Pyrite

The mineral pyrite ( PY-ryte), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.

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.

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 actual gold. A substantial proportion of this is "invisible gold" incorporated into the pyrite. It has been suggested that the presence of both gold and arsenic is a case of coupled substitution. However, as of 1997, the chemical state of the gold remained controversial.

Etymology The name pyrite is derived from the Greek πυρίτης λίθος (pyritēs lithos), 'stone or mineral which strikes fire', in turn from πῦρ (pŷr), '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 is now called pyrite. By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulfide minerals.

Uses

Historical

The oldest definitive evidence for fire making (i.e. igniting a new fire) dates to ~400,000 years ago at a Neanderthal site in Suffolk, England, where burnt soil was found along with fire-cracked flint handaxes and two fragments of iron pyrite, used to strike sparks with flint. At other sites in France dating from 50,000 years ago onwards, dozens of Neanderthal hand axes exhibit use-wear traces suggesting they were struck with pyrite to produce sparks. Ötzi, a well-preserved natural mummy of a man who lived in the Ötztal Alps between 3350 and 3105 BCE, carried fire-making material in the form of tinder fungus with flint and pyrite for creating sparks. The Kaurna people of South Australia have used pyrite with flintstone and a form of tinder made of stringybark as a traditional method of starting fires. Pyrite has been used since classical times to manufacture copperas (ferrous 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, new methods of 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. Marcasite jewelry, using small faceted pieces of pyrite, often set in silver, has been made since ancient times and was popular in the Victorian era. When the term became common in jewelry making, "marcasite" referred to all iron sulfides including pyrite, and not to the eponymous orthorhombic FeS2 mineral marcasite, which is lighter in color, brittle and chemically unstable, and thus not suitable for jewelry making. Pyrite gained a brief popularity in the 16th and 17th centuries as a source of ignition in early firearms, most notably the wheellock, where a sample of pyrite was placed against a circular file to strike the sparks needed to fire the gun. 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.

Present day 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 (1,004 °F); at around 700 °C (1,292 °F), pS2 is about 1 atm. A newer commercial use for pyrite is as the cathode material in Energizer brand non-rechargeable lithium metal batteries. Pyrite is a semiconductor material with a band gap of 0.95 eV. Pure pyrite is naturally n-type, in both crystal and thin-film forms, potentially due to sulfur vacancies in the pyrite crystal structure acting as n-dopants. Pyrite has been proposed as an abundant, non-toxic, inexpensive material in low-cost photovoltaic solar panels due to its high absorption of visible light . Synthetic iron sulfide can be used with copper sulfide to create the photovoltaic material. More recent efforts are working toward thin-film solar cells made entirely of pyrite. Pyrite specimens are also very popular in mineral collecting. Among the sites that provide the most sought-after specimens are Soria and La Rioja provinces (Spain). In terms of monetary value, China constitutes the largest market for imported unroasted iron pyrites worldwide, at $47 million, making up 65% of global imports. It is also the fastest growing, with a CAGR of +27.8% from 2007 to 2016.

Research In July 2020 scientists reported that they have observed a voltage-induced transformation of normally diamagnetic pyrite into a ferromagnetic material, which may lead to applications in devices such as solar cells or magnetic data storage. Researchers at Trinity College Dublin, Ireland have demonstrated that FeS2 can be exfoliated into few-layers just like other two-dimensional layered materials such as graphene by a simple liquid-phase exfoliation route. This is the first study to demonstrate the production of non-layered 2D-platelets from 3D bulk FeS2. Furthermore, they have used these 2D-platelets with 20% single walled carbon-nanotube as an anode material in lithium-ion batteries, reaching a capacity of 1000 mAh/g close to the theoretical capacity of FeS2. In 2021, a natural pyrite stone was crushed and pre-treated followed by liquid-phase exfoliation into two-dimensional nanosheets, which showed capacities of 1200 mAh/g as an anode in lithium-ion batteries.

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