Chapter 7

Minerals past, present, and future

Mineral evolution

According to current thinking, when the Universe began the only elements that existed were hydrogen, helium, and traces of lithium. There were no minerals. Millions of years after the Big Bang came the formation of the first stars within which all the other elements were made via nuclear reactions. Then as giant stars exploded in the first supernovae, it is proposed that the first crystalline matter, the first minerals in fact, formed in the cooling stellar envelopes. This would have been a carbon-rich environment, with diamond and graphite likely to have been the most abundant crystalline phases, together with lesser amounts of carbides, nitrides, oxides, and magnesium silicates. How did we get from this dozen or so primeval minerals to the more than 4,000 minerals known today? Strange as it may seem, it has only been in the last few years that this question has been systematically addressed through the concept of ‘mineral evolution’ as proposed by a group of mineralogists in the USA led by Bob Hazen of the Carnegie Institution of Washington.

In what we might call the ‘theory of mineral evolution’, it has been proposed that the Earth (itself accreted from a cloud containing the detritus of various supernovae and circling our early Sun) has evolved through three eras which can themselves be further subdivided to give a total of ten stages, with each adding to the total inventory of mineral species. The three eras have been named as the eras of planetary accretion (Stages 1 and 2), crust and mantle reworking (Stages 3, 4, and 5), and biologically mediated mineralogy (Stages 6, 7, 8, 9, and 10).

During Stage 1, approximately 4.6 billion years ago, about 60 mineral species formed as condensates from the Solar Nebula. These were the essential planet forming materials such as iron–nickel alloys, sulphides, phosphides, and the oxides and silicates which are stable to high temperatures. They would soon have clumped together to form the small planetary bodies known as ‘planetismals’. Stage 2 would involve some of these bodies becoming large enough to partially melt and, as a result, for their minerals to undergo alteration processes forming new minerals. This would have led to an enlarged inventory of about 250 minerals; those minerals found today in the diverse types of meteorites that fall to Earth.

Stage 3, the beginning of the crust and mantle reworking era (c.4.0–4.5 billion years ago) would have involved igneous processes such as volcanism, planetary degassing, fractional crystallization, metamorphism, and the beginnings of plate tectonics with its large-scale water–rock interactions. As discussed in Chapter 3, the formation of the different types of igneous rocks was described as an ‘evolution’ by N. L. Bowen in his classic work. All of these processes, which would have included a major role for volatiles such as water and carbon dioxide, might have brought the total number of mineral species to around 500.

Stage 4 (at an estimated time 3.5–4.0 billion years ago) requires the planet to have sufficient inner heat to melt its original basalt rock crust so as to form granite-type rocks and even pegmatites (coarsely crystalline, late-forming rocks produced by fractional crystallization and containing rare elements such as lithium, beryllium, boron, niobium, tantalum, and uranium). At this point the mineral inventory would total about 1,000 species. The final stage (Stage 5) of this second era is associated with fully developed plate tectonics. Volcanism associated with spreading centres and with subduction zones would have led to very large-scale water–rock interactions creating new minerals. Furthermore, there would have been the uplift associated with mountain-building leading to exposure at the Earth’s surface of the minerals in what had been deeply buried rocks. In this way our total number of minerals reaches around 1,500. Much of what we have described up to this point appears to hold true for the other rocky planets that we have begun to explore in our Solar System, but on Earth this number of 1,500 falls very far short of the total of around 4,400. So what makes the Earth so different? The answer is biologically mediated mineralogy.

During the first designated stage of the third era (Stage 6; 2.5–3.9 billion years ago), the Earth’s atmosphere was still without oxygen, and it is suggested that few new minerals were added at this time. The situation changed dramatically as a result of what is termed the ‘Great Oxidation Event’ (GOE) caused by the development of photosynthesis by certain bacteria and therefore of an oxygen-rich atmosphere (Stage 7; 1.9–2.5 billion years ago). This probably began about 2.4 billion years ago, at which time atmospheric oxygen may have risen to a little more than 1 per cent of present-day levels. More than half (about 2,500) of all known minerals are produced when other minerals are exposed to oxygen and oxygen-carrying waters at the surface of the Earth. Examples include the numerous clay minerals formed by the weathering of silicates, or the sulphates and hydroxides formed by alteration of sulphides. These would not have developed in an oxygen-poor environment, that is before the GOE changed the face of our planet.

The billion years or so following this rapid growth in the number of mineral species (Stage 8; 1.0–1.9 billion years ago) has been called the ‘Intermediate Ocean’ and is associated with relatively little activity that would give rise to new minerals. The oxygen-rich zone in the uppermost waters of the oceans may have deepened, but there is no evidence of dramatic developments. The ninth stage (0.542–1.0 billion years ago) is distinguished by at least two global glaciations referred to as ‘Snowball Earth’ episodes. It is still debated as to whether ice ever completely covered the Earth, but it certainly dominated for periods of more than 10 million years. Beneath the ice, volcanic activity continued and would have contributed to the mineral inventory, as would activity during interglacial periods with a likely rapid increase in the generation of new clay minerals.

The final stage (Stage 10) embraces the whole of what geologists call the Phanerozoic. As discussed in Chapter 5, the Phanerozoic spans from 541 million years ago until the present day. The clear fossil evidence for living forms which characterizes the start of the Phanerozoic was particularly associated with mineral skeletons made of carbonate, phosphate, and silica mineral matter. These were easily preserved, providing a rich and varied ‘fossil record’. Later, when life became established on land as well as in the seas, the rise of land plants would have led to the first soils; these conditions contributed further members to the families of clay minerals, completing the story of mineral evolution.

The emergence of life

Any parallels between mineral evolution and the evolution of life forms can only be taken so far. We are not seeing the driving force that is provided by natural selection in Darwinian evolution, or the common thread of an inheritable molecule (DNA). And whereas around 98 per cent of the living organisms that have ever existed are now extinct, minerals must be very rarely, if ever, lost from our list of 4,000 plus species. There is, however, one period in Earth history where the biological and mineralogical may truly have converged. The evidence that minerals played a key role in the emergence of life on Earth is very strong. Just a few of the ideas put forward in this complex and diverse field of research will convey some aspects of current thinking.

The complex series of steps leading to the emergence of life must have started with the formation of the organic molecules required as life’s building blocks, such as amino acids, lipids, and sugars. In what was probably the most significant laboratory experiment ever conducted in the quest for understanding life’s beginnings, Chicago University student Stanley Miller and his supervisor Harold Urey showed that such molecules could be synthesized when a simple mixture of gases is subjected to electrical sparks simulating a lightning strike. The result was a so-called ‘primordial soup’ rich in the essential components needed for life. But then the question became: ‘How did the simple molecules formed in this way, and greatly diluted in Earth’s earliest oceans, come together to build more complex molecules and eventually form self-replicating entities? Some sorts of templates would have been needed and the perfect answer is provided by the surfaces of minerals. For example, the surfaces of common minerals such as quartz and calcite have been shown experimentally to select and concentrate specific biologically important amino acids, the building blocks of proteins.

A role for minerals as catalysts for biochemical reactions and templates in the emergence of complex biomolecules is now widely accepted. However, many different routes have been proposed for the emergence of the first living organisms; significantly, almost all hypotheses have major roles for minerals. Some routes, such as proposed by Joe Smith, Ian Parsons, and co-workers, involve minerals that have biomolecule-sized cavities in their crystal structures or weathered surfaces; they could have acted both as templates and as catalysts for biochemical reactions. Good examples of such minerals are the zeolites, which are framework silicates with cavities and tunnels in their crystal structures that can accommodate organic molecules and promote reactions between them, as shown by their widespread use as catalysts in industries such as oil refining (see Chapter 6). Others involve clay minerals such as montmorillonite in the formation of the first self-replicating genetic molecules. The experimentally demonstrated capacity of montmorillonite to catalyse the construction of more complex, longer chain biomolecules from RNA (ribonucleic acid, which some believe to have been a necessary precursor of DNA), is strongly suggestive of a key role for minerals. Probably the most extreme case where a critical role is envisaged for clay minerals has been advocated by the Glasgow chemist, Graham Cairns-Smith. He has argued that the driving force for the transition from geochemistry (minerals) to biochemistry (biomolecules) was a form of ‘natural selection’ operating initially just on inorganic materials. In effect, it is suggested that the varying stacking sequences found in the layering of clay minerals could retain and transmit information in a similar, but very much cruder, fashion to that now transmitted by DNA.

Another direction taken in the ‘emergence debate’ focuses attention on a very different group of minerals, the metal sulphides; this approach took its initial inspiration from the hydrothermal vents discovered on the ocean floor in the proximity of mid-ocean ridges. Here, hot fluids (~350°C) expelled into the ocean waters as ‘black smokers’ release a stream of metal sulphide particles onto the ocean floor and build ‘chimneys’ from the deposited sulphides. Both micro- and macro-organisms utilize chemical energy available in these environments for their metabolisms. Advocates of a central role for sulphide minerals also point to the central role played by transition metal sulphide clusters in key microbial enzymes and, hence, in the metabolic chemistry of many microorganisms. Two notable hypotheses are associated with the names of the German chemist Gunter Wachtershauser and an originally Glasgow-based scientist, Mike Russell. Both propose that life emerged through abiotic chemical reactions catalysed by metal sulphide minerals and involving fluids coming from depth in the Earth, probably emerging on the floor of an early ocean. Both also propose that the first organism was a so-called autotroph, a life form that can manufacture its own biomolecules from small molecules.

Wachtershauser suggested that pyrite (FeS₂) formation from the reaction of iron monosulphide (FeS) with hydrogen sulphide (H₂S) provided an energy source for the first life forms and a route to forming key organic molecules such as formic acid. He also suggested that pyrite could act as a catalyst for a wide range of reactions that would produce simple biomolecules. Russell and co-workers envisage hydrothermal fluids mixing with ocean water on the sea floor, rapidly precipitating iron sulphide (FeS, the mineral mackinawite) to form bubbles; these bubbles would serve as primitive membranes which could control and catalyse essential biomolecule-generating chemical reactions.

Many experiments have been conducted over the past two decades that demonstrate the capacity of minerals to catalyse reactions that must have been important for the eventual construction of the biomolecules needed for living organisms. There are also tantalizing clues regarding the transition from non-living to living worlds in the ways in which minerals can act as templates for the construction of complex molecules.

Resources for the future

Theories of evolution, whether it is evolution of minerals, or of biochemical systems leading to the first living organisms, are intellectually challenging, but generally focus our attention on the past. For the present and the near future, our greatest ‘mineralogical’ concerns are about practical issues centred on minerals as resources. They are also about the impact on our environment of the extraction, processing, and utilization of mineral resources, and disposal of the associated waste products.

As we have outlined in earlier chapters of this book, minerals are the sources of the majority of our material needs apart from food, water, and certain forms of energy, and even those rely heavily on mineral-derived products for their production and utilization. Great demands will be placed on mineral supplies in the coming years, so a key question is ‘Will the Earth be able to provide our future mineral needs?’ The answer to this question depends on the particular resources being considered. Metals are a good case in point. Iron and aluminium have become the principal metals for the transport and construction industries and for manufacturing many other essential products. Reserves of these two metals are vast, and technologies for their extraction are well developed, as are systems for their recycling. The other (geologically) abundant metals—magnesium, manganese, silicon, and titanium—also have large reserves and established patterns of use which look set to continue.

Twenty-five years ago, the reserves of the (geologically) scarce metals were thought to be running out. However, advances in exploration methods and improvements in recovery of metals from the mined ores and in recycling have greatly improved the situation in recent years. The precious metals are likely to continue to hold their allure in jewellery and as investments, and see a growth in technological applications.

New alloys may add to the demand for ferro-alloy metals. The base metals may, in some cases such as the toxic metals lead and mercury, see declining use because of environmental concerns. Others such as copper may find new uses. However, it is in the case of the special metals where demand could outstrip supply. Metals such as niobium, tantalum, germanium, gallium, indium, beryllium, and the rare earth elements (REE) are of crucial importance for modern industries, especially electronics. Niobium (also called columbium), as well as being used in high-strength alloys, is used for special types of magnets (superconducting magnets). Tantalum is used in electronic components which are essential for mobile telephones, DVD players, and computers. REE are to be found in every car, computer, smartphone, energy efficient fluorescent lamp, and colour television.

Because of their roles in advanced technologies, the special metals can also be strategically important. This is illustrated by the REE, for which the world supply has been effectively limited to several mining districts in China. This dependence was already a cause for concern when, a decade ago, the Chinese government introduced export quotas restricting the supply of REE outside China. Initially, supply matched demand but in 2010, a reduction in quotas of 40 per cent over 2009 levels was introduced which also led to very large price increases (over 1,500 per cent) in just a few months. Not surprisingly, these developments have prompted worldwide exploration efforts in attempts to find supplies of REE from outside China. Controversially, rich deposits appear to be located in unspoilt areas such as South Greenland. Another, but contrasting example of the impact that having deposits of a strategic metal can inflict on a country is provided by the Democratic Republic of Congo. Here, the tantalum that is mined (as ores referred to locally as ‘coltan’, a name derived from the minerals columbite-tantalite) and exported to European and American markets, is cited by experts as a key factor in financing civil wars in that region. In the mining itself, child labour is commonly used in appalling working conditions.

What of mineral resources other than metals? The minerals used for fertilizer, chemical, and building materials exist in large quantities and reserves will be adequate for many years. Marine potassium and phosphate deposits are extensive and sufficient to meet world needs far into the 21st century. The oceans and saline lakes are likely to serve increasingly as sources of chemical minerals. Building materials will always be available, even if local shortages may arise, and much the same can be said of most industrial minerals, even though some, such as the asbestos minerals, may be replaced by more environmentally friendly alternatives. Energy sources are always going to be problematic, given the great and growing energy demands of modern societies. Only one source is directly dependent upon minerals for its energy and that is nuclear power. Although a certain amount of secrecy surrounds the mining of uranium ores, there is no suggestion that serious shortages are about to arise, and substantial deposits are known on all the continents.

Returning to the question of whether the Earth’s mineral resources are enough to meet our future needs, in many cases the answer is clearly ‘yes’. In cases such as certain of the scarce metals, exploration in more hostile terrains may be a way forward. In particular, exploration and exploitation of deeper parts of the continental crust may be a way to supply our needs. Our knowledge of the subsurface at depths greater than ~1,500 feet is extremely limited; new techniques and programmes aimed at remotely mapping these deeper regions should reveal new deposits, but these will also pose new challenges for mining methods. Another way of addressing this question is to consider potential sources of metals where the amounts (percentages) of the metal are lower than those currently being economically mined. This can be illustrated by the example of copper. Figure 23 is a hypothetical depiction of copper production in the centuries ahead. One axis of this graph shows the percentage of copper in the mined ore, mining beginning with the exploitation of sulphide mineral ores with several per cent copper down to about one-tenth of a per cent copper content. After these have been exhausted, attention could turn to mining of deep ocean manganese nodules and metal-rich deep ocean muds containing smaller amounts of the metal. As a last resort, common rocks such as basalts, containing very low concentrations of copper, could be mined. However, the decreasing economic viability of following the pathway shown by the arrows on this diagram is emphasized by the information on the other axis showing the estimated energy required to recover a pound (0.454 kg) of copper from these different sources. As the percentage of copper (the ‘grade’) decreases, the cost of extraction in energy terms (and financially) increases. In addition, recovering copper from sulphide minerals or oxides (as in nodules) is much easier than recovering it from solid solution in rock-forming silicate minerals. The latter requires crossing what has been called a ‘mineralogical barrier’, with new extraction technologies being needed. What is shown in Figure 23 is conjecture of course, but it highlights the point that recovering scarce metals will be more difficult and expensive in the future.

23. A hypothetical illustration of sources of copper for future exploitation; the percentages of copper available from different sources are plotted against rising costs of extraction expressed in terms of Btu (British Thermal Units) per pound (lb) weight

An epilogue

Our introduction to minerals has taken us on a journey in space and time; from deep within the Earth to the outer regions of the Solar System and beyond, and from the beginning of time through prehistory and history to the present. Minerals have always been, and always will be, central to our efforts to understand our planet, its past and its possible futures. Today, we are at the threshold of a new understanding of the Earth’s surface processes which integrates the mineralogical, geochemical, and biological realms at the molecular scale. The emergent field of ‘molecular environmental science’ should provide new insights into the way the planet ‘works’ that could be comparable to the revolutionary advances seen in human biology associated with the genetic code. These ideas are associated with what is now being called ‘Earth System Science’. However, one of the great founders of the science of geology, James Hutton, was the first to see the Earth as a ‘system’. In 1785 he wrote:

A theory is thus formed, with regard to a mineral system. In this system, hard and solid bodies are to be formed from soft bodies, from loose or incoherent material, collected together at the bottom of the sea; and the bottom of the ocean is to be made to change its place with relation to the centre of the earth, to be formed into land above the level of the sea, and to become a country fertile and inhabited.

Regarding the practical applications of our knowledge of minerals and of Earth Systems, the health and well-being of humankind are linked to minerals as both sources of essential nutrients or as potential poisons, and as the providers of the raw materials which are vital for our survival. Our dependence on minerals as resources can only increase in importance as world population grows from its present nearly 7 billion to the more than 9 billion predicted by the year 2050. For many of the challenges facing humanity in the coming decades, minerals will play a central role. Truly it can be said that ‘minerals matter’.