Physical properties of minerals

Many minerals are readily identified by their physical properties, but identification of other minerals may require instruments designed to examine details of their chemical composition or crystal structure. The characteristic physical properties of some minerals, such as hardness, malleability, and ductility, make them commercially useful.

Color, Streak, and Luster

Minerals have diagnostic physical properties resulting from their chemistry and crystal structure. Physical properties of minerals include color, streak, luster, crystal shape, cleavage, fracture, hardness, and density, or specific gravity. Several minerals have additional diagnostic physical properties, including tenacity, magnetism, luminescence, and radioactivity. Some minerals, notably halite (common table salt), are easily identified by their taste. Other minerals such as calcite effervesce or fizz when they come into contact with hydrochloric acid.

Color is an obvious physical property, but it is one of the least diagnostic for mineral identification. In some minerals, color results from the presence of major elements in the chemical formula; in these minerals, color is a diagnostic property. For example, malachite is always green, azurite is always blue, and rhodochrosite and rhodonite are always pink. In other minerals, color is the result of trace amounts of chemical impurities or defects in the crystal lattice structure. Depending on the impurities, a particular species of mineral can have many different colors. For example, pure quartz is colorless, but quartz may be white (milky quartz), pink (rose quartz), purple (amethyst), yellow (citrine), brown (smoky quartz), green, blue, or black.

Streak is the color of the mineral in powdered form. Streak is more definitive than mineral color, because although a mineral may have several color varieties, the streak will be much more consistent in color. Streak is best viewed after rubbing the mineral across an unglazed porcelain tile. The tile has a hardness of approximately 7, so minerals with a hardness of greater than 7 will not leave a streak, although their powdered color may be studied by crushing a small piece. This property is useful because it effectively reveals the color of very thin pieces of the mineral, which tends to be much more uniform than the color of large pieces.

Luster refers to the reflectivity of the mineral surface. There are two major categories of luster: metallic and nonmetallic. Metallic minerals include metals (such as native copper and gold), as well as many metal sulfides, such as pyrite and galena. Nonmetallic lusters can be described as vitreous or glassy (characteristic of quartz and olivine), resinous (resembling resin or amber, characteristic of sulfur and some samples of sphalerite), adamantine or brilliant (diamond), greasy (appearing as if covered by a thin film of oil, including nepheline and some samples of massive quartz), silky (in minerals with parallel fibers, such as malachite, chrysotile asbestos, or fibrous gypsum), pearly (similar to an iridescent pearl-like shell, such as talc), and earthy or dull (as in clays).

Crystal Shape, Cleavage, and Fracture

Crystal shape is the outward expression of the internal three-dimensional arrangement of atoms in the crystal lattice. Crystals are formed in a cooling or evaporating fluid as atoms begin to slow down, move closer, and bond together in a particular geometric lattice. If minerals are unconfined and free to grow, they will form well-shaped, regular crystals. Conversely, if growing minerals are confined by other, surrounding minerals, they may have irregular shapes. Some of the common shapes or growth habits of crystals include acicular (or needlelike, as in natrolite), bladed (elongated and flat like a knife blade, as in kyanite), blocky (equidimensional and cube-like, such as galena and fluorite), and columnar or prismatic (elongated or pencil-like, such as quartz and tourmaline). Other crystal shapes are described as pyramidal, stubby, tabular, barrel-shaped, or capillary.

Cleavage is the tendency for minerals to break in smooth, flat planes along zones of weaker bonds in their crystal structure. Cleavage is one of the most important physical properties in identifying minerals because it is so closely related to the internal crystal structure. Cleavage is best developed in minerals that have particularly weak chemical bonds in a given direction. In other minerals, differences in bond strength are less pronounced, so cleavage is less well developed. Some minerals have no planes of weakness in their crystal structure; they lack cleavage and do not break along planes. Cleavage can occur in one direction (as in the micas, muscovite, and biotite) or in more than one direction. The number and orientation of the cleavage planes are always the same for a particular mineral. For example, orthoclase feldspar has two directions of cleavage at right angles to each other.

Fracture is irregular breakage that is not controlled by planes of weakness in minerals. Conchoidal fracture is a smooth, curved breakage surface, commonly marked by fine concentric lines, resembling the surface of a shell. Conchoidal fracture is common in broken glass and quartz. Fibrous or splintery fracture occurs in asbestos and sometimes in gypsum. Hackly fracture is jagged with sharp edges and occurs in native copper. Uneven or irregular fracture produces rough, irregular breakage surfaces.

Hardness, Density, and Specific Gravity

Hardness is the resistance of a mineral to scratching or abrasion. Hardness is a result of crystal structure; the stronger the bonding forces between the atoms, the harder the mineral. The Mohs hardness scale, devised by German mineralogist Friedrich Mohs in 1822, is a series of ten minerals arranged in order of increasing hardness. The minerals on the Mohs hardness scale are talc (the softest mineral known), gypsum, calcite, fluorite, apatite, potassium feldspar (orthoclase), quartz, topaz, corundum, and diamond (the hardest mineral known). A mineral with a higher hardness number can scratch any mineral of equal or lower hardness number. The relative hardness of a mineral is easily tested using a number of common materials, including the fingernail (a little over 2), a copper coin (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).

Density is defined as mass per unit volume (typically measured in terms of grams per cubic centimeter). In a very informal sense, density refers to the relative weight of samples of equal size. Quartz has a density of 2.6 grams per cubic centimeter, whereas a “heavy” mineral such as galena has a density of 7.4 grams per cubic centimeter (about three times as heavy). Specific gravity (or relative density) is the ratio of the weight of a substance to the weight of an equal volume of water at 4 degrees Celsius.

The terms “density” and “specific gravity” are sometimes used interchangeably, but density requires the use of units of measure (such as grams per cubic centimeter), whereas specific gravity is without unit. Specific gravity is an important aid in mineral identification, particularly when studying valuable minerals or gemstones, which might be damaged by other tests of physical properties. The specific gravity of a mineral depends on the chemical composition (type and weight of atoms) as well as the manner in which the atoms are packed together.

Tenacity

Tenacity is the resistance of a mineral to bending, breakage, crushing, or tearing. A mineral may be brittle (breaks or powders easily), malleable (may be hammered out into thin sheets), ductile (may be drawn out into a thin wire), sectile (may be cut into thin shavings with a knife), flexible (bendable, and stays bent), and elastic (bendable but returns to its original form). Minerals with ionic bonding, such as halite, tend to be brittle. Malleability, ductility, and sectility are diagnostic of minerals with metallic bonding, such as gold. Chlorite and talc are flexible, and muscovite is elastic.

Magnetism and Luminescence

Magnetism causes minerals to be attracted to magnets. Magnetite and pyrrhotite are the only common magnetic minerals; they are called ferromagnetic. Lodestone, a type of magnetite, is a natural magnet. When in a strong magnetic field, some minerals become weakly magnetic and are attracted to the magnet; these minerals are called paramagnetic. Examples of paramagnetic minerals include garnet, biotite, and tourmaline. Other minerals are repelled by a magnetic field and are called diamagnetic minerals. Examples of diamagnetic minerals include gypsum, halite, and quartz.

Luminescence is the term for the emission of light by a mineral. Minerals that glow or luminesce in ultraviolet light, X-rays, or cathode rays are fluorescent minerals. The glow is the result of the mineral changing invisible radiation to visible light, which happens when the radiation is absorbed by the crystal lattice and then reemitted by the mineral at lower energy and longer wavelength. Fluorescence occurs in some specimens of a mineral but not all. Examples of minerals that may fluoresce include fluorite, calcite, diamond, scheelite, willemite, and scapolite. Some minerals will continue to glow or emit light after the radiation source is turned off; these minerals are phosphorescent. Some minerals glow when heated, a property called thermoluminescence, present in some specimens of fluorite, calcite, apatite, scapolite, lepidolite, and feldspar. Other minerals luminesce when crushed, scratched, or rubbed, a property called triboluminescence, present in some specimens of sphalerite, corundum, fluorite, and lepidolite and, less commonly, in feldspar and calcite.

Radioactivity and Electrical Characteristics

Radioactive minerals contain unstable elements that change spontaneously to other kinds of elements, releasing subatomic particles and energy. Some elements come in several different forms, differing by the number of neutrons present in the nucleus. These different forms are called isotopes, and one isotope of an element may be unstable (radioactive), whereas another isotope may be stable (not radioactive). Radioactive isotopes include potassium-40, rubidium-87, thorium-232, uranium-235, and uranium-238. Examples of radioactive minerals include uraninite and thorianite.

Some minerals also have interesting electrical characteristics. Quartz is a piezoelectric mineral, meaning that when squeezed, it produces electrical charges. Conversely, if an electrical charge is applied to a quartz crystal, it will change shape and vibrate as internal stresses develop. The oscillation of quartz is the basis for its use in digital quartz watches.

Study Techniques

In many cases, the physical properties of minerals can be studied using relatively common, inexpensive tools. The relative hardness of a mineral may be determined by attempting to scratch one mineral with another, thereby bracketing the unknown mineral’s hardness between that of other minerals on the Mohs hardness scale. Streak may be determined by rubbing the mineral across an unglazed porcelain tile to observe the color (and sometimes odor) of the streak, if any. Color and luster are determined simply by observing the mineral.

The angles between adjacent crystal faces may be measured using a goniometer. There are several types of goniometers, but the simplest is a protractor with a pivoting bar, which is held against a large crystal so that the angles between faces can be measured. There are also reflecting goniometers, which operate by measuring the angles between light beams reflected from crystal faces.

Density can be determined by measuring the mass of a mineral and determining its volume (perhaps by measuring the amount of water it displaces in a graduated cylinder), then dividing these two measurements. Specific gravity (SG) is density relative to water. It is usually determined by first weighing the mineral in air and then weighing it while it is immersed in water. When immersed in water, it weighs less because it is buoyed up by a force equivalent to the weight of the water displaced. A Jolly balance, which works by stretching a spiral spring, can measure specific gravity. For tiny mineral specimens weighing less than 25 milligrams, a torsion balance (or Berman balance) is useful for accurate determinations. Heavy liquids, such as bromoform and methylene iodide, are also used to determine the specific gravity of small mineral grains. The mineral grain is placed into the heavy liquid, and then acetone is added to the liquid until the mineral grain neither floats nor sinks (that is, until the specific gravity of the mineral and the liquid are the same). Then, the specific gravity of the liquid is determined using a special balance called a Westphal balance, which uses a calibrated float to measure the density of the liquid.

An ultraviolet light source (with both long and short wavelengths) is used to determine whether minerals are fluorescent or phosphorescent. A portable ultraviolet light can be used to prospect for fluorescent minerals. Thermoluminescence can be triggered by heating a mineral to 50 to 100 degrees Celsius. Radioactivity is measured using a Geiger counter or scintillometer.

Commercial Applications

The physical properties of minerals affect their usefulness for commercial applications. Minerals with great hardness—such as diamonds, corundum (sapphire and ruby), garnets, and quartz—are useful as abrasives and in cutting and drilling equipment. Other minerals are useful because of their softness, such as calcite, which is used in cleansers because it will not scratch the surface being cleaned. Also, calcite in the form of marble is commonly used for sculpture because it is relatively soft and easy to carve. Talc (hardness 1) is used in talcum powder because of its softness.

Most metals, such as copper, are ductile, which makes them useful for the manufacture of wire. Copper is one of the best electrical conductors. Copper is also a good conductor of heat and is often used in cookware. Gold is the most malleable and ductile mineral. Because of its malleability, gold can be hammered into sheets so thin that 300,000 of them would be required to make a stack 1 inch high. Because of its ductility, 1 gram of gold (about the weight of a raisin) can be drawn into a wire more than a mile and a half long. Gold is the best conductor of heat and electricity known, but it is generally too expensive to use as a conductor; however, it is widely used in computers because of its corrosion resistance and because it takes only a small amount of gold to wire a computer connection.

Other minerals are valuable because they do not conduct heat or electricity. They are used as electrical insulators or for products subjected to high temperatures. For example, kyanite, andalusite, and sillimanite are used in the manufacture of spark plugs and other high-temperature porcelains. Muscovite is also useful because of its electrical and heat-insulating properties; sheets of muscovite are often used as an insulating material in electrical devices.

Cleavage is the property responsible for the use of graphite as a dry lubricant and in pencils. Graphite has perfect cleavage in one direction and is slippery because microscopic sheets of graphite slide easily over one another. A “lead” pencil is actually a mixture of graphite and clay; it writes by leaving tiny cleavage flakes of graphite on the paper.

The color of the streak (or crushed powder) of many minerals makes them valuable as pigments. Hematite has a red streak and is used in paints and cosmetics. Silver is essential as a raw material for photographic films and papers because, in the form of silver halide, it is light-sensitive and turns black. After developing and fixing, metallic silver remains on the film to form the negative. No other element is as useful for photography, and at one time there was real concern about the supply of photographic silver. Digital photography has greatly relieved fears of silver shortages.

The uranium-bearing minerals (uraninite, carnotite, torbernite, and autunite) are used as sources of uranium, which is important because its nucleus is susceptible to fission (splitting or radioactive disintegration), producing tremendous amounts of energy. This energy is used in nuclear power plants to generate electricity. Pitchblende, a variety of uraninite, is a source of radium, which is used as a source of radioactivity in industry and medicine. The high specific gravity of barite makes it a useful additive to drilling muds to prevent oil well gushers or blowouts. It is also opaque to X-rays and is used in medicine for “barium milkshakes” before patients are X-rayed so that the digestive tract will show up clearly.

Principal Terms

cleavage: the tendency for minerals to break in smooth, flat planes along zones of weaker bonds in their crystal structure

crystal: a solid bounded by smooth planar surfaces that are the outward expression of the internal arrangement of atoms; crystal faces on a mineral result from precipitation in a favorable environment

density: in an informal sense, the relative weight of mineral samples of equal size; it is defined as mass per unit volume

luminescence: the emission of light by a mineral

luster: the reflectivity of the mineral surface; there are two major categories of luster: metallic and nonmetallic

Mohs hardness scale: a series of ten minerals arranged in order of increasing hardness, with talc as the softest mineral known (1) and diamond as the hardest (10)

tenacity: the resistance of a mineral to bending, breakage, crushing, or tearing

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