Page:The American Cyclopædia (1879) Volume XI.djvu/605

 MINERALOGY 58T ters, and also hardness and specific gravity, may be due partly or entirely to the state of chemical combination. It has been shown that the superior hardness and specific gravity of the epidote group of minerals, as compared with the scapolite group, may be explained by supposing that the molecule of the former is more condensed than that of the latter. Classification of Minerals. The explanations above given embody the leading principles upon which the numerous minerals found in nature are distinguished from each other and arranged in related groups. The unit in min- eralogy is the species. A mineral species must have a definite composition and individual characteristics of form, sufficient to establish its difference from all others. The mode of occurrence may be gaseous, fluid, or solid ; the nitrogen and oxygen of the atmosphere, water, and mercury are all native minerals, as well as the solid substances. But definiteness of composition is a necessary characteristic, and marks the difference between minerals and rocks. While the latter are composed of min- eral substances, the indefiniteness of their con- stitution prevents their classification and de- scription by the accurate methods known in mineralogy. Even in the latter science a cer- tain latitude in composition is necessarily al- lowed, as minerals are seldom perfectly pure. Elements foreign to those which properly com- pose the mineral are nearly always present, and when their amount is large in proportion to the whole, it may be a question whether a new species should be made. The tendency of the best authorities is to restrict the number of species as much as possible, and to describe the modifications, where the usual characteris- tics of the mineral are not much altered, as varieties. Thus under pyroxene Prof. Dana describes 21 varieties, and under amphibole 20. Tourmaline has already been cited as a case of extreme variation in chemical composition, and calcite in crystalline form, the variation in both cases being in remarkably well characterized species. In the fifth edition of Dana's "Min- eralogy " (1868), 838 species are described, and the number of varieties is probably two or three times as great. The classification of these spe- cies is based upon chemical composition ; com- pounds of one kind, as silicates or sulphides, being placed together and subdivided into groups having the same general symbol, or the same crystalline form, or some common physi- cal character. The arrangement according to composition will be understood by referring to the table of elements given above. Six general divisions are made : 1. Native elements, including any element in the pure state, and any compound of two elements in the same series and group. There are 20 elements known, forming 25 mineral species. Gold, silver, platinum, iridium, palladium, mercury, copper, lead, arsenic, antimony, bismuth, tel- lurium, sulphur, selenium, carbon, nitrogen, and oxygen are certainly found native ; while iron, zinc, and tin, though reported, are some- what doubtful, if meteoric iron is excluded as not having been subjected to terrestrial con- ditions. When elements from two or more groups are united in a mineral, we are brought to the study of compounds, which forms all the remaining part of mineralogy, including five divisions. 2. All compounds in which the negative element is taken from the arsenic or sulphur group (series II. in the table). This di- vision therefore includes phosphides, arsenides, antimonides, bismuthides, sulphides, selenides, tellurides, and double compounds, as sulph-an- timonides, sulpho-bismuthides, &c. ; in all, 110 species. 3. Compounds in which the negative element is taken from group A, series III., and therefore this division comprises all chlorides, bromides, and iodides, numbering 23 species. 4. Compounds containing the negative element of group B, series III., or fluorides, 13 in num- ber. 5. Oxygen compounds, the negative ele- ment being taken from group C, series III. This division exceeds all others in the number of its species (587) and in the abundance of its minerals, which form probably more than nine tenths of the globe. 6. Those compounds of hydrogen and carbon which are called "or- ganic," of which 73 species are recognized. In addition to the above, more than 100 new species have been reported since 1868, and though some of these may not be sustained, the interest taken in mineralogy as a speculative science is rapidly extending our knowledge of minerals and the discovery of new species. A general classification of species having been made according to chemical composition, as above explained, groups are formed, each of which contains minerals of one type. A min- eral type includes species which closely resem- ble each other in crystalline form, and have a related elementary composition. Thus eight similar compounds of protoxides and deutoxides are found to crystallize in the isometric system, and are all of the " spinel type." Crystallized minerals containing ferric anhydride assume either inclined or hemihedral forms, and there- fore constitute a well marked type. Amor- phous minerals are necessarily classed with those crystalline species which they resemble in composition, as their lack of definite form is looked upon not as a characteristic, but as the lack of one. This mode of ranking them does no violence to the theory held by some that they are formed from matter in the colloidal state. No uniform system of comparison has yet been discovered which will suit the re- quirements of all classes of minerals. Each element appears to have a definite form, which it tends to assume under all circumstances ; and if the strength of this tendency varies with each one, the form of any given species will either be that of some dominant element, or a compound one resulting from the inter- action of all the substances contained in it. But nothing is known of such a scale of crys- tallographic forces except that, in the some-