Page:The American Cyclopædia (1879) Volume XI.djvu/601

 MINERALOGY 583 MINERALOGY, the science which treats of the composition, structure, formation, and classi- fication of minerals. The term therefore cov- ers both descriptive mineralogy and mineral- ology, which is the study of the laws in accord- ance with which minerals are formed. All objects in nature consist of certain substances recognized as elementary bodies, which exist either as individual wholes, when they are called native elements, or combined with one another. All the native elements belong to the mineral kingdom, and also all combinations of elements which do not pass through the cy- cle of change called growth. The combina- tions of the elements which man produces all belong to the mineral kingdom, since he is not able to impart the principle of growth. When his products are homogeneous in composition and structure, they are, strictly speaking, arti- ficial minerals ; and chemists are able to repro- duce a great number of the combinations found in nature. The study of minerals presents three general classes of characteristics: chemical com- position, crystalline form, and physical proper- ties. I. CHEMISTRY OF MINERALS. In combi- ning, the elements exhibit a strict subjection to certain fixed modes of union, and these modes are the laws of chemical combination, which are still very imperfectly understood. Chem- ists recognize two kinds of units. Thje small- est possible particle of an elementary substance is called an atom. These atoms seem to exist in a state of polarity, and to possess electrical attraction and repulsion, by means of which they effect union with each other and with the atoms of other elements. They are not always able to exist by themselves, but the atoms of some elements act in pairs or triplets, or in some other degree of union. This combina- tion of atoms, whether composed of the atoms of more than one element, or of one only, is called a molecule. Molecules have the power of cohesion, and by their aggregation masses of matter are formed. Both of these units are used in mineralogy. Every true mineral is formed of innumerable molecules cohering to- gether, and each of these molecules is com- posed of one or more atoms of each element contained in the mineral, according to the pro- portion in which it is present. While there is unending diversity in the composition of min- erals, it is found that the elements always unite in some simple proportion or ratio. Three kinds of ratios are used in mineralogy. The percentage ratio is the one in which analy- ses are always published. It assumes the weight of each molecule to be 100, and ex- presses the proportionate quantity of each ele- ment in the molecule in parts of 100. Lime, for instance, contains 71 '43 per cent, of calci- um and 28'57 per cent, of oxygen. The atomic ratio is the ratio between the number of atoms of each element in the molecule, and is ob- tained by simply dividing the percentage ratio of each element by its atomic weight. When the symbol of a mineral is given, the atomic ratio may be ascertained by simple inspection of the symbol. In lime, the symbol of which is CaO, the molecule is composed of 1 calcium and 1 oxygen, and the atomic ratio is there- fore 1:1. In andalusite, which is composed of AlaO&Si, the atomic ratio of the aluminum, oxygen, and silicon is 2 : 5 : 1 ; while if the oxygen is divided between the aluminum and silicon, the compound will be considered as formed of two radicals, alumina and silica, and the atomic ratio of these will be 1 : 1, there being one of each. The third method of com- parison is the oxygen ratio; it consists in a comparison of the number of oxygen atoms contained in the different oxygen compounds present. In andalusite, for instance, the alu- mina has three oxygen atoms and the silica two. The O ratio is therefore 3 : 2. The ex- planations so far given relate to the old method of writing chemical symbols. The new chem- istry reaches the same results by a different mode of reasoning. Every binary compound consists of one positive and one negative ele- ment. Every ternary compound consists of one positive element, a second which is nega- tive to the first but positive to the third, and finally a third which is negative to both the others. The number of negative atoms in a binary compound is found to vary with the different elements, each element having the power to fix a certain number of atoms of a more negative element ; this power is called its atomicity or quantivalence. All of the sta- ble binary compounds of hydrogen are found to contain one atom of hydrogen and one of the other element, whatever it is; and hydro- gen is therefore taken as the standard. By comparing the other elements with it, it is found that 23 of them have a combining pow- er equal either to 1, 3, or 5 hydrogen atoms ; and these are therefore called univalent, triva- lent, or quinquivalent. These never form sta- ble saturated compounds with any even num- ber of negative univalent atoms, and they are therefore called perissads, from the Greek word for odd numbers. The remaining 40 ele- ments have a combining power which is 2, 4, or 6 times that of hydrogen, and they are therefore bivalent, quadrivalent, and sextiva- lent. These form the general class of artiads, and are never saturated when combined with an odd number of negative univalent atoms. The highest possible combining power of an element is called its atomicity, but this is not always the most common form of its occur- rence, which is often one of the lower de- grees ; this prevalent form is its quantiva- lence. The oxygen ratio, although it was used with the best results in mineralogy long before the new chemical theories were established, is nothing more than the expression of the rela- tive quanti valences of the different elements contained in a mineral. Oxygen is in all cases a negative element, and the number of its atoms which are combined with one atom of any other element, taken in connection with