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Pyrite

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

                                   Pyrite
            An alluring mass of relatively worthless fool's gold

                            A mass of intergrown, striated pyrite crystals

                                            Euhedral cubic pyrite crystals
                                   General
                                                          Category Mineral
                                  Chemical formula iron disulfide (FeS[2])
                               Identification
                                                    Colour Pale, dull gold
           Crystal habit Cubic, faces may be striated, but also frequently
        octahedral and pyritohedron. Often inter-grown, massive, radiated,
                                       granular, globular and stalactitic.
                                       Crystal system Isometric; bar 3 2/m
                                             Melting Point 1,177-1,188 °C
                                                             Cleavage Poor
                                Fracture Very uneven, sometimes conchoidal
                                               Mohs Scale hardness 6 - 6.5
                                               Luster Metallic, glistening
                                                   Refractive index Opaque
                                   Streak Greenish-black to brownish-black
                                              Specific gravity 4.95 - 5.10
                                                          Fusibility 2.5-3
                                             Solubility insoluble in water
                                                    Magnetism paramagnetic

   The mineral pyrite, or iron pyrite, is iron sulfide, FeS[2]. It has
   isometric crystals that usually appear as cubes. The cube faces may be
   striated (parallel lines on crystal surface or cleavage face) as a
   result of alternation of the cube and pyritohedron faces. Pyrite also
   frequently occurs as octahedral crystals and as pyritohedra (a
   dodecahedron with pentagonal faces). It has a slightly uneven and
   conchoidal fracture, a hardness of 6–6.5, and a specific gravity of
   4.95–5.10. It is brittle, meaning it breaks or powders easily. Its
   metallic luster and pale-to-normal brass-yellow hue have earned it the
   nickname fool's gold, but ironically, small quantities of actual gold
   are sometimes found in pyrite. In fact, such auriferous pyrite is a
   valuable ore of gold.

   Pyrite is the most common of the sulfide minerals. It is usually found
   associated with other sulfides or oxides in quartz veins, sedimentary
   rock and metamorphic rock, as well as in coal beds, and as the
   replacement mineral in fossils.

   Pyrite exposed to the environment during mining and excavation reacts
   with oxygen and water to form sulfuric acid, resulting in acid mine
   drainage. This results from the action of Thiobacillus bacteria, which
   generate their energy by using oxygen to oxidize ferrous iron (Fe^2+)
   to ferric iron (Fe^3+). The ferric iron in turn reacts with pyrite to
   produce ferrous iron and sulfuric acid. The ferrous iron is then
   available for oxidation by the bacteria; this cycle can continue until
   the pyrite is exhausted.

   Pyrite is used for the production of sulfur dioxide, e.g. for the paper
   industry, and in the manufacture of sulfuric acid, though such
   applications are declining in importance.

   Pyrites can show negative resistance and have experimentally been used
   in oscillator circuits as radio detectors .

   The name pyrite is from the Greek word "Pyr" meaning "fire". This is
   likely due to the sparks that result when pyrite is struck against
   steel. This capacity made it popular for use in early firearms such as
   the wheellock.

   In her historical fiction book The Valley of Horses, Jean Auel's
   heroine Ayla is described as inventing a new method of creating fire by
   striking a pyrite-like material against flint.

Pyrite and marcasite

   Pyrite is often confused with the mineral marcasite, a name derived
   from the Arabic word for pyrite, due to their similar characteristics.
   Marcasite is a polymorph of pyrite, which means it has the same formula
   as pyrite but a different structure and, therefore, different symmetry
   and crystal shapes. The formal oxidation states are, however, the same
   as in pyrite because again the sulfur atoms occur in persulfide-like
   pairs. Marcasite/pyrite is probably the most famous polymorph pair next
   to the diamond/ graphite pair.

   Marcasite is metastable relative to pyrite and will slowly be changed
   to pyrite if heated or given enough time. Marcasite is relatively rare,
   but may be locally abundant in some types of ore deposits, such as
   Mississippi Valley-type Pb-Zn deposits. Marcasite appears to form only
   from aqueous solutions.

   Pyrite is often used in jewellery such as necklaces and bracelets, but
   although the two are similar, marcasite cannot be used in jewellery as
   it has a tendency to crumble into powder. Adding to the confusion
   between marcasite and pyrite is the use of the word marcasite as a
   jewellery trade name. The term is applied to small polished and faceted
   stones that are inlaid in sterling silver, but even though they are
   called marcasite, they are actually pyrite.

Formal oxidation states for pyrite, marcasite, and arsenopyrite

   From the perspective of classical inorganic chemistry, which assigns
   formal oxidation states to each atom, pyrite is probably best described
   as Fe^2+S[2]^2-. This formalism recognizes that the sulfur atoms in
   pyrite occur in pairs with clear S-S bonds. These persulfide units can
   be viewed as derived from hydrogen persulfide, H[2]S[2]. Thus pyrite
   would be more descriptively called iron persulfide, not iron disulfide.
   In contrast, molybdenite, MoS[2], features isolated sulfide (S^2-)
   centers. Consequently, the oxidation state of molybdenum is Mo^4+, or
   Mo(IV). The mineral arsenopyrite has the formula FeAsS. Whereas pyrite
   has S[2] subunits, arsenopyrite has AsS units, formally derived from
   deprotonation of H[2]AsSH. Analysis of classical oxidation states would
   recommend the description of arsenopyrite as Fe^3+AsS^3-. Of course
   these formalisms ignore covalency, which is strongly implied by the
   semi-conducting behaviour of this family of inorganic solids.

Related minerals and varieties

   Bravoite is a nickeloan variety of pyrite, with >50% substitution of
   Ni^2+ for Fe^2+ within pyrite. Bravoite is not a formally recognised
   mineral, and is named after Peruvian scientist, Jose J. Bravo
   (1874-1928) .
   Retrieved from " http://en.wikipedia.org/wiki/Pyrite"
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