The Many Faces of Fool's Gold

Pyrite, an iron sulfide, may be worthless to gold miners, but the mineral has great utility in everything from fertilizer to electronics.

Agriculture Chemistry Technology Metallurgy

Current Issue

This Article From Issue

May-June 2016

Volume 104, Number 3
Page 174

DOI: 10.1511/2016.120.174

The glittering golden mineral pyrite, an iron sulfide (FeS2), is known to most people as fool’s gold, something that promises great value but is intrinsically worthless. But pyrite is, has been, and will be important to you and the rest of humankind: It is the mineral that made the modern world.

Pyrite crystals most commonly take on cubic formations; some are smooth (<em>top left</em>), but many are striated (<em>top right</em>). Irregular pentagonal dodecahedrons (<em>bottom right</em>) are the crystal form found next most often. Octahedrons capped with cubic faces (<em>bottom left</em>) are the least common natural crystal shape. <strong>Top left and bottom right images courtesy of the author and Oxford University Press; top right, courtesy of J. Murowchick; bottom left, courtesy of Carlos Millan.</strong>
Ad Right

This influence extends from human evolution and culture, through science and industry, to ancient, modern, and future Earth environments, and the origins and evolution of early life on the planet.

The role of pyrite in fire-lighting is a feature of all ancient civilizations. It led to the development of the modern chemical, pharmacological, and armament industries, in which pyrite continues to play a vital role. Even today, pyrite is still used in the manufacture of sulfuric acid—the most abundantly manufactured chemical on the planet. The production of medicines based on pyrite, such as the alums, paralleled the development of the chemical industry and can be shown to be the origin of Big Pharma, the industrial production of medicines.

Pyrite crystals stand out in nature, and the ancients were naturally curious as to how these were formed. This led to the idea that pyrite formation occurred deep within the Earth and the mineral was brought to the surface by volcanoes. And yet many occurrences of pyrite in nature do not appear to be related to volcanism at all. Pyrite can be found in soils and sediments throughout the Earth as myriads of microscopic crystals. This pyrite is formed by bacteria that remove oxygen from sulfate in the water, producing sulfide that reacts with iron to form pyrite. More than 90 percent of the pyrite on Earth is formed by microbiological processes. Bacteria also catalyze pyrite’s oxidation and breakdown.

Pyrite is already playing a significant role in frontier areas of science and technology, such as nanotechnology and energy conversion. The remarkable chemical and physical properties of this mineral ensure that it will continue to do so. Likewise, the widespread distribution of huge pyrite concentrations throughout both the land and the oceans of the Earth will ensure that pyrite remains an important source of raw materials needed by a future 10 billion human beings.

Crystal Shapes

The pyrite unit cell, the building block of the pyrite crystal, is basically a cubic structure. Cubes are also the most common pyrite crystal form. The rate of growth of the crystal faces is different on each face, and square prisms are more probable results than perfect cubes. In the extreme case this feature may result in the formation of pyrite wires. This explains the formation of balls of radiating pyrite crystals, commonly found in limestone and chalk, where they are produced from the sulfur and iron in groundwater. The individual pyrite crystals have simply grown into elongated forms that radiate from a center.

A broken surface of a 5-centimeter diameter pyrite nodule shows radiating pyrite crystals. <strong>Image courtesy of the author and Oxford University Press.</strong>

The next most common crystal of pyrite is the pentagonal dodecahedron, but it turns out not to be regular; the interfacial angles are not all 102 degrees.

Octahedra are the least common natural pyrite crystals. And they are not, in fact, all perfect: The tips are flattened off with cubic faces. The reason for this formation was first suggested by the great Japanese mineralogist Ichiro Sunagawa in 1957, and I further developed this theory in 2012. In order for a crystal to grow, the concentrations of the dissolved constituents must exceed the solubility product of the mineral. Pyrite is a very insoluble mineral, so its solubility product is very low—so low that the concentrations of iron and sulfur in solution are virtually immeasurable. Nucleation is the key here: It refers to the first stage of the formation of crystals, when atoms and molecules initially coalesce. Pyrite will not nucleate from solution unless there is 100 billion times more iron and sulfur in solution than the equilibrium concentration.

The dodecahedral face requires the greatest amount of energy and thus tends to be preferred at the highest saturations. The octahedral face is the next highest, and the most stable cubic face the least. So in a situation where the supply of nutrients is limited, crystal growth depletes the concentration of the dissolved components and the crystal faces change with time. The octahedral crystal will grow until the nutrients in solution are used up, and then the cubic faces will take over. So most octahedra are capped by cube faces.

My research group designs pyrite crystals with various shapes, with applications in the Earth and environmental sciences and in materials science. For example, if we understood what controlled the shape of a natural pyrite crystal, we would know what the environment was like when the crystal was formed millions of years ago.

By varying the concentrations of dissolved iron and sulfur, and the hydrodynamics of the solution, a vast array of forms of pyrite crystals can be produced. This explains how a chemically simple mineral such as pyrite may exhibit the greatest variation in natural crystal forms in the mineral kingdom.

Pyrite Rasberries

One of the most common forms of pyrite in nature is as small, globular aggregates of pyrite crystals called framboids, because they look like tiny raspberries. Pyrite framboids are mostly invisible to the naked eye, with diameters usually around 0.01 millimeter. Framboids are found in rocks, especially sediments, of all ages. The oldest reported pyrite framboids may be from 2.9-billion-year-old sediments from South Africa. They are therefore extremely stable configurations and can last over eons of geologic time.

 group of pyrite framboids (<em>a</em>) shows various ordering patterns of the individual pyrite crystals. A close-up of a single framboid (<em>b</em>) shows the typical subspherical form and partial ordering of the 0.001-millimeter pyrite crystals. <strong>From D. Rickard, 2012, <em>Sulfidic Sediments and Sedimentary Rocks</em>; courtesy of Elsevier.</strong>

The abundance of pyrite framboids is quite extraordinary. A guesstimate of the total number of framboids in the world suggests that there are around 1030, which is 10 billion times the number of sand grains in the world, or about 1 million times the number of stars in the universe. Today, some 1012 pyrite framboids are being formed every second.

In the early 20th century, improved microscopy showed that these spherules consisted of aggregates of pyrite crystals less than 0.001 millimeter in size. So each framboid may contain more than 1 million tiny crystals of pyrite, each of which has a similar shape and size. Not only that, but they are often beautifully organized and arranged in the framboid.

Detailed studies by my group revealed that framboids are not truly spherical but have flattened faces. They did not grow like normal crystals but aggregated together under the influence of their surface electrical charges. Because these crystals are so small, with 50 million of them usually needed to make up 1 gram of pyrite, these tiny surface electrical forces are sufficient to stick the crystals together.

The Earliest Chemical Industry

It may not be generally appreciated how important pyrite has been and still is to the world economy and to providing the basics for our current civilization. Pyrite continues to be mined worldwide and is a major source of sulfur, the basic constituent of sulfuric acid. Sulfuric acid has become one of the most important industrial chemicals, and more of it is made each year than any other manufactured chemical.

Sulfuric acid is such an important commodity chemical that a nation’s industrial strength can be indicated by its sulfuric acid production. World production in 2004 was about 180 million tons. Sulfuric acid is used in the chemical industry for production of detergents, synthetic resins, dyestuffs, pharmaceuticals, petroleum catalysts, insecticides, and antifreeze, as well as in various processes such as oil-well acidicizing, aluminum reduction, paper sizing, and water treatment. It is used in the manufacture of pigments and includes paints, enamels, printing inks, coated fabrics, and paper. The list is endless and includes the production of explosives, cellophane, acetate and viscose textiles, lubricants, nonferrous metals, and batteries.

Sulfuric acid is a relatively recent manufactured chemical. Prior to this, the important analogous chemical substances were the sulfate salts of iron, copper, and aluminum, known to the ancients as the vitriols. These occurred in the lists of minerals compiled by the Sumerians 4,000 years ago. They were used as mordants in the dyeing industry. In order for natural dyes to be fixed in the cloth—and not be washed out during the next rainy day—it is necessary to treat the cloth with a mordant. The mordants widely used in dyeing were solutions of the vitriols. The demand for vitriols could not be satisfied from natural supplies, and industries developed to manufacture this substance from pyrite. It became particularly important in late medieval England, when much of the nation’s wealth was dependent on the wool trade. The production of one mordant, pure alum, from pyrite has been described as the point of origin of the modern chemical industry, because the process required not only the manufacture of a chemical substance but also its purification.

Big Pharma

The manufacture of artificial drugs—in contrast to the use of natural remedies—can be traced back to pyrite and strike-a-lights. It is not a big step to drop pyrite from a strike-a-light into the fire. The result is the formation of sulfur oxide gases with their characteristic burnt smell. These sulfur oxide gases, apart from being poisonous in high doses, can clear clogged-up noses and are very useful in fumigation.

Pyrite is a major source of sulfur, the basic constituent of sulfuric acid, which is one of the most important industrial chemicals, and made in greater amounts each year than any other manufactured chemical.

By 300 CE sulfur was being produced from pyrite in China’s Shanxi, Hebei, Henan, Hunan, and Sichuan provinces. One of the earliest descriptions of the medicinal use of sulfur was in The Pharmacopeia of the Heavenly Husbandsman, compiled in the Western Han period (206 BCE–24 CE), which cataloged the medicines invented some 3,500 years earlier by the legendary emperor Shen Nong.

Medical sulfur had to be produced from pyrite in the absence of deposits of natural sulfur. Sulfur was used mainly in creams, to alleviate conditions such as scabies, ringworm, psoriasis, eczema, and acne. The mechanism of action is unknown—although sulfur does oxidize slowly to sulfurous acid, which in turn (through the action of sulfite) acts as a mild reducing and antibacterial agent.

The use of alum in medicine has been documented for more than 2,000 years since the Babylonians listed it in one of the first pharmacopeias. The main medicinal use of alum was, as it still is today, as an astringent to improve wound healing. The modern styptic used to close up razor nicks occurring after wet shaving is alum-based.It helps reduce swelling of the skin around healing sores. It has also been used as an emetic to treat someone who has ingested a poison.

Pyrite Feeds the World

We have seen that pyrite is the raw material from which sulfuric acid can be made, and a major use of sulfuric acid in modern economies is in the production of fertilizers. About 60 percent is currently consumed for fertilizer manufacture, especially superphosphates, ammonium phosphate, and ammonium sulfates.

During the early part of the Industrial Revolution, sulfur in Europe was sourced from natural sulfur deposits associated with volcanic fumaroles in Sicily. In 1839 the Sicilian deposits came into the hands of a French company, which raised the price threefold. This led to other countries reverting to pyrite as a source of sulfur. Roasting of pyrite produces sulfur oxide gases, and these can be dissolved in water to produce sulfuric acid. Byproducts of the process include copper metal from the pyrite and an iron-based slag that is used in road-building.

The United Verde massive pyrite deposit in Jerome, AZ, shows multicolored rocks resulting from the oxidation of pyrite. For scale, the mine entryway at center left is approximately human-sized. <strong>Image courtesy of the author and Oxford University Press.</strong>

It has been estimated that the population of Great Britain was constrained to around 6 million in preindustrial times due to the limitations of agricultural productivity. This compares with more than 60 million today. The excess 54 million people are fed by postindustrial technological advances. This step increase in agricultural productivity was fueled by the development of industrial fertilizers. This, in turn, caused a consequent exponential increase in the demand for sulfuric acid, sulfur, and pyrite.

A collapsed slope in the Falun mine in Sweden shows the effect of 300 years of oxidation on a massive pyrite ore. Sulfate stalactites of iron (<em>green</em>), copper (<em>turquoise</em>), calcium, magnesium, zinc, and lead (<em>white</em>) protrude from a mass of iron oxide ocher. The temperature in the mine can reach 50 degrees Celsius. <strong>From D. Rickard and R. O. Hallberg, 1973, <em>De Små gruvarbetarna i Falun. Sevens Naturvetenskap</em>, pp. 102–107.</strong>

Pyrite reserves are distributed throughout the world, and known deposits have been mined in about 30 countries. Currently global pyrite production is about 14 million tons per year, and about 85 percent of this is in China. This production is equivalent to around 7 million tons of sulfur annually with a value of about $160 million. This amounts to around 10 percent of the total world sulfur production. Most of this sulfur is used in sulfuric acid manufacture, and most of the sulfuric acid is used to make fertilizers. In this context, pyrite continues to be a major factor in food production.

Crystal Diffraction

The year 2014 was designated the International Year of Crystallography by the General Assembly of the United Nations. The reason is that the science of crystallography is little appreciated by the general public or understood by fellow scientists, apart from the crystallographers themselves. And yet this science has won more Nobel Prizes over the past century than any other subdiscipline. Of the 136 Nobel Prizes in science and medicine that have been awarded since 1901, more than 100 have directly involved crystallography. The golden crystals of pyrite have played a key role in the development of crystallography, ultimately permitting atoms themselves to be counted, imaged, and probed.

If you look at the surface of a CD or DVD disk at an angle, you will see a shimmering spectrum of colors on the surface of the disk as bands of luminous greens and blues seem to radiate out from the center of the disk. The grooves on the disk are diffracting the light that is being reflected from its silver surface. Diffraction occurs when a wave encounters an obstacle. As the wave hits an object, new waves are produced at all points along the wave front. These waves propagate spherically, and thus light can appear to bend as it passes an object. If there is a narrow slit, light will appear to bend around both edges of the slit. And if the width of the slit approaches the wavelength of the light, the light waves emitted from the slit edges will either be in phase or out of phase: If the diffracted waves are in phase (that is, their peaks and troughs are coincident), then the resultant intensity is increased; if the diffracted waves are out of phase, then the peaks are canceled out by the troughs and no light is seen. In the case of light, the troughs and ridges are represented by a series of bands. These depend on the wavelength of the incident beam and the density of the slits in the object. The diffraction effect is seen on the fine grooves of a CD disk but not on a grill, for example. In a typical diffraction grating, the number of slits ranges from a few tens to a few thousand per millimeter. Note that because there is a relationship between the wavelength of light and the slit width, each wavelength of the incident beam is sent in a slightly different direction. This can produce a spectrum of colors from white light illumination, visually similar to the operation of a glass prism; this is the shimmering, multicolored effect on the CD surface. The upshot of all this is that by measuring the angle of the emitted light from a diffraction grating and its wavelength, we can calculate the size and number of the slits in the grating that produced the spectrum.

In 1912 Max Laue reported that x-rays were diffracted by crystals. Laue’s great insight was to realize that because x-rays have wavelengths similar to that of the distances between atoms in crystals, the atoms would diffract the x-ray waves, producing bands of more and less intense x-rays. As with the CD and other diffraction gratings, the distances between the x-ray bands and their intensities depend on the distances between the atoms in the crystal. X-rays exited in a pattern determined by the atomic structure.

The technique was seized upon by W. H. Bragg and W. L. Bragg. The Braggs realized that the angles and wavelength of the x-rays diffracted by a crystal would be functions of the positions of the planes of atoms in the crystal. Because there are several such planes in any crystal, this would enable the atomic structure of the crystal to be computed.

Pyrite was one of the first crystalline materials investigated by the Braggs. They used it to demonstrate that x-rays behaved in the same manner as light and not as a series of particles. In 1914, W. L. Bragg succeeded in solving the pyrite structure and confirmed a theoretical mathematical model of pyrite.

Pyrite helped support the foundations of x-ray crystallography, because it showed how the method could be used to determine the structure of a more complex substance. This ultimately led to the determination of the structure of DNA in 1953 by Francis Crick and James Watson, based on Rosalind Franklin’s x-ray crystallographic analyses.

Pyrite and the Electronics Industry

Pyrite is a semiconductor; that is, it is neither a conductor like metal nor an insulator like most rocks. Semiconductors such as pyrite can switch between being a good conductor or insulator under the effects of electric fields or light, or by doping the material with traces of impurities. In pyrite, only a small amount of energy is required to release electrons from being chained to the atomic nuclei so that they can move freely in the material and conduct electricity. In other words, a small amount of energy will switch pyrite from behaving like an insulator to behaving like a conductor.

A suspension of tiny pyrite crystals might be sprayed onto solar panels like paint.

Satisfying the increased demand for electricity will be one of the fundamental problems faced by humankind over the next 50 years. It is estimated that more than 30 million billion watts of extra power will be required by 2050, and supplying this by fossil-fuel generation not only is improbable but also would have a considerable impact on the Earth’s climate. The obvious solution is to capture the energy from the Sun using solar panels. However, current silicon-based solar panels are expensive. The energy cost, amortized over the 20-year lifetime of the panel, is around twice as much as that of wind- and natural gas–generated electricity. This is where pyrite comes in as the most cost-efficient alternative solar panel material to conventional silicon.

Pyrite absorbs 100 times as much light as the present major solar cell material, silicon. A thin layer of pyrite, just 0.1 millionth of a meter in thickness, theoretically absorbs almost 90 percent of the solar radiation, whereas thicker current silicon-based systems harvest less than 20 percent. Silicon, although it is the second most abundant element in the Earth’s crust, is expensive to extract. The cost of extraction is about $1.7 per kilogram, or more than 50 times as much as pyrite. Because only a very thin layer of pyrite is required to collect the sunlight, suspensions of tiny pyrite crystals, such as those that constitute the ubiquitous pyrite framboids, might be mixed in a solvent and sprayed onto panels like paint. Considerable research is going on worldwide at present to synthesize pyrite crystals and films with various compositions in order to produce an optimal solar energy collector.

The other way to help resolve the world energy gap is to find a better way to store electricity. Electric automobiles are wonderful, except for the fact that they are at present limited to a 100-mile working distance and a 24-hour charging cycle. Portable computers are fantastic—for eight hours until the battery runs out. Pyrite is a source material for sulfuric acid, and one use of it is in car batteries: It is the acid in the lead-acid battery. These lead-acid batteries are still used in automobiles, even though the technology is ancient, because they are rechargeable. However, these lead-acid batteries are cumbersome and not suitable for many applications where a small solid-state battery is required. The problem with these small batteries is that they are not especially powerful or, in many cases, rechargeable. There have been many recent advances in battery technology. One of the most familiar is the development of lithium batteries. In the Energizer series of lithium batteries, lithium metal is the anode (the negative electrode), and pyrite is the cathode (the positive electrode). This pyrite has been ground down to 0.1-millimeter particles and stuck on aluminum foil in the battery. The battery works by a redox reaction whereby the lithium metal is oxidized to produce lithium sulfide and the pyrite is reduced to iron. The redox reaction produces electrons, which we use as electricity. The lithium batteries are popular because they are relatively light, so the amount of energy per gram is optimized. At present these basically are not rechargeable, and the development of rechargeable lithium batteries is a major international target of technological research.

Pyrite is an attractive material for the electronics industry: It is widely distributed, cheap, and readily available. It has some environmental benefits in terms of the amount of energy required in transport and manufacture. All of these attributes are the same as those that originally placed pyrite at the core of early industrial development. It is interesting to speculate that the 21st century will see the burgeoning of a pyrite-driven electronics industry, just as earlier periods witnessed the development of pyrite-driven chemical, pharmaceutical, and explosives industries.

Invisible Gold

A microscopic image (below) shows gold occurring as tiny blebs entirely enclosed within a pyrite grain. In fact, pyrite is often associated with gold. The solutions in the Earth that transport iron and sulfur to form pyrite are also likely to transport other metals, including gold. Pyrite is slightly oxidized relative to other metal sulfide minerals. The slightly more oxidized environment in which pyrite precipitates also destroys the sulfide complexes that keep the gold in solution, and the gold precipitates as a metal. For these reasons, most gold deposits in the world contain pyrite as a more or less abundant mineral. In the case of so-called invisible gold, tiny precipitated gold particles have been trapped in the growing crystal of pyrite. The amount of gold within the grain shown above is probably around 1 percent by weight, because gold is about four times as heavy as pyrite. A ton of this pyrite would then contain around 10,000 grams of gold, with a present-day value of more than $400,000. It is worth mining if the gold can be extracted from within the pyrite.

A photomicrograph, taken with a blue filter, shows small blebs of yellow gold (arrow) within a 0.3-millimeter pyrite crystal. <strong>Image courtesy of the author and Oxford University Press.</strong>

Gold dissolves in cyanide solutions, and more than 90 percent of gold production is based on a cyanide process. In some low-grade ores, the crushed rock is piled into heaps and sprayed with a dilute cyanide solution. After about six months the liquor exiting the heap carries sufficient gold in solution to be collectable. Of course, cyanide is highly poisonous, and although efforts are made to contain and denature it, accidents and escapes continue to occur.

One of the problems with gold extraction is that much of the gold is included within pyrite and related minerals. This means that chemical leachates, such as cyanide, cannot get at the gold because it is protected by the pyrite. In the past the only way to release the gold was to roast the pyrite at 700 degrees Celsius or to digest it in strong acid in an autoclave at elevated temperatures and pressures. Both of these processes are very expensive. In 1986 industrial-scale testing of bio-oxidation of refractory gold ores was introduced by Gencor in South Africa. In this process the crushed ores are initially subjected to the attention of pyrite-oxidizing microorganisms in tank fermenters. The process exposes more of the gold, and subsequent cyanidation of the bio-oxidized concentrates can result in increases in gold recovery from just 30 percent to more than 95 percent. The use of microorganic reactors to prepare pyritic gold ores for cyanidation has become widespread, and numerous mines in South Africa, Australia, and North America have various operating systems.

The future of bioleaching of ores looks bright as ore grades become poorer and metal prices become higher, and there is a considerable amount of research going on at present into this technology. There is particular interest in the potential of real in situ bioleaching, where the ore deposit is not mined but fractured and the leach solutions are pumped down. The problem with this idea at present is the difficulty in recovering the solutions—they tend to disappear down fractures in the Earth’s crust, never to be seen again. It seems that whatever the future of this technology, pyrite will be at the heart of it.

This article is excerpted and adapted from Pyrite: A Natural History of Fool’s Gold by David Rickard, with permission from Oxford University Press. © Oxford University Press 2016.