The mineral pyrite, or iron pyrite, also known as fool's gold, is an
iron sulfide with the chemical formula FeS2 (iron(II) disulfide).
Pyrite is considered the most common of the sulfide minerals.
Pyrite's metallic luster and pale brass-yellow hue give it a
superficial resemblance to gold, hence the well-known nickname of
fool's gold. The color has also led to the nicknames brass, brazzle,
and Brazil, primarily used to refer to pyrite found in coal.
The name pyrite is derived from the Greek πυρίτης (pyritēs),
"of fire" or "in fire", in turn from πύρ (pyr), "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, c. 1550, 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, but has also been
identified in the sclerites of scaly-foot gastropods.  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–type gold
deposits, arsenian pyrite contains up to 0.37% gold by weight.
2 Formal oxidation states for pyrite, marcasite, and arsenopyrite
4 Crystal habit
6 Distinguishing similar minerals
7.1 Acid drainage
7.2 Dust explosions
7.3 Weakened building materials
8 Pyritised fossils
11 Further reading
12 External links
An abandoned pyrite mine near
Pernek in Slovakia.
Pyrite enjoyed brief popularity in the 16th and 17th centuries as a
source of ignition in early firearms, most notably the wheellock,
where the cock held a lump of pyrite against a circular file to strike
the sparks needed to fire the gun.
Pyrite has been used since classical times to manufacture copperas,
that is, iron(II) sulfate. Iron pyrite was heaped up and allowed to
weather (an example of an early form of heap leaching). The acidic
runoff from the heap was then boiled with iron to produce iron
sulfate. In the 15th century, such leaching began to replace the
burning of sulfur as a source of sulfuric acid. By the 19th century,
it had become the dominant method.
Pyrite remains in commercial use for the production of sulfur dioxide,
for use in such applications as the paper industry, and in the
manufacture of sulfuric acid. Thermal decomposition of pyrite into FeS
(iron(II) sulfide) and elemental sulfur starts at 540 °C; at around
700 °C pS2 is about 1 atm.
A newer commercial use for pyrite is as the cathode material in
Energizer brand non-rechargeable lithium batteries.
Pyrite is a semiconductor material with a band gap of 0.95 eV.
During the early years of the 20th century, pyrite was used as a
mineral detector in radio receivers, and is still used by 'crystal
radio' hobbyists. Until the vacuum tube matured, the crystal detector
was the most sensitive and dependable detector available – with
considerable variation between mineral types and even individual
samples within a particular type of mineral.
Pyrite detectors occupied
a midway point between galena detectors and the more mechanically
complicated perikon mineral pairs.
Pyrite detectors can be as
sensitive as a modern 1N34A germanium diode detector.
Pyrite has been proposed as an abundant, inexpensive material in
low-cost photovoltaic solar panels. Synthetic iron sulfide was
used with copper sulfide to create the photovoltaic material.
Pyrite is used to make marcasite jewelry.
Marcasite jewelry, made from
small faceted pieces of pyrite, often set in silver, was known since
ancient times and was popular in the Victorian era. At the time
when the term became common in jewelry making, "marcasite" referred to
all iron sulfides including pyrite, and not to the orthorhombic FeS2
mineral marcasite which is lighter in color, brittle and chemically
unstable, and thus not suitable for jewelry making.
does not actually contain the mineral marcasite.
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, MoS2, features isolated
sulfide (S2−) centers and the oxidation state of molybdenum is Mo4+.
The mineral arsenopyrite has the formula FeAsS. 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−.
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. It belongs to the crystallographic space group Pa3
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. The unit
cell is composed of a Fe face-centered cubic sublattice into which the
S ions are embedded. The pyrite structure is also used by other
compounds MX2 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.
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 Mössbauer
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 (1⁄4 1⁄4 1⁄4)
etc., the S− ions in FeS2 are shifted from these high symmetry
positions along <111> 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
). 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 and 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 C3i and C3, 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. 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 inadequately accounted for by a
strictly ionic treatment.
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 from that of pyrite. Despite its name a chalcopyrite does
not contain dianion pairs, but single S2− sulfide anions.
Dodecahedron- shaped crystals from Italy.
Pyrite usually forms cuboid crystals, sometimes forming in close
association to form raspberry-shaped masses called framboids. However,
under certain circumstances, it can form anastamozing filaments or
Pyrite can also form almost perfect
dodecahedral shapes known as pyritohedra and this suggests an
explanation for the artificial geometrical models found in Europe as
early as the 5th century BC.
Cattierite (Co S2) and vaesite (Ni S2) are similar in their structure
and belong also to the pyrite group.
Bravoite is a nickel-cobalt bearing variety of pyrite, with
> 50% substitution of Ni2+ for Fe2+ within pyrite. Bravoite is
not a formally recognised mineral, and is named after Peruvian
scientist Jose J. Bravo (1874–1928).
Distinguishing similar minerals
It is distinguishable from native gold by its hardness, brittleness
and crystal form. Natural gold tends to be anhedral (irregularly
shaped), whereas pyrite comes as either cubes or multifaceted
Pyrite can often be distinguished by the striations which,
in many cases, can be seen on its surface.
Chalcopyrite is brighter
yellow with a greenish hue when wet and is softer (3.5–4 on Mohs'
Arsenopyrite is silver white and does not become more
yellow when wet.
A pyrite cube (center) has dissolved away from a host rock, leaving
behind trace gold.
Iron pyrite is unstable in the natural environment: in nature it is
always being created or being destroyed. Iron pyrite exposed to air
and water decomposes into iron oxides and sulfate. This process is
hastened by the action of
Acidithiobacillus bacteria which oxidize the
pyrite to produce ferrous iron and sulfate. These reactions occur more
rapidly when the pyrite is in fine crystals and dust, which is the
form it takes in most mining operations.
Sulfate released from decomposing pyrite combines with water,
producing sulfuric acid, leading to acid rock drainage. An example of
acid rock drainage caused by pyrite is the 2015
Gold King Mine waste
Pyrite oxidation is sufficiently exothermic that underground coal
mines in high-sulfur coal seams have occasionally had serious problems
with spontaneous combustion in the mined-out areas of the mine. The
solution is to hermetically seal the mined-out areas to exclude
In modern coal mines, limestone dust is sprayed onto the exposed coal
surfaces to reduce the hazard of dust explosions. This has the
secondary benefit of neutralizing the acid released by pyrite
oxidation and therefore slowing the oxidation cycle described above,
thus reducing the likelihood of spontaneous combustion. In the long
term, however, oxidation continues, and the hydrated sulfates formed
may exert crystallization pressure that can expand cracks in the rock
and lead eventually to roof fall.
Weakened building materials
Building stone containing pyrite tends to stain brown as the pyrite
oxidizes. This problem appears to be significantly worse if any
marcasite is present. The presence of pyrite in the aggregate used
to make concrete can lead to severe deterioration as the pyrite
oxidizes. In early 2009, problems [clarification needed] with
Chinese drywall imported into the
United States after Hurricane
Katrina were attributed to oxidation of pyrite. In the United
States, in Canada, and more recently in Ireland, where
it was used as underfloor infill, pyrite contamination has caused
major structural damage. Modern tests for aggregate materials
certify such materials as free of pyrite.
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Pyrite and marcasite commonly occur as replacement pseudomorphs after
fossils in black shale and other sedimentary rocks formed under
reducing environmental conditions. However, pyrite dollars or
pyrite suns which have an appearance similar to sand dollars are
pseudofossils and lack the pentagonal symmetry of the animal.
As a replacement mineral in an ammonite from France
Pyrite from Ampliación a Victoria Mine, Navajún, La Rioja, Spain
Pyrite from the Sweet Home Mine, with golden striated cubes intergrown
with minor tetrahedrite, on a bed of transparent quartz needles
Radiating form of pyrite
Paraspirifer bownockeri in pyrite
Pink fluorite perched between pyrite on one side and metallic galena
on the other side
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Wikimedia Commons has media related to Pyrite.
Educational article about the famous pyrite crystals from the Navajun
How Minerals Form and Change "
Pyrite oxidation under room conditions".
Poliakoff, Martyn (2009). "Fool's Gold". The Periodic Table of Videos.
University of Nottingham.