The mineral pyrite, or iron pyrite, also known as fool's gold, is an iron sulfide with the
formula FeS2. This mineral'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. Pyrite is the most common of the sulfide minerals.
The name pyrite is derived from the Greek πυρίτης, "of fire" or "in fire", in
turn from πύρ, "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–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.
Uses
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. 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 sulfide) and elemental sulfur
starts at 550 °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 diode detector.
Pyrite has been proposed as an abundant, inexpensive material in low cost photovoltaic solar panels.
Synthetic iron sulfide is used with copper sulfide to create the experimental photovoltaic
material. Pyrite is used to make marcasite jewelry.
Marcasite jewelry, made from small faceted pieces of pyrite, often set in silver, was
popular in the Victorian era. Marcasite jewelry does not contain marcasite, which is a medieval
term that encompassed both pyrite and the mineral now called 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 centers because 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−. Crystallography
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, 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 etc., the S- ions in FeS2 are shifted
from these high symmetry positions along <111> axes to reside on 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 = 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 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 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 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 from that
of pyrite. Despite its name a chalcopyrite does not contain dianion pairs, but single
S2- sulfide anions. Crystal habit
Pyrite usually forms cuboid crystals, sometimes forming in close association to form raspberry-like
framboids. However, under certain circumstances, it can form anastamozing filaments or T-shaped
crystals. Pyrite can also form dodecahedral crystals and this suggests an explanation
for the artificial geometrical models found in Europe as early as the 5th century BC.
Varieties Cattierite and vaesite 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.
Distinguishing similar minerals It is distinguishable from native gold by
its hardness, brittleness and crystal form. Natural gold tends to be anhedral, whereas
pyrite comes as either cubes or multifaceted crystals. Chalcopyrite is brighter yellow
with a greenish hue when wet and is softer. Arsenopyrite is silver white and does not
become more yellow when wet. Hazards
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.
Acid drainage Sulfate released from decomposing pyrite combines
with water, producing sulfuric acid, leading to acid rock drainage and potentially acid
rain. Dust explosions
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 oxygen.
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 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. Pyritised fossils
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.
References
Further reading American Geological Institute, 2003, Dictionary
of Mining, Mineral, and Related Terms, 2nd ed., Springer, New York, ISBN 978-3-540-01271-9
Mineral galleries External links
How Minerals Form and Change "Pyrite oxidation under room conditions".
Poliakoff, Martyn. "Fool's Gold". The Periodic Table of Videos. University of Nottingham.