Page:The American Cyclopædia (1879) Volume XI.djvu/604

 586 MINERALOGY protoxides, like water and zincic oxide, take the hexagonal instead of the isometric form, this fact leads mineralogists to look upon these minerals as being composed of condensed mole- cules containing three atoms of each element, and to write their symbols, H 8 3 and Zu 3 O s , rather than reject the theory. This theory does not account for all the examples of poly- morphism, nor can they be accounted for with- out greater knowledge of the crystalline sys- tems. Certain forms in some of the systems, when placed in a particular position, are iden- tical, both in position and angle of the faces, with others in entirely different systems. Nevertheless, none of the efforts to reduce the crystallographic systems below six have been successf ul. It is, however, established that min- erals may be isomorphous with others crystal- lizing in a different system, when their angles are nearly similar. The variations in the six systems depend upon the relative length of their axes ; and when the axial dimensions of two minerals in different systems are nearly the same, they may enter into chemical or physical combination without violence to their individual laws of formation. The likelihood of such replacement is increased by the fact that the crystallographic forms of minerals, though precise in general, are not perfectly uniform in angle. Even the most important and distinctive angular measurements vary de- cidedly, and since a certain flexibility thus ex- ists, the entrance of a different though similar mineral may take place without altering the angles beyond their ordinary limits. The ex- treme variation of axial dimensions which may take place is shown by the common and very well marked mineral calcite, the forms of which include 48 different rhombohedrons and 88 scalenohedrons, besides pyramids and prisms. If the extreme positive rhombohedrons were represented graphically on the same scale, one would be 112 times as long as the other. Yet these extremes are so intimately connected by gradually progressive steps as to forbid any classification of them. Many theories have been proposed to account for the exact forms assumed by minerals, but two of them will be sufficient to indicate the tendency of spec- ulation. One is chemical. It supposes that the elementary atoms and molecules have defi- nite forms, and that when two elements com- bine, their molecules take a form which is dependent on the forces that produce the combination. The introduction of a third ele- ment may produce a complete rearrangement of the molecules and an entirely new form. The other theory is based on physical laws. It has been suggested that minerals crystal- lizing in the isometric system may be com- posed of spherical molecules, that being the form which any body free to move must take when acted on equally in all directions. Min- erals crystallizing in the other systems are made up of ellipsoidal molecules, and the form is tetragonal or rhombic, according as the lateral axes are conjugate axes or conjugate diame- ters of an ellipsoid. These axes and diame- ters are equal in all the systems except the triclinic, where they are unequal, and the ver- tical axis is at right angles to the other two in all but the monoclinic and triclinic systems. The hexagonal form may be produced by an ellipsoidal molecule in which three conjugate diameters form the axes on which the faces are laid. These axes are called crystallogenic, to distinguish them from the ordinary crystallo- graphic axes, which are entirely distinct. Mol- ecules are supposed to be governed by the laws of polarity, the opposite ends of the conjugate axes or diameters representing the north and south poles. By grouping them according to the known electrical laws, many of the remark- able compound forms can be imitated, and an interesting insight gained into the probable constitution of minerals. Local circumstances will sometimes alter the intensity of attraction between the molecules in favor of some one of the crystallogenic axes, and a distorted form will result. A more general modification of molecular relations produces secondary planes. What these local circumstances are is not known, but the character of the mother liquor, or of the solid matrix in which the mineral is formed, is certainly one of them. Laboratory experiments prove this, and in nature we find aragonite assuming different modifications ac- cording as it is found in iron mines or in gyp- sum clays ; minerals collected from one locality often present a general likeness, and may differ from the same species found in another region. Since a crystal increases by successive addi- tions to a minute molecular nucleus, any vari- ations in the intensity of the uniting force must produce alternate zones of strong and weak attraction. These pulsations of the formative force are the cause of cleavage, which is due to the lessened tenacity of the mineral along those lines which represent the period of weak action during the pulsation. III. PHYSICAL CHARACTERISTICS. Fracture, taste, odor, po- larization, electrical properties, and transpa- rency are among the least decisive peculiarities of minerals. Streak is a very important char- acter in all classes. Lustre is of great impor- tance in distinguishing the two kinds, metallic and non-metallic minerals. The value of the other physical characters depends upon the kind of mineral under examination. Among those possessing metallic lustre, the hardness, specific gravity, color, and state of aggregation are far more serviceable than with those of non-metallic lustre. The origin of physical properties is unknown, but it is certain that some of them, as transparency, polarization, and refraction, depend upon the relations of the molecules toward light ; lustre, color, and streak may have a similar origin, varied by the operation of the forces which formed the min- eral. To these forces, tenacity, ductility, and state of aggregation may also probably be as- cribed. Some of the above mentioned charac-