as a fertilizer.
Nitric acid HNO3
Nitric acid, otherwise known as aqua fortis or spirit of nitre, is a colourless, corrosive liquid, a toxic acid which can cause severe burns. If the solution contains more than 86% nitric acid, it is referred to as fuming nitric acid, and can be separated into two kinds of fuming acids, white fuming nitric acid and red fuming nitric acid.
Commonly used as a laboratory reagent, nitric acid is used in the manufacture of explosives such as nitroglycerin, trinitrotoluene (TNT) and Cyclotrimethylenetrinitramine (RDX), as well as fertilizers such as ammonium nitrate.
It has additional uses in metallurgy and refining as it reacts with most metals, and in organic syntheses. When combined with hydrochloric acid, it forms aqua regia, one of the few reagents capable of dissolving gold and platinum.

Reactions of Nitrogen

Under normal conditions nitrogen will not react with air, water, halogens, acids or bases.

Occurrence of Nitrogen

Nitrogen is the largest single component of the Earth’s atmosphere (78.082% by volume of dry air, 75.3% by weight in dry air).
14N is created as part of the fusion processes in stars, and is estimated to be the 7th most abundant chemical element (by mass) in our universe.
Compounds that contain this element have been observed by astronomers, and molecular nitrogen has been detected in interstellar space by David Knauth and coworkers using the Far Ultraviolet Spectroscopic Explorer. Molecular nitrogen is a major constituent of Titan’s thick atmosphere, and occurs in trace amounts of other planetary atmospheres.
Nitrogen is present in all living tissues as proteins, nucleic acids and other molecules. It is a large component of animal waste (for example, guano), usually in the form of urea, uric acid, and compounds of these nitrogenous products.

Isotopes of Nitrogen

13N [6 neutrons]
Abundance: synthetic
Half life: 9.965 minutes [ Electron Capture ]
Decay Energy: 2.220 MeV
Decays to 13C.
14N [7 neutrons]
Abundance: 99.634%
Stable with 7 neutrons
15N [8 neutrons]
Abundance: 0.366%
Stable with 8 neutrons
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Rino Safrizal
Jejaring Kimia Updated at: Maret 25, 2011

Maret 23, 2011

PYROLUSITE [ Oxides and Hydroxides : Rutile ]

Maret 23, 2011 0
  • MnO2, manganese Oxide
  • A major ore of manganese and as a mineral specimen
Pyrolusite is the most common manganese mineral and is an important ore. Manganese is a strategically valuable metal since it is an essential ingredient in steel and other alloys. The mining term "wad" is used to indicate ores that are a mixture of several manganese oxides such as pyrolusite, psilomelane and others that are difficult to distinguish.
Pyrolusite is an oxidation product of weathered manganese minerals and also forms from stagnant shallow marine and freshwater bog and swamp deposits. Minerals such as rhodochrosite, rhodonite and hausmannite are often replaced by pyrolusite.
Pyrolusite has some interesting habits dispite its common occurrence as dull, sooty, black masses and/or earthy forms. Possibly its most popular form is its dendritic habit that forms wonderfully detailed, fern-like patterns on the surfaces of rocks such as sandstone. These dendrites are so amazing that they have often been mistaken for fossil plants. Another popular habit is its acicular or hair-like crystal aggregates that produce nice tufts of "hair", or meadows of shiny black pyrolusite fibers. Often specimens of pyrolusite are very difficult to distinguish from other manganese oxides. Thus, as a consequence of its more abundant distribution, pyrolusite is the default name for black, hair-like manganese crystals or powdery black alteration products of manganese minerals in general.

Physical Characteristics

Colour: steel gray to a solid black in earthy specimens
Luster: metallic to dull in weather or thinly crusted specimens
Transparency: crystals are opaque, translucent in only thin splinters
Crystal System: tetragonal; 4/m 2/m 2/m
Crystal Habits: typically massive and compact forms, but also fibrous, acicular, columnar, concretionary, scaly and earthy forms are well known. A thin dendritic habit is commonly seen encrusted on sandstones and siltstones and will form wonderful fern or tree like patterns that are often mistaken for fossil plants. A variety with large, easily visible crystals is called polianite, and occurs as minute prismatic crystals with a square or rectangular cross-section and a wedge-shaped terminations
Cleavage: good in two directions forming prisms, but rarely seen except in rare large crystals
Fracture: conchoidal to uneven
Hardness: 6 in individual crystals, but aggregates can be as soft as 4 or 5 and massive or earthy forms will mark paper and leave powder on fingers (a hardness under 2)
Specific Gravity: 4.4 - 5.1 (average for metallic minerals)
Streak: black
Associated Minerals: limonite, hematite, quartz, manganite, psilomelane and other manganese and iron oxide minerals
Major Occurrences: include nice specimens from Germany; iron mines in Minnesota and Michigan and at Lake County, New Mexico, USA. Pyrolusite is mined in many countries around the world with the most productive countries being Georgia and Ukraine of the former USSR, India, China, South Africa, Brazil, Australia and Gabon. Polianite occurs in abundance at the Kisenge Mine, in Zaire
Best Indicators: habits, luster, softness, colour and streak
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Rino Safrizal
Jejaring Kimia Updated at: Maret 23, 2011

COBALTITE [ Sulfides and Sulfosalts : Cobaltite ]

Maret 23, 2011 0
(Co, Fe)AsS, cobalt arsenic Sulfide
Important ore of cobalt and as mineral specimens
Cobaltite although rare is still an important and valuable ore of cobalt, a strategically and industrially useful metal. The symmetry of cobaltite is somewhat in dispute. Its structure is very similar to the structure of pyrite, FeS2. The sulfur to sulfur link (S-S) in pyrite is replaced by an arsenic to sulfur link (As-S) in cobaltite. If the position of the arsenic is not ordered then the symmetry is the same as pyrite’s symmetry which is in the isometric class, 2/m bar 3. However it appears from some x-ray spectroscopy studies that the arsenic is ordered there by breaking the higher symmetry and giving cobaltite a symmetry of the orthorhombic class, 2/m 2/m 2/m. But the debate is not settled yet.
Regardless of its actual symmetry, cobaltite forms isometric looking crystals. Either from really being isometric or from simply having such a similar structure to pyrite, cobaltite’s crystals mimic those of pyrite. Although the crystal habits are similar to pyrite, cobaltite can not be confused with pyrite which is brassy yellow in contrast to cobaltite’s silver gray or white colour. Skutterudite on the other hand is also white and forms similar crystals although it has poor cleavage.
Often deposits of cobaltite will have a weathering crust of minerals such as erythrite, Co3(AsO4)2-8(H2O). Since cobalt is a strong colouring metal, minerals like erythrite are strongly coloured, in this case a pink to bright purple. Miners called these colourful minerals "cobalt blooms" and used them as indicators of the presence of cobalt ores, such as cobaltite. Good crystals are usually common when cobaltite deposits are found and are a treasure for collectors.

Physical Characteristics

Colour: white to silver gray
Luster: metallic
Transparency: crystals are opaque
Crystal System: it has been described as isometric; 2/m bar 3, but its actual structure is perhaps orthorhombic; m m 2 although the last word has not been said on this subject
Crystal Habits: include cubes, octahedrons, pyritohedrons and combinations of these isometric forms. If cobaltite is actually orthorhombic than these forms are either pseudocubes etc or they are pseudomorphs from a truly isometric phase which existed at higher temperature and/or pressure. Cobaltite is also commonly massive and granular
Cleavage: distinct in three directions forming cubes
Fracture: uneven to subconchoidal
Hardness: 5.5
Specific Gravity: approx. 6.0 - 6.4+ (heavier than average for metallic minerals)
Streak: dark gray
Other: Striations on cube faces
Associated Minerals: silver, chalcopyrite, pyrite, erythrite, skutterudite and other cobalt minerals
Major Occurrences: include cobalt, Ontario, Canada; Zaire; Siegerland, Germany; Skutterud, Norway; Tunaberg, Sweden; Sonora, Mexico; England; Boulder, Colorado and other USA localities
Best Indicators: crystal habit, cleavage, colour, streak, association with erythrite and luster
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Rino Safrizal
Jejaring Kimia Updated at: Maret 23, 2011

SIDERITE [ Carbonates : Calcite ]

Maret 23, 2011 0
FeCO3, iron Carbonate
A minor ore of iron and as mineral specimens
Siderite is named for the Greek word for iron, sideros. The word siderite is used in a number of rarer minerals and except in the fact that they all contain iron, they are otherwise unrelated to siderite. They include: Alumopharmacosiderite, arseniosiderite, barium-alumopharmacosiderite, barium-pharmacosiderite, chalcosiderite, erythrosiderite, pharmacosiderite, phosphosiderite and sodiumpharmacosiderite. A variety of siderite forming sphericules is known as sphaerosiderite.
Siderite forms series with the closely related minerals rhodochrosite, MnCO3 and magnesite, MgCO3. These minerals can have some substitution of their metal ions and the cutoff between them and siderite is at 50% iron. Sometimes variety names are given to intermediate members of these minerals such as mangansiderite for an iron rich rhodochrosite or magniosiderite for a magnesium rich siderite. But this can be confusing.
Siderite is roughly the equivalent of calcite but with iron replacing the calcium. The iron has little effect on most structural properties such as cleavage and crystal form; but does effect properties such as density, colour, hardness and electro-chemical properties. Siderite is fairly easy to distinguish from calcite by its higher specific gravity and hardness and less vigorous reaction to acids. It can be difficult to distinguish from dolomite however.
Siderite and calcite can form similar crystal habits, although siderite is far less diverse. The typical habit for siderite is the rhombohedron. Most of the time crystals are found in the standard rhomb shape but noticeably flattened rhombs and rhombs with curved surfaces are also well known. Other typical habits include the scalahedron, crusts, sphericules, concretions and complicated botyroidal masses. Siderite forms mostly in sedimentary and hydrothermal environments, although it is also found in some igneous pegmatites.
Siderite is a common mineral although excellent, attractive crystal specimens are sometimes hard to find. But many crystals, when colourfully iridescent, make a wonderful, colourful, satiny, shimmering mineral specimen. The iridescence is probably caused by a surface alteration to the iron oxide, goethite. Further alteration of siderite crystals can result in pseudomorphs. The amorphous iron oxide limonite, forms pseudomorphs that are complete replacements of siderite crystals. Siderite can be a nice collection mineral. It comes in just enough of a variety of crystal forms and colours that make collecting this mineral very interesting.

Physical Characteristics

Colour: gray, yellow, yellowish brown, greenish-brown, reddish brown and brown. Some specimens show an iridescence probably caused by surface alteration to goethite
Luster: vitreous to pearly or silky in some specimens
Transparency: Crystals are usually translucent or virtually opaque
Crystal System: trigonal; bar 3 2/m
Crystal Habits: commonly curved rhombohedrons that are sometimes flattened to appear bladed, rarely scalahedral. Many aggregate forms are also found such as botryoidal, sphericules (sphaerosiderite), concretionary, stalactitic, vein-filling and earthy
Cleavage: perfect in 3 directions forming rhombs
Fracture: conchoidal to uneven
Hardness: fairly variable going from 3.5 - 4.5
Specific Gravity: 3.9+ (relatively heavy)
Streak: white
Other: Becomes magnetic when heated, effervesces slightly in contact with strong acids or with warm acids
Associated Minerals: include iron sulfides and quartz, cerussite, ankerite, dolomite, goethite, cryolite, limonite, barite, pyrite and sphalerite
Major Occurrences: numerous and include the Harz Mountains, Germany; several mines in Cornwall, England as well as Peru; Biera Baixa, Portugal; Tatasi, Bolivia; Minas Gerais, Brazil; Lorraine, France; Bohemia, Czech Republic; Broken Hill, New South Wales, Australia; Tsumeb, Otavi, Namibia; the mines of Franklin, New Jersey; San Bernardino County, California; Flambeau Mine, Ladysmith, Wisconsin, Antler Mine, Arizona and Connecticut, USA and Rapid Creek, Yukon Territory; Francon Quarry, Montreal and Mont Saint-Hilaire, Quebec, Canada
Best Indicators: crystal habit, slight reaction to acids, cleavage, colour and higher than average density
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Rino Safrizal
Jejaring Kimia Updated at: Maret 23, 2011

MAGNETITE [ Oxides and Hydroxides : Spinel ]

Maret 23, 2011 0
Fe3O4, iron Oxide
Major ore of iron and as mineral specimens
Magnetite is a natural magnet, hence the name, giving it a very nice distinguishing characteristic. Explaining the magnetism is not easy but here is a go at it. Remember, electricity produces magnetic fields just as magnetism produces electic fields. Magnetite is a member of the spinel group which has the standard formula A(B)2O4. The A and B represent usually different metal ions that occupy specific sites in the crystal structure. In the case of magnetite, Fe3O4, the A metal is Fe +2 and the B metal is Fe +3; two different metal ions in two specific sites. This arrangement causes a transfer of electrons between the different irons in a structured path or vector. This electric vector generates the magnetic field.

Physical Characteristics

Colour: black
Luster: metallic to dull
Transparency: Crystals are opaque
Crystal System: isometric; 4/m bar 3 2/m
Crystal Habits: typically octahedrons but rarely rhombododecahedron and other isometric forms, most commonly found massive or granular. Twinning of octahedrons into spinel law twins is seen occassionally
Cleavage: absent although octahedral parting can be seen on some specimens
Fracture: conchoidal
Hardness: 5.5 - 6.5
Specific Gravity: 5.1+ (average for metallic minerals)
Streak: black
Other: Magnetism stronger in massive examples than in crystals, striations on crystal faces (not always seen)
Associated Minerals: talc and chlorite (schists), pyrite and hematite
Major Occurrences: include South Africa, Germany, Russia and many locallities in the USA
Best Indicators: magnetism, crystal habit and streak
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Rino Safrizal
Jejaring Kimia Updated at: Maret 23, 2011

PYRITE [ Sulfides : Pyrite ]

Maret 23, 2011 0
FeS2, iron Sulfide
A very minor ore of sulfur for sulfuric acid, used in jewellery under the trade name "marcasite" and as mineral specimens
Pyrite is the classic "Fool’s Gold". There are other shiny brassy yellow minerals, but pyrite is by far the most common and the most often mistaken for gold. Whether it is the golden look or something else, pyrite is a favorite among rock collectors. It can have a beautiful luster and interesting crystals. It is so common in the earth’s crust that it is found in almost every possible environment, hence it has a vast number of forms and varieties.
Bravoite is the name given to a nickel-rich iron sulfide. It is closely related to pyrite but contains up to 20% nickel. Some mineral books treat it as a variety of pyrite.
Pyrite is a polymorph of marcasite, which means that it has the same chemistry, FeS2, as marcasite; but a different structure and therefore different symmetry and crystal shapes. Pyrite is difficult to distinguish from marcasite when a lack of clear indicators exists.
Pyrite’s structure is analogous to galena’s structure with a formula of PbS. Galena though has a higher symmetry. The difference between the two structures is that the single sulfur of galena is replaced by a pair of sulfurs in pyrite. The sulfur pair are covalently bonded together in essentially an elemental bond. This pair disrupts the four fold symmetry that a single atom of sulfur would have preserved and thus gives pyrite a lower symmetry than galena.
Although pyrite is common and contains a high percentage of iron, it has never been used as a significant source of iron. Iron oxides such as hematite and magnetite, are the primary iron ores. Pyrite is not as ecomonical as these ores possibly due to their tendency to form larger concentrations of more easily mined material. Pyrite would be a potential source of iron if these ores should become scarce.
Pyrite has been mined for its sulfur content though. During WWII, sulfur was in demand as a strategic chemical and North American native sulfur mines were drying up. A sulfide deposit near Ducktown Tenn. was found to be able to mine pyrite and other sulfides such as pyrrhotite and pentlandite and produce the needed sulfur as well as iron and other metals. The sulfur was used in the production of sulfuric acid, an important chemical for industrial purposes. Now most sulfur production comes from H2S gas recovered from natural gas wells.

Physical Characteristics

Colour: brassy yellow
Luster: metallic
Transparency: Crystals are opaque
Crystal System: isometric; bar 3 2/m
Crystal Habits: include the cube, octahedron and pyritohedron (a dodecahedron with pentagonal faces) and crystals with combinations of these forms. Good interpenetration twins called iron crosses are rare. Found commonly in nodules. A flattened nodular variety called "Pyrite Suns" or "Pyrite Dollars" is popular in rock shops. Also massive, reniform and replaces other minerals and fossils forming pseudomorphs or copies
Cleavage: very indistinct
Fracture: conchoidal
Hardness: 6 - 6.5
Specific Gravity: approx. 5.1+ (heavier than average for metallic minerals)
Streak: greenish black
Other: Brittle, striations on cubic faces caused by crossing of pyritohedron with cube. (note - striations on cube faces also demonstrate pyrite’s lower symmetry). Pyrite unlike gold is not malleable
Associated Minerals: quartz, calcite, gold, sphalerite, galena, fluorite and many other minerals. Pyrite is so common it may be quicker to name the unassociated minerals
Major Occurrences: include Illinois and Missouri, USA; Peru; Germany; Russia; Spain; and South Africa among many others
Best Indicators: crystal habit, hardness, streak, luster and brittleness
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Rino Safrizal
Jejaring Kimia Updated at: Maret 23, 2011

Maret 13, 2011

DOLOMITE [ Carbonates : Dolomite ]

Maret 13, 2011 0
CaMg(CO3)2, calcium magnesium Carbonate
In some cements, as a source of magnesium and as mineral specimens
Dolomite, which is named for the French mineralogist Deodat de Dolomieu, is a common sedimentary rock-forming mineral that can be found in massive beds several hundred feet thick. They are found all over the world and are quite common in sedimentary rock sequences. These rocks are called appropriately enough dolomite or dolomitic limestone. Disputes have arisen as to how these dolomite beds formed and the debate has been called the "Dolomite Problem". Dolomite at present time, does not form on the surface of the earth; yet massive layers of dolomite can be found in ancient rocks. That is quite a problem for sedimentologists who see sandstones, shales and limestones formed today almost before their eyes. Why no dolomite? Well there are no good simple answers, but it appears that dolomite rock is one of the few sedimentary rocks that undergoes a significant mineralogical change after it is deposited. They are originally deposited as calcite/aragonite rich limestones, but during a process call diagenesis the calcite and/or aragonite is altered to dolomite. The process is not metamorphism, but something just short of that. Magnesium rich ground waters that have a significant amount of salinity are probably crucial and warm, tropical near ocean environments are probably the best source of dolomite formation.
Dolomite in addition to the sedimentary beds is also found in metamorphic marbles, hydrothermal veins and replacement deposits. Except in its pink, curved crystal habit dolomite is hard to distinguish from its second cousin, calcite. But calcite is far more common and effervesces easily when acid is applied to it. But this is not the case with dolomite which only weakly bubbles with acid and only when the acid is warm or the dolomite is powdered. Dolomite is also slightly harder, denser and never forms scalenohedrons (calcite’s most typical habit).
Dolomite differs from calcite, CaCO3, in the addition of magnesium ions to make the formula, CaMg(CO3)2. The magnesium ions are not the same size as calcium and the two ions seem incompatible in the same layer. In calcite the structure is composed of alternating layers of carbonate ions, CO3, and calcium ions. In dolomite, the magnesiums occupy one layer by themselves followed by a carbonate layer which is followed by an exclusively calcite layer and so forth. Why the alternating layers? It is probably the significant size difference between calcium and magnesium and it is more stable to group the differing sized ions into same sized layers. Other carbonate minerals that have this alternating layered structure belong to the Dolomite Group. Dolomite is the principle member of the Dolomite Group of minerals which includes ankerite, the only other somewhat common member.
Dolomite forms rhombohedrons as its typical crystal habit. But for some reason, possibly twinning, some crystals curve into saddle-shaped crystals. These crystals represent a unique crystal habit that is well known as classical dolomite. Not all crystals of dolomite are curved and some impressive specimens show well formed, sharp rhombohedrons. The luster of dolomite is unique as well and is probably the best illustration of a pearly luster. The pearl-like effect is best seen on the curved crystals as a sheen of light can sweep across the curved surface. Dolomite can be several different colours, but colourless and white are very common. However it is dolomite’s pink colour that sets another unique characteristic for dolomite. Crystals of dolomite are well known for their typical beautiful pink colour, pearly luster and unusual crystal habit and it is these clusters that make very attractive specimens.

Physical Characteristics

Colour : often pink or pinkish and can be colourless, white, yellow, gray or even brown or black when iron is present in the crystal
Luster : pearly to vitreous to dull
Transparency : crystals are transparent to translucent
Crystal System : trigonal; bar 3
Crystal Habits : include saddle shaped rhombohedral twins and simple rhombs some with slightly curved faces, also prismatic, massive, granular and rock forming. Never found in scalenohedrons
Cleavage : perfect in three directions forming rhombohedrons
Fracture : conchoidal
Hardness : 3.5 - 4
Specific Gravity : 2.86 (average)
Streak : white
Other : Unlike calcite, effervesces weakly with warm acid or when first powdered with cold HCl
Associated Minerals : include calcite, sulfide ore minerals, fluorite, barite, quartz and occasionally with gold
Major Occurrences : include many localities throughout the world, but well known from sites in Midwestern quarries of the USA; Ontario, Canada; Switzerland; Pamplona, Spain and in Mexico
Best Indicators : typical pink colour, crystal habit, hardness, slow reaction to acid, density and luster
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Rino Safrizal
Jejaring Kimia Updated at: Maret 13, 2011

CINNABAR [ Sulfides and Sulfosalts ]

Maret 13, 2011 0
HgS, mercury Sulfide
Primary ore of mercury, a pigment and as a minerals specimen
Cinnabar is a colourful mineral that adds a unique colour to the mineral colour palette. Its cinnamon to scarlet red colour can be very attractive. Well shaped crystals are uncommon and the twinned crystals are considered classics among collectors. The twinning in cinnabar is distinctive and forms a penetration twin that is ridged with six ridges surrounding the point of a pryamid. It could be thought of as two scalahedral crystals grown together with one crystal going the opposite way of the other crystal. Cinnabar was mined by the Roman Empire for its mercury content and it has been the main ore of mercury throughout the centuries. Some mines used by the Romans are still being mined today. Cinnabar shares the same symmetry class with quartz but the two form different crystal habits.

Physical Characteristics

Colour : bright scarlet or cinnamon red to a brick red
Luster : adamantine to submetallic in darker specimens
Transparency : crystals are translucent to transparent
Crystal System : trigonal; 32
Crystal Habits : individual, well formed, large crystals are scarce; crusts and crystal complexes are more common; may be massive, or in capilary needles. Crystals that are found tend to be the six sided trigonal scalahedrons that appear to have opposing three sided pyramids. It also forms modified rhombohedrons, prismatic and twinned crystals as discribed above.
Cleavage : perfect in three directions, forming prisms
Fracture : uneven to splintery
Hardness : 2 - 2.5
Specific Gravity : approx. 8.1+ (very heavy for a non-metallic mineral)
Streak : red
Other : silghtly sectile and crystals can be striated
Associated Minerals : realgar, pyrite, dolomite, quartz, stibnite and mercury
Major Occurrences : include Almaden, Spain; Idria, Serbia; Hunan Prov., China and California, Oregon, Texas, and Arkansas, USA
Best Indicators : crystal habit, density, cleavage, softness and colour
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Rino Safrizal
Jejaring Kimia Updated at: Maret 13, 2011

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