What Is Pyrite?

In this chapter I show how pyrite was at the heart of our early understanding of the composition of substances and how it was central to the acceptance of the revolutionary idea that substances have fixed compositions. This, in turn, was the evidential basis for the modern atomic theory. Taxonomists will argue that naming things accurately is important since otherwise no-one will know what you are talking about. They would disagree with Shakespeare that a rose would smell as sweet whatever it was called on the grounds that you would not know that a rose was being described. Even so, the only reason things are named is because of need. Thus Homer did not have a word for blue because he never needed it: the blue sea became wine-dark, for example. By contrast, contemporary ancient Egyptians had a word for blue because they used the blue mineral lapis lazuli for decoration. The mineral pyrite has been employed by humankind for millennia, and it needed a name. Its long history means that a variety of terms have been used to describe it, often reflecting the technology available at the time. In order to understand the role that pyrite has played in the past, we need to interpret the various names given to this mineral by earlier authorities. This problem is compounded since its history is determined by ancient texts and these were commonly written down by scribes from direct dictation. The scribes rendered the sounds of words as best they could within the limitations of the current orthography. Before the advent of printing, copyists made reproductions of these original texts according to the customs and mores of their local culture. The texts that have come down to us are usually the result of the work of several generations of copyists, and the interpretations become like a game of Chinese whispers. Whether or not a word in an ancient text means pyrite is, at best, a matter of relating it to a description that reflects key properties of the mineral. At worst it may mean probing the etymology of the word and considering its context.

Microbes and Minerals

Pyrite consists of two elements—iron and sulfur—but considerations of pyrite formation have mainly concerned sulfur. Iron is extremely abundant in the Earth; in fact, it is the fourth most abundant element on Earth and is less localized in its distribution. By contrast, sulfur is the 17th most abundant element in the Earth’s crust, and there are about 100 times more iron than sulfur. The interest in the primary role of sulfur in pyrite formation continues to the present day. In the Old Latin of early Republican Rome (i.e., before c. 200 BCE), sulfur was called sulpur or burning stone (i.e., brimstone). The p was pronounced with a puff of air. This puff was transliterated with an h following the p. When the f sound was introduced into classical Latin, p was often changed to ph in Latin words of Greek origin. Sulpur, however, had no Greek roots. The Greeks called it θείον (thion), which gave rise to our prefix, thio-. Sulfur had been written as sulphur in Old Latin, with the h indi­cating the puff of air after the p, but when the f sound was introduced this gave the mistaken impression that sulphur was originally a Greek word. At the end of classical times (around 27 BCE) the spelling was altered to sulfur, which is the spelling that usually appears in Latin dictionaries. In the last millennium, the element has traditionally been spelled sulphur in the United Kingdom and countries where UK rule held sway. By contrast, US English has continually used the correct sulfur spelling. The fountainhead of all chemical definitions worldwide, the International Union of Pure and Applied Chemistry, adopted the spelling sulfur in 1990. Finally, the UK authorities admitted their error and the UK Royal Society of Chemistry Nomenclature Committee recommended the correct spelling in 1992. In 2000 the authority determining quality and standards in UK schools decreed UK children should be taught the sulfur spelling. The sulphur spelling still occurs, but at best this is a literary affectation.

Pyrite and the Global Environment

The two basic processes concerning pyrite in the environment are the formation of pyrite, which usually involves reduction of sulfate to sulfide, and the destruction of pyrite, which usually involves oxidation of sulfide to sulfate. On an ideal planet these two processes might be exactly balanced. But pyrite is buried in sediments sometimes for hundreds of millions of years, and the sulfur in this buried pyrite is removed from the system, so the balance is disturbed. The lack of balance between sulfide oxidation and sulfate reduction powers a global dynamic cycle for sulfur. This would be complex enough if this were the whole story. However, as we have seen, both the reduction and oxidation arms of the global cycle are essentially biological—specifically microbiological—processes. This means that there is an intrinsic link between the sulfur cycle and life on Earth. In this chapter, we examine the central role that pyrite plays, and has played, in determining the surface environment of the planet. In doing so we reveal how pyrite, the humble iron sulfide mineral, is a key component of maintaining and developing life on Earth. In Chapter 4 we concluded that Mother Nature must be particularly fond of pyrite framboids: a thousand billion of these microscopic raspberry-like spheres are formed in sediments every second. If we translate this into sulfur production, some 60 million tons of sulfur is buried as pyrite in sediments each year. But this is only a fraction of the total amount of sulfide produced every year by sulfate-reducing bacteria. In 1982 the Danish geomicrobiologist Bo Barker Jørgensen discovered that as much as 90% of the sulfide produced by sulfate-reducing bacteria was rapidly reoxidized by sulfur-oxidizing microorganisms. Sulfate-reducing microorganisms actually produce about 300 million tons of sulfur each year, but about 240 million tons is reoxidized. The magnitude of the sulfide production by sulfate-reducing bacte­ria can be appreciated by comparison with the sulfur produced by volcanoes. As discussed in Chapter 5, it was previously supposed that all sulfur, and thus pyrite, had a volcanic origin. In fact volcanoes produce just 10 million tons of sulfur each year.

Crystals and Atoms

According to one magic crystal website, pyrite is a highly protective stone blocking and shielding you from negative energy. This may originate from Pietro Maria Canepario, who in 1619 cited Avicenna as stating that “if pyrite is worn on an infant’s neck, it defends him from all fear.” Other New Age sources maintain that pyrite can be beneficial when planning large business concepts because placing a piece on the desk energizes the area around it. Pyrite also reduces fatigue and is good for students because it is thought to improve memory and recall and to stimulate the flow of ideas. So you are certainly reading the right book . . . The magical properties of pyrite stem at least partly from the occurrence of pyritized ammonites (Figure 4.1) in ancient Egypt. Ammonites are fossils of coiled mollusks that became extinct at the same time as the dinosaurs at the end of the Mesozoic Era, about 60 million years ago. Ammonites got their name because they resemble coiled ram’s horns and the Egyptian god Amun (or Amon, Ammon, etc.) usually wore ram’s horns. The person responsible for this flight of fancy was Pliny the Elder, who called these fossils ammonis cornua or horns of Ammon. The golden pyritized ammonites were prized as lucky charms and worn as amulets in ancient Egypt. They are common today and may be readily collected from the beach at Charmouth in southern England, particularly after a storm has caused more fresh rock from the cliffs to tumble down onto the beach. The bright golden crystals of pyrite have fascinated humankind through the ages. The crystals display a variety of distinct shapes that make them extremely attractive. Indeed, pyrite may display the greatest variety of crystal forms of any common mineral. The great American mineralogist James Dwight Dana described eighty-five different forms, and the founder of geochemistry, Victor Moritz Goldschmidt, drew line drawings of almost 700 different pyrite crystals. In this chapter I show how the explanation of this extraordinary diversity of pyrite crystal shapes (or habits, formally) has helped reveal the nature of the material universe.

Pyrite and the Origins of Civilization

Pyrite is an often-overlooked material today although it has been instrumental in enabling many aspects of our modern culture and industry. This bright, brassy mineral is the most abundant metal sulfide in the Earth’s crust and provides a marked chemical contrast to the duller silicates and oxides that constitute most rocks. Most people today are familiar with the mineral, even though they do not know its details, because it stands out in the natural environment and because of the connection with fool’s gold. Pyrite has been a source of both metals and sulfur since ancient times, and both of these commodities have been key to our civilization. The mineral is easily decomposed by heat with the production of sulfur, sulfur oxide gases, and a metal-rich slag. It oxidizes readily in aerated water to form red and yellow ochers that may be used as pigments. It commonly occurs with other valuable metals that may be extracted by leaching or heating with various fluxes. In summary, it is an exceptional mineral whose benefits were readily available to primitive societies and have led to the development of our modern civilization. One of the extraordinary facets of our modern civilization is that we take lighting fires for granted. All you need is a cheap match. However, this is a relatively recent invention. So how did the early Victorians and their predecessors light their fires? Old films and television series, the so-called costume dramas, rarely, if ever, show people lighting fires. One reason for this was that lighting a fire could be a long process, so once it was lit, it was kept going. Even I remember that letting the fire go out was a heinous crime in the days before central heating, when our house was heated by a coal fire. The fire was kept going during the coldest winter weeks: it was banked up at night with coal, which kept it nicely smoldering while we slept. Its heat prevented the water pipes in the house from freezing during the iciest nights and subsequently bursting when they were warmed up again.

Acid Earth

The atmosphere and much of the rivers, lakes, and oceans of the Earth are oxygenated. Any pyrite that comes into contact with these environments becomes unstable and breaks down. The process is called oxidation. It is an exothermic process and, as described in Chapter 5, this process was thought to heat the Earth. It is the opposite of reduction, which we discussed with regard to the microbial formation of sulfide from sulfate in Chapter 6. The counterintuitive concept important here is that oxidation is a chemical process that does not necessarily need oxygen. This idea—that you can oxidize things in the absence of oxygen—is one that most natural scientists are aware of but that they need a couple of nudges occasionally to remind themselves about. This means that pyrite oxidizes not only in oxygenated environments—although that is what we are most familiar with—but also in oxygen-free environments. Among the products of pyrite oxidation are large quantities of acid. Although this happens naturally during rock weathering, the intervention of humankind has led to an enormous increase in the exposure of pyrite to the atmosphere. This has produced contamination of the atmosphere, groundwater, and watercourses on a regional scale. It has also increased the amount of uncontrolled coal burning in coal seams, coal mines, and coal waste tips worldwide, making whole towns uninhabitable and laying waste to large areas. In this chapter I consider in more detail what exactly the process of pyrite oxidation is and how it affects the Earth’s environment today, as well as the problems it stores up for humanity in the future. In chemical terms, oxidation does not mean just the addition of oxygen. Oxidation is a reaction that involves the removal of one or more electrons from a compound because of a chemical reaction. One of the most familiar oxidation reactions is combustion, where substances burn in air to produce heat. The way to put out such a fire is to restrict oxygen access using a chemical foam or fire blanket. Since this reaction with oxygen was the best known, the process was called originally called oxidation.

Hell and Black Smokers

Most of the important metal ores in medieval and ancient times were pyrite-rich sulfides. These pyrite-rich ores were a major source of a suite of valuable commodities such as sulfur, arsenic, copper, lead, zinc, and nickel, as well as some gold and silver. This is why in 1725 Henckel could devote a 1,000-page volume to pyrites, sensu lato. Because of its relative abundance, its potential economic importance, and its exotic composition compared with the rock-forming minerals, pyrite has played a key role through the ages in developing ideas of how minerals and ore deposits form. During the last century, pyrite became an even more important mineral in discussions of ore genesis because it is also a key component of sediments. This led to conflicting theories of ore genesis, in which the ore minerals were formed in the sediments or introduced later, often by processes related to volcanism. The conflict between adherents of these theories continues to this day. Pyrite constituted a key, but sometimes uncomfortable, mineral in ancient theories of mineral formation. It was relatively common and often economically important. However, it contained sulfur as a key constituent and this contrasted it to many other common minerals and rocks in that this meant that pyrite could be changed by heating. Heating released sulfur from pyrite, leaving a residue of stony slag. The ancients also recognized sulfur as a special material since it occurred in solid, liquid, and gaseous form, rather like water. Any theory of mineral formation needed to explain how this protean element got into pyrite. This problem was compounded by the fact, discussed in Chapter 3, that for some unknown reason the ancients did not know that pyrite contained iron. Ancient theories of mineral formation divide into three categories: (a) the Genesis theory: that all minerals were formed by God during the creation of the Earth; (b) the Aristotelian theory: that all minerals were formed at depth in the Earth through the interactions of the four basic elements; and (c) the Alchemical theory: that minerals were formed from combinations of mercury and sulfur.

Foole’s Gold

This classic opening gambit at the stereotypical drinks party always throws me. I have been a professor at a university for most of my life, so the easiest answer is that I teach. This is true, but it disguises the reality that much of my waking time has been concerned with research. If I admit this, then it becomes necessary to explain what I actually research. One of my pet subjects is pyrite. But if I let on that I research pyrite, my interlocutors look at me as though I am one of those wonderful beings who haunt the bowels of natural history museums as world experts on a rare species of toad. As with toads, most people in the world have heard of pyrite. They know it is a mineral or stone, and most know that it is also called fool’s gold, a familiar theme of moral tales and nursery stories. So the idea of someone studying pyrite is not altogether the stuff of IgNobel prizes. Within the time limits imposed by decent conversation I cannot explain that pyrite is the mineral that made the modern world. I cannot refer them to a book about it since there has not been one published about pyrite since 1725. This book is an attempt to rectify the situation. In it I contend that pyrite has had a disproportionate and hitherto unrecognized influence on developing the world as we know it today. 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 book is aimed at making the subject accessible to the general reader. It is not a scientific monograph, since these handle only the science and are really directed at the converted: the high priests of the cathedral of science and technology and their aspirant novices. It is also not aimed at being a textbook in the conventional sense: textbooks are generally aimed at specific academic courses and ultimately pave the way for the students to understand the monographs.

Full Circle

The thesis in this book is that pyrite has been a key material in the development of our civilization and culture. It has figured in the foundation of nations and key industries, in the development of science, and in our current understanding of the nature of matter. It has played a key role in the development of our culture mostly through its use in the most important of human inventions: the taming of fire. Pyrite has determined the nature of the Earth’s surface environment and the origin and evolution of life itself. I have discussed how pyrite affects our present environment through its key role in the great biogeochemical cycles of fundamental substances like oxygen and carbon and how this has continued through over 4,000 million years of Earth history. This long history of the centrality of pyrite to the Earth system has enabled confident predictions about how pyrite is going to affect future Earth environment through acidification of atmo­spheres, rivers, and soils and eutrophication of the oceans, for example. Pyrite has played a central role in the development of humankind for the entire 200,000 years of the existence of Homo sapiens sapiens and this is unlikely to end now. It seems incontrovertible that pyrite will play a similar role in future human development as it has for the last 200,000 years. In this chapter I return to some of the themes from previous chapters and show how pyrite is still influencing our society and how this is likely to continue into the future. Gold occurs naturally in two basic forms: visible gold and invisible gold. Invisible gold was a term used by one of the greatest of 20th-century gold prospectors, John Livermore, to describe gold that “would not pan”—that is, gold that did not appear in the prospector’s pan when the crushed rock or natural gravel was gently swirled around with water. Livermore discovered invisible gold in Nevada, which led to the 1980s gold rush in that state that has, to date, produced gold to a value of over US$85 billion.

Pyrite and the Origins of Life

If you have been reading this book since the beginning, you will not be surprised by now to find that you have come across a chapter documenting the involvement of pyrite in the origin of life. This is because you will have read in this book how pyrite has been at the root of many fundamental discoveries about the nature of our world. So you do not suffer more than eyebrow-raising surprise and maybe a gentle throat-clearing in learning that pyrite is contributing to our current understanding of the origins of life. By contrast, if you have dived in at Chapter 9 you probably look at the title of this chapter with disbelief. After all, what could be the connection between a common glitzy mineral and the origin of life? The more diligent reader will have already learned that pyrite formation is intimately associated with biology because most of it is produced by bacteria that extract their oxygen from sulfate and produce hydrogen sulfide. This relationship is so overweening today that pyrite formation controls many fundamental aspects of the Earth’s environment. So what happens if we extend this line of inquiry back to the beginnings of geologic time? We have already seen that the characteristics of ancient pyrite are one of the main sources of information about the nature of the early Earth. The consequence of this is that we know quite a bit about the relationship between pyrite and early life on Earth. In this chapter, we further explore this and review the laboratory work that implicates pyrite itself in the original syntheses of the self-replicating biomolecules that assembled to produce Earth’s first life forms. The thesis that life developed from nonbiological chemistry is a very old idea stretching back through Anaximander in 6th-century BCE Greece to the Vedic writings of ancient India around 1500 BCE and is often called abiogenesis.