Chapter 1 The mineral world

In 2004, I had the honour of being the lecturer at a ‘Friday Evening Discourse’ at the Royal Institution in London. These public lectures, aimed at the popularization of science, were initiated by Michael Faraday in 1826, and were probably the first such lectures anywhere in the world. My lecture was entitled ‘Minerals, Molecules and Maladies’; and, indeed, my first word was ‘minerals’. I pointed out that the first experience many of us have of minerals comes from those seen in pebbles collected on a beach during a seaside holiday, or the fine specimens of minerals on display at a local museum. Although the study of minerals begins with their collection and identification, my lecture was about the new developments in what I would describe as ‘modern mineralogy’, the central theme of this book.

The study of minerals (mineralogy) is the most fundamental aspect of the Earth and environmental sciences. Minerals existed long before any forms of life. They have played an important role in the origin and evolution of life, and interact with biological systems in ways we are only now beginning to understand. Mineralogy is arguably also the oldest of all of the practical sciences. The first manufacture of fire that could be called upon as needed depended, in part, on the sparks produced on striking minerals such as pyrite. Although we cannot be sure of the date when the first mineral-based fire strikers were used, the earliest generally accepted evidence of fire production dates back 500,000 years to Homo erectus populations, and Homo sapiens was an expert fire starter by 40,000 to 50,000 years ago, during the Palaeolithic period.

That the use of minerals was the key to human development is shown by our use of the terms ‘Stone Age’, ‘Bronze Age’, and ‘Iron Age’. Stone tools were fashioned by our ancestors more than 30,000 years ago from hard, fine-grained rocks such as flint. By 9000 bc, clay minerals were being fired to make pottery, leading to other ceramic arts such as brickmaking and glassmaking by 3500 bc. By 3000 bc copper was being extracted from its ores, as were other metals (silver, lead, zinc, antimony), some of which could be used to make alloys. The smelting methods developed to extract these metals led the way for the development of the more demanding metallurgy needed to extract the most useful of all metals, iron. (The Roman writer Pliny described iron as ‘the most useful and most fatal instrument in the hand of man’.) In primitive societies, an expert on minerals (a ‘mineralogist’) was surely required to seek out the raw materials for the production of bronze (an alloy of copper and tin) and, later, of iron. Today, minerals are essential raw materials for our technologically advanced societies. We are surrounded by structures, machines, and devices made using mineral raw materials. It can truly be said of essential resources that ‘if it cannot be grown, it must be mined’.

Traditional mineralogy has been about describing, analysing (chemically and in terms of crystal structure), naming, and classifying minerals. It has also been concerned with the properties of minerals (such as optical, magnetic, or electrical properties). Modern mineralogy is more concerned with the roles different minerals play in the plate tectonic cycle on the one hand (in the Earth’s interior) and the weathering–transport–deposition cycles (at the Earth’s surface) on the other, and about how minerals transform under different conditions and the rates at which they change. Where minerals interact with the living world, surfaces and interfaces are central to understanding—for example, in the roles microbes play in both mineral formation and breakdown. Minerals can also be critical for human health, providing essential nutrients or releasing poisons such as arsenic (the ‘mineral–human interface’). A more indirect influence of minerals on human well-being leads us to considering the topic of mineral resources—their formation, exploitation, and scarcity.

So what exactly do we mean by ‘mineral’. As with many things in the natural world, the perfect definition is elusive, but a reasonable working definition is:

a solid material, formed by natural processes, that has a regular arrangement of its constituent atoms which sets limits to its range of chemical composition and commonly gives it a characteristic crystal shape.

Here, the ‘regular arrangement of … atoms’ is what we term the ‘crystal structure’ of a mineral, and it is commonly illustrated using the kind of ‘ball and spoke’ model shown for galena (PbS) in Figure 1a (as further discussed below). We should note here that a ‘crystal shape’ may not be obvious in some cases; the crystals may be too small to see, or not well developed, if developed at all. Also, a small number of minerals fall outside this definition, such as opal which is a form of silica lacking the ‘regular arrangement of its constituent atoms’ (being therefore described as ‘amorphous’, ‘non-crystalline’, or at least ‘poorly crystalline’). Other substances excluded by this definition are coals and bitumens, and also amber. These have no regular atomic structures or well-defined compositional limits. Also generally excluded are materials which are liquids at room temperature such as petroleum and, of course, water (but elemental mercury, a liquid above −39°C and familiar from the mercury thermometer, is classed as a mineral). We also exclude here commonplace uses of the word mineral meaning an effervescent soft drink or a substance needed by the body for good health (as in ‘vitamins and minerals’).

The distinction between a mineral and a rock is also important to clarify. Rocks are generally composed of a number of different minerals. For example, granites typically contain the potassium and sodium aluminium silicate minerals of the feldspar family, along with layer silicates of the mica family and silica itself, as the mineral quartz (see Table 1). The individual mineral grains in rocks such as granites will mostly range in size from a few centimetres in diameter to a millimetre or less, and will be intimately intergrown together. Although quartzite is made up, as the name implies, almost entirely of quartz, it is a rock not a mineral. It is rocks, themselves comprising minerals, which make up ocean floors or mountain ranges. Although the analogy should not be taken too far, we can liken the Earth to a human body, with rocks the limbs or organs, and minerals the different kinds of cells. Just as living cells are the fundamental components of the body, so minerals are the fundamental components of the Earth.

There are approximately 4,400 known minerals, with new mineral species being discovered every year (adding around 50 a year to the list). Proposed new minerals have to be approved by a Commission of the International Mineralogical Association. To be approved, data on the chemical composition, basic crystallography, and certain key properties have to be submitted, along with a proposed mineral name (see Box 1).

As well as the actual mineral species, some minerals occur in such different forms that they justify varietal names. For example, amethyst, jasper, agate, chalcedony, and rose quartz are all varieties of quartz. The differences between them relate to properties such as colour or crystallinity; differences that are caused by the presence of very minor impurities, or due to formation under different conditions.

Although there are thousands of minerals, very few are commonplace. In their mineralogy textbook, Darby Dyar, Mickey Gunter, and Dennis Tasa suggest a list of ‘big ten minerals’. Apart from calcite (calcium carbonate) which is the dominant mineral in limestones, and quartz (silicon dioxide) which dominates in sandstones as well as being important in rocks such as granites, the other members of this list are all silicates (a group discussed in detail below). They comprise the olivines (magnesium iron silicates), pyroxenes (calcium, magnesium, iron silicates), amphiboles (calcium, magnesium, iron silicates which also contain bonded water), the mica minerals muscovite (potassium, aluminium silicates with bonded water) and biotite (potassium, magnesium, iron, aluminium silicates with bonded water), and the main minerals of the feldspar group. The feldspars are alumino-silicates with dominantly potassium (orthoclase), sodium (albite), or calcium (anorthite) as essential constituents. Alternative lists to this one could be proposed on the basis of different criteria, such as the most economically important minerals. Those above are suggested to be the ‘big ten’ minerals because they are by far the most abundant components of the rocks that make up the Earth’s crust and, immediately below that, the upper mantle.

Table 1 Mineral groups

Group	Examples
Native elements	gold (Au), copper (Cu), sulphur (S), diamond and graphite (both carbon, C)
Sulphides(including arsenides)	pyrite (FeS₂), pyrrhotite (Fe_1-xS), chalcopyrite (CuFeS₂), galena (PbS), sphalerite (ZnS), mackinawite (FeS), arsenopyrite (FeAsS)
Oxides(including hydroxides)	hematite (Fe₂O₃), magnetite (Fe₃O₄), rutile (TiO₂), corundum (Al₂O₃), cuprite (Cu₂O), goethite (FeOOH)
Carbonates	calcite (CaCO₃), magnesite (MgCO₃), dolomite ((Ca,Mg)CO₃)
Sulphates	gypsum (CaSO₄·H₂O), barite (BaSO₄)
Phosphates	apatite (CaPO₄)
Halides	halite (NaCl), fluorite (CaF₂)
Silicates:
Framework	quartz (SiO₂), feldspar minerals—orthoclase feldspar (KAlSi₃O₈), plagioclase feldspar(CaAl₂Si₂O₈—NaAlSi₃O₈)
Layer	mica minerals—muscovite (K₂Al₄Si₆Al₂O₂₀(OH,F)₄) biotite (K₂(Mg,Fe)₆Si₆Al₂O₂₀(OH)₄) clay minerals—kaolinite (Al₄Si₄O₁₀(OH)₈)
Ring	beryl—Be₃Al₂Si₆O₁₈
double chain	amphibole minerals—anthophyllite ((Mg,Fe)₇ Si₈O₂₂(OH,F)₂) tremolite Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
single chain	pyroxene minerals—enstatite (MgSiO₃), diopside ((Ca,Mg) Si₂O₆)
island	olivine ((Mg,Fe)₂ SiO₄)

Box 1 Mineral names

Some minerals have names derived from antiquity, such as galena which comes from a Latin word for lead ore, and chalcopyrite from the greek chalkos meaning copper. Others have names associated with a particular property (the magnetism of magnetite, or the rose colour of rhodocrosite). However, the majority of minerals are named either after the locality where they were first discovered (aragonite from Aragon in Spain, montmorillonite from Montmorillon in France, or mackinawite from the Mackinaw Mine, Washington, USA) or after people. In the early years of characterizing and describing minerals, this may have been a leading cultural figure such as a writer or philosopher (goethite named for the German poet and philosopher, Goethe). Latterly, minerals have been named after mineralogists, in recognition of their contributions to the subject (noting that it is not acceptable for the person reporting a new mineral to name it after themselves—such a suggestion would be described by the British as ‘not cricket’). For obvious reasons, the same name cannot be used for more than one mineral, and this has led to some inventiveness when the obvious name has already been used. For example, the silver mineral smithite was named for an eminent British mineralogist; this led to scientists later wishing to honour J. V. Smith, an equally distinguished mineralogist, to name a mineral joesmithite. Another hazard faced by those honoured in this way is that of having a name ‘discredited’. If further work shows that the original study leading to a new mineral was flawed, the name has to be removed from the list of recognized mineral species and cannot ever be used again. The way in which we name minerals may seem arcane to the lay person, and there have been proposals for inventing whole new systems. But rather like attempts to simplify the spelling of English or introduce a new international language like Esperanto, they have never been adopted. Here I must own up to having a vested interest, in that the mineral vaughanite was named in my honour by Canadian scientists in 1989.

As we will see in Chapter 2, the characterization of a particular mineral involves determining its chemical composition and crystallographic properties. Until the middle of the 20th century, chemical analysis required taking the pure mineral, dissolving it in acid, and applying the methods used in the ‘wet chemical analysis’ of that solution, a procedure familiar to all who have taken chemistry courses in school or college. Remarkable results were achieved by the countless hours of patient analytical work carried out by early practitioners. Mineral analysis was revolutionized by the invention of the electron microprobe, an instrument in which a beam of electrons is focused using magnetic lenses so as to strike a flat polished surface of a mineral grain, stimulating the emission of X-rays. The energies and intensities of these X-rays are characteristic of the elements present, and can be used to obtain a quantitative chemical analysis of a volume of solid material as small as a few cubic micrometres (millionths of a metre; also called ‘microns’).

Until the early years of the 20th century, crystallography was about observing the external symmetry of natural crystals and measuring the angles between the faces of crystals. Revolutionary advances were achieved with the discovery that the interaction between a beam of X-rays and a crystalline solid could be used to determine the arrangement of the atoms in that solid. Minerals were the first crystalline solids to be studied in this way—a path which led to the greatest scientific discovery of the 20th century, the determination of the crystal structure of DNA.

The routine identification of minerals in a ‘hand specimen’ (which, as the name implies, is a sample of a size that fits neatly into the hand) in the field or laboratory relies upon a variety of simply observed properties. These include crystal morphology, hardness, density, presence of crystal planes along which the mineral can be ‘cleaved’ or ‘fractured’, lustre, and, in a very few cases, mineral magnetism. Colour may be diagnostic (the blues and greens of some copper minerals) but can be misleading. Quartz, for example, can be transparent and colourless, pink, purple, yellow, or black, and there is a range of non-transparent forms with darker colours.

In the field, it may be possible to make a definite, or at least a preliminary, identification by visual observation or with a simple test. If the mineral occurs as well-developed crystals, it should be possible to use the presence of symmetry elements to assign it to one of the seven crystal systems (see below). Some minerals have distinctive crystal shapes known as habits which may be, for example, needle-like, fibrous, platey, tabular, etc. The hardness of a mineral can be simply assessed as to whether it can scratch or be scratched by a fingernail, a copper coin, or a steel penknife blade. Minerals can break along particular, regular planes of weakness called cleavage planes, seen by inspection, or by actually causing the specimen to cleave on applying a penknife blade. A parting plane is simply such a plane that is less well developed than a cleavage plane, whereas a fracture is an irregular breakage surface. Properties such as lustre are described by terms such as metallic, resinous, pearly, glassy, or adamantine (diamond-like), and which are self-explanatory. A few minerals have one or more very distinctive properties which make identification straightforward. For example, the black metallic iron oxide mineral magnetite can deflect a compass needle due to its magnetic properties. One of the forms of the iron sulphide mineral pyrrhotite has similar behaviour but is brassy and metallic, and very different in appearance to magnetite. Density is rarely definitive, but the barium sulphate mineral baryte is unusually dense for a non-metallic mineral.

Moving from the field to the laboratory means that all minerals can be identified with certainty, provided they are present in sufficient quantity (although with modern analytical methods, even very small amounts can be identified). The methods described in Chapter 2 are extremely powerful for characterization that goes well beyond giving the mineral concerned a name. Compositions in terms of major, minor, and trace element content, crystal structure, or surface chemistry can all be determined in detail, as we shall see.

A note about crystal structures and compositions

The two essential characteristics of any mineral are its chemical composition and its crystal (‘atomic’) structure. We need to emphasize some points which are central to a proper understanding of minerals. The first concerns crystal structure, by which we mean the regular arrangement of atoms referred to above in the definition of the word ‘mineral’. Many mineral ‘phases’ (the term used to describe a specific composition and structure) can undergo a ‘phase transformation’ when subjected to a change in conditions such as an increase in temperature or pressure. A phase transformation is the way in which the structure responds to changing conditions; for example, by adopting a more compact arrangement of its atoms as they are forced together at high pressure. The best-known example of a high pressure phase transformation is that involving carbon (see Figures 1b and 1c). At atmospheric pressure, carbon occurs as the mineral graphite with a structure in which layers of carbon atoms form six-membered rings with quite strong bonds between them. But between the layers, the bonds are extremely weak. At very high pressures a different structure becomes stable, that of diamond. In diamond, each carbon atom is bonded to four other carbon atoms at the corners of a tetrahedron, and each of these carbon atoms is bonded to another four carbon atoms which are themselves bonded to another four carbon atoms (and so on in all directions). This structure is an exceptionally strong arrangement and this is reflected in the fact that diamond is by far the hardest known naturally occurring substance. This is a dramatic contrast with graphite, which is one of the softest of all minerals. Whereas phase transformations at high pressures are to more compact crystal structures, transformations at high temperatures are to more open structures.

As we shall discuss in later chapters, phase transformations are of great importance for understanding our planet, given the high temperatures and pressures in the interior of the Earth. Also shown, as Figure 1d, is a synthetic form of carbon consisting of 60 carbon atoms forming a ball. This material is called buckminsterfullerene after the American designer and architect, R. Buckminster Fuller and because of the fancied resemblance of this molecule to his architectural invention, the geodesic dome. Another synthetic form of carbon is named graphene and is essentially a monolayer of carbon atoms like a single layer of atoms in the structure of graphite (see Figure 1b). These synthetic nanomaterials (see Chapter 4 for further discussion) have the potential for wide-ranging practical applications.

Minerals are not pure, simple chemical compounds of the kind we might find in a jar in a chemistry laboratory. Take the example of the mineral pyrite (FeS₂). We could go to our laboratory, take some powdered pure iron and pure sulphur, seal them in a glass tube from which we exclude any air, and heat this mixture up to several hundred degrees. On cooling and breaking open the glass tube, we would find a single ‘phase’ to be present which (on examination using methods discussed below) we could identify as pure ‘synthetic’ pyrite (FeS₂). Scientists, myself included, undertake such experiments either to find out the temperature conditions under which particular minerals are stable, or to have a sample of pure pyrite to use in other experiments.

1. Crystals and crystal structures: (a) the crystal structure of galena, PbS, shown as both a ‘ball and spoke’ figure with black spheres representing atoms of lead and white spheres of sulphur, as well as a visualization of the structure as ‘edge-sharing’ PbS₆ octahedra; (b) ‘ball and spoke’ representation of the structure of graphite

1. (c) of diamond, and (d) of buckminsterfullerene

1. (e) one of Haüy’s original figures showing how a crystal of calcite can be considered as built from rhombohedral units; (f ) an octahedron built by stacking cubelets

There is an important difference between our synthetic pyrite and a natural sample of pyrite (or any other mineral) collected in the field or from a mine. The latter will always contain impurities. Some of these impurities may be minute grains of other mineral phases trapped within the pyrite, others may be held as atoms of the impurity element actually within the crystal structure of the pyrite. Although some of these may occur in vacant ‘spaces’ within the structure, more commonly they will be found replacing some of the iron (Fe) or sulphur (S) atoms in the pyrite. In the extreme case, a large percentage or even all of the Fe or S atoms will be replaced, as can happen with nickel (Ni) replacing the iron in pyrite to give the mineral vaesite (NiS₂). In fact, it is possible to find in nature every composition between FeS₂ and NiS₂. This is represented in mineral formulae by writing (Fe,Ni)S₂ and it is an example of what is known as a ‘solid solution’. Many examples of this are found in nature and it is behaviour that is an especially important feature of the ‘big ten’ minerals, as can be seen from an example such as the olivines ((Mg,Fe)SiO₄; see also Table 1). As well as being important for understanding the chemistry of many mineral groups, impurities may provide important clues about the way certain minerals have formed, or may be important for economic or environmental reasons. Pyrite, for example, may contain small but economically important amounts of gold (ironic given that it is sometimes called ‘fool’s gold’), or may contain arsenic (substituting for sulphur), presenting a hazard for human health.

Crystallography

It will be evident from the definition of ‘mineral’ given above that the study of minerals is also the study of crystals. Indeed, many mineralogists have also been crystallographers. Long before the discovery of X-rays and our ability to probe the ordered arrangement of atoms in crystals, scholars were attempting to understand the nature and properties of crystalline minerals. One such scholar was the Frenchman René Haüy who published his ideas in 1784. A story told about Haüy, considered by many to be the ‘father of crystallography’, was that whilst examining a group of crystals of calcite (CaCO₃) in the mineral collection belonging to a friend, he dropped the specimen which broke apart along a single plane. His forgiving friend presented him with the broken crystal which he took away and attempted to break (‘cleave’) in other directions. In this he succeeded, in his words, in ‘extracting its rhomboid nucleus’ from the crystal. This led him to propound the view that continued cleaving would eventually lead to the smallest possible unit, or building block, by repetition of which the whole crystal is built up (Figure 1e).

We now know that there is indeed a fundamental building block of any crystal that we call the unit cell, not obtainable in quite the way in which Haüy envisioned because it is at the atomic scale, being the smallest group of atoms from which the crystal can be built up by repetition in three dimensions. Shown in Figure 1e is one of Haüy’s original figures and Figure 1f shows how a macroscopic octahedral crystal can be built up by stacking cubelets. Of course, in an actual crystal there would be billions of such cubelets. The unit cell of a crystal of galena (PbS) is a cube which can be stacked in this way to form an octahedron, or simply a cube. In Figure 1a, the origin of the cube crystal of galena can be seen in the arrangement of the lead (Pb) and sulphur (S) atoms shown in a ‘ball and spoke’ figure. This is one of the ways of representing the arrangements of the atoms in a crystalline solid. Also shown in this figure is the way in which each lead atom is surrounded by six sulphur atoms at the corners of an octahedron, and how these octahedra are stacked together to build up the structure. The mineral halite, sometimes known as rock-salt (NaCl), has the same crystal structure as galena. (We should note that there are some subtleties regarding the precise definition of the ‘unit cell’ best explored by further reading; a good account is given in the book by Putnis, details of which are given in the Further reading).

The link between the unit cell and the crystal is particularly made when we talk of symmetry, which is the property that distinguishes crystals from most other natural materials. It is the determination of the elements of symmetry of a crystal that enables the mineralogist to assign it to one of seven crystal systems, an important step in routine identification. The unit cells of the seven systems are illustrated in Figure 2, along with their key symmetry elements which can be planes of symmetry, axes of symmetry or a centre of symmetry. A plane of symmetry is also known as a mirror plane whereby the crystal on one side of a plane slicing through the crystal is a mirror image of that on the other side. These planes are shown by the heavier lines in the figures, which represent the intersection of the mirror plane and the surface of the crystal. (One such plane is ‘shaded in’ in Figure 2b). An axis of symmetry can be envisaged by thinking of holding up a large crystal between thumb and forefinger and rotating it through 360°. If, to the observer, the crystal appears the same 2, 3, 4, or 6 times during one rotation, it is said to have a two-, three-, four-, or sixfold axis of symmetry. In the figures, the symmetry axes are shown as lines passing through the centre of the crystal, and with appropriate symbols on their ends (hexagon, square, triangle, or lozenge for 6-, 4-, 3-, 2-fold axes). A centre of symmetry is present if each face has a similar parallel face on the opposite side of the crystal.

As well as the elements of symmetry, the unit cells shown in Figure 2 are characterized by the dimensions along, and angular relationships between, what we can define as their x, y, and z axes. The triclinic unit cell (Figure 2a) is the least symmetric, having only a centre of symmetry as symbolized by the open circle. Here, the dashed lines can be considered the x, y, and z axes; the lengths of the unit cell edges along these three axes are the unit cell parameters and labelled a, b, and c (where a is a length along the x axis, b along the y axis, and c along the z axis). In the triclinic case, these lengths are all different. Furthermore, none of the angles between the axes (labelled a, b, c) are right angles. This means that the angles between a triclinic crystal’s edges or faces are never 90°. In a monoclinic crystal (Figure 2b) the parameters a, b, and c are again unequal but, whereas the angle between the x and z axes (b) is greater than 90°, the angles between x and y, and between y and z are both 90°. Also shown here is the 2-fold axis and, at right angles to it, a plane of symmetry, a combination characteristic of monoclinic crystals. For the other five crystal systems, the angles between x, y, and z axes are always 90°. Whereas in orthorhombic crystals the a, b, and c parameters are all different, in tetragonal crystals, a and b parameters are the same and differ from the c dimension (Figures 2c, 2d). Of course, these crystals have many more planes and axes of symmetry and this reaches the extreme in the cubic system (Figure 2e) where a = b = c. The trigonal (or rhombohedral) and the hexagonal systems have a z axis which is, respectively, 3-fold or 6-fold and with a c dimension that differs from a and b which are themselves equal. To deal with the 3-fold and 6-fold rotation in these systems, a fourth axis in the x–y plane is introduced and labelled ‘u’ (so that x, y, and u are at 120°).

2. Unit cells of the seven crystal systems illustrating their axes and planes of symmetry. The latter are shown as the darker lines on the surfaces of the unit cells but shaded in for emphasis in the monoclinic case, (b); the axes of symmetry are the lines extending out and marked with symbols to show whether 2-, 3-, 4-, or 6-fold axes. The unit cells are: (a) triclinic; (b) monoclinic; (c) orthorhombic; (d) tetragonal; (e) cubic; (f) trigonal, and (g) hexagonal in symmetry

The seven crystal systems are just the beginning as regards crystallography, one of the oldest formal branches of science. Subdivision of the seven crystal systems on the basis of symmetry elements gives us the 32 crystal classes. Here, whereas each system has a minimum requirement in terms of symmetry needed to ‘qualify for membership’, other classes within that system have additional symmetry elements, but not sufficient to qualify for membership of another system. For example, in the cubic system, the 4-fold axes seen in the cube itself (Figure 2e) are not essential for membership of the system. The essential elements are the four 3-fold axes that, in the cube, pass through the opposite ‘corners’ of the crystal. Therefore, a lower symmetry crystal belonging to the cubic system can be in the form of a tetrahedron, which has four triangular faces giving a shape familiar from some packaging of milk or other drinks. Also, as discussed below, the basic building block of all silicate minerals is the SiO₄ tetrahedron (see Figure 3a). One other point worth emphasizing is that a particular crystal shape may be found in several different systems. A simple example is provided by the octahedron, already illustrated for the cubic case in Figure 1f. If the same shape is modified by ‘stretching’ along the z axis, lengthening the c dimension so that it is now greater than the a and b dimensions, the octahedron becomes a tetragonal rather than a cubic octahedron.

3. The silicate mineral crystal structures showing: (a) the basic SiO₄ building block as both ‘ball and spoke’ model and as a tetrahedron; (b) a six-membered ring of linked tetrahedra

3. (c) a chain and double chain of linked tetrahedra

Mineral families

An important aspect of the development of mineralogy, as with other natural sciences, has been concerned with classification. Minerals are classified in terms of membership of families, better termed mineral groups. The main groups are shown in Table 1. As with many classification schemes, there are various ways we could choose to organize the ~4,400 known mineral species into groups. We could, for example, regard all of the copper-containing minerals as members of one group. This would involve naturally occurring copper metal itself (‘native’ copper), copper combined with anions (i.e. negatively charged atoms) of elements such as sulphur, oxygen, chlorine as sulphides, oxides, and chlorides of copper. There would also be copper in combination with more complex anions such as sulphate (SO₄), carbonate (CO₃), and silicate (SiO₄). However, both in terms of mineral chemistry and natural occurrence, it is more useful to classify minerals such that all of the sulphides form a group, and likewise all of the oxides, carbonates, silicates, and so on.

3. (d) a complete sheet of linked tetrahedra

In Table 1, only the main groups are shown with some important examples for each group. It is common practice to include closely related minerals within a major group; for example, including hydroxides (OH containing minerals) in with the oxides, or including chemically related minor minerals within a group (selenides and tellurides within the sulphide group). The silicates are a special case when it comes to classification, given their overwhelming dominance of the ‘top ten’ minerals which is a consequence, in turn, of their dominance in the rocks of the Earth’s crust and upper mantle. All silicates contain tetrahedral building blocks in which a silicon atom is located at the centre of four oxygen atoms, thus forming a SiO₄ unit (see Figure 3a). These units can be represented by a ‘ball and spoke’ figure, or simply as a tetrahedron if the intention is to show how they link together to build up the structures of the different silicates. Tetrahedra can remain unconnected as ‘islands’ held together by the other atoms in the crystal; in the olivines these are atoms of magnesium and/or iron. There may be pairs of tetrahedra joined by sharing a single oxygen, rings with different numbers of units linking together, or single chains as in the pyroxene minerals (see Figures 3b, 3c). The amphibole family has double chains, and the micas and clay minerals complete sheets of linked tetrahedra (Figures 3c, 3d). Linkage via the sharing of all four corner oxygen atoms produces the framework structure of quartz. In some cases, AlO₄ tetrahedral units may also be involved in building up the structures instead of SiO₄ units; this is the case for the framework structures of the feldspar minerals, the most abundant of all minerals in the rocks of the Earth’s crust. The most important silicate minerals and their categorization on the basis of structure are shown in Table 1.

In our discussion of minerals, I will focus on those that are either the most abundant, as noted above in terms of the ‘top ten minerals’, or of particular interest as regards mineral resources or Earth processes (such as those associated with the deep interior). The great majority of minerals, and even whole groups of minerals, are relatively rare and will not feature in our discussions. Nevertheless, the principles involved in the methods of study, and the processes of formation of these rarer minerals are essentially the same as for many of the commoner minerals which are discussed in this book. There are comprehensive mineral classification schemes, as well as listings of most known minerals, detailed in the recommended further reading at the end of this book.