1922 Encyclopædia Britannica/Mineralogy

by Leonard James Spencer

MINERALOGY (see 18.509). — During the war period of 1914-8 much attention was given in all countries to the development of home resources of various minerals of economic value, and to meet new circumstances new sources of supply were developed. Further, there was an increased demand for certain kinds of minerals, for example those which yield the rarer metals used in the hardening of steel. Much of the mineralogical literature of the period has therefore been of an economic character, e.g. a long series of “Special Reports on the Mineral Resources of Great Britain” has been issued by the Geological Survey; and many recent text-books give prominence to the practical uses of minerals. Fortunately, however, pure science has not been altogether neglected. Many new facts have been recorded, and new methods of investigation have been devised. A review of the recent scientific literature is given in the Mineralogical Society's series of “Mineralogical Abstracts.”

X Rays and Crystal-Structure.—The new X-ray method of investigating the internal structure of crystals has been applied with much success to the study of minerals (see Crystallography). The material for examination has usually been prepared as definitely orientated crystal plates, but it is now found that results can be obtained with a fine powder, i.e. an aggregate of minute crystals or fragments of crystals with all possible orientations. The method can therefore be used for the purpose of distinguishing between the crystalline and the amorphous or colloidal states. Stress has recently been laid on the importance of the colloidal forms of minerals, and some authors have separated these as distinct species, to which special names have been applied, from the corresponding crystalline forms possessing the same chemical composition. It has, however, not hitherto been possible, by optical means alone, to distinguish with certainty a colloidal form from a microcrystalline mineral which is opaque or which is cubic in crystallization.

Microscopical Examination of Opaque Minerals.—A new method for the investigation of opaque minerals has recently been borrowed from metallography, in which polished sections are examined under the microscope in reflected light. This method has proved to be especially useful for the study of metallic ores, and it consequently finds an economic application in the valuation of ore-deposits. The several mineral-species of which the ore is composed can be distinguished, and their relations to one another determined; e.g. the order of their deposition, and whether they are of primary or secondary origin. The technique of the subject (called mineralography or mineragraphy) is dealt with in the recent text-books of J. Murdoch, Microscopical Determination of Opaque Minerals (New York, 1916) and of W. M. Davy and C. M. Farnham, Microscopic Examination of the Ore Minerals (New York, 1920). The process of grinding and polishing the sections presents certain difficulties owing to the extreme differences of hardness of the several minerals that may be present. The prepared section is illuminated vertically by means of a right-angle prism placed in the tube of the microscope above the objective. Details of structure can be brought out by etching the section with various chemical reagents. The various characters (colour, hardness, relief) of the minerals, together with their behaviour towards reagents, help in their determination. But in many cases ordinary simple tests made on fragments detached from the polished surface are more reliable. Electrical tests can be made with quite simple apparatus: for example, the electrical conductivity can be determined with a dry cell and voltmeter using needles as terminals on the polished surface. Certain optical determinations can also be made in reflected polarized light; but whilst the use of polarized light is of prime importance for the examination of transparent minerals (e.g. in thin slices of rocks), it has only a limited application in the case of opaque minerals. It is, however, possible to determine whether a crystal is isotropic or anisotropic, and in the latter case to determine the directions of the principal axes of refringence and of absorption.

One result of this study of opaque minerals is to draw attention to the extremely intimate association and intergrowth of many of the ore-minerals; this is well shown in the numerous photomicrographs published by American workers in economic geology. What to all appearances by ordinary methods is a homogeneous mineral may be found by the new method to be really heterogeneous; and, in fact, several supposed mineral-species have been proved to be mixtures, and well-developed crystals have in certain cases been found to contain enclosures of other minerals. The method is thus of use for ascertaining the degree of purity of material collected for exact chemical analysis when the formula of a species is to be established. The long-debated question as to how silver exists in argentiferous galena (lead-ore) has been studied by this method. Galena containing 0.10 to 0.35% of silver shows definite spots of tetrahedrite and argentine, whilst specimens containing more silver show evidence of later addition of proustite or pyrargyrite in the form of veinlets.

Mineral Transformations. —In synthetical mineralogy a large amount of experimental work has been done, more especially in the Geophysical Laboratory of the Carnegie Institution at Washington. Many minerals and allied compounds have been prepared artificially in silicate and salt fusions. The conditions necessary for their formation and their ranges of stability—either when alone or when in the presence of other compounds—have been studied in detail. One important result obtained by experimenting over wide ranges of temperature has been to show that practically all compounds known as minerals exist in several polymorphous forms. A long-known example of this is given by the three minerals andalusite, fibrolite and kyanite, all of which are composed of aluminium silicate (Al₂SiO₅), but which differ from one another in crystalline form and physical characters. Recent work has shown that silica (SiO₂) undergoes a remarkable series of changes in its crystalline structure and physical characters when it is submitted to different degrees of temperature. The changes with increasing temperature are:—

α-quartz (tetartohedral hexagonal), passing at 575° C. into
β-quartz (hemihedral hexagonal), passing at 870° C. into
β-tridymite (holohedral hexagonal), passing at 1,470° C. into
β-cristobalite (cubic) melting at 1,625° C.

These transformations are reversible, but with falling temperature they take place very slowly. Molten silica unless cooled very slowly solidifies as a glass. β-tridymite when quickly cooled undergoes a change at 163° C. (β₂-tridymite to β₁-tridymite), and at 117° C. passes over into α-tridymite, which is optically biaxial and probably orthorhombic in crystallization, being identical with the naturally occurring tridymite. Similarly, β-cristobalite when quickly cooled changes at about 180-270° C. into α-cristobalite, which is optically biaxial (pseudo-cubic) and identical with the cristobalite occasionally found in volcanic rocks.

Now these and many other similar changes give information as to the conditions of temperature under which various minerals were formed in nature, thus providing a “geological thermometer.” For example, the presence of tridymite, or of pseudomorphs of the more stable quartz after tridymite, establishes that the rock in which they occur must have been formed at a temperature between 870° and 1,470° C. The quartz of certain veins and that of granite present differences in structure which indicate that the former was formed below 575° C. and the latter above this temperature. Or, again, the presence of orthorhombic copper-glance (β-Cu₂S) as a pseudomorph after cubic α-Cu₂S proves that the ore-deposit in which it occurs was formed at a temperature higher than 91° C.

Chemical Composition.—The chemical composition of many minerals is still imperfectly understood, and even for some quite common species there are doubts as to the correct empirical formulae. This is especially the case in the large division of the silicates, a satisfactory classification of which is still wanting. Many attempts have within recent years been made to gain some idea as to the constitution of the silicates; there has been much experimental work and plenty of speculation, but with no very definite results. In certain groups, e.g. the felspars and the garnets, the composition can be satisfactorily expressed on the assumption of the isomorphous mixing of different chemical molecules. But attempts to extend this principle to all silicates often lead to highly complex hypothetical molecules, the existence of which can only be regarded as doubtful. Alternative suggestions have been put forward, such as the “mass effect” of large molecules and the “solid solution” of certain other substances in the main mass of the crystal. Experiments with silicate fusions show that various substances can be taken up, or dissolved, in certain amounts, giving on solidification apparently homogeneous crystals.

As an example, the recent discussion on the composition of nephelite (see 19.383) may be cited. Analyses of this mineral invariably show an excess of silica over that required by the orthosilicate formula NaAlSiO₄, and sodium is always partly replaced by an equivalent amount of potassium in the varying ratio (Na:K) of 5.5:1 to 3:1, whilst a small amount of calcium is also present. Here the excess of silica (the ratio SiO₂:Al₂O₃ ranging from 2.1:1 to 2.2:1) has been assumed to be present in “solid solution” in an isomorphous mixture of NaAlSiO₄ and KAlSiO₄. The higher ratio of 2.2:1 is regarded as the “saturation ratio” of the silica, for when it is exceeded visible albite is found with the nephelite. Another view expresses the composition of nephelite as an isomorphous mixture in varying proportions of the following molecules, in one of which SiO₄ is replaced by Si₃O₈:—

$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	NaAlSiO4	(known as an artificial compound; soda-nephelite below 1,248° C., and as the triclinic felspar carnegieite above this inversion-temperature).
	KAlSiO4	(the mineral kaliophilite).
	NaAlSi3O8	(hypothetical isomer of albite).
	Ca½AlSiO4	(hypothetical isomer of anorthite).

Or, again, the following series of “normal” nephelites, each representing a double compound of alumotrisilicic acid (H₂Al₂Si₃O₁₀) and alumodisilicic acid (H₂Al₂Si₂O₈), has been suggested:

K2Na8	Al10Si11O42 = K2Al2Si3O10+4	Na2Al2Si2O8.
K2Na9	Al11Si12O46 = K2Al2Si3O10+4½	Na2Al2Si2O8.
K2Na10	Al12Si13O50 = K2Al2Si3O10+5	Na2Al2Si2O8.
K2Na11	Al13Si14O54 = K2Al2Si3O10+5½	Na2Al2Si2O8.

Whilst for a slightly more basic type is given the (highly improbable) formula:—

K₄Na₁₈Al₂₂Si₂₃O₉₀ = 2K₂Al₂Si₂½O₁₀+9Na₂Al₂Si₂O₈.

Another, less complex, example is afforded by the mineral pyrrhotite (see 22.696). In its meteoric equivalent, known as troilite, the composition is quite simple, namely FeS, but in the terrestrial mineral there is always an excess of sulphur, as represented by the formulae Fe₇S₈, Fe₁₁S₁₂, etc., or in general Fe_nS_n+1. When pyrites (FeS₂) is heated it dissociates at 565° C. into pyrrhotite and free sulphur, and the composition of the resulting pyrrhotite varies with the temperature and the pressure of the surrounding sulphur-vapour. It is therefore concluded that the excess sulphur is absorbed by the ferrous sulphide, or in other words present in “solid solution,” the formula being written FeS(S)_x.. The absence of any excess of sulphur in the troilite of meteorites is accounted for by the coexistence of free iron. Radioactivity.—The strong radioactivity of uranium minerals affords a ready means of recognizing these valuable ores in the search for them by prospectors. The mineral may be wrapped up with a photographic plate, which is afterwards developed; but a simpler and quicker test is that with a quite simple (home-made) gold-leaf electroscope. A piece of the mineral to be tested is placed on the cap of the electroscope, which is then charged with electricity, readily developed by rubbing glass or vulcanite (say the mouth-piece of a tobacco-pipe): if the mineral contains uranium (and consequently radium), the gold leaves will soon come together. It is always well to make a comparative test, timing the rate of collapse, with a piece of ordinary stone.

Determinations of the ratio of the amount of uranium to the amounts of the various products of its decay (radium, helium, lead, etc.) present in various radioactive minerals give (knowing the rate of the decay) some idea of the period of time during which these products have been accumulating. In this way estimates have been made of the age in years of these minerals and even of the age of the earth; but, of course, many unknown factors must have been omitted from such calculations. Lead of radioactive origin, or isotopic lead the final product of the decay of uranium is found to vary slightly in its atomic weight according to the uranium mineral from which it is extracted.

To radioactivity is ascribed the well-known “pleochroic haloes”—tiny spots or borders of deeper colour surrounding microscopic inclusions—long ago observed in certain rock-forming minerals (cordierite, andalusite, mica, etc.) when micro-sections of rocks are examined in polarized light. The long and continued emission of X-rays from zircon or other mineral grains has caused a change in colour of the surrounding mineral for distances varying from 0.002 to 0.04 mm. A study of these has again given some information as to the age of the minerals. In this connexion it may be mentioned that much experimental work on the coloration of minerals has been done within recent years by exposing the minerals to the action of radiations of various kinds, including ultra-violet rays, cathode rays, Röntgen rays and the rays emitted by radium salts. For example, some diamonds acquire a green colour, and fluorspar becomes blue when placed in contact with radium bromide.

New Minerals.—In descriptive mineralogy a considerable number of new minerals have been named, but unfortunately in many cases not completely determined and described. A few of the more prominent and well-established of these are:—

Alamosite, lead metasilicate, PbSiO₃, found at Alamos, Sonora, Mexico, as radially fibrous masses with snow-white colour and adamantine lustre. Crystals are monoclinic, and the mineral is analogous to wollastonite (CaSiO₃).

Amosite, a variety of monoclinic amphibole-asbestos rich in iron (FeO 32.44%) and consisting essentially of ferrous silicate with but little magnesia. It is of the “cross-fibre” type and resembles crocidolite in its mode of occurrence, thus differing from the ordinary type of amphibole-asbestos. It is found over a wide area in northeastern Transvaal and is named from the Amosa asbestos mine, this word being formed of the initial letters of the company “Asbestos Mines of South Africa.”

Betafite, a hydrated titano-columbate of uranium containing UO₃ 26.28%. It occurs in pegmatites near Betafo in Madagascar as sharply developed octahedral crystals with the edges truncated by faces of the rhombic-dodecahedron. Curiously flattened crystals are also found.

Carnotite, hydrated vanadate of uranium and potassium, K₂O·V₂O₄·2U₂O₃·3H₂O, occurring as a canary-yellow crystalline powder impregnating sandstones over a wide area in western Colorado and the adjoining portions of Utah and New Mexico. In Colorado it has been collected on a large scale for the extraction of vanadium, uranium and radium. It has also been found in South Australia and in Pennsylvania; and an allied mineral (tyuyamunite, containing calcium in place of potassium) is known from Tyuya-Muyun in Russian Turkestan.

Inyoite, hydrated calcium borate, 2CaO·3B₂O₃·13H₂O, found as large, colourless, monoclinic crystals in the borate deposits of Inyo county, Cal., and in the gypsum mines at Hillsborough in New Brunswick.

Lorandite, sulpharsenite of thallium, TlAsS₂, forming transparent, monoclinic crystals with a carmine-red colour and adamantine lustre. It is found with realgar at Allchar in Macedonia, and is one of the few minerals that contain the rare element thallium as an essential constituent (Tl 59.5%).

Margarosanite, metasilicate of lead and calcium, PbCa₂(SiO₃)₃, occurring as colourless or snow-white, platy masses and anorthic crystals with pearly lustre. It has been found at Franklin in New Jersey and at Långban in Sweden.

Otavite, basic carbonate of cadmium (Cd 61.5%), occurring as small, pearly white, curved rhombohedra on copper and lead ores at Otavi in South-West Africa. The only cadmium mineral previously known is the sulphide, greenockite.

Patronite, vanadium sulphide, VS₄, forming dark, greenish-black, compact masses. It occurs abundantly at Minasragra, Cerro de Pasco, Peru, where it is a valuable ore of vanadium. It weathers very readily with the production of various highly coloured vanadium compounds; even on material kept in collections there is a slow growth of blue and green efflorescences.

Spencerite, hydrated basic zinc phosphate, Zn₃(PO₄)₂·Zn(OH)₂·3H₂O, forming pearly white, scaly cleavage masses and small monoclinic crystals. It has been found in some abundance forming large stalactites in a cavern near Salmo, British Columbia.

Stichtite, hydrated basic carbonate of magnesium and chromium, MgCO₃·5Mg(OH)₂·2Cr(OH)₃·4H₂O, occurring as foliated masses with pearly lustre and bright lilac colour. This colour forms a striking contrast to the bright green serpentine in which the mineral is embedded. It has been found in western Tasmania, in the Transvaal, and near Black Lake in Quebec.

Tarbuttite, basic zinc phosphate, Zn₃(PO₄)₂·Zn(OH)₂, forming colourless, or faintly coloured green or red, anorthic crystals, with a perfect cleavage in one direction. It has been found in considerable quantity at the Rhodesia Broken Hill mine in northern Rhodesia.

Thortveitite, silicate of scandium, yttrium, etc. (Sc,Y)₂O₂·2SiO₂, occurring as large orthorhombic crystals of prismatic habit in pegmatite in southern Norway and Madagascar. This is the only mineral known to contain the rare element scandium in large amount.

Tungstenite, tungsten sulphide, WS₂, forming minute scales resembling molybdenite and graphite in appearance. It is found intimately intermixed with other ores in the Little Cottonwood district, Utah.

References.—Details of descriptive mineralogy are collected in Appendices 1-3 of Dana's System of Mineralogy (New York 1899-1915); and numerical data respecting the constants of minerals are tabulated in the international Tables annuelles de constantes et données numériques (4 vols., Paris 1912, etc.). A new work of a comprehensive character is C. Doelter, Handbuch der Mineralchemie (3 vols., Dresden and Leipzig 1912, etc.). A number of elementary text-books have been published, e.g. F. H. Hatch, Mineralogy (London 1912); G. A. J. Cole, Outlines of Mineralogy for Geological Students (London 1912); A. F. Rogers, Introduction to the Study of Minerals (New York 1912); A. H. Phillips, Mineralogy, an Introduction to the Theoretical and Practical Study of Minerals (New York 1912); E. H. Kraus and W. F. Hunt, Mineralogy, an Introduction to the Study of Minerals and Crystals (New York 1920). A popular book with coloured plates is L. J. Spencer, The World's Minerals (London 1911). Books of an economic character are H. Ries, Economic Geology (4th ed., New York 1916); T. Crook, Economic Mineralogy, a Practical Guide to the Study of Useful Minerals (London 1921); B. Dammer and O. Tietze, Die nutzbaren Mineralien (2 vols., Stuttgart 1913-4); O. Stutzer, Die wichtigsten Lagerstätten der “Nicht-Erze” (Berlin 1911, etc.). New journals are Fortschritte der Mineralogie, Kristallographie und Petrographie, ed. by G. Linck (Jena since 1911); Beiträge zur Kristallographie und Mineralogie, ed. by V. Goldschmidt (Heidelberg since 1914).

(L. J. S.)