mineral, 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.
A mineral, which by definition must be formed through natural processes, is distinct from the synthetic equivalents produced in the laboratory. Artificial 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 chemical compounds. Homogeneity is determined relative to the scale on which it is defined. A specimen that appears homogeneous to the unaided eye, 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. 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. 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.
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, such 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, New Jersey, U.S., the site of its occurrence as the dominant ore mineral for zinc (Zn); and sillimanite (Al2SiO4) is in honour of the American chemist Benjamin Silliman. Since 1960 the Commission on New Minerals and Mineral Names of the International Mineralogical Association has reviewed descriptions of new minerals and proposals for new mineral names and has attempted to remove inconsistencies. Any new mineral name must be approved by this committee, and the type material is usually stored in a museum or university collection.
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, or magmatic, in which minerals crystallize from a melt, (2) sedimentary, in which minerals are the result of sedimentation, a process whose raw materials are particles from other rocks that have undergone weathering or erosion, (3) metamorphic, 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, and (4) hydrothermal, in which minerals are chemically precipitated from hot solutions within 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, which adds to a specimen’s aesthetic beauty. Similarly, geodes, 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
Nearly all minerals have the internal ordered arrangement of atoms and ions that is the defining characteristic of crystalline solids. 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.
The external shape, or morphology, of a crystal is perceived as its aesthetic beauty, and its geometry reflects the internal atomic arrangement. 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. (For example, a sixfold rotation occurs when the crystal repeats itself each 60°—that is, 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, where 1 is equivalent to a centre of symmetry (or inversion), 2 is equivalent to a mirror plane, and 3 is equivalent to a threefold rotation axis plus a centre of symmetry. When the axis of the crystal is vertical, 4 is characterized by two top faces with identical faces upside down underneath. 6 is equivalent to a threefold rotation axis with a mirror plane perpendicular to the axis.
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. 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 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. An imaginary mirror plane (or symmetry plane) can also be used to separate a crystal into halves. In a perfectly developed crystal, the halves are mirror images of one another.
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 order of overall symmetry content, beginning with the system with the highest and most complex crystal symmetry, they are isometric (or cubic), hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. (Many sources list seven crystal systems by dividing the hexagonal crystal system into two parts—trigonal and hexagonal.)
|crystal system||symmetry content*||crystal class**|
|i, 1A2, 1m||2/m|
|i, 3A2, 3m||2/m2/m2/m|
|i, 1A4, m||4/m|
|1A4, 2A2, 2m||42m|
|i, 1A4, 4A2, 5m||4/m2/m2/m|
|1A3 (= i + 1A3)||3|
|1A3, 3A2, 3m (1A3 = i + 1A3)||32/m|
|1A6 (= 1A3 + m)||6|
|i, 1A6, 1m||6/m|
|1A6, 3A2, 3m (1A6 = 1A3 + m)||6m2|
|i, 1A6, 6A2, 7m||6/m2/m2/m|
|3A2, 3m, 4A3 (1A3 = 1A3 + i)||2/m3|
|3A4, 4A3, 6A2||432|
|3A4, 4A3, 6m||43m|
|3A4, 4A3, 6A2, 9m (1A3 = 1A3 + i)||4/m32/m|
|*Abbreviations used in column 2: i = inversion (or centre of symmetry); A = axis of rotation; A2 = axis of twofold rotation; A3 = axis of threefold rotation; A4 = axis of fourfold rotation; and A6 = axis of sixfold rotation; A = axis of rotoinversion; A3 = axis of threefold rotoinversion; A4 = axis of fourfold rotoinversion; A6 = axis of sixfold rotoinversion; m = mirror, or symmetry, plane.|
|**Symbolic representation used in column 3: rotation axes are shown as 1, 2, 3, 4, or 6 (in which 2 = twofold rotation, 3 = threefold rotation, etc.); rotoinversion axes are shown as 3, 4, or 6 (in which 3 is a threefold rotoinversion axis, etc.); centre of symmetry i is equivalent to 1; mirrors are represented by m; rotation axes perpendicular to mirror planes are shown by the notation 2/m, 4/m, or 6/m, in which 2/m is a twofold axis perpendicular to a mirror, etc.|
|Source: Modified from C. Klein and C.S. Hurlbut, Jr., Manual of Mineralogy, copyright © 1985 John Wiley and Sons, Inc., reprinted with permission of John Wiley and Sons.|
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 and tetragonal systems 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 system 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. 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.
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.