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PYRITE [ Sulfides : Pyrite ]

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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.
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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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