mineral
Introduction
naturally occurring homogeneous solid with a definite chemical composition and a highly ordered atomic arrangement; it is usually formed by inorganic processes. There are several thousand known mineral species, about 100 of which constitute the major mineral components of rocks; these are the so-called rock-forming minerals.
General considerations
Definition
A mineral, which by definition must be formed through natural processes, is distinct from the synthetic equivalents produced in the laboratory. Man-made versions of minerals, including emeralds, sapphires, diamonds, and other valuable gemstones, are regularly produced in industrial and research facilities and are often nearly identical to their natural counterparts.
By its definition as a homogeneous solid, a mineral is composed of a single solid substance of uniform composition that cannot be physically separated into simpler compounds. Homogeneity is determined relative to the scale on which it is defined. A specimen that megascopically appears homogeneous, for example, may reveal several mineral components under a microscope or upon exposure to X-ray diffraction techniques. Most rocks are composed of several different minerals; e.g., granite consists of feldspar, quartz, mica, and amphibole. In addition, gases and liquids are excluded by a strict interpretation of the above definition of a mineral. Ice, the solid state of water (H2O), is considered a mineral, but liquid water is not; liquid mercury, though sometimes found in mercury ore deposits, is not classified as a mineral either. Such substances that resemble minerals in chemistry and occurrence are dubbed mineraloids and are included in the general domain of mineralogy.
Since a mineral has a definite composition, it can be expressed by a specific chemical formula. Quartz (silicon dioxide), for instance, is rendered as SiO2, because the elements silicon (Si) and oxygen (O) are its only constituents and they invariably appear in a 1:2 ratio. The chemical makeup of most minerals is not as well defined as that of quartz, which is a pure substance. Siderite, for example, does not always occur as pure iron carbonate (FeCO3); magnesium (Mg), manganese (Mn), and, to a limited extent, calcium (Ca) may sometimes substitute for the iron. Since the amount of the replacement may vary, the composition of siderite is not fixed and ranges between certain limits, although the ratio of the metal cation to the anionic group remains fixed at 1:1. Its chemical makeup may be expressed by the general formula (Fe, Mn, Mg, Ca)CO3, which reflects the variability of the metal content.
Minerals display a highly ordered internal atomic structure that has a regular geometric form (see Figure 1-->). Because of this feature, minerals are classified as crystalline solids. Under favourable conditions, crystalline materials may express their ordered internal framework by a well-developed external form, often referred to as crystal form or morphology (see Figure 2-->). Solids that exhibit no such ordered internal arrangement are termed amorphous. Many amorphous natural solids, such as glass, are categorized as mineraloids.Traditionally, minerals have been described as resulting exclusively from inorganic processes; however, current mineralogic practice often includes as minerals those compounds that are organically produced but satisfy all other mineral requirements. aragonite (CaCO3) is an example of an inorganically formed mineral that also has an organically produced, yet otherwise identical, counterpart; the shell (and the pearl, if it is present) of an oyster is composed to a large extent of organically formed aragonite. Minerals also are produced by the human body: hydroxylapatite 【Ca5(PO4)3(OH)】 is the chief component of bones and teeth, and calculi are concretions of mineral substances found in the urinary system.
Nomenclature
While minerals are classified in a logical manner according to their major anionic (negatively charged) chemical constituents into groups such as oxides, silicates, and nitrates, they are named in a far less scientific or consistent way. Names may be assigned to reflect a physical or chemical property, such as colour, or they may be derived from various subjects deemed appropriate, as, for example, a locality, public figure, or mineralogist. Some examples of mineral names and their derivations follow: albite (NaAlSi3O8) is from the Latin word (albus) for “white” in reference to its colour; goethite (FeO · OH) is in honour of Johann Wolfgang von Goethe, the German poet; manganite (MnO · OH) reflects the mineral's composition; franklinite (ZnFe2O4) is named after Franklin, N.J., U.S., the site of its occurrence as the dominant zinc (Zn) mineral; and sillimanite (Al2SiO4) is in honour of the American chemist Benjamin Silliman. Since 1960, an international committee of nomenclature has reviewed descriptions of new minerals and proposals for new mineral names and has attempted to remove inconsistencies.
Occurrence and formation
Minerals form in all geologic environments and thus under a wide range of chemical and physical conditions, such as varying temperature and pressure. The four main categories of mineral formation are (1) igneous (igneous rock), or magmatic, in which minerals crystallize from a melt; (2) sedimentary (sedimentation), in which minerals are the result of the processes of weathering, erosion, and sedimentation; (3) metamorphic (metamorphism), in which new minerals form at the expense of earlier ones owing to the effects of changing—usually increasing—temperature or pressure or both on some existing rock type (metamorphic minerals are the result of new mineral growth in the solid state without the intervention of a melt, as in igneous processes); and (4) hydrothermal, in which minerals are chemically precipitated from hot solutions within the Earth. The first three processes generally lead to varieties of rocks in which different mineral grains are closely intergrown in an interlocking fabric. Hydrothermal solutions, and even solutions at very low temperatures (e.g., groundwater), tend to follow fracture zones in rocks that may provide open spaces for the chemical precipitation of minerals from solution. It is from such open spaces, partially filled by minerals deposited from solutions, that most of the spectacular mineral specimens have been collected. If a mineral that is in the process of growth (as a result of precipitation) is allowed to develop in a free space, it will generally exhibit a well-developed crystal form (see Figure 2-->), which adds to a specimen's aesthetic beauty. Similarly, geodes (geode), which are rounded, hollow, or partially hollow bodies commonly found in limestones, may contain well-formed crystals lining the central cavity. Geodes form as a result of mineral deposition from solutions such as groundwater.The nature of minerals
Morphology
Nearly all minerals have the internal ordered arrangement of atoms and ions that is the defining characteristic of crystalline (crystal) solids (see Figure 1-->). Under favourable conditions, minerals may grow as well-formed crystals, characterized by their smooth plane surfaces and regular geometric forms. Development of this good external shape is largely a fortuitous outcome of growth and does not affect the basic properties of a crystal. Therefore, the term crystal is most often used by material scientists to refer to any solid with an ordered internal arrangement, without regard to the presence or absence of external faces.Symmetry elements
The external shape, or morphology, of a crystal is perceived as its aesthetic beauty, and its geometry reflects the internal atomic arrangement (see Figure 2-->). The external shape of well-formed crystals expresses the presence or absence of a number of symmetry elements. Such symmetry elements include rotation axes, rotoinversion axes, a centre of symmetry, and mirror planes.A rotation axis is an imaginary line through a crystal around which it may be rotated and repeat itself in appearance one, two, three, four, or six times during a complete rotation. A sixfold rotation axis is illustrated in Figure 3A. When rotated about this axis, the crystal repeats itself each 60° (six times in a 360° rotation).
A rotoinversion axis combines rotation about an axis of rotation with inversion. Rotoinversion axes are symbolized as 1, 2, 3, 4, and 6. 1 is equivalent to a centre of symmetry (or inversion, i), 2 is equivalent to a mirror plane, 3 is equivalent to a threefold rotation axis plus a centre of symmetry, 4 is not composed of other operations and is unique, and 6 is equivalent to a threefold rotation axis with a mirror plane perpendicular to the axis. The morphological expression of a fourfold rotoinversion axis is illustrated in Figure 3B.
A centre of symmetry exists in a crystal if an imaginary line can be extended from any point on its surface through its centre and a similar point is present along the line equidistant from the centre (see Figure 3C). This is equivalent to 1, or inversion. There is a relatively simple procedure for recognizing a centre of symmetry in a well-formed crystal. With the crystal (or a wooden or plaster model thereof) laid down on any face on a tabletop, the presence of a face of equal size and shape, but inverted, in a horizontal position at the top of the crystal proves the existence of a centre of symmetry.
A mirror plane is an imaginary plane that separates a crystal into halves such that, in a perfectly developed crystal, the halves are mirror images of one another. A single mirror in a crystal, also called a symmetry plane, is illustrated in Figure 3D.
Morphologically crystals can be grouped into 32 crystal classes that represent the 32 possible symmetry elements and their combinations. These crystal classes, in turn, are grouped into six crystal systems. In decreasing overall symmetry content, beginning with the system with the highest and most complex crystal symmetry, they are isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. The systems may be described in terms of crystallographic axes used for reference. The c axis is normally the vertical axis. The isometric system exhibits three mutually perpendicular axes of equal length (a1, a2, and a3). The orthorhombic (orthorhombic system) and tetragonal systems (tetragonal system) also contain three mutually perpendicular axes; in the former system, all the axes are of different lengths (a, b, and c), and, in the latter, two axes are of equal length (a1 and a2) while the third (vertical) axis is either longer or shorter (c). The hexagonal system contains four axes: three equal-length axes (a1, a2, and a3) intersect one another at 120° and lie in a plane that is perpendicular to the fourth (vertical) axis of a different length. Three axes of different lengths (a, b, and c) are present in both the monoclinic and triclinic systems (triclinic system). In the monoclinic system, two axes intersect one another at an oblique angle and lie in a plane perpendicular to the third axis; in the triclinic system, all axes intersect at oblique angles.
The grouping of the 32 possible crystal classes (point group) among the crystal systems is shown in Table 1-->. Column 1 of the table lists the six crystal systems; column 2 gives the total symmetry content of each of the 32 crystal classes; and column 3 gives a symbolic representation for each of the 32 combinations of symmetry elements known as the Hermann-Mauguin, or international, notation. This compact and very useful notation is discussed in professional literature, such as the references given in the Bibliography (feldspar).
Three different crystals with distinctively dissimilar symmetry contents, as expressed by their external morphology, are given in Figure 4. Figure 4A shows a well-formed monoclinic crystal with symmetry content i, 1A2, and 1m (2/m); Figure 4B features a crystal in the tetragonal system with symmetry content i, 1A4, and 1m (4/m); and Figure 4C shows a crystal in the isometric system having the highest possible symmetry content of 3A4, 4A3, 6A2, and 9m (4/m32/m). (See Table 1-->.) Photographs of some well-formed crystal groups are given in Figure 5-->. twinning
If two or more crystals form a symmetrical intergrowth, they are referred to as twinned crystals. A new symmetry operation (called a twin element), which is lacking in a single untwinned crystal, relates the individual crystals in a twinned position. There are three twin elements that may relate the crystals of a twin: (1) reflection by a mirror plane (twin plane), (2) rotation about a crystal direction common to both (twin axis) with the angular rotation typically 180°, and (3) inversion about a point (twin centre). An instance of twinning is defined by a twin law that specifies the presence of a plane, an axis, or a centre of twinning. If a twin has three or more parts, it is referred to as a multiple, or repeated, twin.
Internal structure
Examining crystal structures
The external morphology of a mineral is an expression of the fundamental internal architecture of a crystalline (crystal) substance—i.e., its crystal structure. The crystal structure is the three-dimensional, regular (or ordered) arrangement of chemical units (atoms, ions, and anionic groups in inorganic materials; molecules in organic substances); these chemical units (referred to here as motifs) are repeated by various translational and symmetry operations (see below). The morphology of crystals can be studied with the unaided eye in large well-developed crystals and has been historically examined in considerable detail by optical measurements of smaller well-formed crystals through the use of optical goniometers. The internal structure of crystalline materials, however, is revealed by a combination of X-ray, neutron, and electron diffraction techniques, supplemented by a variety of spectroscopic methods, including infrared, optical, Mössbauer, and resonance techniques. These methods, used singly or in combination, provide a quantitative three-dimensional reconstruction of the location of the atoms (or ions), the chemical bond types and their positions, and the overall internal symmetry of the structure. The repeat distances in most inorganic structures and many of the atomic and ionic motif sizes are on the order of 1 to 10 angstroms (1 angstrom 【Å】 is equivalent to 10-8 centimetre 【cm】 or 3.94 × 10-9 inch) or 10 to 100 nanometres (nm; 1 nm is equivalent to 10-7 cm or 10 Å).
Space groups
Symmetry elements that are observable in the external morphology of crystals, such as rotation and rotoinversion axes, mirror planes, and a centre of symmetry, also are present in their internal atomic structure. In addition to these symmetry elements, there are translations and symmetry operations combined with translations. (Translation is the operation in which a motif is repeated in a linear pattern at intervals that are equal to the translation distance 【commonly on the 1 to 10 Å level】.) Two examples of translational symmetry elements are screw axes (combining rotation and translation) and glide planes (combining mirroring and translation). The internal translation distances are exceedingly small and can be seen directly only by very high magnification electron-beam techniques, as used in a transmission electron microscope, at magnifications of about 600,000×. The 32 combinations of the translation-free symmetry elements are the crystal classes listed in Table 1--> above. When all possible combinations of translational elements compatible with the 32 crystal classes (also known as point groups) are considered, one arrives at 230 possible ways in which translations, translational symmetry elements (screw axes and glide planes), and translation-free symmetry elements (rotation and rotoinversion axes and mirror planes) can be combined. These translation and symmetry groupings are known as the 230 space groups (space group), representing the various ways in which motifs can be arranged in an ordered three-dimensional array. The symbolic representation of space groups is closely related to that of the Hermann-Mauguin notation of point groups as listed in column 3 of Table 1-->. A detailed discussion of space groups, their derivation, and notation is beyond the scope of this article. For more specific information, consult the books on mineralogy cited in the Bibliography (feldspar).Illustrating crystal structures
As in the case of the illustrations of the external morphology of crystals given above (Figures 2 through 5), the three-dimensional arrangement of crystal structures must be presented on a two-dimensional page. A common method of illustration involves projecting the crystal structure onto a planar surface, as in Figure 6, which portrays the structure of a form of silicon dioxide (SiO2) known as tridymite. The structural motif units in this case are SiO4 tetrahedrons composed of a silicon atom surrounded by four oxygen atoms. To further aid the visualization of complex crystal structures, models of such structures can be built or obtained commercially. Models of this sort reproduce the internal atomic arrangement on an enormously enlarged scale (e.g., one angstrom might be represented by one centimetre).
polymorphism
Polymorphism is the ability of a specific chemical composition to crystallize in more than one form. This generally occurs as a response to changes in temperature or pressure or both. The different structures of such a chemical substance are called polymorphic forms, or polymorphs. For example, the element carbon (C) occurs in nature in two different polymorphic forms, depending on the external (pressure and temperature) conditions. These forms are graphite, with a hexagonal structure, and diamond, with an isometric structure. The composition FeS2 occurs most commonly as pyrite, with an isometric structure, but it is also found as marcasite, which has an orthorhombic internal arrangement. The composition 2 (silica) is found in a large number of polymorphs, among them quartz, tridymite, cristobalite, coesite, and stishovite. The stability field (conditions under which a mineral is stable) of these SiO2 polymorphs can be expressed in a stability diagram, with the external parameters of temperature and pressure as the two axes. In the general quartz field, there is additional polymorphism leading to the notation of high quartz and low quartz, each form having a slightly different internal structure. The diagram clearly indicates that cristobalite and tridymite are the high-temperature forms of SiO2, and indeed these SiO2 polymorphs occur in high-temperature lava flows. The high-pressure forms of SiO2 are coesite and stishovite, and these can be found in meteorite craters, formed as a result of high explosive pressures upon quartz-rich sandstones, and in very deep-seated rock formations, as from the Earth's upper mantle.
Chemical composition
The chemical composition of a mineral is of fundamental importance because its properties greatly depend on it. Such properties, however, are determined not only by the chemical composition but also by the geometry of the constituent atoms and ions and by the nature of the electrical forces that bind them. Thus, for a complete understanding of minerals, their internal structure, chemistry, and bond types must be considered.
Various analytical techniques may be employed to obtain the chemical composition of a mineral. Quantitative chemical analyses conducted prior to 1947 mainly utilized so-called wet analytical methods, in which the mineral sample is first dissolved. Various compounds are then precipitated from the solution, which are weighed to obtain a gravimetric analysis. Since 1947 a number of analytical procedures have been introduced that provide faster but somewhat less accurate results. Most analyses performed since 1960 have made use of instrumental methods such as optical emission, X-ray fluorescence, atomic absorption spectroscopy, and electron microprobe analysis. Relatively well-established error ranges have been documented for these methods, and samples must be prepared in a specific manner for each technique. A distinct advantage of wet analytical procedures is that they make it possible to determine quantitatively the oxidation states of positively charged atoms, called cations (e.g., Fe2+ versus Fe3+), and to ascertain the amount of water in hydrous minerals. Instrumental techniques, on the other hand, cannot provide this type of information.
To ensure an accurate chemical analysis, the selected sample must contain only one mineral species (i.e., the one for which the analysis is being done) and must not have undergone alteration processes. Since it is frequently difficult, and at times impossible, to obtain as much as 0.1 to 1 gram of “clean” material for analysis, the results should be accompanied by specifications on the amount of impurities present. To reduce the effect of the impurities, an instrumental technique, such as electron microprobe analysis, is often employed. In this method, quantitative analysis in situ may be performed on mineral grains only 1 micrometre (10-4 centimetre) in diameter.
Mineral formulas
Elements may exist in the native (uncombined) state, in which case their formulas are simply their chemical symbols: gold (Au), carbon (C) in its polymorphic form of diamond, and sulfur (S) are common examples. Most minerals, however, occur as compounds consisting of two or more elements; their formulas are obtained from quantitative chemical analyses and indicate the relative proportions of the constituent elements. The formula of sphalerite, ZnS, reflects a one-to-one ratio between atoms of zinc and those of sulfur. In bornite (Cu5FeS4), there are five atoms of copper (Cu), one atom of iron (Fe), and four atoms of sulfur. There exist relatively few minerals with constant composition; notable examples include quartz (SiO2) and kyanite (Al2SiO5). Minerals of this sort are termed pure substances. Most minerals display considerable variation in the ions that occupy specific atomic sites within their structure. For example, the iron content of rhodochrosite (MnCO3) may vary over a wide range. As ferrous iron (Fe2+) substitutes for manganese cations (Mn2+) in the rhodochrosite structure, the formula for the mineral might be given in more general terms—namely (Mn, Fe)CO3. The amounts of manganese and iron are variable, but the ratio of the cation to the negatively charged anionic group remains fixed at one Mn2+ or Fe2+ atom to one CO3 group.
Compositional variation
As stated above, most minerals exhibit a considerable range in chemical composition. Such variation results from the replacement of one ion or ionic group by another in a particular structure. This phenomenon is termed ionic substitution, or solid solution. Three types of solid solution are possible, and these may be described in terms of their corresponding mechanisms—namely, substitutional, interstitial, and omission.
Substitutional solid solution is the most common variety. For example, as described above, in the carbonate mineral rhodochrosite (MnCO3), Fe2+ may substitute for Mn2+ in its atomic site in the structure.
The degree of substitution may be influenced by various factors, with the size of the ion being the most important. Ions of two different elements can freely replace one another only if their ionic radii differ by approximately 15 percent or less. Limited substitution can occur if the radii differ by 15 to 30 percent, and a difference of more than 30 percent makes substitution unlikely. These limits, calculated from empirical data, are only approximate.
The temperature at which crystals grow also plays a significant role in determining the extent of ionic substitution. The higher the temperature, the more extensive is the thermal disorder in the crystal structure and the less exacting are the spatial requirements. As a result, ionic substitution that could not have occurred in crystals grown at low temperatures may be present in those grown at higher ones. The high-temperature form of KAlSi3O8 (sanidine), for example, can accommodate more sodium (Na) in place of potassium (K) than can microcline, its low-temperature counterpart.
An additional factor affecting ionic substitution is the maintenance of a balance between the positive and negative charges in the structure. Replacement of a monovalent ion (e.g., Na+, a sodium cation) by a divalent ion (e.g., Ca2+, a calcium cation) requires further substitutions to keep the structure electrically neutral.
Simple cationic or anionic substitutions are the most basic types of substitutional solid solution. A simple cationic substitution can be represented in a compound of the general form A+X- in which cation B+ replaces in part or in total cation A+. Both cations in this example have the same valence (+1), as in the substitution of K+ (potassium ions) for Na+ (sodium ions) in the NaCl (sodium chloride) structure. Similarly, the substitution of anion X- by Y- in an A+X- compound represents a simple anionic substitution; this is exemplified by the replacement of Cl- (chlorine ions) with Br- (bromine ions) in the structure of KCl (potassium chloride). A complete solid-solution series involves the substitution in one or more atomic sites of one element for another that ranges over all possible compositions and is defined in terms of two end-members. For example, the two end-members of olivine 【(Mg, Fe)2SiO4】, forsterite (Mg2SiO4) and fayalite (Fe2SiO4), define a complete solid-solution series in which magnesium cations (Mg2+) are replaced partially or totally by Fe2+.
In some instances, a cation B3+ may replace some A2+ of compound A2+X2-. So that the compound will remain neutral, an equal amount of A2+ must concurrently be replaced by a third cation, C+. This is given in equation form as 2A2+ ⇆ B3+ + C+; the positive charge on each side is the same. Substitutions such as this are termed coupled substitutions. The plagioclase feldspar series exhibits complete solid solution, in the form of coupled substitutions, between its two end-members, albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Every atomic substitution of Na+ by Ca2+ is accompanied by the replacement of a silicon cation (Si4+) by an aluminum cation (Al3+), thereby maintaining electrical neutrality: Na+ + Si4+ ⇆ Ca2+ + Al3+.
The second major type of ionic substitution is interstitial solid solution, or interstitial substitution. It takes place when atoms, ions, or molecules fill the interstices (voids) found between the atoms, ions, or ionic groups of a crystal structure. The interstices may take the form of channellike cavities in certain crystals, such as the ring silicate beryl (Be3Al2Si6O18). Potassium, rubidium (Rb), cesium (Cs), and water, as well as helium (He), are some of the large ions and gases found in the tubular voids of beryl.
The least common type of solid solution is omission solid solution, in which a crystal contains one or more atomic sites that are not completely filled. The best-known example is exhibited by pyrrhotite (Fe1 - xS). In this mineral, each iron atom is surrounded by six neighbouring sulfur atoms. If every iron site in pyrrhotite was occupied by ferrous iron, its formula would be FeS. There are, however, varying percentages of vacancy in the iron site, so that the formula is given as Fe6S7 through Fe11S12, the latter being very near to pure FeS. The formula for pyrrhotite is normally written as Fe1 - xS, with x ranging from 0 to 0.2. It is one of the minerals referred to as a defect structure, because it has a structural site that is not completely occupied.
chemical bonding
Electrical forces are responsible for binding together the atoms, ions, and ionic groups that constitute crystalline solids. The physical and chemical properties of minerals are attributable for the most part to the types and strengths of these binding forces; hardness, cleavage, fusibility, electrical and thermal conductivity, and the coefficient of thermal expansion are examples of such properties. On the whole, the hardness and melting point of a crystal increase proportionally with the strength of the bond, while its coefficient of thermal expansion decreases. The extremely strong forces that link the carbon atoms of diamond, for instance, are responsible for its distinct hardness. Periclase (MgO) and halite (NaCl) have similar structures; however, periclase has a melting point of 2,800° C (5,072° F), whereas halite melts at 801° C (1,474° F). This discrepancy reflects the difference in the bond strength of the two minerals: since the atoms of periclase are joined by a stronger electrical force, a greater amount of heat is needed to separate them.
The electrical forces, called chemical bonds, can be divided into five types: ionic, covalent, metallic, van der Waals, and hydrogen bonds. Classification in this manner is largely one of expediency; the chemical bonds in a given mineral may in fact possess characteristics of more than one bond type. For example, the forces that link the silicon and oxygen atoms in quartz exhibit in nearly equal amount the characteristics of both ionic and covalent bonds. As stated above, the electrical interaction between the atoms of a crystal determine its physical and chemical properties. Thus, classifying minerals according to their electrical forces will cause those species with similar properties to be grouped together. This fact justifies classification by bond type.
Ionic bonds (ionic bond)
Atoms have a tendency to gain or lose electrons so that their outer orbitals become stable; this is normally accomplished by these orbitals being filled with the maximum allowed number of valence electrons. Metallic sodium, for example, has one valence electron in its outer orbital; it becomes ionized by readily losing this electron and exists as the cation Na+. Conversely, chlorine gains an electron to complete its outer orbital, thereby forming the anion Cl-. In the mineral halite, NaCl (common, or rock, salt), the chemical bonding that holds the Na+ and Cl- ions together is the attraction between the two opposite charges. This bonding mechanism is referred to as ionic, or electrovalent (see Figure 7A).
Ionically bonded crystals typically display moderate hardness and specific gravity, rather high melting points, and poor thermal and electrical conductivity. The electrostatic charge of an ion is evenly distributed over its surface, and so a cation tends to become surrounded with the maximum number of anions that can be arranged around it. Since ionic bonding is nondirectional, crystals bonded in this manner normally display high symmetry (see Table 2-->).Covalent bonds (covalent bond)
In the discussion of the ionic bond, it was noted that chlorine readily gains an electron to achieve a stable electron configuration. An incomplete outer orbital places a chlorine atom in a highly reactive state, so it attempts to combine with nearly any atom in its proximity. Because its closest neighbour is usually another chlorine atom, the two may bond together by sharing one pair of electrons. As a result of this extremely strong bond, each chlorine atom enters a stable state.
The electron-sharing, or covalent, bond is the strongest of all chemical bond types. Minerals bonded in this manner display general insolubility, great stability, and a high melting point. Crystals of covalently bonded minerals tend to exhibit lower symmetry than their ionic counterparts because the covalent bond is highly directional, localized in the vicinity of the shared electrons (see Table 2-->).The Cl2 molecules formed by linking two neighbouring chlorine atoms are stable and do not combine with other molecules. Atoms of some elements, however, have more than one electron in the outer orbital and thus may bond to several neighbouring atoms to form groups, which in turn may join together in larger combinations. Carbon, in the polymorphic form of diamond, is a good example of this type of covalent bonding. There are four valence electrons in a carbon atom, so that each atom bonds with four others in a stable tetrahedral configuration (see Figure 7B). A continuous network is formed by the linkage of every carbon atom in this manner. The rigid diamond structure results from the strong localization of the bond energy in the vicinity of the shared electrons; this makes diamond the hardest of all natural substances. Diamond does not conduct electricity, because all the valence electrons of its constituent atoms are shared to form bonds and therefore are not mobile (see Table 2-->).Metallic bonds (metallic bond)
Bonding in metals is distinct from that in their salts, as reflected in the significant differences between the properties of the two groups. In contrast to salts, metals display high plasticity, tenacity, ductility, and conductivity. Many are characterized by lower hardness and have higher melting and boiling points than, for example, covalently bonded materials. All these properties result from a metallic bonding mechanism that can be envisioned as a collection of positively charged ions immersed in a cloud of valence electrons (see Figure 7C). The attraction between the cations and the electrons holds a crystal together. The electrons are not bound to any particular cation and are thus free to move throughout the structure. In fact, in the metals sodium, cesium, rubidium, and potassium, the radiant energy of light can cause electrons to be removed from their surfaces entirely. (This result, known as the photoelectric effect, is utilized in light meters.) Electron mobility is responsible for the ability of metals to conduct heat and electricity. The native metals are the only minerals to exhibit pure metallic bonding (see Table 2-->).Van der Waals (van der Waals forces) bonds
Neutral molecules may be held together by a weak electric force known as the van der Waals bond. It results from the distortion of a molecule so that a small positive charge develops on one end and a corresponding negative charge develops on the other (see Figure 7D). A similar effect is induced in neighbouring molecules, and this dipole effect propagates throughout the entire structure. An attractive force is then formed between oppositely charged ends of the dipoles. Van der Waals bonding is common in gases and organic liquids and solids, but it is rare in minerals. Its presence in a mineral defines a weak area with good cleavage and low hardness. In graphite, carbon atoms lie in covalently bonded sheets with van der Waals forces acting between the layers.
Hydrogen bonds
In addition to the four major bond types described above, there is an interaction called hydrogen bonding. This takes place when a hydrogen atom, bonded to an electronegative atom such as oxygen, fluorine, or nitrogen, is also attracted to the negative end of a neighbouring molecule. A strong dipole-dipole interaction is produced, forming a bond between the two molecules. Hydrogen bonding is common in hydroxides and in many of the layer silicates—e.g., micas and clay minerals.
Physical properties
The physical properties of minerals are the direct result of the structural and chemical characteristics of the minerals. Some properties can be determined by inspection of a hand specimen or by relatively simple tests on such a specimen. Others, such as those determined by optical and X-ray diffraction techniques, require special and often sophisticated equipment and may involve elaborate sample preparation. In the discussion that follows, emphasis is placed on those properties that can be most easily evaluated with only simple tests.
Crystal habit and crystal aggregation
The external shape (habit) of well-developed crystals can be visually studied and classified according to the crystal systems and crystal classes listed in Table 1-->. The majority of crystal occurrences, however, are not part of well-formed single crystals but are found as crystals grown together in aggregates. Examples of some descriptive terms for such aggregations, illustrated in Figure 8, are given here: granular, an intergrowth of mineral grains of approximately the same size; lamellar, flat, platelike individuals arranged in layers; bladed, elongated crystals flattened like a knife blade; fibrous, an aggregate of slender fibres, parallel or radiating; acicular, slender, needlelike crystals; radiating, individuals forming starlike or circular groups; globular, radiating individuals forming small spherical or hemispherical groups; dendritic, in slender divergent branches, somewhat plantlike; mammillary, large smoothly rounded, masses resembling mammae, formed by radiating crystals; botryoidal, globular forms resembling a bunch of grapes; colloform, spherical forms composed of radiating individuals without regard to size (this includes botryoidal, reniform, and mammillary forms); stalactitic, pendent cylinders or cones resembling icicles; concentric, roughly spherical layers arranged about a common centre, as in agate and in geodes (geode); geode, a partially filled rock cavity lined by mineral material (geodes may be banded as in agate owing to successive depositions of material, and the inner surface is often covered with projecting crystals); and oölitic (oölite), an assemblage consisting of small spheres resembling fish roe. cleavage and fracture
Both these properties represent the reaction of a mineral to an external force. Cleavage is breakage along planar surfaces, which are parallel to possible external faces on the crystal. It results from the tendency of some minerals to split in certain directions that are structurally weaker than others. Some crystals exhibit well-developed cleavage, as seen by the planar cleavage in mica; perfect cleavage of this sort is characterized by smooth, shiny surfaces. In other minerals, such as quartz, cleavage is absent. Quality and direction are the general characteristics used to describe cleavage. Quality is expressed as perfect, good, fair, and so forth; cleavage directions of a crystal are consistent with its overall symmetry (see Table 1-->).Some crystals do not usually break in any particular direction, reflecting roughly equal bond strengths throughout the crystal structure. Breakage in such minerals is known as fracture. The term conchoidal is used to describe fracture with smooth, curved surfaces that resemble the interior of a seashell; it is commonly observed in quartz and glass. Splintery fracture is breakage into elongated fragments like splinters of wood, while hackly fracture is breakage along jagged surfaces.
lustre
The term lustre refers to the general appearance of a mineral surface in reflected light. The main types of lustre, metallic and nonmetallic, are distinguished easily by the human eye after some practice, but the difference between them cannot be quantified and is rather difficult to describe. Metallic refers to the lustre of an untarnished metallic surface such as gold, silver, copper, or steel. These materials are opaque to light; none passes through even at thin edges. Pyrite (FeS2), chalcopyrite (CuFeS2), and galena (PbS) are common minerals that have metallic lustre. Nonmetallic lustre is generally exhibited by light-coloured minerals that transmit light, either through thick portions or at least through their edges. The following terms are used to distinguish the lustre of nonmetallic minerals: vitreous, having the lustre of a piece of broken glass (this is commonly seen in quartz and many other nonmetallic minerals); resinous, having the lustre of a piece of resin (this is common in sphalerite 【ZnS】); pearly, having the lustre of mother-of-pearl (i.e., an iridescent pearllike lustre characteristic of mineral surfaces that are parallel to well-developed cleavage planes; the cleavage surface of talc 【Mg3Si4O10(OH)2】 may show pearly lustre); greasy, having the appearance of being covered with a thin layer of oil (such lustre results from the scattering of light by a microscopically rough surface; some nepheline 【(Na, K)AlSiO4】 and milky quartz may exhibit this); silky, descriptive of the lustre of a skein of silk or a piece of satin and characteristic of some minerals in fibrous aggregates (examples are fibrous gypsum 【CaSO4 · 2H2O】, known as satin spar, and chrysotile asbestos 【Mg3Si2O5(OH)4】); and adamantine, having the brilliant lustre of diamond, exhibited by minerals with a high refractive index comparable to diamond and which as such refract light as strongly as the latter (examples are cerussite 【PbCO3】 and anglesite 【PbSO4】).
colour
Minerals occur in a great variety of colours. Because colour varies not only from one mineral to another but also within the same mineral (or mineral group), the observer must learn in which minerals it is a constant property and can thus be relied on as a distinguishing criterion. Most minerals that have a metallic lustre vary little in colour, but nonmetallic minerals can demonstrate wide variance. Although the colour of a freshly broken surface of a metallic mineral is often highly diagnostic, this same mineral may become tarnished with time. Such a tarnish may dull minerals such as galena (PbS), which has a bright, bluish lead-gray colour on a fresh surface but may become dull upon long exposure to air. bornite (Cu5FeS4), which on a freshly broken surface has a brownish bronze colour, may be so highly tarnished on an older surface that it shows variegated purples and blues; hence, it is called peacock ore. In other words, in the identification of minerals with a metallic lustre, it is important for the observer to have a freshly broken surface for accurate determination of colour.
A few minerals with nonmetallic lustre display a constant colour that can be used as a truly diagnostic property. Examples are malachite, which is green; azurite, which is blue; rhodonite, which is pink; turquoise, which gives its name to the colour turquoise, a greenish blue to blue-green; and sulfur, which is yellow. Many nonmetallic minerals have a relatively narrow range of colours, although some have an unusually wide range. Members of the plagioclase feldspar series range from almost pure white in albite through light gray to darker gray toward the anorthite end-member. Most common garnets show various shades of red to red-brown to brown. Members of the monoclinic pyroxene group range from almost white in pure diopside to light green in diopside containing a small amount of iron as a substitute for magnesium in the structure through dark green in hedenbergite to almost black in many augites. Members of the orthopyroxene series (enstatite to orthoferrosilite) range from light beige to darker brown. On the other hand, tourmaline may show many colours (red, blue, green, brown, and black) as well as distinct colour zonation, from colourless through pink to green, within a single crystal. Similarly, numerous gem minerals such as corundum, beryl, and quartz occur in many colours; the gemstones cut from them are given varietal names. In short, in nonmetallic minerals of various kinds colour is a helpful, though not a truly diagnostic (and therefore unique), property.
Hardness
Mohs hardness scale and observations on hardness of some additional materialsHardness (H) is the resistance of a mineral to scratching. It is a property by which minerals may be described relative to a standard scale of 10 minerals known as the Mohs scale of hardness. The minerals that make up the Mohs scale are listed in Table 3 (Mohs hardness scale and observations on hardness of some additional materials). The degree of hardness is determined by observing the comparative ease or difficulty with which one mineral is scratched by another or by a steel tool.
Mohs hardness scale and observations on hardness of some additional materialsFor measuring the hardness of a mineral, several common objects that can be used for scratching are helpful, such as a fingernail, a copper coin, a steel pocketknife, glass plate or window glass, the steel of a needle, and a streak plate. The approximate hardness of these materials is listed next to the minerals of the Mohs scale in Table 3 (Mohs hardness scale and observations on hardness of some additional materials).
Because there is a general link between hardness and chemical composition, these generalizations can be made:
1. Most hydrous minerals are relatively soft (H \\< 5).
2. Halides, carbonates, sulfates, and phosphates also are relatively soft (H \\< 51/2).
3. Most sulfides are relatively soft (H \\< 5), with marcasite and pyrite being examples of exceptions (H \\< 6 to 61/2).
4. Most anhydrous oxides and silicates are hard (H \\> 51/2).
Because hardness is a highly diagnostic property in mineral identification, most determinative tables use relative hardness as a sorting parameter.
Tenacity
Several mineral properties that depend on the cohesive force between atoms (and ions) in mineral structures are grouped under tenacity. A mineral's tenacity can be described by the following terms: malleable, capable of being flattened under the blows of a hammer into thin sheets without breaking or crumbling into fragments (most of the native elements show various degrees of malleability, but particularly gold, silver, and copper); sectile, capable of being severed by the smooth cut of a knife (copper, silver, and gold are sectile); ductile, capable of being drawn into the form of a wire (gold, silver, and copper exhibit this property); flexible, bending easily and staying bent after the pressure is removed (talc is flexible); brittle, showing little or no resistance to breakage, and as such separating into fragments under the blow of a hammer or when cut by a knife (most silicate minerals are brittle); and elastic (elasticity), capable of being bent or pulled out of shape but returning to the original form when relieved (mica is elastic).
specific gravity
Specific gravity (G) is defined as the ratio between the weight of a substance and the weight of an equal volume of water at 4° C. Thus a mineral with a specific gravity of 2 weighs twice as much as the same volume of water. Since it is a ratio, specific gravity has no units.
The specific gravity of a mineral depends on the atomic weights of all its constituent elements and the manner in which the atoms (and ions) are packed together. In mineral series whose species have essentially identical structures, those composed of elements with higher atomic weight have higher specific gravities. If two minerals (as in the two polymorphs of carbon, namely, graphite and diamond) have the same chemical composition, the difference in specific gravity reflects variation in internal packing of the atoms or ions (diamond, with a G of 3.51, has a more densely packed structure than graphite, with a G of 2.23).
Measurement of the specific gravity of a mineral specimen requires the use of a special apparatus. An estimate of the value, however, can be obtained by simply testing how heavy a specimen feels. Most people, from everyday experience, have developed a sense of relative weights for even such objects as nonmetallic and metallic minerals. For example, borax (G = 1.7) seems light for a nonmetallic mineral, whereas anglesite (G = 6.4) appears heavy. Average specific gravity reflects what a nonmetallic or metallic mineral of a given size should weigh. The average specific gravity for nonmetallic minerals falls between 2.65 and 2.75, which is seen in the range of values for quartz (G = 2.65), feldspar (G = 2.60 to 2.75), and calcite (G = 2.72). For metallic minerals, graphite (G = 2.23) feels light, while silver (G = 10.5) seems heavy. The average specific gravity for metallic minerals is approximately 5.0, the value for pyrite. With practice using specimens of known specific gravity, a person can develop the ability to distinguish between minerals that have comparatively small differences in specific gravity by merely lifting them.
Although an approximate assessment of specific gravity can be obtained by the hefting of a hand specimen of a specific monomineral, an accurate measurement can only be achieved by using a specific gravity balance. An example of such an instrument is the Jolly balance, which provides numerical values for a small mineral specimen (or fragment) in air as well as in water. Such accurate measurements are highly diagnostic and can greatly aid in the identification of an unknown mineral sample.
magnetism
Only two minerals exhibit readily observed magnetism: magnetite (Fe3O4), which is strongly attracted to a hand magnet, and pyrrhotite (Fe1-xS), which typically shows a weaker magnetic reaction. Ferromagnetic (ferromagnetism) is a term that refers to materials that exhibit strong magnetic attraction when subjected to a magnetic field. Materials that show only a weak magnetic response in a strong applied magnetic field are known as paramagnetic (paramagnetism). Those materials that are repelled by an applied magnetic force are known as diamagnetic (diamagnetism). Because minerals display a wide range of slightly different magnetic properties, they can be separated from each other by an electromagnet. Such magnetic separation is a common procedure both in the laboratory and on a commercial scale.
Fluorescence
Some minerals, when exposed to ultraviolet light, will emit visible light during irradiation; this is known as fluorescence. Some minerals fluoresce only in shortwave ultraviolet light, others only in longwave ultraviolet light, and still others in either situation. Both the colour and intensity of the emitted light vary significantly with the wavelengths of ultraviolet light. Due to the unpredictable nature of fluorescence, some specimens of a mineral manifest it, while other seemingly similar specimens, even those from the same geographic area, do not. Some minerals that may exhibit fluorescence are fluorite, scheelite, calcite, scapolite, willemite, and autunite. Specimens of willemite and calcite from the Franklin district of New Jersey in the United States may show brilliant fluorescent colours.
Solubility in hydrochloric acid
The positive identification of carbonate minerals (carbonate mineral) is aided greatly by the fact that the carbon-oxygen bond of the CO3 group in carbonates becomes unstable and breaks down in the presence of hydrogen ions (H+) available in acids. This is expressed by the reaction 2H+ + CO2-/3 → H2O + CO2, which is the basis for the so-called fizz test with dilute hydrochloric acid (HCl). Calcite, aragonite, witherite, and strontianite, as well as copper carbonates, show bubbling, or effervescence, when a drop of dilute hydrochloric acid is placed on the mineral. This “fizz” is due to the release of carbon dioxide (CO2). Other carbonates such as dolomite, rhodochrosite, magnesite, and siderite will show effervescence only in hot hydrochloric acid.
radioactivity
Minerals containing uranium (U) and thorium (Th) continually undergo decay reactions in which radioactive isotopes of uranium and thorium form various daughter elements and also release energy in the form of alpha and beta particles and gamma radiation. The radiation produced can be measured in the laboratory or in the field using a Geiger counter or a scintillation counter. A radiation counter therefore is helpful in identifying uranium- and thorium-containing minerals, such as uraninite, pitchblende, thorianite, and autunite.
Several rock-forming minerals contain enough radioactive elements to permit the determination of the time elapsed since the radioactive material was incorporated into the mineral (see dating: Principles of isotopic dating (dating).)
Classification of minerals
Since the middle of the 19th century, minerals have been classified on the basis of their chemical composition. Under this scheme, they are divided into classes according to their dominant anion or anionic group (e.g., halides, oxides, and sulfides). Several reasons justify use of this criterion as the distinguishing factor at the highest level of mineral classification. First, the similarities in properties of minerals with identical anionic groups are generally more pronounced than those with the same dominant cation. For example, carbonates have stronger resemblances to one another than do copper minerals. Secondly, minerals that have identical dominant anions are likely to be found in the same or similar geologic environments. Therefore, sulfides tend to occur together in vein or replacement deposits, while silicate-bearing rocks make up much of the Earth's crust. Third, current chemical practice employs a nomenclature and classification scheme for inorganic compounds based on similar principles.
Investigators have found, however, that chemical composition alone is insufficient for classifying minerals. Determination of internal structures, accomplished through the use of X rays, allows a more complete appreciation of the nature of minerals. Chemical composition and internal structure together constitute the essence of a mineral and determine its physical properties; thus, classification should rely on both. Crystallochemical principles—i.e., those relating to both chemical composition and crystal structure—were first applied by the British physicist W. Lawrence Bragg and the Norwegian mineralogist Victor Moritz Goldschmidt in the study of silicate minerals. The silicate group was subdivided in part on the basis of composition but mainly according to internal structure. Based on the topology of the SiO4 tetrahedrons, the subclasses include framework, chain, and sheet silicates, among others. Such mineral classifications are logical and well-defined.
The broadest divisions of the classification used in the present discussion are (1) native elements, (2) sulfides, (3) sulfosalts, (4) oxides and hydroxides, (5) halides, (6) carbonates, (7) nitrates, (8) borates, (9) sulfates, (10) phosphates, and (11) silicates.
Native elements (native element)
Apart from the free gases in the Earth's atmosphere, some 20 elements occur in nature in a pure (i.e., uncombined) or nearly pure form. Known as the native elements, they are partitioned into three families: metals, semimetals, and nonmetals (see Table 4-->). The most common native metals, which are characterized by simple crystal structures, make up three groups: the gold group, consisting of gold, silver, copper, and lead; the platinum group, composed of platinum, palladium, iridium, and osmium; and the iron group, containing iron and nickel-iron. Mercury, tantalum, tin, and zinc are other metals that have been found in the native state. The native semimetals are divided into two isostructural groups (those whose members share a common structure type): (1) antimony, arsenic, and bismuth, with the latter two being more common in nature, and (2) the rather uncommon selenium and tellurium. Carbon, in the form of diamond and graphite, and sulfur are the most important native nonmetals.Metals (metal)
gold, silver, and copper are members of the same group (column) in the periodic table of elements (see periodic law) and therefore have similar chemical properties. In the uncombined state, their atoms are joined by the fairly weak metallic bond. These minerals share a common structure type, and their atoms are positioned in a simple cubic closest-packed arrangement. Gold and silver both have an atomic radius of 1.44 angstroms (Å), which enables complete solid solution to take place between them. The radius of copper is significantly smaller (1.28 Å), and as such copper substitutes only to a limited extent in gold and silver. Likewise, native copper contains only trace amounts of gold and silver in its structure.
Owing to their similar crystal structure, the members of the gold group display similar physical properties. All are rather soft, ductile, malleable, and sectile; gold, silver, and copper serve as excellent conductors of electricity and heat and exhibit metallic lustre and hackly fracture. These properties are attributable to their metallic bonding. The gold-group minerals crystallize in the isometric system and have high densities as a consequence of cubic closest packing.
In addition to the elements listed above, the platinum group also includes the rarer mineral alloys platiniridium and iridosmine. The members of this group are harder than the metals of the gold group and also have higher melting points.
The iron-group metals are isometric and have a simple cubic packed structure (see Figure 9A). Its members include pure iron, which is rarely found on the surface of the Earth, and two species of nickel-iron (kamacite and taenite), which have been identified as common constituents of meteorites. The atomic radii of iron and nickel are both approximately 1.24 Å, and so nickel is a frequent substitute for iron. The terrestrial core is thought to be composed largely of such iron-nickel alloys.
Semimetals (metalloid)
The semimetals antimony, arsenic, and bismuth have a structure type distinct from the simple-packed spheres of the metals. In these semimetals, each atom is positioned closer to three of its neighbouring atoms than to the rest. The structure of antimony and arsenic, composed of spheres that intersect along flat circular areas, is shown in Figure 9B.
The covalent character of the bonds joining the four closest atoms is linked to the electronegative nature of the semimetals, reflected by their position in the periodic table. Members of this group are fairly brittle, and they do not conduct heat and electricity nearly as well as the native metals. The bond type suggested by these properties is intermediate between metallic and covalent; it is consequently stronger and more directional than pure metallic bonding, resulting in crystals of lower symmetry.
Nonmetals (nonmetal)
The native nonmetals diamond, graphite, and sulfur are structurally distinct from the metals and semimetals. The structure of sulfur (atomic radius = 1.04 Å), usually orthorhombic in form, may contain limited solid solution by selenium (atomic radius = 1.16 Å).
The polymorphs (polymorphism) of carbon, graphite and diamond, display dissimilar structures, resulting in their differences in hardness and specific gravity. In diamond, each carbon atom is bonded covalently in a tetrahedral arrangement, producing a strongly bonded and exceedingly close-knit but not closest-packed structure (see Figure 9C). The carbon atoms of graphite, however, are arranged in six-membered rings in which each atom is surrounded by three close-by neighbours located at the vertices of an equilateral triangle (see Figure 9D). The rings are linked to form sheets that are separated by a distance exceeding one atomic diameter. Van der Waals forces act perpendicular to the sheets, offering a weak bond, which, in combination with the wide spacing, leads to perfect basal cleavage and easy gliding along the sheets.
Sulfides (sulfide mineral)
This important class includes most of the ore minerals. The similar but rarer sulfarsenides are grouped here as well (see Table 5-->). Sulfide minerals consist of one or more metals combined with sulfur; sulfarsenides contain arsenic replacing some of the sulfur.Sulfides are generally opaque and exhibit distinguishing colours and streaks. (Streak is the colour of a mineral's powder.) The nonopaque varieties (e.g., cinnabar, realgar, and orpiment) possess high refractive indices, transmitting light only on the thin edges of a specimen.
Few broad generalizations can be made about the structures of sulfides, although these minerals can be classified into small groups according to similarities in structure. Ionic and covalent bonding are found in many sulfides, while metallic bonding is apparent in others as evidenced by their metal properties. The simplest and most symmetric sulfide structure is based on the architecture of the sodium chloride structure. A common sulfide mineral that crystallizes in this manner is the ore mineral of lead, galena. Its highly symmetric form consists of cubes modified by octahedral faces at their corners. The structure of the common sulfide pyrite (FeS2) also is modeled after the sodium chloride type; a disulfide grouping is located in a position of coordination with six surrounding ferrous iron atoms (see Figure 1-->). The high symmetry of this structure is reflected in the external morphology of pyrite (see Figure 2-->). In another sulfide structure, sphalerite (ZnS), each zinc atom is surrounded by four sulfur atoms in a tetrahedral coordinating arrangement. In a derivative of this structure type, the chalcopyrite (CuFeS2) structure, copper and iron ions can be thought of as having been regularly substituted in the zinc positions of the original sphalerite atomic arrangement. arsenopyrite (FeAsS) is a common sulfarsenide that occurs in many ore deposits. It is the chief source of the element arsenic.
Sulfosalts (sulfosalt)
There are approximately 100 species constituting the rather large and very diverse sulfosalt class of minerals. Some common examples are listed in Table 5-->. The sulfosalts differ notably from the sulfides and sulfarsenides with regard to the role of semimetals (metalloid), such as arsenic (As) and antimony (Sb), in their structures. In the sulfarsenides, the semimetals substitute for some of the sulfur in the structure, while in the sulfosalts they are found instead in the metal site. For example, in the sulfarsenide arsenopyrite (FeAsS), the arsenic replaces sulfur in a marcasite- (FeS2-) type structure. In contrast, the sulfosalt enargite (Cu3AsS4) contains arsenic in the metal position, coordinated to four sulfur atoms. A sulfosalt such as Cu3AsS4 may also be thought of as a double sulfide, 3Cu2S · As2S5.Oxides (oxide mineral) and hydroxides
These classes consist of oxygen-bearing minerals; the oxides combine oxygen with one or more metals, while the hydroxides are characterized by hydroxyl (OH)- groups.
The oxides are further divided into two main types: simple and multiple. Simple oxides contain a single metal combined with oxygen in one of several possible metal:oxygen ratios (X:O): XO, X2O, X2O3, etc. Ice, H2O, is a simple oxide of the X2O type that incorporates hydrogen as the cation. Although SiO2 (quartz and its polymorphs) is the most commonly occurring oxide, it is discussed below in the section on silicates because its structure more closely resembles that of other silicon-oxygen compounds. Two nonequivalent metal sites (X and Y) characterize multiple oxides, which have the form XY2O4.
Unlike the minerals of the sulfide class, which exhibit ionic, covalent, and metallic bonding, oxide minerals generally display strong ionic bonding. They are relatively hard, dense, and refractory.
Examples of several oxides are given in Table 6-->. These minerals generally occur in small amounts in igneous and metamorphic rocks and also as preexisting grains in sedimentary rocks. Several oxides have great economic value, including the principal ores of iron (hematite and magnetite), chromium (chromite), manganese (pyrolusite, as well as the hydroxides, manganite and romanechite), tin (cassiterite), and uranium (uraninite).Members of the hematite group are of the X2O3 type and have structures based on hexagonal closest packing of the oxygen atoms with octahedrally coordinated (surrounded by and bonded to six atoms) cations between them. corundum and hematite share a common hexagonal architecture (see Table 1-->). In the ilmenite structure, iron and titanium occupy alternate Fe-O and Ti-O layers.The XO2-type oxides are divided into two groups. The first structure type, exemplified by rutile, contains cations in octahedral coordination with oxygen. The second resembles fluorite (CaF2); each oxygen is bonded to four cations located at the corners of a fairly regular tetrahedron, and each cation lies within a cube at whose corners are eight oxygen atoms. This latter structure is exhibited by uranium, thorium, and cerium oxides, whose considerable importance arises from their roles in nuclear chemistry.
The spinel-group minerals have type XY2O4 and contain oxygen atoms in approximate cubic closest packing. The cations located within the oxygen framework are octahedrally (sixfold) and tetrahedrally (fourfold) coordinated with oxygen (see Figure 10).
The (OH)- group of the hydroxides generally results in structures with lower bond strengths than in the oxide minerals. The hydroxide minerals tend to be less dense than the oxides and also are not as hard. All hydroxides form at low temperatures and are found predominantly as weathering products, as, for example, from alteration in hydrothermal veins. Some common hydroxides are brucite 【Mg(OH)2】, manganite 【MnO · OH】, diaspore 【α-AlO · OH】, and goethite 【α-FeO · OH】. The ore of aluminum, bauxite, consists of a mixture of diaspore, boehmite (γ-AlO · OH—a polymorph of diaspore), and gibbsite 【Al(OH)3】, plus iron oxides. Goethite is a common alteration product of iron-rich occurrences and is an iron ore in some localities.
Halides (halide mineral)
Members of this class are distinguished by the large-sized anions of the halogens chlorine, bromine, iodine, and fluorine. The ions carry a charge of negative one and easily become distorted in the presence of strongly charged bodies. When associated with rather large, weakly polarizing cations of low charge, such as those of the alkali metals, both anions and cations take the form of nearly perfect spheres. Structures composed of these spheres exhibit the highest possible symmetry.
Pure ionic bonding is exemplified best in the isometric halides, for each spherical ion distributes its weak electrostatic charge over its entire surface. These halides manifest relatively low hardness and moderate-to-high melting points. In the solid state they are poor thermal and electric conductors, but when molten they conduct electricity well.
Halogen ions may also combine with smaller, more strongly polarizing cations than the alkali metal ions. Lower symmetry and a higher degree of covalent bonding prevail in these structures. Water and hydroxyl ions may enter the structure, as in atacamite 【Cu2Cl(OH)3】.
The halides consist of about 80 chemically related minerals with diverse structures and widely varied origins. The most common are halite (NaCl), sylvite (KCl), chlorargyrite (AgCl), cryolite (Na3AlF6), fluorite (CaF2), and atacamite. The structure of sodium chloride is illustrated in Figure 11A. By the arrangement of the ions, it is evident that no molecules are present in the structure. Each cation and anion is in octahedral coordination with its six closest neighbours. The NaCl structure is found in the crystals of many XZ-type halides, including sylvite (KCl) and chlorargyrite (AgCl). Some sulfides and oxides of XZ type crystallize in this structure type as well—for example, galena (PbS), alabandite (MnS), and periclase (MgO).
Several XZ2 halides have the same structure as fluorite (CaF2), which is shown in Figure 11B. In fluorite, calcium cations are positioned at the corners and face centres of cubic unit cells. (A unit cell is the smallest group of atoms, ions, or molecules from which the entire crystal structure can be generated by its repetition.) Each fluorine anion is in tetrahedral coordination with four calcium ions, while each calcium cation is in eightfold coordination with eight fluorine ions that form the corners of a cube around it.
Uraninite (UO2) and thorianite (ThO2) are two of the several oxides that have a fluorite-type structure.
Carbonates
The carbonate minerals (carbonate mineral) contain the anionic complex (CO3)2-, which is triangular in its coordination—i.e., with a carbon atom at the centre and an oxygen atom at each of the corners of an equilateral triangle. These anionic groups are strongly bonded, individual units and do not share oxygen atoms with one another. The triangular carbonate groups are the basic building units of all carbonate minerals and are largely responsible for the properties particular to the class.
Carbonates are frequently identified using the effervescence test with acid (see above Physical properties: Solubility in hydrochloric acid (mineral)). The reaction that results in the characteristic fizz, 2H+ + CO2−/3 → H2O + CO2, makes use of the fact that the carbon-oxygen bonds of the CO3 groups are not quite as strong as the corresponding carbon-oxygen bonds in carbon dioxide.
The common anhydrous carbonates are divided into three groups that differ in structure type: calcite, aragonite, and dolomite. The copper carbonates azurite and malachite are the only notable hydrous varieties (see Table 7-->).The members of the calcite group that are given in the table share a common structure type. It can be considered as a derivative of the NaCl structure in which CO3 groups substitute for the chlorine ions and calcium cations replace the sodium cations. As a result of the triangular shape of the CO3 groups, the structure is rhombohedral instead of isometric as in NaCl. The CO3 groups are in planes perpendicular to the threefold c axis, and the calcium ions occupy alternate planes and are bonded to six oxygen atoms of the CO3 groups.
Members of the calcite group exhibit perfect rhombohedral cleavage. The composition CaCO3 most commonly occurs in two different polymorphs: rhombohedral calcite with calcium surrounded by six closest oxygen atoms and orthorhombic aragonite with calcium surrounded by nine closest oxygen atoms.
When CO3 groups are combined with large divalent cations (generally with ionic radii greater than 1.0 Å), orthorhombic structures result. This is known as the aragonite structure type. Members of this group include those with large cations: BaCO3, SrCO3, and PbCO3. Each cation is surrounded by nine closest oxygen atoms.
The aragonite group displays more limited solid solution than the calcite group. The type of cation present in aragonite minerals is largely responsible for the differences in physical properties among the members of the group. Specific gravity, for example, is roughly proportional to the atomic weight of the metal ions.
Dolomite 【CaMg(CO3)2】, kutnahorite 【CaMn(CO3)2】, and ankerite 【CaFe(CO3)2】 are three isostructural members of the dolomite group. The dolomite structure can be considered as a calcite-type structure in which magnesium and calcium cations occupy the metal sites in alternate layers. The calcium (Ca2+) and magnesium (Mg2+) ions differ in size by 33 percent, and this produces cation ordering with the two cations occupying specific and separate levels in the structure. Dolomite has a calcium-to-magnesium ratio of approximately 1:1, which gives it a composition intermediate between CaCO3 and MgCO3.
Nitrates (nitrate and iodate minerals)
The nitrates are characterized by their triangular (NO3)- groups that resemble the (CO3)2- groups of the carbonates, making the two mineral classes similar in structure. The nitrogen cation (N5+) carries a high charge and is strongly polarizing like the carbon cation (C4+) of the CO3 group. A tightly knit triangular complex is created by the three nitrogen-oxygen bonds of the NO3 group; these bonds are stronger than all others in the crystal. Because the nitrogen-oxygen bond has greater strength than the corresponding carbon-oxygen bond in carbonates, nitrates decompose less readily in the presence of acids.
Nitrate structures analogous to those of the calcite group result when NO3 combines in a 1:1 ratio with monovalent cations whose radii can accommodate six closest oxygen neighbours. For example, nitratite (NaNO3), also called soda nitre, and calcite exhibit the same structure, crystallography, and cleavage. The two minerals differ in that nitratite is softer and melts at a lower temperature owing to its lesser charge; also, sodium has a lower atomic weight than calcium, causing nitratite to have a lower specific gravity as well. Similarly, nitre (KNO3), also known as saltpetre, is an analogue of aragonite. These are two examples of only seven known naturally occurring nitrates.
Borates (borate mineral)
Minerals of the borate class contain boron-oxygen groups that can link together, in a phenomenon known as polymerization, to form chains, sheets, and isolated multiple groups (see Figure 12). The silicon-oxygen (SiO4) tetrahedrons of the silicates polymerize in a manner similar to the (BO3)3- triangular groups of the borates. (For further information on such structures, see below Silicates (mineral)). A single oxygen atom is shared between two boron cations (B3+), thereby linking the BO3 groups into extended units such as double triangles, triple rings, sheets, and chains. The oxygen atom is able to accommodate two boron atoms because the small boron cation has a bond strength to each oxygen that is exactly one-half the bond energy of the oxygen ion.
Although boron is usually found in triangular coordination with three oxygens, it also occurs in fourfold coordination in tetrahedral groups. In addition, boron may exist as part of complex anionic groups such as 【B3O3(OH)3】2-, consisting of one triangle and two tetrahedrons (see Figure 12E). Complex infinite chains of tetrahedrons and triangles are found in the structure of colemanite 【CaB3O4(OH)3 · H2O】; a complex ion composed of two tetrahedrons and two triangles, 【B4O5(OH)4】2-, is present in borax 【Na2B4O5(OH)4 · 8H2O】 (see Figure 12F).
Sulfates (sulfate mineral)
This class is composed of a large number of minerals, but relatively few are common. The most frequently occurring sulfates are listed in Table 8-->. All contain anionic (SO4)2- groups in their structures. These anionic complexes are formed through the tight bonding of a central S6+ ion to four neighbouring oxygen atoms in a tetrahedral arrangement around the sulfur. This closely knit group is incapable of sharing any of its apical oxygen atoms with other SO4 groups; as such the tetrahedrons occur as individual, unlinked groups in sulfate mineral structures.Members of the barite group constitute the most important and common anhydrous sulfates. They have orthorhombic symmetry with large divalent cations bonded to the sulfate ion (see Table 8-->). In barite (BaSO4), each barium ion is surrounded by 12 closest oxygen ions belonging to seven distinct SO4 groups. anhydrite (CaSO4) exhibits a structure very different from that of barite since the ionic radius of Ca2+ is considerably smaller than Ba2+. Each calcium cation can only fit eight oxygen atoms around it from neighbouring SO4 groups.
gypsum (CaSO4 · 2H2O) is the most important and abundant hydrous sulfate.
Phosphates (phosphate mineral)
Although this mineral class is large, most of its members are quite rare. Of the phosphates listed in Table 9-->, only apatite 【Ca5(PO4)3(F, Cl, OH)】, the most important and abundant, can be considered as truly common. The members of this group are characterized by tetrahedral anionic (PO4)3- complexes, which are analogous to the (SO4)2- groups of the sulfates. The phosphorus ion, with a valence of positive five, is only slightly larger than the sulfur ion, which carries a positive six charge.Silicates (silicate mineral)
The silicates, owing to their abundance on the Earth, constitute the most important mineral class. Approximately 25 percent of all known minerals and 40 percent of the most common ones are silicates; the igneous rocks that make up more than 90 percent of the Earth's crust are composed of virtually all silicates.
The fundamental unit in all silicate structures is the silicon-oxygen (SiO4)4- tetrahedron. It is composed of a central silicon cation (Si4+) bonded to four oxygen atoms that are located at the corners of a regular tetrahedron (see Figure 13). The terrestrial crust is held together by the strong silicon-oxygen bonds of these tetrahedrons. Approximately 50 percent ionic and 50 percent covalent, the bonds develop from the attraction of oppositely charged ions as well as the sharing of their electrons.
The positive charge (+4) of each silicon cation is satisfied by its four bonds to oxygen atoms. Each oxygen ion (O2-), however, contributes only one-half of its total bonding energy to a silicon-oxygen bond, so it is capable of also bonding to the silicon cation of another tetrahedron. The SiO4 tetrahedrons thereby become linked by shared oxygen atoms; this is referred to as polymerization. The degree and manner of polymerization are the bases for the variety present in silicate structures.
The silicates can be divided into groups according to structural configuration, which arises from the sharing of one, two, three, or all oxygen ions of a tetrahedron (see Figure 14). Nesosilicates have isolated groups of SiO4, while sorosilicates (sorosilicate) contain pairs of SiO4 tetrahedrons linked into Si2O7 groups. Ring silicates, also known as cyclosilicates (cyclosilicate), are closed, ringlike silicates; the sixfold variety has composition Si6O18. Silicates that are composed of infinite chains of tetrahedrons are called inosilicates; (inosilicate) single chains have a unit composition of SiO3 or Si2O6, whereas double chains contain a silicon to oxygen ratio of 4:11. Phyllosilicates (phyllosilicate), or sheet silicates, are formed when three oxygen atoms are shared with adjoining tetrahedrons. The resulting infinite flat sheets have unit composition Si2O5. In structures where tetrahedrons share all their oxygen ions, an infinite three-dimensional network is created with an SiO2 unit composition. Minerals of this type are called framework silicates or tectosilicates.
As a major constituent of the Earth's crust, aluminum follows only oxygen and silicon in importance. The radius of aluminum, slightly larger than that of silicon, lies close to the upper bound for allowable fourfold coordination in crystals. As a result, aluminum can be surrounded with four oxygen atoms arranged tetrahedrally, but it can also occur in sixfold coordination with oxygen. The ability to maintain two roles within the silicate structure makes aluminum a unique constituent of these minerals. The tetrahedral AlO4 groups are approximately equal in size to SiO4 groups and therefore can become incorporated into the silicate polymerization scheme. Aluminum in sixfold coordination may form ionic bonds with the SiO4 tetrahedrons. Thus, aluminum may occupy tetrahedral sites as a replacement for silicon and octahedral sites in solid solution with elements such as magnesium and ferrous iron.
Several ions may be present in silicate structures in octahedral coordination with oxygen: Mg2+, Fe2+, Fe3+, Mn2+, Al3+, and Ti4+ (see Table 10-->). All cations have approximately the same dimensions and thus are found in equivalent atomic sites, even though their charges range from positive two to positive four. solid solution involving ions of different charge is accomplished through coupled substitutions, thereby maintaining neutrality of the structures.Nesosilicates
The silicon-oxygen tetrahedrons of the nesosilicates are not polymerized; they are linked to one another only by ionic bonds of the interstitial cations. As a result of the isolation of the tetrahedral groups, the crystal habits of these minerals are typically equidimensional so that prominent cleavage directions are not present. The size and charge of the interstitial cations largely determine the structural form of the nesosilicates. The relatively high specific gravity and hardness that are characteristic of this group arise from the dense packing of the atoms within the structure. Substitution of aluminum for silicon is normally quite low. Examples of common nesosilicates are given in Table 11-->.Sorosilicates (sorosilicate)
These minerals contain sets of two SiO4 tetrahedrons joined by one shared apical oxygen (see Figure 14). A silicon-to-oxygen ratio of 2:7 is consequently present in their structures. More than 70 minerals belong to the sorosilicate group, although most are rare. Only the members of the epidote group and vesuvianite are common. Both independent (SiO4)4- and double (Si2O7)6- groups are incorporated into the epidote structure, as is reflected in its formula: Ca2(Al, Fe)Al2O(SiO4)(Si2O7)(OH).
Cyclosilicates (cyclosilicate)
Silicon-oxygen tetrahedrons are linked into rings in cyclosilicate structures, which have an overall Si:O ratio of 1:3. There are three closed cyclic configurations with the following formulas: Si3O9, Si4O12, and Si6O18. The rare titanosilicate benitoite (BaTiSi3O9) is the only mineral that is built with the simple Si3O9 ring. axinite 【(Ca, Fe, Mn)3Al2(BO3)(Si4O12)(OH)】 contains Si4O12 rings, along with BO3 triangles and OH groups. The two common and important cyclosilicates, beryl (Be3Al2Si6O18) and tourmaline (which has an extremely complex formula), are based on the Si6O18 ring.
Inosilicates (inosilicate)
This class is characterized by its one-dimensional chains and bands created by the linkage of SiO4 tetrahedrons. Single chains may be formed by the sharing of two oxygen atoms from each tetrahedron, resulting in a structure with an Si:O ratio of 1:3. Two such chains that are aligned side by side with alternate tetrahedrons sharing an additional oxygen atom form bands of double chains (see Figure 14). These structures have an Si:O ratio of 4:11. Two significant rock-forming mineral families display these structure types: the single-chain pyroxenes (pyroxene) and the double-chain amphiboles (amphibole). Some common examples are given in Table 12-->.The amphiboles and pyroxenes share the same cations and have many similar crystallographic, chemical, and physical properties: the colour, lustre, and hardness of analogous species are alike. A distinguishing factor between the two groups, the presence of the hydroxyl radical in the amphiboles, generally gives the double-chain members lower specific gravities and refractive indices than their single-chain analogues. Their crystal habits also are different: amphiboles exhibit needlelike or fibrous crystals, while pyroxenes take the form of stubby prisms. In addition, the different chain structures of the two groups result in different cleavage angles.
Pyroxenes occur in high-temperature igneous (igneous rock) and metamorphic rocks. They crystallize at higher temperatures than their amphibole counterparts. A pyroxene formed early in the cooling of an igneous melt or in a metamorphic fluid may later combine with water at a lower temperature to form amphibole.
Phyllosilicates (phyllosilicate)
These minerals display a two-dimensional framework of infinite sheets of SiO4 tetrahedrons (see Figure 14). An Si:O ratio of 2:5 results from the sharing of three oxygen atoms in each tetrahedron. Sixfold symmetry is exhibited in undistorted sheets. The silicate sheet framework is largely responsible for the following properties of the phyllosilicates: platy or flaky habit, single pronounced cleavage, low specific gravity, softness, and possible flexibility and elasticity of cleavage layers. Most minerals of this group contain hydroxyls positioned in the middle of the sixfold rings of tetrahedrons.
Many soil constituents, produced through rock weathering, possess a sheet structure. Phyllosilicate properties contribute greatly to the ability of soils to release and retain plant food, to reserve water from wet to dry seasons, and to accommodate organisms and atmospheric gases.
The phyllosilicate class includes several important mineral groups (see Table 13-->).Tectosilicates
Almost 75 percent of the Earth's crust is composed of minerals with the three-dimensional framework of the tectosilicates. All oxygen atoms of the SiO4 tetrahedrons of members of this class are shared with nearby tetrahedrons, creating a strongly bound structure with an Si:O ratio of 1:2 (see Figure 14). The most important tectosilicates are listed in Table 14-->. Other than the zeolite group, which can accommodate water owing to the open nature of its structure, all members listed in the table are anhydrous.Mineral associations and phase equilibrium
The preceding sections provided an overview of major mineral groups but did not treat minerals as part of assemblages in rock types nor discuss the experimental study of minerals and rock occurrences. petrology, the scientific study of rocks, is concerned largely with identifying individual minerals in rocks, along with their abundance, grain size, and texture, because rocks typically consist of a variety of minerals. Such information is essential to an understanding of the history of any rock.
Petrological research requires a strong understanding of the principles of mineralogy and mineral identification and a thorough familiarity with the theoretical and experimental studies of rock origins. The present section focuses on phase equilibrium, upon which the link between the study of minerals and the study of rocks is largely based.
A phase is a homogeneous substance that has a fixed composition and uniform chemical and physical properties. Only a mineral that displays no solid solution may therefore be considered a phase. Quartz (SiO2), for example, is a low-temperature phase in the Si-O2 (SiO2) system, and kyanite (Al2SiO5) is a high-pressure phase in the Al2O3-SiO2 (Al2SiO5) system. The term phase region is used when a mineral exhibits compositional variation, as in the solid-solution series between forsterite and fayalite. A phase may exist as a solid, liquid, or gas: H2O, for example, occurs in the form of ice (solid), water (liquid), and steam (gas).
Equilibrium refers to the stable coexistence of two or more phases and is established relative to time. If two phases in a mixture of water and ice coexist so that the amount of each is fixed indefinitely, they are said to be in equilibrium. The minerals of some rocks have existed together since their formation for periods of several million years, yet one cannot always ascertain if these rock constituents are in equilibrium or are still undergoing changes.
A determining factor of the equilibrium state of minerals is the presence (or absence) of a reaction rim, which is a region separating two or more minerals consisting of the products of a reaction between them. The absence of any observable reaction rims between minerals that physically touch each other suggests that they were in equilibrium at the time when the rock formed. Additional chemical data regarding elemental distribution between the minerals is necessary to verify this assumption. In contrast, the presence of megascopically or microscopically visible rims indicates that some minerals were not in equilibrium. Garnet, for example, may react with coexisting biotite to produce a chlorite rim between them, revealing that the two minerals were not always in equilibrium. An experimental petrologist must assign some period of time after which the absence of further changes between phases will indicate that equilibrium has been reached. The time period is variable, depending on the speed of the reactions involved and in part on the patience of the investigator; it may range from a few hours to several years.
Components are the minimum number of independent chemical species that are necessary to describe the compositions of all the phases present in a system. The compound H2O is generally used as the sole component defining the H2O system, although H2 and O2 define the chemical system as well. In examinations of the stability fields of MgSiO3 (enstatite), MgSiO3 is normally used as the component rather than the three elements, Mg, Si, and O, or the two oxides, MgO and SiO2. The three components generally used in the pyroxene system CaO-MgO-FeO-SiO2 are CaSiO3-MgSiO3-FeSiO3.
Assemblage and the phase rule
In the early stages of the study of a rock, the constituent minerals of the rock must be identified. Orthoclase, albite, quartz, and biotite may be found in an igneous granite. By examining the granite's texture, one may conclude that the four minerals crystallized at approximately the same elevated temperature and that orthoclase-albite-quartz-biotite is its mineral assemblage. The term assemblage is frequently applied to all minerals included in a rock but more appropriately should be used for those minerals that are in equilibrium (and are known more specifically as the equilibrium assemblage). The granite discussed above may display surficial cavities that are lined by several clay minerals and limonite (a hydrous iron oxide). The original high-temperature granite was altered to form the low-temperature clay minerals and limonite; there are consequently two distinct assemblages present in the rock: the high-temperature orthoclase-albite-quartz-biotite assemblage and the low-temperature assemblage of clay minerals and limonite.
Metamorphic rocks also may contain separate assemblages. A shale that at low temperatures was composed of a sericite-kaolin-dolomite-quartz-feldspar assemblage can become metamorphosed at higher temperatures to produce a garnet-sillimanite-biotite-feldspar assemblage.
An assemblage thus consists of minerals that formed under the same or quite comparable conditions of pressure and temperature. In practice, minerals that physically touch one another with no reaction rims or alteration products are included in the assemblage. It is likely that the minerals satisfying these conditions are in equilibrium, but additional chemical tests are commonly necessary to define the equilibrium assemblage without ambiguity.
Phase systems are governed by a phase rule, which defines the number of minerals that may coexist in equilibrium: F = C - P + 2, where F is the variance, or number of degrees of freedom, C is the number of independent components, and P is the number of phases. Applying this rule to a three-phase, three-component system, F is 2. This indicates that two parameters—e.g., pressure and temperature—may be varied independently of one another without altering the number of phases.
Phase diagrams (phase diagram)
Phase (or stability) diagrams are used to illustrate the conditions under which certain minerals are stable. The following are examples of phase diagrams employed in the study of igneous, metamorphic, and sedimentary rocks. For more detailed discussions of the applications of phase diagrams in petrology, consult the books cited in the Bibliography (mineral).
Use in igneous petrology
In the field of igneous petrology, the researcher commonly employs a phase equilibrium approach to compare the mineral assemblages found in naturally occurring and synthetic rocks. Much can be learned from studying the melting of an igneous rock and the reverse process, the crystallization of minerals from a melt (liquid phase). Graphic representations of systems with a liquid phase are called liquidus diagrams. A three-component system (quartz 【SiO2】–kalsilite 【KAlSiO4】–forsterite 【Mg2SiO4】) is illustrated through a liquidus diagram in Figure 15. The dashed contours of the diagram, called isotherms (isotherm), represent temperatures at which a mineral melts. They define what is known as a liquidus surface. As temperatures decrease, the minerals will crystallize in the manner defined by the arrows on the boundaries separating the different mineral phases. A careful study of the crystalline products formed upon the cooling of melts of specific compositions allows the igneous petrologist to compare such results with minerals observed in natural igneous rocks.
Use in metamorphic petrology
Pressure-temperature (P-T) phase diagrams are applied in the study of the conditions under which metamorphic rocks originate. They illustrate the equilibrium relationships among various mineral phases in terms of pressure and temperature. A composite P-T diagram that includes reaction curves for several minerals is given in Figure 16. The minerals that are separated by a reaction curve may exist in equilibrium at the conditions occurring along the line. The reaction curves for Al2SiO5 and for muscovite + quartz ⇆ potassium-feldspar + sillimanite + H2O are significant in metamorphic rocks that have a high aluminum oxide (Al2O3) content as compared to other components (e.g., calcium oxide 【CaO】, magnesium oxide 【MgO】, and ferrous oxide 【FeO】). Shales (shale) enriched in clay minerals contain a rather large amount of aluminum oxide, and during metamorphism of the shale mineral reactions and recrystallization occur. In their metamorphic form, shales appear as pelitic schists (schist), and these may include significant amounts of sillimanite, muscovite, and quartz. Such a schist may have equilibrated under the pressure and temperature conditions indicated by the shaded portion of Figure 16.
Theoretical calculations are combined with experimental observations to arrive at phase diagrams such as the one shown in Figure 16. In laboratory experiments conducted on the three polymorphs of Al2SiO5, chemicals of high purity are most often used as the starting materials, but extremely pure minerals may be substituted. A specimen of gem-grade kyanite that does not contain any inclusions may be reacted at high temperatures to form sillimanite (see Figure 16). The placement of the reaction curve between the kyanite and sillimanite fields is determined by the first instance of sillimanite formation from kyanite and also the initial stage of the reverse reaction. X-ray powder diffraction and optical microscopic techniques are employed to estimate the conditions under which these reactions commence, but the experimental methods are subject to some degree of uncertainty. Therefore, the reaction curves, although commonly drawn as narrow lines, may actually represent wider reaction zones. Also, the naturally occurring system is more complex than the simplified version devised in the laboratory, and so difficulty arises when attempting to relate the two. A P-T diagram such as the one given in Figure 16 serves mainly as a tool in evaluating the conditions of metamorphism, such as pressure and temperature.
Use in sedimentary petrology
Phase diagrams can also be helpful in the assessment of physical and chemical conditions that prevailed during the deposition of a chemical sedimentary sequence. Atmospheric conditions are characterized by low temperatures and pressures, and under such conditions stability fields of minerals can often conveniently be expressed in terms of Eh (oxidation potential) and pH (the negative logarithm of the hydrogen ion concentration 【H+】; a pH of 0–7 indicates acidity, a pH of 7–14 indicates basicity, and neutral solutions have a pH of 7). For a discussion of the construction of such Eh-pH diagrams (Eh–pH diagram), see the references cited in the Bibliography (mineral).
Stability relations for some iron oxides and iron sulfides at atmospheric conditions, 25° C and one atmosphere pressure, are given in Figure 17. (One standard atmosphere of pressure equals 760 millimetres, or 29.92 inches, of mercury.) A high Eh value corresponds to a compound stable under oxidizing conditions, such as hematite, while a low Eh value indicates a mineral that occurs in reducing environments, such as magnetite. Pyrite and pyrrhotite, two sulfide minerals, occur at low Eh values and at pH values of 4–9. Lines separating the fields of an Eh-pH diagram represent conditions under which the two minerals may exist in equilibrium. Hematite and magnetite, for example, are often found together in iron-bearing sediments; line ab of Figure 17 indicates the Eh and pH values at which their coexistence can take place. Eh-pH diagrams are valuable in providing information regarding the chemical and physical environments that existed during atmospheric weathering and during chemical sedimentation and diagenesis of sediments deposited by water at temperatures of 25° to about 100° C and approximately one atmosphere pressure. Τhe coexistence of hematite and magnetite common in Precambrian iron-bearing rocks (those formed from 3.96 billion to 570 million years ago) may enable investigators to estimate variables such as Eh and pH that prevailed in the original ancient sedimentary basin.
Additional Reading
Standard mineralogical reference works include W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (1962–63), with a 2nd ed. in progress (1978– ); Manual of Mineralogy (after James D. Dana), 20th ed. by Cornelis Klein and Cornelius S. Hurlbut, Jr. (1985); Annibale Mottana, Rodolfo Crespi, and Giuseppe Liborio, Simon and Schuster's Guide to Rocks and Minerals (also published as The Macdonald Encyclopedia of Rocks and Minerals, 1978; originally published in Italian, 1977); and Michael Fleischer and Joseph A. Mandarino, Glossary of Mineral Species, 1991, 6th ed. (1991). Useful texts and monographs include Harvey Blatt, Sedimentary Petrology (1982); Richard V. Dietrich and Brian J. Skinner, Rocks and Rock Minerals (1979); Robert M. Garrels and Charles L. Christ, Solutions, Minerals, and Equilibria, 2nd ed. (1990); Paul C. Hess, Origins of Igneous Rocks (1989); Cornelis Klein, Minerals and Rocks: Exercises in Crystallography, Mineralogy, and Hand Specimen Petrology (1989); Anthony R. Philpotts, Principles of Igneous and Metamorphic Petrology (1990); and Tibor Zoltai and James H. Stout, Mineralogy: Concepts and Principles (1984).