A Guide to Familiar
Minerals, Gems, Ores,
and Rocks

Crystal Form is critical in mineral identification as it reflects the structure of the very molecules of the mineral. It is also the most difficult characteristic to use and one that requires the most careful study. Yet crystals are so magnificent in their beauty and symmetry it is sometimes hard to believe they are natural. The study of how they are formed reveals mathematical relationships as amazing and as beautiful as the crystals themselves. Perfect crystals are rare, and some are of great value. However, a fragment or an imperfect crystal will yield basic data to the experienced mineralogist. Sometimes crystals develop in clusters, or as twins. They reveal distortions, inclusions, and other interruptions in their development. A very simple outline or six systems of crystals is given . . . . as a bare introduction to the science of crystallography:

Cubic [Isometric] System includes crystals in which the three axes [common to five of six systems] are of equal length and are at right angles to one another, as in a cube--examples: galena, garnet, pyrite, and halite.

Tetragonal System has two axes of equal length and one unequal. All three axes are at right angles to one another, as in zircon, rutile, and cassiterite.

Hexagonal System has three equal axes at 120Á angles arranged in one plane and one more axis of a different length at right angles to these, as in quartz, beryl, calcite, tourmaline, and cinnabar.

Orthorhombic System has crystals with three axes all at right angles, but all of different length. Examples: sulfur, barite, celestite, staurolite, and olivine.

Monoclinic System has three unequal axes, two of which are not at right angles. The third makes a right angle to the plane of the other two, as in orthoclase, gypsum, micas, augite, epidote, and hornblende.

Triclinic System has three unequal axes but none forms a right angle with any other. Examples: plagioclase feldspars, rhodonite, and chalcanthite. [p. 17-18]

Minerals are chemicals. They are chemical elements or compounds found naturally in the crust of the earth. They are inorganic, in contrast to organic chemicals [made mainly of carbon, hydrogen, and oxygen] typical of living things. Some minerals have a fixed chemical composition. Others are a series of related compounds in which one metallic element may wholly or partly replace another . . . . Only rarely will a single physical or chemical property identify a mineral. Usually more characteristics must be used . . . . Some are inherent and reliable; others are variable and must be used with care. You can easily learn to use the simpler physical and chemical tests. Identification of many rare minerals often requires expensive laboratory equipment and detailed chemical and optical tests which only an expert can make. [p. 13]

Elements are the building blocks of all materials, including minerals and rocks. About 100 elements are known. A dozen or so were known in ancient times; the latest were found in atom-splitting experiments. All are made up of protons, neutrons, and electrons. These, combined, form atoms of matter. The atoms in turn join to form molecules--the smallest particles usually produced in chemical reactions. When temperatures are high, molecules may break down into atoms or atom groups. With slow cooling these may join together, in regular order, to from crystals. Most minerals are crystalline, being formed from cooling mixtures, liquids, or vapors in the crust or the earth.

The arrangement of an atom's electrons determines with what other elements it will combine, and in what proportions. The physical conditions in molten materials also set the pattern by which chemical elements form different minerals. The science of physical chemistry has much to reveal about how, why, and when minerals forms. [p. 14]

Optical Properties of minerals are used mainly by experts, but amateurs should know about them because they are fundamental in precise mineral identification. Optical identification is highly accurate and can be used with particles of microscopic size. X-rays sent through thin fragments or powders produce a pattern dependent on the structure of the molecules and so are an aid to identification. Pieces of minerals or rocks are mounted on slides, then ground till paper thin. These thin sections are examined through ordinary and polarized light. The bending of light as it passes through the minerals gives patterns that aid in identification. Fragments of minerals can be immersed in transparent liquids of different density to measure their index of refraction. This is distinct for each mineral and is related to its crystal system . . . . Thus an expert can tell if a diamond or emerald is real or false without doing any damage to the stone. [p. 15]

Specific Gravity is the relative weight of a mineral compared to the weight of an equal volume of water. Since the weight of an equal volume of water is identical with the mineral's loss in weight when weighted in water, specific gravity (Sp. Gr.) is quickly determined. A corundum crystal weighing 2.0 oz. dry weights 1.5 oz. when suspended in water. The loss (o.5 oz.) divided into the dry weight gives a specific gravity of 4.0. This may seem odd, because corundum contains only aluminum (Sp. Gr. 2.5) and oxygen, a gas . . . . Below are some average figures.

Borax 1.7 - Talc 2.8 - Corundum 4.0

Sulfur 2.0 - Muscovite 2.8 - Rutile 4.2

Halite 2.1 - Tremolite 3.0 - Barite 4.5

Stilbite 2.2 - Apatite 3.2 - Zircon 4.7

Gypsum 2.3 - Crocidolite 3.3 - Zincite 5.5

Serpentine 2.5 - Topaz 3.5 - Cassiterite 7.0

Orthoclase 2.6 - Rhodochrosite 3.6 - Cinnabor 8.0

Quartz 2.7 - Staurolite 3.7 - Uraninite 9.5

Calcite 2.7 - Siderite 3.9 - Gold 19.3

Aluminum 2.5

[p. 19]

Cleavage is the way some minerals split along planes related to the molecular structure of the mineral and parallel to possible crystal faces. The perfection of cleavage is described in five steps from poor (as in bornite) to fair, good, perfect, and eminent (as in micas). The types of cleavage are usually described by the number and direction of cleavage planes . . . . [p. 20]

Fracture is the breakage of a mineral specimen in some way other than along cleavage planes. Not all minerals show good cleavage; most show fracture. Fresh fractures show the minerals' true color. Five to seven types of fracture are recognized . . . . Conchoidal fracture: obsidian; Uneven Fracture: arsenopyrite; Earthy fracture: clay. [p. 20]

Color is the first of three characteristics that have to do with the way a mineral looks. In most metallic ores it is a surface clue in identification. But in quartz, corundum, calcite, fluorite, agate, garnet, tourmaline, and others it is often due to impurities and may vary greatly. So use color with caution and use only the color of a freshly broken surface. Note that surface tarnish on some metallic ores; it differs from the true color, but can be used for identification also. [p. 21]

Streak is the color of the powdered mineral best seen when the mineral is rubbed against a streak plate of unglazed porcelain (the back of a tile is excellent). In metallic ores the streak may differ from the color and so is worth noting. [p. 21]

Luster depends on the absorption, reflection, or refraction of light by the surface of a mineral. It is often an aid in identification. About a dozen terms are used, most of which are self-explanatory: adamantine (brilliant), like diamond; vitreous (glassy), like quartz; and metallic (like metal), like galena. The prefix sub- is used when the characteristic is less clear. Other lusters to note: dull, earthy, silky, greasy, pearly, resinous. [p. 21]

Ultraviolet light is invisible. Its waves are too short to be detected by the eye. However, some minerals, when exposed to this light, are "excited"--they absorb the ultraviolet light and emit longer light rays which we see as colors. Minerals which do this are fluorescent. If they continue to emit light after the ultraviolet rays have been cut off, they are phosphorescent (like the luminous dial of a watch). A quartz lamp is a fine source of ultraviolet light of short (about 1/10,000 in.) wavelength. The argon light gives longer ultraviolet rays. Not all minerals fluoresce when exposed to ultraviolet light. Uranium minerals do; so does scheelite, an ore of tungsten, and other tungsten minerals. Other minerals may fluoresce because of impurities, and still other fluoresce when from one locality and not when from another; this makes the search for fluorescent minerals exciting . . . . [p. 22]

Magnetism occurs in a few minerals. Lodestone (a form of magnetite) is a natural magnet. An alnico magnet will attract bits of magnetite and pyrrhotite. A few manganese, nickel, and iron-titanium ores become magnetic when heated by a blowpipe. [p. 23]

Electrical Properties of minerals are varied. Thin slabs of quartz crystal control radio frequencies. Crystals of sulfur, topaz, and other minerals develop an electric charge when rubbed. Tourmaline crystals, when heated, develop opposite charges at opposite ends of the crystal. [p. 23]

Heat may raise the temperature of a mineral till it will fuse in a blowpipe flame..... [p. 23]

M i s c e l l a n e o u s

Titanium is a metal with a future and its minerals will be of increasing importance. Light weight and a high melting point give it importance in rocket construction. Now used in steel alloys, as a cutting tool (titanium carbide), and in white paints. Titanium is abundant, making up 0.6 per cent of the earth's crust. Its ores are found principally in southeastern US and Arkansas, and in India, Norway, France, Switzerland, and Brazil. [pg. 58]

Graphite is one of the world's softest minerals. Diamond is the hardest. Both are carbon [C]. Graphite occurs in igneous and metamorphic rocks--schists and marbles. It may form when high temperature veins cut coal deposits, and an artificial form is made in electric furnaces. Graphite is earthy, or forms scaly or flaky crystals with a metallic luster, greasy and flexible. H. 1; Sp. Gr. 2.0. Commercial deposits in this country are limited to three mines in Alabama, Texas, and Rhode Island; these cannot compete with the rich deposits of Korea, Ceylon, Mexico, and Madagascar. Graphite is used for dry and wet lubrication and for electrical and chemical purposes. Its best known use is as "lead" in lead pencils, where it is usually mixed with other materials to give various degrees of hardness. Graphite is a strategic mineral. Its latest use is as a moderator to slow down neutrons in atomic piles. [p. 62]

Fossils are the remains, prints, or other indications of former plant or animal life found naturally buried in rock. World-wide studies over the past century indicate that the older the rocks, the simpler the types of plant and animal fossils found in them. The fossils have therefore been used to establish the age of the rock which encloses them. Fossils show that many thousands of kinds of plants and animals, common in the past, no longer exist, and that most of those living today resemble strongly the fossil forms found in relatively recent rocks.

In addition to telling the details of life in the past and the story of such unique animals as giant dinosaurs and titanotheres, fossils also tell of past climates. Colonial corals in Greenland rocks attest to warmer conditions in the past than today, and imprints of fir and spruce in unconsolidated clays near the surface record the penetration of glacial cold far to the south. Fossils are also used to determine the marine or fresh-water origin of rocks.

The occurrence of fossils is both rare and common. Only a tiny fraction of the total number of living things has ever been preserved as fossils, and yet certain layers of rock or strata are made almost entirely of shells, teeth, plant remains, and even of bone. [p. 130]

Fossils are preserved in many ways. The simplest is the intact preservation of the hard parts of a plant or an animal . . . . Wood, bone, teeth, and other hard parts are preserved in tact for relatively short periods.

In another type of fossilization, buried plant or animal materials decompose, leaving a residual film of carbon behind. This may mark the form of a leaf or of some simple animal. On a larger scale this process is responsible for our great deposits of coal.

Sometimes buried material is gradually replaced by silica and other material like calcite, dolomite, or pyrite from solutions which permeate the rock in a process called petrifaction. These replacements form another very common type of fossil.

Probably the most spectacular of all replacements is that of wood by agate or opal as a result of the action of hot, silica-bearing waters. This forms petrified wood. The replacement may be so minute and complete that even the details of cellular structure are preserved. The best-known examples are preserved in the Petrified Forest National park in Arizona. [p. 131]

Molds and casts are very common fossil forms. They are impressions, and so differ from intact preservation and replacement. A footprint, as that of a dinosaur, is a good example of a mold. The impression left in soft mud or silt may harden before more sediment fills it in and provides material for a new layer of rock. If the sediment later consolidates and the rock is eventually broken open , the original imprint will be found below and filling it will be a cast of the underside of the dinosaur's foot.

When shells are buried in sand or mud, a mold of the outer surface of the shell is formed. Percolating waters may dissolve the shell material completely, and the mold will then be the fossil record. Later, percolating waters may refill the cavity with calcium carbonate or silica, forming a cast which will on its outer surface completely duplicate the external form of the shell.

Paleontology, the study of plant and animal fossils and their histories, is an important and exciting branch of the science of geology. The study of small fossil forms (which are known as microfossils) has recently yielded a great deal of new information. [p. 132]

Sedimentary rocks are extremely varied, differing widely in texture, color, and composition. Nearly all are made of materials that have been moved from a place of origin to a new place of deposition. The distance moved may be a few feet or thousands of miles. Running water, wind, waves, currents, ice, and gravity move materials on the surface of the earth by action that takes place only on or very near the surface. In total these rocks cover about three-quarters of the earth's surface.

Unconsolidated mud or sand is usually referred to as a sediment, while consolidated materials are called sedimentary rocks. Rocks made up of grains of particles are called clastic; they may range from less than a thousandth of an inch to huge boulders. Other sedimentary rocks are of chemical or organic origin. Most sedimentary rocks form in layers or strata; many contain fossils. Major sedimentary strata form slowly over millions of years. [p. 121]