Chapter 4

Earth’s surface and the cycling of minerals

Although the movement of tectonic plates seems to us very slow, with ‘continents’ drifting at most a few centimetres in a year, these processes are responsible for most of the drama we associate with the ‘dynamic Earth’. This includes earthquakes, tsunamis, and volcanic eruptions. As we have seen, there is a cycling of minerals taking place on a very large scale as part of the plate tectonic cycle, with material brought up from the mantle forming new ocean floor and that material, in turn, eventually being consumed by subduction. There is also a cycling of minerals that is less dramatic but of great importance to our understanding of Planet Earth. It is also part of our common human experience, wherever we live on Earth, and has great practical as well as theoretical importance. This is the cycling of minerals at or very near the surface of the Earth, cycling particularly associated with the weathering and erosion by rain, wind, and frost action of rocks exposed at the surface. Some of the minerals in those rocks may be dissolved during weathering, others may be transported in the flowing water of streams and rivers, by glaciers, or as fine mineral dusts in the atmosphere, eventually being deposited elsewhere as sediments. This is part of what geologists call the rock cycle (which is also necessarily a ‘mineral cycle’), but one involving the rocks of the Earth’s crust.

As seen in Figure 13, we can think of the complete rock cycle as beginning with igneous rocks formed by crystallization from melts; when exposed at the Earth’s surface, these rocks will weather and be eroded and transported to form sediments. These unconsolidated sediments can then be buried and slowly transformed into solid sedimentary rocks (the process of lithification). Deep burial of such rocks can subject them to greatly increased pressures and temperatures, causing their mineral components to change in various ways in the processes of metamorphism already discussed in Chapter 3. As noted there, this may involve the minerals in these rocks changing in the sizes and shapes of their crystals, or more importantly, transforming into new minerals. Eventually, they may be so deeply buried as to start melting, taking the cycle full circle back to igneous rocks. As shown in Figure 13, it is also possible for the cycle to be broken with a return to an earlier point in the cycle; so sedimentary rocks can be exposed to weathering and erosion at the Earth’s surface to form new sediments, as can metamorphic rocks.

The overall rock cycle can be thought of as having two parts. The first is driven by the heat coming from the Earth’s interior and involves interactions between the mantle and crust. The minerals formed in this part of the cycle crystallize from melts (magmas) or very hot waters (hydrothermal waters). The second is driven primarily by the heat from the Sun and involves interactions between exposed crust and the waters of the hydrosphere or gases of the atmosphere. As we note above, and in Figure 13, the rocks formed in this part of the cycle are deposited at Earth’s surface temperatures as sediments. These may be detrital in origin (such as the grains of quartz in a sandstone), may come from evaporation of waters containing dissolved mineral components (such as the rock-salt in a salt bed), or have a biological origin (such as the calcite originally deposited by the coral animals in a ‘reef’ limestone).

The biological processes involved in the formation of the calcium carbonate minerals (calcite and aragonite) making up many limestones are a reminder of the importance of the biosphere in the cycling of minerals at the Earth’s surface. It is also true that minerals have a vital role in sustaining life on Earth. For this reason, the region of the surface and near-surface that is essential to sustaining life is now called the critical zone. More will be said about minerals and the biosphere in Chapter 5.

13. The rock cycle

Weathering

The weathering of minerals and rocks at the surface of the Earth depends on a number of factors. As well as the nature of the mineral itself, these include climate, vegetation, micro- and macro-organisms such as bacteria or earthworms, slope of the ground, and time since exposure of the mineral at the Earth’s surface.

A good example of mineral weathering is provided by the products of weathering of a typical granite, probably the best known of all igneous rocks. The minerals which comprise a typical granite are feldspars, micas, and quartz. During weathering of the orthoclase feldspar (KAlSi₃O₈), minor amounts of potassium (K) and silica (SiO₂) go into solution and the feldspar reacts to form a potassium-bearing clay mineral; the plagioclase feldspar ((Na,Ca)Al_1-2Si_3-2O₈) similarly may form a clay mineral but one containing sodium (Na) and calcium (Ca), and silica along with Na and Ca go into solution; the micas (muscovite and biotite) form clay minerals plus haematite (Fe₂O₃), soluble magnesium (Mg) and silica, and minor soluble potassium (K). The quartz (SiO₂) will survive even intense weathering largely unaltered. The golden sands of beaches in granite areas (such as western Cornwall, England) are the products of this survival, being almost entirely quartz grains.

The example of granite weathering highlights the importance of clay minerals as breakdown products of feldspars and micas. They can also form from the breakdown of other silicates such as the olivines, pyroxenes, and amphiboles. Clay minerals are silicates containing aluminium and also bonded water in their structures, and, in most cases, other elements such as sodium, potassium, calcium, magnesium, or iron. All clay minerals are layer or ‘phyllo’ silicates (see Table 1; and Figure 14) with crystal structures in which SiO₄ tetrahedral units share oxygen atoms to form layers, and AlO₆ or MgO₆ octahedral units also link together to form layers. Various combinations of layer sequences are possible with, in some cases, charged atoms (ions) such as K⁺, Ca²⁺, Mg²⁺ between certain of the layers (as interstitial ions) to bond them together. Clay minerals are always of very small particle size and their structures and compositions give them distinctive properties. As well as being important components of soils, these properties make clays the critical raw materials for a range of industries, the oldest of which is the manufacture of pottery. As early in human history as 9000 bc, clays were being fired to make pottery. Brickmaking and other ceramic arts followed by 3500 bc. Modern industries use clay minerals in numerous products. For example, kaolinite (Al₄Si₄O₁₀(OH)₈), also known as china clay, is not only used in manufacturing fine porcelain but in providing the coating on the surface of the high quality paper used in glossy magazines. Another important property of many clay minerals is a capacity to take up impurities such as ions or molecules from solution. This gives clay minerals in soils a key role as the carriers of plant nutrients, and also applications in certain industrial processes where impurities need to be removed from contaminated liquids. One of the oldest such processes is that of fulling, which is the removal of grease from wool or other organic fibres. Because of this application, natural deposits of the clays used in this way are known as ‘fuller’s earth’.

The breakdown of silicate minerals dominates the weathering stage of the rock cycle. The only other minerals that compete with silicates in terms of crustal abundance are the carbonates of limestone rocks. Limestones are made up almost entirely of calcium carbonate (calcite). Magnesium may replace half of the calcium in calcite to give dolomite, CaMg(CO₃)₂, the name given to the mineral, a rock made of that mineral (more correctly called dolostone) and, incidentally, the ‘Dolomites’ mountain range in the Italian Alps built mainly of that rock. The behaviour of carbonates during weathering is controlled by their solubility in rainwater. Uptake by rainwater of carbon dioxide from the atmosphere means that it is not pure H₂O but weak carbonic acid (H₂CO₃) which can dissolve carbonates. This is the origin of the many distinctive features of limestone landscapes which include spectacular cave systems formed by limestone dissolution. Although much less abundant, one other group of minerals must be mentioned in any discussion of weathering; these are the sulphides, especially the iron sulphide, pyrite (FeS₂).

Sulphide minerals occur in substantial amounts in the wastes from the mining of many metals and the mining of coal. They can also be minor components in common rocks, especially certain shales. Sulphides are not stable when exposed to oxygen or oxygenated waters at Earth’s surface; they break down to produce sulphuric acid and, initially, to take the iron or other metals into solution in the acidified waters. The name given to this phenomenon when the sulphide mineral source is minewastes is Acid Mine Drainage (AMD); when the source is the exposure of certain common rocks, it is Acid Rock Drainage (ARD). Nearer to the source, the waters produced can be highly acidic, comparable to automobile battery acid, or even more extreme. Bodies of water filling former open pit mine workings can be sufficiently acid to dissolve discarded metal objects. When the waters of AMD systems mix with other surface waters, they cause acidification, killing off aquatic life and blighting whole stretches of waterways.

14. The crystal structure of the clay mineral montmorillonite (Al₄(Si₄O₁₀)₂(OH)₄.nH₂O); a fragment showing the relationships between layers made up of tetrahedrally and octahedrally coordinated atoms and the interlayer which normally contains weakly held water (‘nH₂O’) which can be exchanged for other molecules

It has been estimated that over 10,000 miles (~16,000 km) of streams and rivers in the USA alone have been seriously polluted by AMD, a legacy from the mining of metals and coal. As well as acidification, such pollution involves large amounts of very fine particulate mineral matter. This is precipitated as the waters downstream become less acid by mixing with unpolluted rivers and streams and can no longer retain metals in solution. For example, the iron held in solution in highly acidic waters will precipitate as iron hydroxide minerals such as orange-red goethite (FeOOH). It is the fine precipitates of such minerals (including examples of the nanominerals discussed below) that cause the waters in many polluted areas to turn into ‘red rivers’. These particles of iron minerals can also transport downstream other elements trapped within their structures or attached to their surfaces, including highly toxic elements such as arsenic or uranium. The uptake of other elements in this way is further considered when we discuss the nature of mineral surfaces later in this chapter.

Nanominerals and nanoparticles

Much of the material derived from the breakdown of the common minerals is of very fine particle size. In some cases, the particle size can be measured in nanometres (nm) where one nanometre is one thousand millionth of a metre. This includes much clay mineral matter and also may include iron oxides or hydroxides such as goethite (FeOOH) or the aluminium hydroxides (AlOOH). The study of these materials, now called ‘nanominerals’, is a relatively recent development in mineralogy. This is partly because their geological importance was not fully appreciated, and partly because techniques to study them were not available. The experiments now possible at synchrotron laboratories, along with scanning probe microscopy methods and advances in electron microscopy, have provided powerful new investigative methods. Two other important aspects of nanominerals need to be discussed here. The first is that much evidence points to nanomaterials (and nanominerals) exhibiting properties that are distinctly different from those of their larger particle size equivalents. The second is that manufactured nanomaterials are now very widely used in industry, prompting much research into their properties, as well as concerns about the impact these materials may have when released into the environment.

Michael Hochella of Virginia Tech (USA), a pioneer of nanomineral studies, has proposed a nanoscale mineral classification scheme. All nanoscale minerals must have at least one dimension in the nano-range (between 1 and 100 nm) but may occur as nanosheets or nanorods as well as nanoparticles. A very important further distinction is drawn between mineral nanoparticles and nanominerals: the former can also exist in sizes that exceed the nano-range (possibly up to the largest dimensions found in minerals) whereas the latter exist only as one of the three types of nanoscale minerals noted above. A very good example of a nanomineral is ferrihydrite (Fe_4-5(OH,O)₁₂), a mineral that is very common in soils and natural waters. It has never been found as particles larger than 20 nm, and is typically 10 nm or less in diameter. It is very probable that many such nanominerals remain to be discovered.

The properties of nanoparticles can be dramatically different compared to their larger-size equivalents. For example, solubility in water may be orders of magnitude greater for nanoparticles compared with larger particles of the same mineral. It may also be that the crystal structure of the mineral that is most stable at atmospheric temperature and pressure in the nanoparticle size range is not the most stable at a larger size. This is the case for the titanium dioxide (TiO₂) minerals where there are three different crystal structure forms (polymorphs) and hence minerals: rutile, anatase, and brookite. Here, anatase, rather than rutile, is the stable phase only for nanoparticle sized grains. Even where particle size does not influence crystal structure, it may influence the preferred crystal shape (e.g. cube versus octahedron or tetrahedron). So, size really does matter!

In part, at least, these differences arise because so many of the atoms in a nanoparticle are at or very near the surface (a 10 nm diameter cube crystal will have 16 per cent of its atoms at the surface). One consequence of this is the very large surface area presented to the environment (the air or natural waters) for reaction and interaction. In particular, processes discussed in detail below where contaminants in waters are taken up (sorbed) by the surfaces of nanoparticles are very important. Toxic elements such as arsenic, lead, cadmium, or radioactive contaminants such as uranium may become attached to these surfaces and transported considerable distances in flowing water. Nanominerals or nanoparticles may also coat larger grains such as those of quartz, causing their surfaces to be active for sorption, or alternatively nanoscale films may coat much larger grains of minerals such as feldspars, inhibiting their weathering.

The subject of ‘nanomineralogy’ is so new that we have no real idea of the total quantities of nanoparticles in the Earth’s surface and near-surface environments, or how they are distributed. It is suspected that most are to be found in the oceans. In the oceans there is also an important role played by nanoparticles of iron minerals. Phytoplankton, important contributors to ocean ecosystems, require iron for their metabolism; this iron had always been assumed to come via input from rivers, particularly the great rivers such as the Amazon. Now it appears that airborne dusts carrying iron minerals provide an input that far exceeds that from rivers.

More surprising occurrences of nanoparticles are those found in minute concentrations in stony meteorites and interplanetary dusts; the latter includes nanodiamonds. They average 3 nm in diameter, although grains as small as 1nm and, therefore, containing fewer than 150 carbon atoms have been observed. Nanodiamonds are believed to represent pre-solar dust, possibly forming in supernovae, although it has also been suggested that they could form directly in the solar nebula. Our unmanned exploration of nearby planets has also revealed evidence of nanoscale ferric oxides on the surface of Mars. Also surprising, and a link to our earlier discussions of ‘Deep Earth’ (in Chapter 3) are the proposed examples of nanoscale particles of the very high pressure minerals wadsleyite and ringwoodite. Here a role is envisaged for nanominerals in the generation of deep focus earthquakes, those at 300–700 km depth in the mantle. The mineral nanoparticles are envisaged as filling ‘anticracks’ which are planes of weakness that do not need to dilate to create empty space, unlike a normal crack. These nanoparticles can easily move past each other without mechanical shearing of individual grains, so that high pressure does not restrict such movement. There is also evidence that the mechanical properties of nanoparticles can play an important role at shallow depths, influencing fault mechanics, and that the mineral particle size in the nano-range may affect compressibility.

The upsurge of interest in natural nanoparticles and nanominerals is partly a result of the development of synthetic nanomaterials and of nanotechnology. Beginning particularly with the nanoscale forms of carbon, such as the spherical cluster of 60 carbon atoms (the C₆₀ phase named buckminsterfullerene or the ‘buckyball’ for short; see Figure 1d) and carbon nanotubes, numerous elements and compounds are now synthesized as ‘nano’ materials for use in industry. Applications range from electronics to environmental clean-up, and medicines to cutting tools. The global nanotechnology market is already estimated to be worth several trillion dollars annually. However, although many will end up in landfills or similar disposal facilities, the impact of these synthetic nanomaterials on the environment is very poorly understood. Indeed, the extent to which both natural and synthetic nanomaterials can enter into biological systems (their so-called ‘bioavailability’) is still poorly understood. This is a subject we return to in Chapter 5.

Something in the air

Probably the most important scientific issue of the 21st century is global warming and its potential impact on sea levels and climate worldwide. The greenhouse gases, particularly carbon dioxide (CO₂), have increased considerably in concentration in the Earth’s atmosphere since the beginning of the industrial revolution as a result of the burning of fossil fuels, especially coal, gas, and oil. The greenhouse gases absorb infrared radiation and, therefore, warm air near to the Earth’s surface, so as to cause an increase in overall average temperatures. Potentially catastrophic consequences of even a modest increase in average temperatures have led governments to introduce policies restricting greenhouse gas emissions. Possible consequences of global warming include melting of polar icecaps, with rises in sea level which could flood many coastal cities, towns, and rural areas, and more frequent extreme weather phenomena such as heatwaves and droughts, or storms and hurricanes.

You might think that minerals have nothing whatever to do with global climate, but that is not the case. Every year, 1,500–2,600 million metric tons of ‘dust’ is transported around the globe. Much of this dust (an important component of the transported solid particles and liquid droplets more generally called aerosols) is mineral matter picked up by winds from soils or sands which are themselves the erosional products of rock weathering. Some is ‘sooty’ matter from burning of fossil fuels or of agricultural waste (biomass) burned as part of a crop cycle. Studies of aerosol particles using the electron microscope (TEM) show that, not surprisingly, the minerals present are dominantly the common rock-forming minerals such as quartz, feldspars, micas, clays, iron oxides, and hydroxides. As well as the sooty materials which may be present as coatings on mineral grains, common components of aerosols include various salts, commonly crystallized from solutions such as ocean spray and, of course, ice as well as water droplets.

Aerosols are of interest to us for a number of reasons. In what are fortunately rare cases, airborne mineral matter can pose a significant health hazard. An example of this is in northern and western China, to the west of Beijing. Here deposits of wind-blown silt known as loess (with particles of 2–63 microns in diameter) cover an area of over 600,000 square kilometres. Chinese loess has a simple mineralogy: angular, blade-shaped quartz grains make up 60–65 per cent of the silt, with not more than 12–15 per cent clay minerals plus minor feldspars, micas, and carbonates. Dust storms are frequent events in this region and, although detailed epidemiological data are very limited, these dusts are certainly responsible for a much greater incidence of respiratory diseases such as silicosis. Furthermore, particulates mobilized and transported by wind may be carried as far away as north-west India, and cause health problems evidenced by high levels of silicosis which has been linked to mineral dusts by finding silica (quartz) in analysed lung tissues.

Globally, mineral matter in aerosols has a relatively poorly understood influence on climate. Mineral particles can themselves either reflect or absorb radiation from the Sun, or may act as nuclei for the formation of water droplets and, in turn, clouds which may reflect or absorb the Sun’s rays. The behaviour of different minerals in these systems will ultimately depend on their fundamental optical properties whereas, of course, mineral grains coated with soot will strongly absorb energy from the Sun. The role of mineral-containing aerosols actually introduces significant uncertainty into climate models.

Surfaces and interfaces

‘The boundary is the best place for acquiring knowledge’ are not the words of a scientist but of the theologian Paul Tillich, but they are very relevant to the study of minerals. This is because the processes controlling the cycling of minerals at the Earth’s surface nearly all occur at the boundary (‘interface’) between the mineral and its environment, whether that be the gases of the atmosphere, or the waters in rivers, lakes, and oceans, or water trapped in soils or underground. Understanding of interfacial phenomena requires an understanding of the mineral surface at the level of individual atoms.

Numerous methods have been developed for characterizing the surfaces of solids; these include the imaging methods discussed in Chapter 2, and various diffraction methods enabling the relative positions of atoms at the mineral surface to be determined. As with many kinds of experiments, the availability of synchrotron radiation has enabled significant advances. A first question we need to consider in studies of mineral surfaces is whether the arrangement of atoms at the surface is like a simple truncation of the bulk (i.e. as though we were to slice, knifelike, through the mineral along a certain plane of atoms). In fact, surfaces are rarely so simple. There are changes in the positions of the atoms at or near the surface which may involve a substantial reorganization (reconstruction) or just a smaller adjustment (relaxation). These changes are about forming the most stable arrangement of atoms at the mineral surface, i.e. about minimizing surface energy. Understanding the nature of the clean surface is an important step towards understanding how the mineral may react when exposed to the air, pure water, or water that contains impurities such as metal ions or organic molecules.

The most important interface in the cycling of minerals at the Earth’s surface is the mineral–water interface. Here, a mineral may dissolve, releasing its constituent elements into solution. This dissolution may continue until the mineral is consumed, or alternatively, lead to an altered surface layer which may protect the surface from further reaction. Other kinds of reactions may occur at the mineral–water interface, especially reactions involving the uptake of atoms or molecules present as contaminants in the water. These are generally referred to as ‘sorption’ and can be studied by adding set amounts of the mineral of interest to a solution containing a known amount of the impurity. Separation of the solution from the solid mineral and chemical analysis of the solution provides a measure of how much of the impurity is removed from solution through interaction with the mineral. In this way, uptake can be measured as a function of variables such as initial concentration of the impurity in solution, or how acid (or alkaline) is the water.

Such overall measurements of uptake tell us nothing about what is happening at the mineral surface at the atomic level. That information can be obtained through spectroscopic methods, notably the X-ray absorption spectroscopies such as EXAFS as described in Chapter 2. Good examples of the possible interactions at the mineral surface are those involving metal atoms (present in the solution as ‘ions’ such as Cu⁺, Cd²⁺, Pb²⁺) and minerals such as the iron oxides and hydroxides (hematite, Fe₂O₃; goethite, FeOOH). The possible types of interactions are illustrated in Figure 15, which shows that, in solution, the metal ions (M) will be surrounded by attached water molecules. ‘Sorption’ of the metal to the mineral surface can involve retaining those water molecules in what is called an outer sphere surface complex or losing some of the water molecules to form an inner sphere surface complex. Inner sphere complexation means that the sorbed metal forms a direct chemical bond with an atom at the mineral surface and so is much more tightly held than an outer sphere complex. With terminology suggestive of the dentists surgery, inner sphere complexation with the metal bonding to just one mineral surface atom is called monodentate, with two surface atoms bidentate, and so on. An important feature of both types of surface complexation is that uptake from solution is limited by sites available on the mineral surface. Once all available surface sites are occupied, no further uptake is possible. That does not apply to the other possibilities for interaction illustrated in Figure 15. They lead to precipitation of material from solution, or of material derived from both the solution and the mineral (co-precipitation) or to replacement of atoms in the mineral involving diffusion of metal atoms into the surface of the mineral.

15. Surface complexation and related interactions with simplified representations of a mineral surface in contact with a liquid containing a metal (M) in solution. The metal is bonded to the oxygen atoms (large grey spheres) of water molecules (hydrogen atoms shown as ‘H’) as a ‘hydration sphere’. Different types of surface complexes, precipitates, or replacement reactions are shown

You might well ask ‘Does all this work on mineral surfaces really matter?’ On both global and local scales the answer is ‘yes’. Much of the material being transported around the globe, whether in the air or in moving water, is attached to mineral substrates. Reactions between mineral substrates and contaminated waters also lead to the formation of minerals through precipitation or replacement reactions. Locally, these mineral surface processes provide key mechanisms for the transport and dispersal, or the localized concentration of major, minor, and trace elements and molecules including those of organic compounds. These substances include toxic elements such as arsenic, cadmium, lead, or mercury, elements from hazardous radioactive wastes such as uranium, radium, neptunium, technetium, and plutonium, and toxic organic compounds from industrial sources. Modelling of mineral surface processes in this context is essential for assessing the risks of pollution, and for remediation of contaminated areas. In fact, ‘sorption’ by mineral substrates is one of the methods we can use to remove toxic contaminants (such as arsenic) from water. At the other extreme from a role in protecting living organisms from poisons, mineral surfaces almost certainly played a role in the emergence of life on Earth, an idea we explore in the final chapter of this book.