INTRODUCTION. THIS book is not intended, in the strict sense of the term, to be a Manual of Mineralogy, but, believing that a concise description of the modes in use for distinguishing between different minerals will assist the student in recognising them, the plan of a mere Glossary has been departed from, and brief hints on the nature of minerals have been introduced. It must be remembered, however, that there is no short cut to a knowledge of minerals. Mineralogy, like the other sciences, demands industry and attention; and to become an accomplished mineralogist much and careful study must be devoted to the subject; an acquaintance with various other branches of science must be brought to bear on it, while, above all, the eye should be rendered familiar by constant inspection with the forms and appearances of minerals, and with their physical properties. This eyeknowledge (as it may be termed) can only be acquired by long and diligent practice, by actual examination and by handling the specimens themselves, no opportunities of doing which should be neglected. To become well versed in mineralogy involves also a knowledge of Physics and Chemistry. By means of the first we make ourselves acquainted with the physical properties of minerals; while the second teaches us the nature of their chemical composition. It appears necessary, therefore, to refer to the bearings of these sciences on mineralogy; but in the limited space to which these remarks must be confined, it is only possible to do so in a very brief manner. xii INTRODUCTION. 1. OF THE PHYSICAL CHARACTERS OF MINERALS. External Form and Structure. Crystallography, or a knowledge of the crystalline forms of minerals, is of the highest importance. It is true minerals frequently occur in an amorphous state; in which case, the particles of which they are composed are arranged according to no definite law; but they very often are crystallized, i. e. assume certain regular and determinate forms called crystals. On To one ignorant of the subject the shapes of these seem to be innumerable; but on closer examination such does not prove to be the case. the contrary, it is found that all these numerous and sometimes complex varieties of crystals may be reduced to some five or six simple types, of which the others are only modifications or variations and even that the complicated forms of crystals may be sometimes actually converted into the typical form by the mechanical process of cleavage. This simple or elementary form to which each particular crystal is capable of being ultimately reduced, has been called, therefore, its primary form. Various systems of crystallography have been proposed by different authors. The classification adopted here is nearly the same as that employed by Brooke and Miller in their admirable edition of Phillips's Manual, and is a modification of the systems of various other crystallographers. These systems, six in number, are called respectively, the Cubical, Pyramidal, Rhombic, Oblique, Anorthic, and Hexagonal or Rhombohedral.* 1. The Cubical System — has three equal axes, intersecting one another at at right angles. Thus, in the cube, the regular octahedron and the rhombic dodecahedron, which belong to this system, the height, and the length, and the breadth of the axes are all equal, and are at right angles to each other. In the cube the axes are drawn from the centres of opposite faces; in the regular octahedron they connect the opposite solid angles; and similarly in the rhombic dodecahedron. 2. Pyramidal System.-In the pyramidal system there are, also, three axes intersecting each other at right angles; but one of these, called the vertical axis, differs in length from the other two, or lateral axes, which are equal. The right square prism, and the octahedron with a square base, belong to this system. In the first the axes connect the centres of opposite faces, and are at right angles to one another. In the octahedron with a square base, which bears the same relation to the right square prism as the regular octahedron does to the cube, the axes connect the opposite solid angles. 3. Rhombic System. In this system there are three unequal axes intersecting one another at right angles. It includes the right rectangular prism, the right rhombic prism, and the octahedron with a rhombic base. These correspond respectively with the following systems employed in Dana's Manual of Mineralogy, 4th edition :-1. Monometric, or Tesseral. 2. Dimetric. 3. Trimetric. 4. Monoclinic. 5. Triclinic. 6. Hexagonal. INTRODUCTION. In the first the axes connect the centres of opposite faces. xiii In the second the vertical axis connects the centres of the basal faces, and the lateral axes connect the centres of the opposite lateral edges. In the octahedron with a rhombic base the axes, as before, connect the opposite solid angles. 4. Oblique System.-This has three unequal axes. The vertical axis is inclined to one of the lateral axes, and at right angles to the other—the two lateral axes being also at right angles to each other. It comprises the right rhomboidal and the oblique rhomboidal prisms. In the second the vertical axis connects the centres of the bases, and the lateral axes the centres of the opposite lateral edges. 5. Anorthic System. In the Anorthic System there are three unequal axes, all intersecting each other obliquely. It comprises the oblique rhomboidal prism. 6. Hexagonal, or Rhombohedral.-In this system there are three equal lateral axes, intersecting at an angle of 60°, and one vertical axis at right angles to them. It comprises the hexagonal prism and the rhombohedron. In the former the vertical axis connects the centres of the bases, and the lateral axes the centres of the opposite lateral edges, or of the lateral faces. In the latter the vertical axis connects two of the solid angles diagonally opposite, and the lateral axes opposite lateral edges. The student will derive great assistance in investigating the primary forms of crystals and their modifications if he make a series of models for himself. Drawings of these, which can be cut out in one piece, and after being stuck on cardboard admit of being fastened together with a very slight degree of trouble, answer the purpose extremely well, and are sold in Germany at a very cheap rate. Besides occurring singly, crystals are sometimes found in twins or in macles. In that case they are divided into two groups. 1st. Those in which the crystals are united in such a way that the axes of the two separate crystals, so united, are parallel to each other; and 2nd, those in which the axes are oblique or inclined to one another. In other instances minerals, instead of crystallizing in the forms which are properly their own, assume pseudomorphous forms; that is to say, forms belonging to some other kind of mineral. This may have happened in several ways. Either the original mineral may have been entirely removed and the newer one deposited in the cast (or the mould) of that which has disappeared, or the original mineral may have been gradually removed atom by atom, and for every particle so carried away portions of another mineral substituted. "Pseudomorphous crystals are distinguished, generally, by their rounded angles, dull surfaces, and often granular composition. They either have no cleavage, or the cleavage is wholly different in direction from that of the mineral imitated. Their surfaces are frequently drusy, or covered with minute crystals. Occasionally the resemblance to real crystals is so perfect, that they are distinguished with difficulty."* * Dana's Mineralogy, vol. i. p. 136. There are other physical characters which furnish extremely useful aids in the identification of minerals. The most important of them will, therefore, now be briefly noticed, nearly in the order in which they are alluded to in the pages of the Glossary, that is as follows: Colour. The colour of a mineral is not, in general, so much to be relied on as some of the other characters. Certain peculiarities in the arrangement of the colours are of importance, thus :— Play of Colours is said to take place when a mineral, on being turned, presents the appearance of several prismatic colours in rapid succession. Examples of this property are afforded by the Diamond, and în a less degree by the Precious Opal. A Change of Colours is of a somewhat similar nature to the play of colours, only the succession of colours is less rapid, and each particular one is spread over a larger surface. Labradorite furnishes a very good example of this. Iridescence is when the prismatic colours appear to be reflected from the interior of a crystal. Opalescence is when a milky or pearly reflection is displayed from the interior of a mineral, as is the case in some kinds of Opal and Cat's-Eye. Tarnish signifies that the colour of the mineral is different from that exhibited by a newly fractured surface. It is, consequently, merely superficial. When the surface of a mineral (as, for example, Columbite) displays the superficial blue colour of tempered steel, it is said to possess the steel-tarnish ; when, as in the Specular Iron Ore of Elba, it exhibits fixed prismatic colours, it is said to be irised. Diaphaneity, or Transparency. The following terms are made use of to express the different degrees in which minerals possess the capacity of transmitting light. 1. Transparent: when the object seen through it appears perfectly distinct, as in Quartz and Gypsum. 2. Subtransparent, or semitransparent: when the outlines of objects seen through it do not appear distinct. 3. Translucent: when only light is transmitted, and objects are not seen, as in Oriental Alabaster. 4. Subtranslucent: when light is only transmitted at the edges. 5. Opaque: when no light is transmittted. Lustre. The kinds of Lustre depending upon the nature of the reflecting surface are six in number, viz. : 1. Metallic, or the lustre of metals; Sub-metallic, denoting that the mineral only possesses the lustre imperfectly. In the determination of minerals it is very important to distinguish the metallic from the non-metallic lustre. 2. Vitreous, or the lustre of broken glass, of which the lustre of Rock Crystal is a good example; Calc Spar, on the other hand, presenting a subvitreous or imperfectly vitreous lustre. 3. Resinous, or the lustre of common rosin; of which Opal and some kinds of Blende are examples. 4. Pearly, or like the lustre of a pearl; as in Talc, Steatite, Brucite, &c. The term metallic-pearly is used to denote when the pearly and submetallic lustre are displayed in the same mineral, as in Hypersthene. 5. Silky, or like silk. It is generally the result of a fibrous structure, as is apparent in fibrous Gypsum and Satin Spar. 6. Adamantine, or like Diamond. When combined in the same mineral with sub-metallic it is called metallic-adamantine, of which Cerusite and Pyrargyrite are examples. The different degrees of intensity of lustre produced by a variation in the quantity of light reflected from the surface are four in number :-- (1.) Splendent: when the surface of the mineral reflects with sufficient brilliancy to give well-defined images, as is the case with Oxide of Tin and Specular Iron. : (2.) Shining when the image produced by reflection from the surface is not well defined, as in Celestine. (3.) Glistening: when the surface reflects the light, but without producing an image, as in Talc, Copper Pyrites, &c. (4.) Glimmering: when the reflection of the light is imperfect, and apparently proceeding from points on the surface, as in Flint, Chalcedony, &c. Optical and Physical Properties. The former of these belong, properly, to the science of Optics, and can be only alluded to here. The principal properties dependent on light, besides those already noticed, employed in the determination of minerals are Refraction, Polarization, and Dichroism. 1. Refraction. It is frequently of importance to know the index of refraction, or the ratio between the sine of the angle of incidence, and that of the angle of refraction; for although there is often some variation in the ratio in the same species (frequently corresponding to a change of colour), yet, as a general rule, each mineral refracting the light in an equal degree has its own index of refraction. Those minerals which refract light most powerfully, or in which the rays passing through them deviate the most from their straight path, afford the most brilliant gems. It is to its high refracting power (2.439 to 2.755) that the Diamond owes its brilliancy. Double Refraction.-Calc Spar and some other minerals present a double image of a point or line seen through them, in every position but one. This is called double-refraction, and a knowledge of whether a mineral possesses this property will enable the observer to refer it at once to its proper crystallographic system. All forms exhibit double refraction, except those belonging to the Cubical System, which have three axes equal to one another. In the Pyramidal and Hexagonal (or Rhombohedral) Systems, in which the |