Iron+Pyrites

The mineral **pyrite**, or **iron pyrite**, is an iron sulfide with the formula [|Fe][|S] 2. This mineral's metallic [|luster] and pale-to-normal, brass-yellow hue have earned it the nickname **fool's gold** because of its resemblance to gold. The color has also led to the nicknames **brass**, **brazzle** and **Brazil**, primarily used to refer to pyrite found in coal. Pyrite is the most common of the sulfide minerals. The name pyrite is derived from the [|Greek] πυρίτης (//puritēs//), "of fire" or "in fire", in turn from πύρ (//pur//), "fire". In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against [|steel] ; [|Pliny the Elder] described one of them as being brassy, almost certainly a reference to what we now call pyrite. By Georgius Agricola 's time, the term had become a generic term for all of the sulfide minerals. Pyrite 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 a replacement mineral in fossils. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and [|arsenic] occur as a coupled substitution in the pyrite structure. In the Carlin, Nevada gold deposit, arsenian pyrite contains up to 0.37 wt% gold.

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 Fe2+S22−. 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 disulfide], H2S2. Thus pyrite would be more descriptively called iron persulfide, not iron disulfide. In contrast, [|molybdenite], [|Mo] S2, features isolated sulfide ( S2− ) centers. Consequently, the oxidation state of molybdenum is Mo4+. The mineral arsenopyrite has the formula Fe [|As] S. Whereas pyrite has S2 subunits, arsenopyrite has AsS units, formally derived from [|deprotonation] of H2AsSH. Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe3+(AsS)3−

Crystallography
Crystal structure of pyrite. In the center of the cell a S22- pair is seen in yellow. Iron-pyrite FeS2 represents the prototype compound of the crystallographic pyrite structure. The structure is simple [|cubic] and was among the first [|crystal structures] solved by [|X-ray diffraction]. [|[26]] It belongs to the crystallographic [|space group] //Pa// 3 and is denoted by the [|Strukturbericht] notation C2. Under thermodynamic standard conditions the [|lattice constant] of stoichiometric iron pyrite FeS2 amounts to 541.87 pm. [|[27]] The [|unit cell] is composed of a Fe [|face-centered cubic sublattice] into which the S ions are embedded. The pyrite structure is also taken by other compounds //MX//2 of [|transition metals] //M// and [|chalcogens] //X// = [|O], [|S] , [|Se] and [|Te]. Also certain [|dipnictides] with //X// standing for [|P], [|As] and [|Sb] etc. are known to adopt the pyrite structure. [|[28]] In the first bonding sphere, the Fe atoms are surrounded by six S nearest neighbours, in a distorted octahedral arrangement. The material is a [|diamagnetic] [|semiconductor] and the Fe ions should be considered to be in a // [|low spin] // [|divalent] state (as shown by Mösbauer spectroscopy as well as XPS), rather than a [|tetravalent] state as the stoichiometry would suggest. The positions of //X// ions in the pyrite structure may be derived from the [|fluorite] structure, starting from a hypothetical Fe2+(S-)2 structure. Whereas [|F] - ions in CaF2 occupy the centre positions of the eight subcubes of the cubic unit cell (¼ ¼ ¼) etc., the S- ions in FeS2 are shifted from these high symmetry positions along axes to reside on (//uuu//) and symmetry-equivalent positions. Here, the parameter //u// should be regarded as a free atomic parameter that takes different values in different pyrite-structure compounds (iron pyrite FeS2: //u//(S) = 0.385 [|[29]] ). The shift from fluorite u=0.25 to pyrite u=0.385 is rather large and creates a S-S distance that is clearly a binding one. This is not surprising as in contrast to F- an ion S- is not a closed shell species. It is isoelectronic with a chlorine //atom//, also undergoing pairing to form Cl2 molecules. Both low spin Fe2+ and the disulfide S22- moeties are closed shell entities, explaining the diamagnetic en semiconducting properties. The S atoms have bonds with three Fe and one other S atom. The site symmetry at Fe and S positions is accounted for by [|point symmetry groups] //C//3//i// and //C//3, respectively. The missing [|center of inversion] at S lattice sites has important consequences for the crystallographic and physical properties of iron pyrite. These consequences derive from the crystal electric field active at the sulfur lattice site, which causes a [|polarisation] of S ions in the pyrite lattice. [|[30]] The polarisation can be calculated on the basis of higher-order [|Madelung constants] and has to be included in the calculation of the [|lattice energy] by using a generalised [|Born-Haber cycle]. This reflects the fact that the covalent bond in the sulfur pair is ill accounted for in the strictly ionic treatment of Madelung theory. Arsenopyrite has a related structure with heteroatomic As-S pairs rather than homoatomic ones. Marcasite also possesses homoatomic anion pairs, but the arrangement of the metal and diatomic anions is different than in a pyrite. Despite its name a chalcopyrite does not contain dianion pairs, but single S2- sulfide anions.