The Encyclopedia Americana (1920)/Mineralogy

MINERALOGY is the science which treats of minerals and especially of the properties of these minerals, their chemical behavior and composition, their crystalline form and structure, their physical characters, their classification and their determination. Mineralogy also considers the part each mineral plays in Nature, its history, its formation and alterations, its variations under different conditions and its relation to other minerals. From a practical standpoint it records the uses of each mineral and the localities in which it has been found.

Mineralogy has many connections with other sciences, especially with crystallography and geology, for minerals are crystals for the most part and must be studied as such, and rocks are only aggregates of minerals and are identified by study of the component minerals. Physics, chemistry and mathematics are fundamental in the study of minerals and minerals are the raw material of the chemist and used by the physicist for the establishment of physical laws. Finally the arts of mining and metallurgy are concerned, the one with the extraction of minerals from the earth, the other with the extraction of metals from minerals.

Minerals arc those substances of definite chemical composition which are found ready made in the crust of the earth and are not directly products of the life or decay of an organism. Usually also they will exhibit definite and characteristic molecular (crystalline) structures.

Although minerals constitute the larger portion by far of the known so-called mineral kingdom, the definition excludes certain portions. Lack of homogeneity excludes asphalt and petroleum; lack of definite chemical composition excludes the natural glasses; ice made in the factory and ruby made in the furnace are not minerals. Coral and pearl are direct products of organic life, therefore not minerals.

The fundamental requirement of definite chemical composition is sometimes apparently waived because other facts, especially proof of definite crystalline structure, have been obtained, although a satisfactory formula has not. For

instance, it is not yet possible to state an unobjectionable formula for tourmaline, and the formulas of the great triclinic feldspars were long a source of confusion. The reasons are manifold, faulty analysis, impurities, replac e ments of one element or group by another, and sometimes, as in the so-called colloid minerals, the originally formed material has taken out other things from solutions which in the dried-out mineral remain inextricably admixed.

The characteristic crystalline structure is so frequent that minerals are sometimes defined as natural crystals. But it is now recognized that crystallinity is dependent on conditions during or preceding solidification, and that, theoretically, like any other chemical substance a mineral may, under different conditions, form in the crystalline state or the amorphous state. Not only is this theoretically true, but there are many so-called &ldquo;gel&rdquo; minerals which are known only in the amorphous state and other minerals which are known both in the amorphous and the crystalline state.

Historically, mineralogy as a science dates from the 18th century only. While the ancients utilized a very considerable number of minerals, some for the metals they contained, others as pigments, others as ornaments, charms and talismans and still others in medicine and the arts, they knew little as to their composition and nothing as to their molecular structure. They classified them, it is true, for there is still extant part of a work, &lsquo;On Stones,&rsquo; written by Theophrastus, who died 286 B.C., while Pliny in his great work on natural history, published 77 A.D., devotes five books to &ldquo;earths, metals, stones and gems.&rdquo;

The greatest contribution to mineralogical knowledge prior to the 18th century was made by Georg Agricola (1494-1555), professor of chemistry at Chemnitz, Saxony, who minutely discussed the known important ores, their mining, concentration and metallurgy; and may be said to have summed up and systematized the knowledge of minerals at that period.

As chemical knowledge increased the compositions of minerals were gradually determined and chemical tests began to replace arbitrary distinctions of structure, color and the like. This is clearly shown in the works of Wallerius, 1747, and Werner, 1798; and from this time the composition became the dominating character.

The existence of a characteristic crystalline structure for most minerals was very gradually recognized and, until the publication of the works of Romè de l'Isle in 1783 and Rènè Just Haüy (who &ldquo;raised mineralogy to the rank of a science&rdquo;), played no part in the study of minerals. From this period the fundamental importance of the crystalline structure and the part it plays in the interpretation of both physical and chemical phenomena have been everywhere recognized.

Mineralogy may conveniently be considered under the following headings: (1) Crystallography; (2) Physical Mineralogy; (3) Chemical Mineralogy; (4) Formation and Occurrence; (5) Uses; (6) Descriptive Mineralogy; (7) Determinative Mineralogy.

1. Crystallography.—Crystallography, although a distinct science, has developed with mineralogy and is so interwoven with it that the two sciences are usually taught and studied

by the same specialists. Crystallography is discussed separately in the articles ; and. The subject need here be only briefly referred to in its relation to mineralogical study.

As previously stated most minerals exhibit in all or some of their occurrences a definite crystalline structure. If the mineral develops &ldquo;well faced&rdquo; crystals the geometric symmetry and constants are obtained by a measurement of the interfacial angles of these crystals. If however these plane-faced crystals are lacking the crystalline structure is studied by the directional characters: cleavage, behavior with polarized light, etch figures, thermal and electrical properties, etc., which yield not only characteristic constants but often the complete symmetry.

Aside from these mentioned constants and symmetry relations resulting from a study of the crystals, interesting problems arise such as the relations between composition and crystalline structure, the causes of variation in crystal habit, and the reasons for vicinal planes and parallel growths.

2. Physical Mineralogy.—Physical mineralogy considers the physical characters of minerals. Many of these are crystal characters since they vary with the direction and have therefore been discussed in the articles on . Other physical characters, however, which are not dependent or notably dependent on the direction the test is made, are, nevertheless, important and some of these may be described as follows:
 * and
 * and
 * and

Lustre, in the mineralogical sense, is not the degree of brilliancy, but the kind of brilliancy. Light reflected from different substances and quite independently of the color, produces different effects; one substance resembles a metal, another glass, another silk, and they are said to possess respectively metallic lustre, vitreous lustre and silky lustre. The determining causes appear to be transparency, structure and refractive power. The most used terms are: Metallic lustre exhibited by those opaque minerals which with the exception of the native metals have a black or nearly black powder. Non-metallic lustre exhibited by all transparent or translucent minerals, which is subdivided into vitreous, adamantine, resinous, pearly, silky and waxy according to the similarity in sheen to glass, diamond, resin, mother of pearl, silk and wax respectively.

Color, by either transmitted or reflected light, depends upon the power of the substance to absorb different proportions of the lights of different wave lengths which together make up the light used. The same substance may, therefore, appear of different colors when viewed with different sources of light; and some minerals are strikingly different as, for instance, alexandrite, which by daylight is bluish to olive green and by lamp or gas light raspberry red. Color is one of the least constant mineral characters and varies with different specimens of the same species. The variation may be due to a few hundredths of 1 per cent of some organic or inorganic substance dissolved in the mineral, or to larger amounts of mechanically included foreign

Color effects may also be due to interference of light, usually as a result of some imperfection in the substance, or, in cut stones, from some purposely chosen shape producing notable dispersion of the white light into its component colors. Such effects are known as play of color, iridescence, opalescence, asterism, etc.

Streak, is the color of the fine powder of the mineral and is nearly constant, no matter how the color of the mass varies.

Hardness, to the mineralogist, means the resistance to abrasion of a smooth surface by a pointed fragment. It is usually, though very crudely, determined by comparison with the following scale introduced by Mohs: (1) Talc; (2) Selenite; (3) Calcite; (4) Fluorite; (5) Apatite; (6) Orthoclase; (7) Quartz; (8) Topaz; (9) Sapphire; (10) Diamond. Intermediate values are window glass 5.5; jeweler's file 6.5; zircon 7.5; chrysoberyl 8.5; carborundum 9.5. The more common procedure is to use pointed fragments of the scale minerals to scratch smooth surfaces of the mineral being tested. Sometimes this is more conveniently reversed and roughly polished plates of the scale minerals are tried by sharp edges or points of the mineral. The members of the scale are not in arithmetical ratio. The average of five attempted comparisons from 9 down give, roughly, sapphire 100, topaz 30, quartz 18, orthoclase 12, apatite 7, fluorite 3½, calcite 2½, gypsum ½. The scale, nevertheless, serves a useful purpose and no convenient substitute has yet been suggested. Elaborate tests with a diamond point, loss of weight by grinding with a standard powder, production of a crack by impact or pressure have been tried but fail to agree even approximately.

The Specific Gravity of a mineral, as of any other chemical substance, is of first rank as a test and is a function of the density of the molecule. As explained in the article comparative molecular volumes are obtained by dividing the molecular weights by the specific gravities.

The specific gravity of a substance is defined as its weight divided by the weight of an equal volume of distilled water at 4° C. The range, in varieties of the same species, is not great and even this is principally due to actual differences in composition. The value is usually obtained by means of a delicate balance provided with attachments for weighing the substance in water, such as a small wooden bench to hold a beaker of distilled water above the scale pan, and a platinum spiral to hold the specimen. Three weighings are needed:

W = weight of the stone. S = weight of the spiral when suspended from the end of the balance frame and immersed in the distilled water. W' = weight of the stone and the spiral suspended in distilled water.

Then, Sp. Gr. = $$\frac{W}{S + W - W'}$$

Instead of absolute weighings, relative weights may be determined on a scale by the stretching of a spring as in the Jolly balance or by the distance the apparatus sinks in water, as in the hydrometer; the results are approximate.

Liquids of high specific gravity such as concentrated solutions of mercuric and potassic

iodide or of silver thallium nitrate, or organic liquids like bromoform or methylene iodide, are often used for quick distinctions between similar appearing substances, one higher in specific gravity than the liquid, the other lower. They may also be conveniently used for certain exact determinations, being equally accurate for minute fragments and coarser material. A liquid is chosen which will float the material; the proper diluent is then stirred in drop by drop until a stage is reached at which the substance, if pushed down, will neither sink nor rise but stay where pushed. The specific gravity of the liquid may then be determined either roughly by dropping in fragments of material of known specific gravity until one is found which just sinks and another which floats, the liquid being of a specific gravity between these; or for more accurate determination a special balance, such as the Westphal, may be used.

Numerous other non-directional characters, some of which, such as fusibility and elasticity, are susceptible of exact determination, are approximately expressed by convenient terms. Fusibility, for instance, is determined in terms of a scale of seven minerals by comparing the effect of the blowpipe flame on small fragments of similar size. Elastic substances are distinguished as elastic and flexible. Tenacity is expressed as brittle, sectile, malleable, ductile or tough. The fracture surface is said to be conchoidal, even, uneven, splintery; and terms are used describing taste, odor and the sense of touch.

Certain characters are limited to a few minerals rather than exhibited by all. Such a character is luminescence or the property of emitting light at ordinary temperatures after being subjected to some exciting influence, such as light, friction, X-rays, ultraviolet light or radium.

3. Chemical Mineralogy.—Minerals are either elements or are formed by the uniting of atoms of different elements in definite proportions in accordance with the laws of chemistry and for either identification or classification their chemical composition is their most important characteristic

The methods of analyses and the calculation of formulæ are in general the same as in the analyses of other definite chemical substances. Much attention has to be paid to securing homogeneous material and in general the problem is complicated by the fact that most minerals are isomorphous mixtures (or mixed crystals) rather than simple salts.

True molecular formulæ are not generally determinable. The empirical formula is calculated from the analysis. Thus, for instance, beryl:

or closely in the ratio 3BeO.Al$2$O$3$.6SiO$2$ or, summing up, Be$2$Al$3$(SiO$2$)O$3$. If the material is an isomorphous mixture, the sum of the ratios of the replaceable elements or groups is considered, for instance, in the sphalerite analysis which follows, the sum of the

in which Zn Fe Cd and Pb are found is 1.040 and the S 1.039.

Such a composition could be expressed either by RS or (Zn.Fe.Cd.Pb)S, the letter R being used to represent a varying group of isomorphic or equivalent elements, and the parentheses with periods between the elements to show that the zinc, iron, etc., taken together accompany one atom of sulphur.

The question whether the water given off during heating is due to the destruction of an acid or basic salt or a hydroxide or is more loosely held as so-called water of crystallization, or is present in solid solution or is adsorbed or adhering atmospheric water, is often difficult to answer. Carefully worked-out water curves showing the loss at frequent intervals of temperature and the rate of loss at each temperature will often assist the judgment.

The chemical testing of minerals in practice is very commonly by the so-called &ldquo;blowpipe tests&rdquo; which possess certain advantages in speed, minute amounts of material needed and in directness of application. Usually no attempt is made to secure a complete qualitative analysis, but merely to determine the dominating constituents. Easy tests exist for most of the common and many of the rare elements, and while group separations, except for instance into volatile and non-volatile, are not practicable the order of testing is such that certain elements are detected and largely removed before the tests for the others are made.

4. The Formation and Occurrence of Minerals.—The history of a mineral, the rôle it has played, is largely told by its occurrence, associates and alterations, and these facts are often illuminated by the successful reproduction of a mineral by a method which does not conflict with the known natural conditions. The processes of mineral formation may be broadly grouped under the headings:

(1) Crystallization from a fluid magma consisting chiefly of silicates but partly of oxides, sulphides, fluorides and ferrates mutually dissolved in each other with certain volatile constituents, chiefly water. By far the greater portion of the earth's crust has formed from such magmas and a comparatively few mineral groups are found to dominate them. Clarke's estimate is

The estimated 7 per cent of accessory includes rarer silicates, elements, sulphides and oxides, sometimes in quantities which are of economic value, especially when they have undergone a natural concentration known as magmatic

as in the important nickel ores of Sudbury, Canada.

(2) Formation by pneumatolysis, that is processes in which gases and vapors especially steam, hydrofluoric, boric, su l phuric and hydrochloric add play a principal part. These gases and vapors are released by the cooling magmas and when charged with dissolved matter deposit it later as new minerals in pegmatite veins, contacts, tin lodes and other places to which they may penetrate. These vapors dissolve, transport and concentrate minerals rare in the rocks which they penetrate; they form new species into the composition of which they enter and they serve as &ldquo;mineralizers,&rdquo; apparently with catalytic action.

(3) Crystallization or precipitation from aqueous solutions. Rain water carrying oxygen and the underground waters with dissolved carbon dioxide and other constituents are the chief agents in the disintegration and alteration of the minerals which are at or near the surface. They take away selectively much of the soda, potash and lime and much less of the magnesia and alumina and silica. The solutions due to this &ldquo;weathering&rdquo; are in part redeposited as cements, in part precipitated in the residual minerals, but much is carried away to rivers, lakes or oceans, and there may form deposits of new minerals, such as carbonate of lime in rivers or underground channels, salt or other minerals of soda in lakes (or if boric acid has been present borates may form) and in land-locked basins, great beds of anhydrite, gypsum and common salt, or, more rarely, as at Stassfurt, salts of potassum and magnesium.

The minerals of veins by their composition and arrangement are shown to be deposits from watery solution, but in most cases not simply solutions of the neighboring rocks in the underground water, but also solutions in the vapors of deep-seated magmas. As the vapors rise into regions of lower pressure and temperature condensation takes place, fluid solutions form, various species separate and are deposited on the walls and may ultimately fill the fissure, forming a vein.

Animal and vegetable organisms often assist in the formation of minerals from watery solutions. The original deposits may not always be strictly mineral species as with coral, shells, diatomaceous earth, but directly or indirectly true species often result, such as limonite, apatite, sulphur and soda nitre. The formation of a mineral may involve very complex agencies such as the combined action of intense pressure from rock folding and of circulating waters often hot and charged with many constituents, including the so-called mineralizing agents. The new minerals are often denser than the originals and many contain constitutional water.

5. The Uses of Minerals.—The mineral industry of this country ranks next to the agricultural, and the value of the minerals considerably exceeds $2,000,000,000 a year. While the principal value of these minerals is for the extraction of particular constituents such as the metals or the substances of use in the chemical industries, there is a large use of the minerals in their natural state not only as constituents of building stones, but as abrasives, fertilizers, fluxes, pigments, refractory materials and in the making of pottery, porcelain, glass, etc. Minerals susceptible of polish and with any

claim to beauty are utilized as precious or ornamental stones.

Minerals are the raw material from which all the metals and all the chemical salts except the organic compounds are made. Not all minerals containing a desired element are utilized, and generally only one or two materials are obtained directly from a mineral. These products are themselves used for the manufacture of others, as for instance the mineral halite or common salt is the indirect source of nearly all of the sodium salts, but is the direct source principally of a crude sodium sulphate from which a multitude of other sails are manufactured.

6. Descriptive Mineralogy.—It may be said that it is the province of descriptive mineralogy to sum up all the results of the study of minerals, as already outlined, into orderly form for each mineral species and to so classify the different species that related minerals shall be grouped together.

The basis of classification may be scientific, or economic or genetic; each for certain purposes being the most satisfactory. Classifications until the 18th century were based on distinctions of structure, color, use, or some fancied similarity, and as has been said were &ldquo;chiefly designed to enable amateurs to arrange their collections in a fixed order.&rdquo; Pliny the naturalist (23-79 A.D.) classified as metals earths, stones and gems. Avicenna nearly 1,000 years later used a very similar classification with many subdivisions based on either external characteristics or easily ascertained properties.

Scientific classifications based on essential characters began as the increased chemical knowledge brought composition and chemical tests to the front. This was instanced in the systems of Wallerius in 1747 and Werner in 1798. The still later realization that most minerals possessed a characteristic molecular structure revealed by its crystals and physical characters followed naturally the discoveries of de l'Isle, Haüy and others of the laws governing crystals; and the methods of examining crystals placed crystalline structure alongside chemical composition as the bases of natural scientific classification. The system of James D. Dana is probably most used throughout the world and &ldquo;follows first the chemical composition and second the crystallographic and other physical characters which indicate more or less clearly the relations of individual species.&rdquo; Eight principal divisions are made from a chemical standpoint as follows:

In subdividing, the chemical composition and crystalline form are considered with the purpose of assembling in groups those minerals which have analogous compositions and closely similar forms. For instance the barite group

under anhydrous sulphates consists of sulphates in which Ba, Ca, Sr and Zn are in the same Mendeléeff group, and which show close similarity in crystal constants.

Descriptive mineralogy also serves to keep in order the nomenclature. Uniformity can only be obtained if after careful consideration the term entitled to priority and otherwise satisfactory is made the name of the species and the host of synonyms and often unessential variety names assembled under it.

7. Determinative Mineralogy.—This subject has already been discussed in a separate article. See.

Bibliography.&mdash; The following is a partial list of comparatively recent works and may be supplemented by the lists following the articles on Crystals and Crystallography.

—Dana, J. D., &lsquo;System of Mineralogy&rsquo; (6th ed., with three appendices, 1899, 1909 and 1915, New York 1892); Hintze, Carl, &lsquo;Handbuch der Mineralogie&rsquo; (Bd. I, 1904; Bd. 11, 1897, Leipzig; still incomplete).

—Bauer, M., &lsquo;Lehrbuch der Mineralogie&rsquo; (2d ed., Stuttgart 1904); Dana-Ford, &lsquo;Manual of Mineralogy&rsquo; (13th ed., New York 1912); Dana, E. S., &lsquo;A Textbook of Mineralogy&rsquo; (New York 1907); Moses, A. J., and Parsons, C. L., &lsquo;Mineralogy, Crystallography and Blowpipe Analysis&rsquo; (5th ed.. New York 1916); Miers, H. A., &lsquo;Mineralogy. An Introduction to the Scientific Study of Minerals&rsquo; (London 1902); Naumann-Zirkel, &lsquo;Elemente der Mineralogie&rsquo; (13th ed., Leipzig 1898); Phillips, A. H., &lsquo;Mineralogy&rsquo; (New York 1912); Rogers, A. F., &lsquo;Introduction to the Study of Minerals&rsquo; (New York 1912); Tschermak, G., &lsquo;Lehrbuch der Mineralogie&rsquo; (6th ed., 1905).

.— Brush-Penfield, &lsquo;Manual of Determinative Mineralogy&rsquo; (16th ed., New York 1906); Eakle, A. S., &lsquo;Mineral Tables&rsquo; (New York 1904}; Fuchs-Brauns, &lsquo;Anleitung zum Bestimmen der Mineralien&rsquo; (6th ed., Giessen 1913); Frazer-Brown, &lsquo;Tables for the Determination of Minerals&rsquo; (6th ed., Philadelphia 1910); Kraus-Hunt. &lsquo;Tables for the Determination of Minerals&rsquo; (New York 1911); Lewis, J. V., &lsquo;Determinative Mineralogy&rsquo; (2d ed., New York 1915); Plattner-Kolbeck &lsquo;Probierkunst mit der Lötrohre&rsquo; (7th ed., Leipzig 1907).

.—Brauns, R., &lsquo;Chemische Mineralogie&rsquo; (Leipzig 1896); Doelter, C., &lsquo;Physikalische-Chemische Mineralogie&rsquo; (Leipzig 1905); Doelter, C, &lsquo;Handbuch der Mineral Chemie&rsquo; (Bd. I, II, III, Dresden 1912-13).

.—Beyschlag-Krusch-Vogt (trans. by S. J. Truscott), &lsquo;The Deposits of the Useful Minerals and Rocks&rsquo; (Vol. I, 1914; Vol. II, 1916; Vol. III, London); Clarke, F. W., &lsquo;The Data of Geochemistry&rsquo; (Bulletin No. 660, United States Geological Survey, 1915); Leith and Meade, &lsquo;Metamorphic Geology&rsquo; (New

York 1915); Lindgren, W., &lsquo;Mineral Deposits&rsquo; (New York 1913); Pirrson, L. V., &lsquo;Rocks and Rock Minerals&rsquo; (New York 1908); Van Hise, C. R., &lsquo;A Treatise on Metamorphism&rsquo; (Monograph 47, United States Geological Survey, 1904); Weinschenck, E., &lsquo;Grundzüge der Gesteinkünde&rsquo; (Freiburg im Briesgau 1905).

.—Daly, R. A., &lsquo;Igneous Rocks and their Origin&rsquo; (New York 1914); Iddings, J. P., &lsquo;Rock Minerals&rsquo; (2d ed, New York 1912); Johannsen, A., &lsquo;Determination of Rock Forming Minerals&rsquo; (New York 1908); Johannsen, A., &lsquo;Manual of Petrographic Methods&rsquo; (New York 1914); Luquer, L. McI., &lsquo;Minerals in Rock Sections&rsquo; (4th ed., New York 1913); Weinschenck-Clark, &lsquo;Petrographic Methods&rsquo; (New York 1912); Wright, F. E., &lsquo;The Methods of Petrographic-Microscopic Research&rsquo; (Carnegie Institution 1911).

.{{&mdash; Murdoch, J., &lsquo;Microscopical Determination of the Opaque Minerals&rsquo; (New York 1916): Schroeder van der Kolk, J. L. C., &lsquo;Tabellen zur mikroskopischen Bestimmung der mineralien nach ihren Brechnungsexponenten&rsquo; (2d ed., Wiesbaden 1906); Seeman, F., &lsquo;Leitfaden der Mineralogischen Bodenanalyse&rsquo; (1914); Winchell-Winchell, &lsquo;Elements of Optical Mineralogy&rsquo; (New York 1908).

{{sc|Rare Minerals}}.—Cahen and Wootten, &lsquo;The Mineralogy of the Rarer Metals&rsquo; (Philadelphia 1912).

{{sc|Gems and Precious Stones}}.—Bauer, Max, &lsquo;Edelsteinkunde&rsquo; (2d ed., Leipzig); Cattelle, W. R., &lsquo;Precious Stones&rsquo; (Philadelphia 1903); Crookes, Sir Wm., &lsquo;Diamonds&rsquo; (New York 1909); Escard, J., &lsquo;Les Pierres Précieuses&rsquo; (Paris 1914); Farrington, O. C., &lsquo;Gems and Gem Minerals&rsquo; (Chicago 1903); Eppler, A., &lsquo;Die Schmuck nnd Edelsteine&rsquo; (Stuttgart 1912); Smith, G. F. H., &lsquo;Gem Stones&rsquo; (New York 1912).

{{sc|The Uses of Minerals}}.—Dammer und Tietze, &lsquo;Die Nutzbaren Mineralien&rsquo; (2 vols., Stuttgart 1913); &lsquo;Mineral Resources of the United States&rsquo; (Annually since 1883. U. S. Geol. Survey); &lsquo;The Mineral Industry&rsquo; (Annually since 1892. New York); Engineering and Mining Journal, especially Annual Review number.

{{sc|History of Mineralogy}}.—Fletcher, L., &lsquo;Guide to Mineral Collection of British Museum&rsquo;; v. Kobell, F., &lsquo; Geschichte der Mineralogie&rsquo; (München 1864).

{{sc|Mineral Synonyms}}.—Chester, A. H., &lsquo;Dictionary of the Names of Minerals&rsquo; (New York 1896).

{{Americana Author|Alfred J. Moses|label=Professor of Mineralogy, Columbia University}}

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