MINERALOGY. 537 MINERALOGY. faces and the methods ol expressing them graphi- cally and symbolically. (See CRYSTALLOdUArHY.) (2) Phi/sical mineralorjii, which descril)es the physical characters of minerals and deals with the properties related to their molecular struc- ture. (3) Chemical miitri/ilogy, which has for its object the determination of the chemical com- position of each mineral si)ecics and the relation in composition between species in the same chemi- cal group. (4) Descriptire miiieralo(!if, which includes the detailed description of each mineral species with respect to its form, structure, physi- cal properties, chemical composition, and geo- graphical and geological occurrence. The <livision of physical mineralogj* is replete with interest- ing problems of cohesion, optics, heat, and elec- tricity, and suggests to the investigator along physical lines many fields for research. The problems connected with chemical mineralogy, while covering a narrower and less varied field than those of physical mineralogj'. are none the less replete with interest. To the chemist work- ing in the field of mineralogy' belongs the ta.sk of determining the part played by the various ele- ments which enter into the composition of the hundreds of mineral species, many of which are rare and exceedingly complex in composition: the phenomena of isomorphism and dimorphism, and the chemical alteration of mineral species under the action of natural agencies, which is known as pseudomorphism. Crystallography. With very few exceptions (mercury and water), minerals are limited to solid substances: that is, they are solid at the present temperature of the earth. In discussing their formation and character, we must, how- ever, revert to the period when the mineral con- stituents of the earth existed in a fiuid or semi- fluid state. When a homogeneous substance passes from a fluid to a solid condition, its par- ticles mutually attract each other along certain definite lines and a solid is built up which shows a definite structural relation between all its in- tegral parts, which relation finds expression in its outward form. Such a solid, formed from a nucleus by the piling up of accretions from with- out, is known as a crystal and is characterized by a regular polyhedral form, bounded by more or less smooth surfaces. A crystal is then the normal form of a mineral which has solidified under ideal conditions and. should its formation be uninterrupted by external agencies, its ap- pearance would be that of a symmetrical geomet- ric solid with smooth faces and sharp edges and angles. Such are the ideal representations, which serve to illustrate the crystallization of mineral species and which are to be found in all text-books on the subject. But inasmuch as the ideal conditions mentioned above are of compara- tively rare occurrence, it is far more ccmimon to find minerals in more or less distorted forms. (Sec Figs. 1 and 2.) Large and well-formed crys- tals are, in general, produced by a slow process of crystallization, whereas a rapid cnoling or concentration of a mineralizing solution tends to form aggregates often resembling the forms of animate nature; such are the frost patterns which form on window panes, the coral-like forms of calcium carbonate to be found in some caves, and many other imitative forms described in the terminology of mineralosr. Miere indi- vidual crystals are entirely lacking, the mineral is said to be massive, although its structure a.s determined by optical and other methods may be distinctly crystalline. Regarding the nature of the crystalline units of accretion, there is at present very little knowledge. They are without' doubt extremely minute and may possibly consist of a number of chemical molecules. Wliatever may be the size or sha|)e of the crystal units or crystal mole- cules, it is sufficient for tlie purpose of discussion to regard them as points. A fuller discussion of this subject will be found under (ue.misthy. The crystal molecules of any chemical substance crystallizing under given conditions are believed to be identical in size and shape. They are never in contact with each other, but are held in equi- librium by attractive and repellent forces acting along lines which difl'er for each type of crystal molecule. A crystal molecule having these lines of crystallizing force at right angles, as shown in Fig. 3, would attract like molecules, which woubl arrange themselves as shown in Fig. 4. The theoretical grouping of molecules lias been dis- cussed by Sohncke, Fedorow, Schijntlies, and Bar- low, who have developed 230 possible groupings. These, however, divide themselves into 32 dis- tinct groups identical with the 32 groups men- tioned under C'RYsrALLOGRAPiiY. If we assume the molecules of a substance to be grouped as shown in Fig. 5, it will be readily seen that the lines of minimum cohesion will be aa and bb rather than iiitii, because the former planes are further separated from the next adjacent parallel plane. This explains in a meas- ure the fact that crystallized substances often tend to break or cleave parallel to a primary crystallographic face. Assuming a crystal mole- cule of any given mineral to be held in equi- librium by forces acting in definite directions, it will be readily seen that the crystal built up from accretions of such molecules will, of neces- sity, present faces which are symmetrically dis- posed with respect to those lines of crystallizing force. Thus we have as a fundamental law of crystallization the principle that a mineral can only crystallize in forms whose symnaetry is referable to one of the 32 groups mentioned in the foregoing paragraph. This is known as the law of symmetry. The number of planes possible from the grouping together of crystal molecules of a substance is invariably greater than the number occurring on any given crystal : and modi- fying planes are common, often running to great complexity, and under unusual conditions pre- dominating over the commoner types. Hence we frequently find great variety of form in crystals of the same sul)stance, as is the case with the min- eral ealcite (q.v. ). It should, however, be noted that crystals of a mineral from a certain locality, which arc presumably formed under the same conditions, show a marked similarity of type and are readily distinguishable from those of the same mineral from a difTerent locality. This variation in type, which is known as crystal habit, is particularly noticeable in large and widely dis- tributed s|)ecies. Certain mineral species exhibit a tendency to join two crystals or two halves of the same crystal in such a manner that some crystallographic plane or axis is common to both. This juxtaposition, which is ordinarily distinguished by reentering angles, is known as lii-inni-nfl. See Fis;. 0. Tt will be readily seen from the above that an accurate knowledge of the occurring crystal forma
