Structure and physical properties of minerals

Minerals—naturally occurring inorganic solids with definite chemical composition and definite crystal structure—are the primary constituents of rocks; they are also found in soil. The variety of minerals is huge, and their myriad applications range from use as gemstones and precious metals to applications in building materials, electronics, food, and pharmaceuticals.

Background

Minerals are the building blocks of rocks, and they have many economic uses. Minerals such as diamond, ruby, emerald, and sapphire are precious gems. Other minerals are valuable metals (gold, silver, platinum, copper) or metal ores, such as hematite (iron), sphalerite (zinc), galena (lead), and bauxite (aluminum). Other minerals are used as salt (halite), lubricants (graphite), abrasives (corundum), and fertilizer (apatite), as well as in pharmaceuticals (sulfur), steel making (fluorite), plaster (gypsum and anhydrite), lime, and portland cement (calcite and dolomite).

A mineral is defined as a naturally occurring, inorganic solid with a definite chemical composition (or range of compositions within certain limits) that can be expressed by a chemical formula, and an orderly internal crystalline structure (its atoms are arranged in a definite pattern which is reflected in the shape of its crystals and in its cleavage). Only substances that meet these precise requirements are considered minerals. As a result, synthetic gems, which may be physically and chemically identical to natural gemstones, are not considered minerals.

Minerals have specific physical properties that result from their chemical composition and crystal structure, and many minerals can be identified by these properties. Physical properties include hardness, color, luster, streak, cleavage, density or specific gravity, and crystal form. Some minerals also have additional diagnostic physical properties, including tenacity, taste, magnetism, electrical properties, luminescence, reaction to hydrochloric acid, and radioactivity.

Hardness

Hardness is a mineral’s resistance to scratching or abrasion and is a result of crystal structure or atomic arrangement. The stronger the chemical bonds between the atoms, the harder the mineral. For example, two minerals may have an identical chemical composition but different crystal structures, such as diamond and graphite, which are both carbon. Diamond is the hardest known mineral, but graphite is so soft that it rubs off on the fingers or a piece of paper (it is used as pencil “lead”). The differences in crystal structure produce the vastly different hardnesses of these two minerals.

Ten minerals have been arranged in order of increasing hardness and are referred to as the Mohs hardness scale, devised in 1822 by a German mineralogist, Friedrich Mohs. The minerals of the Mohs hardness scale, in order from softest to hardest, are: (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) potassium feldspar (orthoclase), (7) quartz, (8) topaz, (9) corundum, and (10) diamond. Minerals can scratch other minerals of the same or lesser hardness. The hardness of minerals can be tested using common materials, including the fingernail (a little over 2), copper (about 3), a steel nail or pocket knife (a little over 5), a piece of glass (about 5.5), and a steel file (6.5).

Color

Althoughcolor is a prominent feature of minerals, it is not a reliable indicator for identifying minerals. The color of some minerals is the result of major elements in their chemical formula, such as the blue color of azurite and the green color of malachite (copper), the pink color of rhodonite and rhodochrosite (manganese), and the yellow color of sulfur. Many minerals come in a variety of colors. Quartz is colorless and transparent when pure, but it may also be white (milky quartz), pink (rose quartz), purple (amethyst), yellow (citrine), brown (smoky quartz), or other colors. Similarly, feldspar and fluorite come in many hues. Color may be caused by impurities, such as iron (pink, green, or greenish yellow), titanium (pink or blue), chromium (red or green), vanadium (green), and nickel (yellow). Milky quartz is white because it contains tiny fluid inclusions. Coloration can be the result of defects in the crystal structure; for example, the purple of amethyst and fluorite and the brown of smoky quartz. Unusual colors may also be induced in minerals by exposing them to radiation, which damages the crystal structure (such as black quartz).

Luster

Luster refers to the “shine,” or quality of reflectivity of light from the mineral’s surface. Minerals can be divided into two luster groups: metallic luster and nonmetallic luster. Metallic minerals include economically valuable metals such as gold, silver, and native copper, and some metal sulfides such as pyrite (FeS₂ or iron sulfide) and galena (PbS or lead sulfide). Nonmetallic minerals include those with vitreous or glassy luster (quartz), earthy or dull luster (kaolinite and other clays), pearly (talc), silky (fibrous minerals such as gypsum, malachite, and chrysotile asbestos), greasy (nepheline), resinous (resembling resin or amber, such as sulfur), and adamantine or brilliant (diamond).

Streak

Streak refers to the color of the mineral in powdered form, viewed after the mineral is rubbed on an unglazed porcelain tile or streak plate. Streak color is more diagnostic than mineral color because it is constant for a particular mineral. A mineral may come in several colors, but its streak is the same color for all. Streak color is not always what one might predict from examining the mineral; a sparkling silver-colored mineral, specular hematite, has a red-brown streak, and pyrite, a golden metallic mineral, has a dark gray streak. Not all minerals have a streak. The streak plate has a hardness of about 7 on the Mohs hardness scale. Minerals harder than this will not leave a streak, but their powdered colors can be studied by crushing a small piece.

Cleavage

Cleavage is one of the most diagnostic physical properties of minerals. cleavage refers to the tendency of some minerals to break along smooth, flat planes that are related to zones of weak bonding between atoms in the crystal structure. Some minerals, however, have no planes of weakness in their crystal structure and therefore lack cleavage. Cleavage is discussed by referring to the number of different sets of planes of breakage and the angles between them. Minerals that have a prominent flat, sheetlike cleavage (such as the micas, muscovite and biotite) have one direction of cleavage, or perfect basal cleavage. This sheetlike cleavage makes muscovite economically valuable; it was once used in window-making material and is still used in some stove windows and in electrical insulation.

Feldspar and pyroxene have two directions of cleavage at right angles to each other, and the amphiboles (hornblende and others) have two directions of cleavage at approximately 60° and 120° to each other. Other minerals have three directions of cleavage. Halite (table salt) and galena have cubic cleavage (three directions of cleavage at right angles to one another) and break into cubes. Calcite has rhombohedral cleavage (three directions of cleavage not at right angles; the angle is about 74 degrees) and breaks into rhombohedrons. Fluorite has four directions of cleavage and breaks into octahedrons with triangular faces. Sphalerite has six directions of cleavage.

Fracture

Irregular breakage in minerals without planes of weak bonding is fracture. There are several types of fracture. Many minerals have uneven or irregular fracture. Conchoidal fracture is characterized by smooth, curved breakage surfaces, commonly marked by fine parallel lines resembling the surface of a shell (seen in quartz, obsidian, and glass). Rocks and minerals with conchoidal fracture were used by American Indians for arrowheads. Hackly fracture is jagged with sharp edges and is characteristic of metals such as copper. Fibrous or splintery fracture occurs in asbestos and sometimes gypsum. Earthy fracture occurs in clay minerals such as kaolinite.

Density and Specific Gravity

Density is defined as mass per unit volume, or how heavy a material is for its size. Specific gravity (or relative density) is commonly used when referring to minerals. Specific gravity expresses the ratio between the weight of a mineral and the weight of an equal volume of water at 4° Celsius. The terms density and specific gravity are sometimes used interchangeably, but density requires the inclusion of units of measure, whereas specific gravity is unitless. Quartz has a specific gravity of 2.65. Barite has a specific gravity of 4.5 (heavy for a nonmetallic mineral), which makes it economically valuable for use in oil and gas well drilling. Metals have higher specific gravity than nonmetals, for example, galena (7.4 to 7.6), and gold (15.0 to 19.3). The high specific gravity of gold allows it to be separated from less dense minerals by panning.

Tenacity, Taste, and Magnetism

Tenacity is the resistance of a mineral to bending, breaking, crushing, or tearing. Minerals may be brittle (break or powder easily), malleable (can be hammered into thin sheets), ductile (can be drawn into a thin wire), sectile (can be cut into thin slivers with a knife), elastic (bend but return to their original form), or flexible (bend and stay bent). Metallic minerals are commonly malleable and ductile (gold, copper). Copper is used for electrical wire because of its ductility.

Some minerals can be identified by taste. Taste is a property of halite (NaCl), used as table salt. Sylvite (KCl, or potassium chloride) has a bitter salty taste and is used as a salt substitute for people with high blood pressure because it does not contain sodium.

Magnetism is a property that causes certain minerals to be attracted to a magnet. Magnetite (Fe₃O₄) and pyrrhotite (Fe_1-xS) are the only common magnetic minerals. Lodestone, a variety of magnetite, acts as a natural magnet. In the presence of a powerful magnetic field, some other iron-bearing minerals become magnetic (garnet, biotite, and tourmaline), whereas other minerals are repelled by the magnet (gypsum, halite, and quartz). Electromagnetic separators are used to separate minerals with different magnetic susceptibilities.

Electrical Properties

Some minerals have electrical properties. Piezoelectricity occurs when pressure is exerted in a particular direction in a mineral (along its polar axis), causing a flow of electrons or electrical current. Piezoelectricity was first detected in quartz in 1881, and it has since been used in a number of applications ranging from submarine detection to keeping time (in quartz watches). When subjected to an alternating electrical current, quartz is mechanically deformed and vibrates; radio frequencies are controlled by the frequency of vibration of the quartz.

Pyroelectricity is caused when temperature changes in a mineral create uneven thermal expansion and deformation. Tourmaline and quartz are pyroelectric.

Luminescence

Luminescence is emission of light from a mineral. Minerals that luminesce or glow during exposure to ultraviolet light, X rays, or cathode rays are fluorescent. If the luminescence continues after the radiation source is turned off, the mineral is phosphorescent. The glow results from impurities in the mineral absorbing invisible, short wavelength radiation and then reemitting radiation at longer wavelengths (visible light). Minerals vary in their ability to absorb different wavelengths of ultraviolet (UV) light. Some fluoresce only in short wavelength UV, some fluoresce only in long wavelength UV, and some fluorescein both types. Fluorescence is unpredictable; not all minerals of a given type fluoresce. Minerals that commonly fluoresce include fluorite, calcite, diamond, scheelite, willemite, hyalite, autunite, and scapolite. Fluorescence has some practical applications in prospecting and mining. Synthetic phosphorescent materials have also been developed for commercial uses.

Some minerals emit light when heated. This property is called thermoluminescence. Thermoluminescent minerals include fluorite, calcite, apatite, scapolite, lepidolite, and feldspar. Minerals that luminesce when crushed, scratched, or rubbed are triboluminescent. This is a property of fluorite, sphalerite, and lepidolite, and less commonly of pectolite, amblygonite, feldspar, and calcite.

Reaction to Hydrochloric Acid

Calcite (CaCO₃) and other carbonate minerals effervesce or fizz in hydrochloric acid, but some will not react unless the acid is heated or the mineral is powdered. Bubbles of carbon dioxide (CO₂) gas are released, and the reaction proceeds as follows:

CaCO₃ + 2 HCI → CaCl₂ + H₂O + CO₂ (gas)

Radioactivity

Radioactive minerals contain unstable elements that alter spontaneously to other kinds of elements, releasing subatomic particles and energy. radioactivity can be detected using Geiger-Müller counters, ionization chambers, scintillometers, and similar instruments. Some elements have several different isotopes, differing by the number of neutrons in the nucleus. Radioactive isotopes include uranium 235 (U²³⁵), uranium 238 (U²³⁸), and thorium 232 (Th²³²). Uranium 235 is the primary fuel for nuclear power plants. Radioactive minerals include uraninite (pitchblende), carnotite, uranophane, and thorianite. Radioactive minerals occur in granites and granitepegmatites, in sandstones, and in black organic-rich shales, and are used for nuclear energy, atomic bombs, coloring glass and porcelain; in photography; and as a chemical reagent. Radioactivity is also used in radiometric dating to determine the ages of rocks and minerals.

Classification of Minerals

Minerals have been classified or grouped in several ways, but classification based on chemical composition is the most widely used. Minerals are grouped into the following twelve categories on the basis of their chemical formulas: native elements, oxides and hydroxides, sulfides, sulfosalts, sulfates, halides, carbonates, nitrates, borates, phosphates, tungstates, and silicates.

Native Elements

Native elements are minerals composed of a single element that is not combined with other elements. About twenty native elements occur (not including atmospheric gases), and they are divided into metals, semimetals, and nonmetals. The native metals include gold (Au), silver (Ag), copper (Cu), iron (Fe), platinum (Pt), and others. They share the physical properties of malleability, hackly fracture, and high specific gravity, along with metallic luster. Their atoms are held together by weak metallic bonds. They are excellent conductors of heat and electricity and have fairly low melting points. The native semimetals include arsenic (As), bismuth (Bi), antimony (Sb), tellurium (Te), and selenium (Se). They are brittle and much poorer conductors of heat. These properties result from bonding intermediate between true metallic and covalent. The native nonmetals include sulfur (S) and two forms of carbon (C), diamond and graphite. These minerals have little in common, but they are distinctive and easily identified. Diamond and graphite are polymorphs, a term meaning “many forms.” Their chemical composition is identical, but they have different crystal structures. Diamond has a tight, strongly bonded structure, whereas graphite has a loose, open structure consisting of sheets of atoms.

Oxides and Hydroxides

Chemically, the oxide and hydroxide minerals consist of metal ions (of either one or two types of metals) combined with oxygen in various ratios, such as Al₂O₃ (corundum) or MgAl₂O₄ (spinel), or metals combined with oxygen and hydrogen, such as Mg(OH)₂ (brucite) or HFeO₂ (goethite). The oxides and hydroxides are a diverse group with few properties in common. Several minerals of great economic importance occur in this group, including the chief ores of iron (magnetite, Fe₃O₄, and hematite, Fe₂O₃), chromium (chromite), manganese (pyrolusite, manganite, psilomelane), tin (cassiterite), and aluminum (bauxite). Some minerals in this group form from molten rock or hydrothermal (hot water) solutions, but others form on or near the surface of the Earth as a result of weathering and may contain water.

Sulfides

Chemically, the sulfides consist of a metal ion combined with sulfur. They are an economically important class of minerals that includes numerous ore minerals. Many of the sulfides are metallic, with high specific gravity, and most are fairly soft. They tend to be brittle, and they have distinctive streak colors. Many sulfides have ionic bonding, but others have metallic bonding, at least in part. Sphalerite has covalent bonding.

Among the sulfides are ores of lead (galena, PbS), zinc (sphalerite, ZnS), copper (chalcocite, Cu₂S; bornite, Cu₅FeS₄; and chalcopyrite, CuFeS₂), silver (argentite, Ag₂S), mercury (cinnabar, HgS), and molybdenum (molybdenite, MoS₂), as well as pyrite (FeS₂), used to manufacture sulfuric acid.

Sulfosalts

The sulfosalts are a type of unoxidized sulfur mineral. They consist of a metal and a semimetal combined with sulfur. There are nearly one hundred sulfosalts, including arsenopyrite (FeAsS), tetrahedrite (Cu₁₂Sb₄S₁₃), and pyrargyrite (Ag₃SbS₂). Some are useful as ore minerals.

Sulfates

The sulfates consist of metal plus a sulfate (SO₄) group. The sulfates are typically soft, and some are translucent or transparent. They include both anhydrous (without water) and hydrous (water-bearing) sulfate minerals. Anhydrous sulfates include barite (BaSO₄), anhydrite (CaSO₄), celestite (SrSO₄), and anglesite (PbSO₄). The hydrous sulfates include gypsum (CaSO₄·2H₂O) and epsomite (MgSO₄·7 H₂O). The structure of gypsum consists of sheets or layers of calcium and sulfate ions separated by water molecules. Loss of water molecules causes the structure of the mineral to collapse into anhydrite, with a decrease in volume and loss of cleavage. The most common sulfate, gypsum is used in the production of plaster of paris, drywall, soil conditioner, and portland cement.

Halides

The halides contain negatively charged halogen ions (chlorine, fluorine, bromine, and iodine), bonded ionically to positively charged ions (such as sodium, potassium, calcium, mercury, and silver). Many have symmetrical crystal structures resulting in cubic cleavage (halite, NaCl, and sylvite, KCl) or octahedral cleavage (fluorite, CaF₂). Many of the halides are water-soluble salts (such as halite and sylvite), and may form from the evaporation of water. Many are transparent or translucent. All are fairly soft and are light in color when fresh. Some of the silver and mercury halides will darken in color on exposure to light, hence their use in photography.

Carbonates

Carbonate minerals contain the carbonate ion, CO₃2-. Carbonate minerals are readily identified by their effervescence in hydrochloric acid, although for some carbonates, the acid must be hot or the mineral must be powdered to obtain the reaction. Some carbonates (such as cerussite, PbCO₃) react to nitric acid. Carbonates include calcite and aragonite (CaCO₃), dolomite (CaMg(CO₃)₂), magnesite (MgCO₃), and siderite (FeCO₃). The colorful malachite (green), azurite (blue), and rhodochrosite (pink) are also carbonates. Most carbonates are fairly soft, and rhombohedral cleavage is common.

Nitrates

The nitrate minerals contain the nitrate ion, NO_3-. Most nitrates are water soluble and are fairly soft. They are light in color, and some are transparent. The nitrates include soda niter (NaNO₃), which is found in desert regions and used in explosives and fertilizer, and niter or saltpeter (KNO₃), which forms as a coating on the walls of caves and is used as a fertilizer.

Borates

The boratescontain boron bonded to oxygen and associated with sodium or calcium, with or without water. Some borates form in igneous deposits, but most are found in dry lake beds in arid areas. Among the borates are borax, kernite, and ulexite. Borax is used for washing, as an antiseptic and preservative, in medicine, and in industrial and laboratory applications.

Phosphates

The phosphate minerals contain the PO₄3-group, bonded to positively charged ions such as calcium, lithium, iron, manganese, lead, and iron, with or without water. Apatite (Ca₅(F,Cl,OH)(PO₄)₃) is the most important and abundant phosphate mineral. It is the primary constituent of bone and is used for fertilizer. Turquoise is a phosphate mineral.

Tungstates

The tungstates contain tungsten (chemical symbol W). Tungstates form a small group of minerals that include wolframite and scheelite (which is fluorescent); both are ores of tungsten.

Silicates

The silicates are the largest group of minerals, and they include the major rock-forming minerals of the Earth’s crust, feldspar and quartz, as well as olivine, pyroxene, amphibole, and the micas. Most are fairly hard, with glassy luster and low to moderate specific gravity; they crush to a light-colored powder. Silicates consist of silicon and oxygen, generally accompanied by other ions such as aluminum, potassium, calcium, sodium, iron, and magnesium. silicate structure is based on the silicate tetrahedron, which consists of four oxygen atoms arranged around one silicon atom. These tetrahedra are arranged in several characteristic patterns that allow the silicates to be classified into a number of groups, including isolated tetrahedra (neosilicates), pairs of tetrahedra (sorosilicates), rings of tetrahedra (cyclosilicates), single and double chains of tetrahedra (inosilicates), sheets of tetrahedra (phyllosilicates), and three dimensional frameworks of silicate tetrahedra (tectosilicates).

Neosilicates tend to be compact and hard, with fairly high specific gravity. Olivine, garnet, zircon, topaz, staurolite, and kyanite are neosilicates. Sorosilicates (or “sister” silicates) include the minerals epidote, prehnite, and hemimorphite. Cyclosilicates are characterized by prismatic, trigonal, tetrahedral, or hexagonal habits. Beryl has rings of six silicate tetrahedra, reflected in its hexagonal (six-sided) crystals. Tourmaline and chrysocolla are also in this group.

Inosilicates (single-chain and double-chain silicates) tend to be fibrous or elongated, with two directions of cleavage parallel to the elongation. They include pyroxenes (including hypersthene, augite, and diopside), pyroxenoids (including wollastonite), and amphiboles (hornblende, tremolite, actinolite, and others).

Phyllosilicates, or sheet silicates, have one prominent direction of cleavage and tend to have a platey or flaky appearance. They are generally soft, have low specific gravity, and may have flexible or elastic sheets. The micas (muscovite, biotite, lepidolite, and phlogopite), and the clay minerals (kaolinite, illite) belong to this group, as do talc, serpentine, chlorite, and others.

The earth’s crust is dominated by tectosilicates, or framework silicates. This is the group that contains feldspar and quartz, the two most abundant minerals in the Earth’s crust. Quartz is chemically the simplest silicate, with the chemical formula SiO₂. Feldspar is a group of minerals, including orthoclase and microcline (two different crystal structures with the formula KAlSi₃O₈) and plagioclase (a solid solution series which ranges in composition from NaAlSi₃O₈ to CaAl₂Si₂O₈). Tectosilicates tend to be of low density and compact habit. Feldspathoids (including nepheline and sodalite) and zeolites (analcime and others) are also in this group.

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