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:—
| 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 FenSn+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.
