Everything about Pyrite totally explained
The
mineral pyrite, or
iron pyrite, is an iron
sulfide with the
formula FeS2. This mineral's metallic
luster and pale-to-normal, brass-yellow hue have earned it the nickname
fool's gold due to its resemblance to
gold. Pyrite is the most common of the
sulfide minerals. The name pyrite is derived from the
Greek πυρίτης (puritēs), “of fire” or "in fire”, from
πύρ (pur), “fire”. This name is likely due to the sparks that result when pyrite is struck against
steel or
flint. This property made pyrite popular for use in early
firearms such as the
wheellock.
Mineralogy
This mineral occurs as
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. Marcasite/pyrite is probably the most famous polymorph pair next to the
diamond/
graphite pair. Appearance is slightly more silver.
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 can't be used in jewellery as it tends 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're called marcasite, they actually contain 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
22-. 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,
H2S
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+. The mineral arsenopyrite has the formula Fe
AsS. Whereas pyrite has S
2 subunits, arsenopyrite has AsS units, formally derived from
deprotonation of H
2AsSH. Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe
3+AsS
3-.
Further Information
Get more info on 'Pyrite'.
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