Chapter 2
Studying minerals
The study of minerals begins with their characterization, identification, and classification. Historically this has been based on properties observable in hand specimens such as crystal morphology (revealing crystallographic properties such as symmetry elements), cleavage, hardness, density, lustre, streak, and, in some cases, colour. For a small number of minerals, other properties may be diagnostic, such as magnetic behaviour. These properties were used as the basis of the first serious attempts at classification, dating back as far as the physician–natural historian Georgius Agricola who, in his work De Natura Fossilium published in 1546, classified about 600 minerals. With the developments in chemistry in the 19th century, it became possible to determine the chemical compositions of minerals and to put classification on a firm footing.
Shining light on minerals
A major advance in studies of both minerals and rocks is associated with the optical microscope. Although single lens magnifying glasses were mentioned by roman writers in the 1st century ad, the first multiple lens device was invented around 1590 by the Dutch spectacle makers Zaccharias and Hans Janssen. It was later improved upon by many inventors including Galileo who, in 1609, worked out the optical principles involved as well as improving the design. Using such instruments, mineral grains or fragments could be examined in the same way as other naturalists might examine a plant fragment or a small insect. A very important advance occurred in 1815, associated with the work of the Scottish scientist William Nicol. It was found that by preparing a very thin slice (30 micrometres = 0.3 mm thick) of a mineral or rock and gluing it to a glass slide, for most minerals, light would pass through (be ‘transmitted’ through to use the correct terminology) the resulting thin section. It was also found that by passing the incident light through a polarizer, similar to the ‘polaroid’ material used in sunglasses, various new optical properties of the minerals studied in plane polarized light could be observed which aided in their identification. If, in addition to polarizing the light before it passes through the mineral, the light has to pass through a second polarizing filter but this one at an angle of 90° to the first (using so-called observation under crossed polars), a further range of diagnostic optical properties could be observed. These and other related developments concerning the ways in which light passes through the minerals in a thin section, revolutionized the study of minerals and rocks because the mineral grains could be readily identified even when very small.
Furthermore, in rocks the shapes and sizes of individual mineral grains and their interrelationships could be studied, providing insights into how various rocks have formed or have been modified since their formation. A photograph of a typical thin section of an igneous rock viewed under the polarizing microscope is shown in Figure 4a. Here, a thin section of a rock from the island of Rhum in north-west Scotland has been photographed under crossed polars. This section shows a rock which has crystallized from a melt. Two minerals are present; the first is the ‘island’ silicate mineral, olivine (see Table 1). All of the rounded and ‘crystal-shaped’ grains of different colours (shades of grey in this black-and-white photograph) are olivine crystals in different orientations (which is the reason for their different colours). The other mineral is the ‘framework’ silicate mineral plagioclase feldspar; this forms the matrix in which the olivine crystals are set. The plagioclase is of two different orientations, with that above a line from the bottom left-hand corner to roughly midway along the right-hand side appearing white, and below that line appearing grey. The olivine must have crystallized first when this rock was formed, followed by the plagioclase feldspar. Note the white bar in the bottom right of this photograph is a 1 mm scale bar.
A limitation of the transmitted light microscope is that the minerals being studied need to be translucent. The great majority of the minerals that make up common rocks, such as all of the silicates and carbonates, do transmit light through a 30 micron thin section. However, most metal sulphide and metal oxide minerals do not; that is to say, they are opaque. Here, the related technique of reflected light microscopy can be used. In this case, a roughly 1–2 cm diameter piece sawn off the sample is mounted in cold-setting plastic resin and a flat surface is ground and polished to give a mirror-like surface. This polished section is studied using a microscope where the light (again, plane polarized) is reflected back to the observer from the polished surface via various magnifying lenses. Observations can again be made under crossed polars, and various optical properties used to identify the minerals, although identification is often not as straightforward as in transmitted light and ancillary measurements (such as of the percentage of the incident light reflected back from the polished surface) may be needed.
As with transmitted light observations, a very valuable aspect of reflected light microscopy is the information that can be provided on the sizes, shapes, textures, and interrelationships of the minerals studied. A photograph taken under the microscope of a polished section in plane polarized light is shown in Figure 4b. The sample here comes from deposits mined for nickel and located at Sudbury, Ontario, Canada. In this case the rounded grey grains are of the iron oxide mineral magnetite, the dominant mineral present is the iron sulphide pyrrhotite; this forms interlocking grains between which there are what appear like ‘veinlets’ of the mineral pentlandite ((Ni,Fe)9S8) which is the source of the nickel. The width of this field of view is 1.2 mm. (Note that the black regions are pits in the polished surface.) The minerals in this sample have crystallized from a predominantly sulphide melt, with the magnetite forming first and then an iron–nickel sulphide phase. On cooling, most of the nickel has diffused to grain boundaries and formed the pentlandite. Because the reflected light technique is mostly used to study ore minerals, it is also known as ore microscopy. It is also possible to combine the advantages of thin and polished sections by preparing a thin section and then polishing its surface. The sample preparation methods needed in order to study minerals under the optical microscope are also required for certain more advanced analytical methods, such as the electron microprobe which is discussed below.
4. Minerals seen using the optical microscope: (a) a thin section photographed in transmitted light and under crossed polars of a rock with olivine crystals in a matrix of plagioclase feldspar
4. (b) a polished section photographed in plane polarized reflected light of an ore sample containing pyrrhotite, pentlandite, and magnetite
X-ray vision
The developments in optical microscopy are part of a story that began in the 16th century and could perhaps have been foreseen. Those associated with a great late 19th century scientific advance were wholly unexpected. X-rays were accidentally discovered in 1895 by the German physicist Roentgen, leading to their familiar use in medical diagnosis. However, it was not until 1912 that von Laue and his co-workers demonstrated that X-rays could not only pass through a crystalline solid such as a mineral, but also produce a characteristic pattern on a photographic plate, a pattern that can be used to determine the arrangement of atoms in the solid, i.e. its ‘crystal structure’ (see Figures 1a, 1b, 1c; also shown in Figure 5a is the most famous diffraction pattern ever recorded, that of a single crystal of DNA). The great breakthrough in this practical application of X-rays came with the work of the father and son team of William and Lawrence Bragg. Bragg’s Law established the mathematical relationship between the wavelength (energy) of X-rays passing through the solid and the distance separating the planes of atoms in the crystalline solid, opening up the whole field of understanding of such solids at the level of individual atoms, and how their ordered arrangements are reflected in the external morphology of crystals. As illustrated in Figure 5b, any crystal can be considered as made up of numerous planes of atoms, with adjacent planes separated by a distance d. It happens that this distance (the ‘d spacing’) is of the same order of magnitude as the wavelength of the X-rays that can be generated in the laboratory and symbolized by λ. This means that when a particular plane of atoms is at the so-called Bragg angle (θ), the X-ray beam will be ‘reflected’ and this X-ray reflection can be detected as spots on a photographic film. So, in the Bragg equation which can be written as ‘λ = 2d sin θ’, the value of λ is known or can be controlled, and of θ can be measured in an experiment to give the value of d. Although this process is not really one of ‘reflection’ but rather of ‘diffraction’, to simplify matters in this discussion it can be thought of as reflection.
When all the d-spacings between all the planes in a given crystal have been measured, and data also collected on the intensities (brightness) of reflections which provide further information on the elemental identities of the atoms in the structure, then the crystal structure of the mineral, or other crystalline solid being investigated, can be calculated.
Amongst the first crystal structures determined were the relatively simple structures of minerals such as halite (NaCl, rock-salt) and the sulphides of lead (PbS, galena; see Figure 1a), zinc (ZnS, sphalerite), and iron (FeS2, pyrite). Much of this work was completed by Lawrence Bragg when he was Professor of Physics at Manchester University. The work of Bragg on the crystal structures of minerals laid the foundations of modern X-ray crystallography. The last major rock-forming silicate mineral structure to be determined was that of feldspar, solved by W. H. Taylor in 1932 (on Christmas Day!). These early years saw painstaking measurements of the relative positions and intensities (‘brightness’) of spots recorded on photographic film in order to elucidate crystal structures. Many months of work were needed to determine more complex structures; in those days, doctoral degrees were commonly awarded for the ‘solving’ of one or two such structures. Today, photographic methods are no longer used; the data are recorded using computer-controlled diffractometers in which X-ray detectors scan the space around the crystal to determine at which angles X-rays from the incident beam have been diffracted and their intensities. The data acquired in this way are analysed and interpreted using standard software. The solving of crystal structures that would have taken many months in the early years can now be completed in a matter of days.
Crystal structure determination requires a single crystal (such as a single ‘cube’ of halite or galena) with a diameter of a millimetre or so. Such a crystal can be mounted (glued) on to the end of a thin glass fibre and then placed in the path of the X-ray beam in a camera or, in modern equipment, a diffractometer. It is also possible to use the data collected when the X-ray beam is directed on to a finely powdered sample of a mineral (or mixture of minerals) spread on a glass slide. In this case, peaks in X-ray intensity are recorded as the detector scans through a range of angles. Again, using Bragg’s Law, these peaks can be converted into spacings between layers of atoms in the structure (interplanar spacings) which are characteristic for the structure and in most cases a ‘fingerprint’ means of identifying the mineral or minerals present in the powder. This method is called x-ray powder diffractometry and is the most widely used method for identifying minerals or other crystalline solids in powder form. Typical data from an X-ray powder diffraction experiment are shown in Figure 5c. As the diffractometer scans through a range of values of θ (measured in degrees), reflection of X-rays produces peaks in the diffraction pattern at values characteristic of the mineral. If there is more than one mineral present, the peak intensities are proportional to the amounts of those minerals present.
5. X-ray diffraction: (a) a diffraction pattern on a photographic film from a single crystal of DNA
5. (b) Bragg diffraction from layers of atoms in a crystal; (c) a typical X-ray powder diffraction pattern for a mixture of quartz (Q) and fluorite (F)
The power of the synchrotron
The radiation employed in an X-ray diffraction experiment is of a single energy (or wavelength). It is electromagnetic energy which, in one of the great paradoxes of modern physics, can be described both as waves or as a stream of particles (‘photons’). It is fundamentally the same as the light we detect with our eyes, except that the latter is of much lower energy (longer wavelength). In fact, electromagnetic energy extends as a continuum from the very long wavelengths and low energies of radio waves, through the infrared, then visible light, then the ultraviolet, and on to the high energies of X-rays and, ultimately, gamma-rays. Electromagnetic radiation in nearly all of these ranges can be used in experiments to probe the nature of materials including minerals. Earlier experiments used what are now called ‘conventional’ sources of radiation, not very different from the domestic light bulb, and certainly on a ‘bench-top’ scale.
However, a major advance took place with the availability of synchrotron radiation, beginning in the 1950s. At a synchrotron, radiation is generated when a linear particle accelerator ‘fires’ electrons at near light speed into a ‘storage ring’ which is several hundred metres in diameter (see Figure 6a ). As the electrons travel around the ring, kept on a curved path by very large magnets and other devices, they emit electromagnetic radiation (photons) at a tangent to the direction of travel of the electrons. This radiation can range in energy from infrared through visible light to the ultraviolet, and low energy (‘soft’) X-rays to high energy (‘hard’) X-rays. The great advantage of synchrotron radiation is its intensity, which is orders of magnitude greater than that from conventional laboratory sources. Also, it is so-called ‘white’ radiation which, as with ‘white light’ (such as daylight), contains contributions from a range of energies in a single beam (as in the rainbow colours that make up white light). This means that in an experiment, a specific energy can be selected to interact with the sample, or a range of energies can be scanned.
The X-rays provided by a synchrotron can be used in diffraction experiments in the same way as those from a conventional laboratory X-ray source. But, as already emphasized, the advantage of synchrotron radiation is its much greater intensity. Using modern detectors, not only can the data needed for determining crystal structures be easily acquired, it can be acquired very rapidly (sometimes in fractions of a second). This means that changes in crystal structure in response to changing conditions can be studied as they occur (in ‘real’ time). At the synchrotron, diffraction data can also be obtained from powder samples and of sufficient quality to be used to determine crystal structures; this is not the case for data obtained using a conventional source. At a synchrotron, experiments can be successfully performed on samples with very low concentrations of the substance of interest, or on very small quantities of sample. The synchrotron beam can also be focused down to a small spot of diameter about one micron or even less, so as to study individual particles, grains, or particular regions of interest in the sample.
Many different types of experiments can be performed at the synchrotron, including the scattering of X-rays from particles suspended in a fluid which can tell us about mechanisms of crystallization and growth from solution. However, along with diffraction, the most important measurements have generally been those involving the absorption of X-rays (or ultraviolet photons) by a powder sample or a slurry. The subject of synchrotron methods is one that is awash with acronyms, most of which need not concern us here. However, two of particular importance are EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectroscopies. In the simplest EXAFS experiments, an X-ray beam emerging from the synchrotron passes through a detector that measures its intensity, and then through the sample before entering another detector which can determine if X-rays have been absorbed by the sample. Between the emerging beam and the first detector is a device called a monochromator (meaning literally ‘one colour’ and a reference to selecting a single colour from a beam of white light which, in this case, is not visible light but ‘white’ X-radiation). This device is used to scan through a range of X-ray energies. At certain energies, X-rays are absorbed by the sample, as seen from the differences in intensities recorded by the detectors placed ‘before’ and ‘after’ the sample. Why are X-rays absorbed by the sample? This occurs when electrons that ‘orbit’ the nucleus of the atom of a particular element of interest (say iron, for example) are ‘kicked’ (or more correctly excited) into a higher energy state by the energy provided by the X-rays. This can only happen at a particular energy that is characteristic of the element concerned, and of the electrons at a particular distance from the nucleus (in what is termed a particular electron shell). What is seen in the experiment, as the monochromator is scanned through a range of energies, is an energy at which the absorption of X-rays dramatically increases; this is an absorption edge (see Figure 6b).
6. Synchrotron radiation showing: (a) an aerial view of the European Synchrotron Radiation Facility
6. (b) a typical X-ray absorption spectrum showing the absorption edge and EXAFS and XANES regions
The presence of the absorption edge at a particular energy tells us that the element concerned is present, but that is not the main reason for interest in EXAFS and XANES spectra. If the element being studied were to be the only element present, and to be in the form of a gas rather than a solid or liquid, then the spectrum (a plot of incident energy against X-ray absorbance) would be a smooth line, rising dramatically at the absorption edge, reaching a maximum and then gently falling away. However, if the element of interest is in a solid or liquid form, on a surface, and especially if it is associated with atoms of other elements, the spectrum will exhibit fine structure. This is the name given to smaller peaks and ‘wiggles’ seen near the absorption edge (the XANES) and in the region beyond the edge (the EXAFS; see Figure 6b). The interpretation of these features, achieved using computer fitting and modelling, provides key information about the nature and environment of the element concerned in the sample under study. Suppose, for example, we are studying iron in a mineral system. The XANES and EXAFS spectra can tell us whether the iron is present as metallic iron, as ferrous or ferric iron, and if it is bonded to other elements, then what it is bonded to (such as oxygen or sulphur) and how many atoms it is bonded to (most commonly four or six) and at what distances. The atoms immediately surrounding the iron in the so-called ‘first shell’ can always be detected and, in many cases, further shells of atoms at greater distances can be pinpointed and their distances away measured. In this way the structure of a solid mineral, even one that is poorly crystalline, can be further defined, or the environment of the iron in a solution or at a surface characterized.
The probing electron
The various forms of electromagnetic radiation are powerful probes of the nature of materials including minerals. Much can be learnt from studying the ways in which a beam of electromagnetic radiation is diffracted, absorbed, or reflected, or causes the emission of other forms of radiation. But samples can also be bombarded with particles and the resulting interactions or emissions studied. The particles used include protons or various ions such as those of oxygen or caesium but the most important, by far, are electrons. Electrons are used in the electron probe microanalyser and in various types of electron microscope.
In the electron probe, a beam of electrons is focused down to a very small diameter using magnetic lenses. The solid mineral sample is prepared as a polished section or polished thin section as described above. When the electrons strike the flat polished surface of the sample, they ‘knock out’ electrons from inner electron shells of the atoms present, causing electrons from outer shells to drop down and fill the ‘holes’ created by this process. The energy lost when this occurs is emitted as X-rays of an energy that is characteristic of the atom concerned. In the electron probe, this radiation is detected, showing the presence of atoms of the element concerned. As well as the energy indicating the presence of the element, the intensity of the emitted X-rays measured against a standard of known composition provides the means to a quantitative analysis of the sample.
Since its development in the 1950s, particularly through the work of Castaing in Paris, it is no exaggeration to say that the electron probe has revolutionized the study of minerals. This is because a volume of mineral as small as a few cubic micrometres (microns) can be analysed. In samples prepared as polished thin sections or polished blocks, individual grains or small areas of a sample can be quantitatively analysed following examination under the optical microscope. By rapidly scanning the beam over an area of the polished surface, it is also possible to produce ‘maps’ of the distribution of major, minor, or trace elements between different areas of the sample. All but the very lightest elements (H, He, Li) can be analysed in concentrations from roughly 0.5 to 100 per cent and in favourable cases, much lower concentrations can be analysed (approaching tens of parts per million). In the 60 years since the first electron probes became commercially available, this instrument has been essential for the discovery of hundreds of new minerals, many of which occur only as small grains first seen under the optical microscope.
If the electron probe has revolutionized mineralogy, the various types of electron microscopes have also had a tremendous impact on the characterization of minerals. In the traditional instrument, the transmission electron microscope (TEM), an electron beam is focused down so as to strike the sample as a very small spot. In this case, however, the specimen must be very thin so that the electron beam can pass through it, producing an image on a fluorescent screen. An appropriately thin specimen can be produced by boring a very small hole in a thin slice of mineral or rock using a beam of ions. There are regions around the edges of the hole, only a few tens of micrometres in extent, which are generally thin enough to allow passage of the electron beam. Alternatively, areas at the edges of very small grains may be suitable, the small grains being produced by crushing of a larger sample or being the natural state of the sample (as for many precipitates; see Figure 7a, where the grain sizes and shapes of a synthetic iron oxide are evident, with a magnified insert showing the fine structure at the edges of some grains). Recent decades have seen further refinements in the preparation of samples for study using the TEM. In particular, samples for putting into the microscope which are too small to be seen with the naked eye can be prepared by slicing through the sample using a focused ion beam (FIB).
The spatial resolution of the TEM, particularly that achievable using a variant of the technique known as High Resolution Transmission Electron Microscopy (HRTEM), is impressive (0.2–0.5 nanometres (nm)). It is sufficient to image features at the level of unit cells (which, for simpler structures, generally have dimensions of the order of 0.5–2.0 nm) if not to image actual atoms. An HRTEM image of the serpentine mineral chrysotile is shown in Figure 7c and clearly illustrates our capability to image at the level of unit cells, making the link between the abstract world of crystallography and our direct observations. In the image of this fibrous (asbestiform) mineral, we are looking down the fibre axis, and the smallest features (dimensions about 0.5 nm) are the unit cells in this structure.
Two other features of the TEM add to its power for studying minerals and many other materials. The first is that the electrons striking the sample can also be diffracted like X-rays, producing a pattern of spots as in an X-ray diffraction experiment. The positions and intensities of these spots provide structural information and a ‘fingerprint’ identification of the material being investigated. For example, the data in Figure 7b confirms that this material is the iron oxide mineral, magnetite. The second feature available on many modern instruments is a capability for using emitted X-rays in chemical analysis. In this case, the X-rays emitted from the sample when struck by the electron beam can be used to obtain a chemical analysis, rather as in the electron probe, although such analyses generally provide only approximate values.
7. Transmission electron microscopy (TEM) data obtained from nanometre-sized magnetite particles with: (a) an image showing grain sizes and shapes (white scale bar is 100 nm) with an inset image at higher magnification (scale bar is 5 nm) showing details of structure; (b) a selected area electron diffraction pattern confirming that the grains are of magnetite; (c) an HRTEM image of the layer silicate mineral chrysotile (‘asbestos’); scale bar is 50 ångstroms (Å)
8. Environmental scanning electron microscope (ESEM) image showing several ‘grub-like’ bacteria on an iron oxide mineral substrate
Another type of electron microscope widely employed in the study of minerals is the scanning electron microscope (SEM). As the name suggests, in this instrument the electron beam scans very rapidly across the sample surface. Secondary electrons are emitted from the surface and these are used to form an image of the sample even if the surface is rough, giving a three-dimensional effect (see Figure 8). Although the resolution is not as great as for TEM (generally it is in the range 10–20 nm) samples can be put in the instrument without special preparation. The X-rays emitted from the bombarded sample can again be used to obtain a semi-quantitative chemical analysis. A disadvantage of the conventional SEM and also of TEM and the electron probe is the need to work with the sample in a vacuum. In recent years, technical advances have led to the development of the ‘environmental (E)SEM’, an instrument where samples which are hydrated or coated with delicate biological materials such as biofilms or microbes, can be studied without being damaged. This is especially important in environmental work. The image shown in Figure 8 is actually from an ESEM and shows an iron-reducing bacterium (Geobacter sulfurreducens) on an iron oxide substrate.
Seeing atoms with scanning probe microscopes
The SEM is primarily an instrument for obtaining a much magnified image of the surface of a sample. Surfaces are a particularly important aspect of modern studies of mineral systems. This is because change generally takes place at surfaces and interfaces, as we will see in later chapters. Clearly the electron microscope methods can provide information about surfaces and interfaces but with certain limitations. Fundamental questions about how the nature of mineral surfaces at atomic level may differ from a simple termination of the bulk crystal structure, how minerals dissolve, or how they may sorb material from solution, could not be properly addressed until another dramatic technique development took place in the 1990s. This was the development of the scanning tunnelling microscope (STM) and the related technique of the atomic force microscope (AFM). Both of these methods are capable of imaging surfaces at ‘atomic resolution’, that is to say, at the level of magnification where individual atoms can be ‘seen’.
These two methods are remarkably simple ‘bench top’ experiments. In the STM, a very sharp tip (like a stylus on an old-fashioned gramophone) is scanned just above the surface of a sample which must be a conducting material. At the same time an electrical potential is applied and a current flows between the tip and the sample. Fluctuations in this current are monitored to build up an image of the surface. In AFM, a sharp tip is mounted on the end of a springy arm (cantilever), and the movement of the cantilever in response to the very small forces between the mineral surface and the tip is monitored during scanning to build up the image. Both methods depend on the ability of the instrument to scan back and forth over nanometre distances; this is achieved using a ‘piezoelectric’ device in which applying a current across a crystal causes it to expand or contract by a very small amount (see Box 2). Both STM and AFM can operate in vacuum, in air, or with the surface in contact with a fluid. Figure 9 shows atomic resolution STM images of two different iron
Box 2 From quartz clock to crystal set
The piezoelectric effect which forms the basis of the STM and AFM methods was first discovered in quartz crystals by the Curie brothers in 1880. In this phenomenon, when certain crystals are compressed along particular directions they acquire an electric field, with one surface of the crystal becoming positively charged and the opposite surface negatively charged. This property has been widely used in technology, in devices ranging from pressure gauges to the crystal pick-ups used for record players. The converse also applies, such that when an electric field is applied to a piezoelectric crystal, its dimensions change very slightly. For example, in a piece (‘bar’) of crystal quartz subjected to a moderate electric field (10,000 volts/m), the change would be equivalent to a bar that is one metre long becoming 1.0000000225 metres long. This phenomenon is at the heart of the scanning probe microscope methods. By using a cleverly designed array of piezoelectric devices to control the movement of a scanning tip when a potential is applied, it is possible accurately to scan over nanometre ranges and, therefore, to image surfaces at ‘atomic resolution’ (Figures 9a, 9b). A more familiar application of the piezoelectric effect in quartz is in the quartz clock (or watch). In this case, a quartz crystal is made to oscillate with a very precise frequency by an electric signal. This oscillation, rather than the swing of a pendulum in a traditional grandfather clock, is used to keep time to far greater accuracy than in a mechanical clock.
Piezoelectricity is just one of a number of examples where new technologies have been developed using a property first observed in a mineral. Another example concerns the early development of radio. At the beginning of the 20th century, researchers discovered that certain semiconducting minerals could be used to detect radio signals. In a very simple radio receiver called a ‘crystal set’, a small piece of galena (PbS) or of pyrite (FeS2) contacted by a fine wire (known as a ‘cat’s whisker’) is central to the receiving device. In the early days of radio, such devices could be made at home very cheaply and bring the benefits of radio to many more people. By the mid 20th century, building a crystal set had become a favourite ‘schoolboy’ (myself included) project.
9. Scanning tunnelling microscope images of: (a) the surface of a crystal of pyrite (FeS2) and (b) the surface of a crystal of pyrrhotite (Fe7S8)
sulphide mineral surfaces. In Figure 9a, the surface is of pyrite (FeS2) with darkened regions due to partial oxidation of the surface. In Figure 9b the surface is of pyrrhotite (Fe7S8) which also shows changes associated with oxidation. Note the nanometre scale of these observations.
The above account provides just a small sample of the numerous methods now available to study minerals. In my view, we too often take for granted our abilities to study the natural world, in particular the mineral world, at a level of magnification that can resolve the contributions of individual atoms. In this context, it is sobering to recall that an atom is as many times smaller compared to a soccer football as a football is compared to planet Earth. Armed with the techniques described above, and many other remarkable methods either available now or under development, we can study all aspects of minerals, their structures, chemistries, surface chemistries, and reactivities.