Pyrite's formula, characteristics - gray pyrites

Pyrite's formula, characteristics - gray pyrites
 Convertible terms: Gray pyrite ,ferric pyrite.
Pyrite - is a most common sulphide in a nature.
Pyrite's name is of Greek origin (pyros - fire) and connected to its ability to produce sparks on mechanical impact.
 Picture of pyrite's cubical crystals joint from Ural, Berezoskoe deposit.
• Crystallographic characteristics
• Forms in natura
• Physical characteristics
• Diagnostics signs
• Mineral's origin
• Deposits of pyrite
• Practical use
• Physical methods of examination
• Crystal-optical characteristics in fine specimen (sections)
• To buy
Pyrite's formula
Chemical composition of pyrites
Theoretical composition - Fe — 46,55 %, S — 53,45 %. Often contains add-mixes in small quantities: Со (Cobalt pyrite), Ni, As, Sb, Se, sometimes Cu, Au, Ag and others. Containing of the last elements is a result of mechanical add-mixes presence in appearance of very fine inclusions of other minerals, sometimes in finely-dispersed condition. In some cases we deal, in fact, with solid pseudo-solutions - crystal-sol.

Mixed crystals or species: bravoite or nickel-pyrite (Ni, Fe, Со) S2, а0 = 5,50 — 5,58*3; villamonite (Си, Ni, Со, Fe) (S,Se)2, а0 = 5,66
Melnikovite  - pyrite is crypto-crystal pyrite of gel-like origin.  Laurite  includes small amount of osmium.
Auerite showing strong non-metallic character, possibly, resulting diamond-like type of connection.

 Crystallographic characteristics
Crystal system
cubic; dido-decahedral b.c. 3L24L3 63PC. Spatial group Ра3 (Т6h). а0 = 5,4066 7A, Z = 4.
Crystal structure of pyrite mineral
Structure of NaCl type Atoms of iron  form face-centered cubic bars (respectively to atoms of sodium in structure of NaCl. Twins atoms of sulfur take place of atoms chlorine, forming face-centered cubic structure too, but deposed to a0/2 in relation to the cation bars. Twins atoms axes are oriented along non-overlapping diagonals of cubic spacial bar. Distance between sulfur atoms, connected in every pair by covalent connection, equal 2,05 А.
Main forms:
Pyrite is quite common in appearance of well formed crystals. Main forms together with  а {100}, о { 111} и е {210} presented also  n {211}, р {221}, s {321}, t {421}, d {110}, m (311}, h {410}, f {310} and g {320}.  The crystal habitus is located in dependency on predominance of one or another edge: cubic, pentagon-dodecahedral, seldom octahedral.
 Pyrite forms presented in nature

In numerous rocks and ores pyrite  is observed as embedded crystals or  rounded grains. Widely spread also pyrite masses of compact aggregate construction. Sometimes druses are formed. 
Crystals appearance. Very common the crystals, mainly cubes, pentagon-dodecahedron or octahedral.

 а - hexahedral (cube) cube а {100}
 b - pentagon-dodecahedron e {210}
  r - octahedron о {111}
 d - octahedron combination (o) and pentagon-dodecahedron (e) - so called mineral icosahedron
Pyrite crystals form:
• а — cube а {100};
• b - pentagon-dodecahedron е {210};
• c - the same form in combination with cube а {100};
• r - octahedron о {111}, with dulled edges of pentagon-dodecahedron;
• d - combination of octahedron (o) and pentagon-dodecahedron (e) - so called mineral icosahedron (combination of octahedron with pentagon-dodecahedron).
Size of the crystals sometimes can reach several tens of centimeters in diameter.
Streak of the edges parallel to cube sides is common (100) : (210), e. g. а : e. This streak located respectively to crystal structure (location of sulfur atoms in the structure) and always oriented  transversely to each next edge, e.g. outer elements of the symmetry are completely correspond to the structure.
For pyrite is very common twins germination on {110}, and rarely on (320).
There are known regular accretions between pyrite and marcasite , tetrahedrite, galena, pyrotine, arsenic-pyrite  and others.
Pyrite crystals, formed in high temperatures, as a rule, lacking the simple formes. The last ones are presented as a cubes, octahedron or {210}. The same is correct for low-temperature formations, however, the crystals, formed in the middle temperatures and depths, are richer with simple forms. The crystals up to 10 cm are observed in the deposits of this type. According to Sanagava, pyrite's crystal habitus depended on the crystal's size. Smaller crystals are mainly cubical, large ones are pentagon-dodecahedron  Thorough studies, conducted by the same author on numerous deposits of Japan, indicated that in metasomatic deposits the cubic pyrite crystals are common for  highly and low-temperature zones.
Pentagon-dodecahedrons are typical for low-temperature, but mineralized zones. Crystals of the pentagon-dodecahedron habitus are formed in intermediate situations. This is in accordance with the sequence of development of pyrite's habitus type. Cubic habitus is typical for weak supersaturations, pentagon-dodecahedron - for greater supersaturations, and octahedrical ones - for the intermediate. Location of pentagon-dodecahedron and octahedrical habitus crystals in veins deposits and cubical ones - in the native rocks, usually as a impregnations, can be interpreted from position of supersaturations.  Certain dependency of ratio between crystals habitus and its admixtures were not determined. In reductive conditions the pyrite's concretion or impregnation is often formed.
In sedimentation condition crypto-crystalline variety of pyrite (melnicoeite), is reserved, which forms mixes with dimorphous modification FeS2 - marcasite. The last mineral is rhombic, was artificially synthesized in acidic environment, while pyrite is formed only in neutral or low-acidic environment. Pyrite can originate with metamorphism from clay sediments, enriched by the organic material. Pyrite is mined for sulfuric acid production, mainly on world famous deposit of Rio-Tinto in Spain.
 Pyrite. Grain aggregate,
Aggregate. Most common are dense, confluent and grain masses, as well as modulated  discharges; coarse-fibered, thin-stalked , radial-radiant formations, often pyrite-like layers of rocks.
In sedimentary rocks there are often observed globular concretions of  pyrite , mainly radial-radical shape and secretions into the shells cavities. Clustered or modulated formations are common in association with other sulphides.

 Physical qualities
• Light color of brass-yellow or straw-yellow, often with yellowish-brown tarnish, sulfur poor; finely- dispersed zole varieties with black color.
• Line is greenish-gray or brownish-black.
Pyrite has strong metallic luster.

• Hardness 6-6.5. Pyrite is fragile, especially if it enriched with gold inclusions.
• Cleavage is quite imperfect on {100} and {111}, sometimes on prism {110} average with angle 87°. Fracture is irregular, sometimes blistered.
Sometimes the jointing on {010} is observed.
• Density 4,9–5,2.
Chemical characteristics
Hardly dissolved in HNO3  , hardly decomposes (easy as a powder), excreting sulfur. Do not dissolve in diluted HCl.
Other characteristics
Pyrite conducts electricity poorly. It relates to paramagnetic minerals. Thermo-electrical. Some of its varieties have detector characteristics.
 Diagnostics signs
Can be easily recognized on color, crystals shape and scratchiness of sides, high hardness (only  one from the widely known sulfides, which can scratch glass). By summary of these signs, it easily distinguishes from the similar on color marcasite, halcopyrite, pyrrotine, arsenic-pyrite, gold  and millerite.
Accompanying minerals. The satellites are  quarts, tiff, halcopyrite, galena,  blende, gold, tellurites of gold, arsenic-pyrite, pyrrotine, wolframite, antimonite .
 Galena, pyrite. Crystals druses

 Origin and location of the mineral

Pyrite is common sulphide in earth crust and originated in very different geological processes: abyssal, hydrothermal, sedimentary, with metamorphism and other.
1. As a finest  impregnations, it is observed in many abyssal rocks. Formed in phase separation events
I majority of cases it is an epigene mineral in relation to silicates, and connected with imposition of hydrothermal occurrences.
2. In contacting- metasomatic deposits it almost constant satellite of sulphides in skarn and  abyssal deposits. In number of cases it is a cobalt- containing. Its formation, as well as other sulphides, connected to hydrothermal stage of contacting- metasomatic processes.
3. As a satellite, it widely spread in hydrothermal deposits of different by their content ores of almost all the types and can be met in paragenesis with different minerals. It is often observed not just in ore bodies, but also in side rocks as a impregnations of well formed crystals, created by metasomatic way (meta-crystals).
4. Also often pyrite can be met in sediment rocks and ores. Very common pyrites and marcasite  concentrations in sandy-clay deposits (often beautiful crystals), deposits of coal, iron, manganese, bauxites and others. Its formation in these rocks and ores is connecting to decomposition of organic residuals without access of free oxygen in deeper areas of water basins. In paragenesis  with it with the same conditions are often observed: marcasite, melnicovite (black powder species of iron  disulfide),siderite (Fe[CO3]) and other.
In oxidizing zones the pyrite, as majority of sulphides, unstable, subjected to oxidizing to  iron protoxide sulphate, which, with free oxygen presence, easily turning to iron oxide sulphate.  The last one, by hydrolysis, decomposes to unsolvable iron hydro-oxide ( limonite) and free sulphuric acid, turning to the solution. By this was the widely known in nature pseudo-morphosis limonite to pyrite are formed.
The pyrite itself is often forming pseudo-morphosis on non-organic residuals ( on wood and various residuals of the organisms), and in endogenous formation the pseudo-morphosis of pyrite to pyrotine are met, magnetite (FeFe2O4),hematite (Fe2O3) and other iron-containing minerals. These pseudo-morphosis, probably, are forming by H2S influence to the minerals.
5. Pyrite can be formed by metamorphism from clay deposits, enriched by organic matter.
6. In volcanic ¬exhalations, sub-volcanic rocks and hydrothermal ¬pyrite deposits (together with halcopyrite¬ and other).
From economical point of view, hydrothermal veins and metasomatic deposits are important.
Because pyrite is widely spread, the deposits, where it found in some quantity are countless. It can be met in various genetic types of deposits, however the main mass is related to endogenous formations.
Largest world deposit is ¬Rio-Tinto (Spain);-
In Russia richest kies deposits widely spread on Ural, where they are connected to meridonal prolonged to hundreds kilometers line of changed acidic and base effusive and sedimentary rocks Sylur-Devonian deposits. Ores bodies, consisted of sulphides, mainly pyrite, usually have shape of vein or lenses deposits. Mark the most important and reach of pyrite deposits: Kalatinskoye in Neviansky district; Degtyarskoe in Sisertsky district; row of Karabash deposits in Kishtimsky district; Bliavinskoe (town Mednogorsk) in Orenburg district and other.
From mineralogical point of view, large interest cause the pyrite crystal druses of famous Berezovsky golden deposit on Ural. Usually they are presented by cubic shapes with heavily dashed sides, to a lower degree - penetagon-dodecahedron and octahedron. The giant crystals sometimes are met (up to 32 kg). Large crystals and their joints are met as a concretions in coals, for example, in Tulunsky (Irkutsk town) and Grizlovsky (Tula district) cuts.
In Transcaucasian deposits the Chiragidzorskoe deposit in Ganzinsky district is related to pyrite ones.
Large hydrothermal deposits co-pyrite Mindigy and Mindola are located in Katanga (Democratic Republic of Congo). Quite well formed crystals and heir joints of decorative collection quality are originated from Logronio deposit in Spain, where they lay in limes.
 Use of pyrite
 Pyrite. Impregnations in argillite. Rostov district
 Pyrite cabochon
Pyrite ores are the main raw material for sulfuric acid production. Average content of sulfur in exploited for this goal ores is varying from 40 to 50%. Ore treatment is occur by burning in special furnaces. Received in this process sulfurous gas SO2 is oxidized by nitrogen oxides in the water steam to H2SO4. An arsenic is unwanted admix in the ores, used for sulfuric acid production.
Often copper, zinc, sometimes gold, Selene and other contained in pyrite ores, can be received as a by-products. Produced as a result of burning so called iron scoria can be used as a dye or as a iron ore. Ores, containing cobalt pyrite, are the source of approximately half of world consumption of cobalt, despite of low content of this element in them (to 0.5-1% in the mineral).
From pyrite of Berezovsky deposit on Ural the inclusions for jewelery are made.
Pyrite  in generally are grained as a cabochon.

 Physical methods of examination
Differential thermo analysis
 Differential thermo analysis Graph of DTA

Main lines on XR screens of pyrite
2,696(8) - 2,417(8) - 2,206(7) — 1,908(6) — 1,629(10) - 1,040(9)
Old methds. Under soldering pipe pyrite cracks, on coal it melts to magnetic ball and the blue flame and smoke are appear with it. Part of the sulfur, which is burning blue, is lost. In solder pipe the part of sulfur is sublimated - the rest is mono-sulphide FeS.
 Crystal-optical characteristics in fine specimen (sections)
In polished sections the pyrite is cream-white, isotropic, but sometimes anisotropic because of sulfur atoms exchange to atoms of iron (according to Gordon-Smith). According to the same author, pyrite, formed with temperature over 135° is  isotropic and characterized by static distribution of iron atoms on the place of sulfur atoms.  (Bellow this temperature usually formed anisotropic pyrites). This quality can be used in geological thermometric.