OF MINERALS A mineral-species, or simple mineral, is completely defined by the statement of its chemical composition and crystalline form. When we are dealing with a definite chemical compound the limi tation of species is easy enough ; thus corundum, cassiterite, ga lena, blende, etc. are quite sharply defined mineral species. But with isomorphous mixtures the division into species, or into sub species and varieties, must be to a certain extent arbitrary, there being no sharp lines of demarcation in many isomorphous groups of minerals. Thus in the minerals garnet and tourmaline the chemical composition varies indefinitely between wide limits, but no corresponding difference can be traced in the crystalline form or in the external characters save colour and specific gravity. Some authors have therefore questioned the advisability of sep arating minerals into species each with distinctive names, and they have attempted to devise chemical names for the different kinds of minerals. Owing, however, to the frequency of poly morphism and isomorphism amongst mineral substances such a system presents many practical difficulties. Thus the three modi fications of titanium dioxide are more simply and conveniently referred to as rutile, anatase and brookite, while to give a purely chemical designation to such a mineral as tourmaline would be quite impracticable.
The practice of giving distinct names to different kinds of min erals dates from very early times (e.g. diamond). The common termination ite (originally itis or ices) was adopted by the Greeks and Romans for the names of stones, the names themselves indi cating some character, constituent, or use of the stone, or the locality at which it was found. For example, haematite, because of the blood-red colour. The custom of naming minerals after per sons is of modern origin; e.g. prehnite, biotite, hailyne, zoisite. Unfortunately there is a lack in uniformity in the termination of mineral names, many long-established names being without the termination ite, e.g. beryl, blende, felspar, garnet, gypsum, quartz, zircon, etc. The termination ine is also often used, e.g. nepheline, olivine, serpentine, tourmaline, etc. ; and many others were intro duced by R. J. flatly without much reason, e.g., anatase, dioptase.
The number of known mineral species differs, of course, accord ing to different authors; roughly there may be said to be about a thousand. The total number of mineral names (apart from chemical names), many of them being applied to trivial varieties or given in error, amount to about 6000.
Minerals may be classified in different ways to suit different purposes, eq., according to their uses, modes of occurrence, system of crystallization, etc. The earlier systematic classifications, being based solely on external characters, were on natural history prin ciples and too artificial to be of any value. J. J. Berzelius, in 1815, was the first to propose a purely chemical system of classi fication; his primary divisions depended on the basic (electro positive) element and the subdivisions on the acid (electro negative) element. This method of classification, though still in
use for metallic ores, is now quite arbitrary. The systematic classifications in use at the present day are modifications in detail of the crystallo-chemical system of G. Rose (1852). Here there are four main divisions, viz. elements; sulphides, arsenides, etc.; halogen compounds; and oxygen compounds : the last, and largest, division is subdivided into oxides and according to the acid (car bonates, silicates, sulphates and chromates, phosphates and arsen ates, etc.) ; in each section isomorphous minerals are grouped to gether. The classifications adopted by different authors differ much in detail, especially in the large section of the silicates.
BIBLIOGRAPHY.-Elementary introductions to the study of minerals are: E. S. Dana, Minerals and how to study them (N.Y., 1895) L. J. Spencer, The World's Minerals (1911, N.Y. 1916). A larger work on popular lines is: R. Brauns, The Mineral Kingdom, Eng. trans. by L. J. Spencer (Stuttgart, 1908-12). Textbooks for students: H. A. Miers, Mineralogy, an Introduction to the Scientific Study of Minerals (1902) ; E. S. Dana, Textbook of Mineralogy (3rd ed., N.Y., 1922) ; and G. Tschermak, Lehrbuch der Mineralogie (9th ed., Vienna, 1923). The standard works of reference for descriptive mineralogy are: J. D. Dana, System of Mineralogy (6th ed., by E. S. Dana, N.Y., 1892, Appendices 1-3, 1899-1915) ; C. Hintze, Handbuch der Mineralogie (Leipzig, 1898, etc.).
For special branches of mineralogy reference may be made to the following: R. Brauns, Chemische Mineralogie (Leipzig, 1896) ; C. Doelter, Handbuch der Mineralchemie (4 vols., Dresden & Leipzig, 1911, etc.) ; H. Rosenbusch, Mikroskopische Physiographie der Miner alien and Gesteine, Band I., Die petrographisch wichtigen Mineralien, 5th ed., by 0. Miigge (Stuttgart, 1925-27) ; J. P. Iddings, Rock Minerals (2nd edit., N.Y., 1911) ; G. P. Merrill, Non-metallic Miner als, their Occurrence and Uses (2nd edit., N.Y., 1910) ; T. Crook, Eco nomic Mineralogy (1921) ; G. J. Brush, Manual of Determinative Mineralogy (16th ed., by S. L. Penfield, N.Y., 1907) ; M. Bauer, Edelsteinkunde (2nd ed., Leipzig, 5909), and Eng. trans. Precious Stones, by L. J. Spencer (1904).
There are many books on topographical mineralogy, for example: Greg and Lettsom, Manual of the Mineralogy of Great Britain and Ireland (1858).
The following scientific journals are devoted to mineralogy: Neues Jahrbuch fur Mineralogie, etc. (Stuttgart, since 1807) ; Tschermaks Mineralogische und petrographische Mitteilungen (Vienna, since 1872) ; The Mineralogical Magazine and Journal of the Mineralogical Society (London, since 1876, including since 1920 Mineralogical Abstracts); Zeitschrift f iir Krystallographie und Mineralogie (Leipzig, since 1877) ; Bulletin de la Societe francaise de Mineralogie (Paris, since 1878), American Mineralogist (Menasha, Wis., since 1916) ; Fortschritte der Mineralogie, Kristallographie und Petrographie (Berlin, since igi I).
(L. J. S.)