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MINERALOGY is the science which treats of minerals and especially of the prop erties of these minerals, their chemical be havior and composition, their crystalline form and structure, their physical characters, their classification and their determination. Mineral ogy also considers the part each mineral plays in Nature, its history, its formation and altera tions, its variations under different conditions and its relation to other minerals. From a prac tical standpoint it records the uses of each mineral and the localities in which it has been found.

Mineralogy has many connections with other sciences, especially with crystallography and geology, for minerals are crystals for the most part and must be studied as such, and rocks are only aggregates of minerals and are identi fied by study of the component minerals. Physics, chemistry and mathematics are funda mental in the study of minerals and minerals are the raw material of the chemist and used by the physicist for the establishment of phys ical laws. Finally the arts of mining and metal lurgy are concerned, the one with the extrac tion of minerals from the earth, the other with the extraction of metals from minerals.

Minerals are those substances of definite chemical composition which are found ready made in the crust of the earth and are not directly products of the life or decay of an organism. Usually also they will exhibit definite and characteristic molecular (crystalline) struc tures.

Although minerals constitute the larger por tion by far of the known so-called mineral kingdom, the definition excludes certain por tions. Lack of homogeneity excludes asphalt and petroleum; lack of definite chemical com position excludes the natural glasses; ice made in the factory and ruby made in the furnace are not minerals. Coral and pearl are direct products of organic life, therefore not min erals.

The fundamental requirement of definite chemical composition is sometimes apparently waived because other facts, especially proof of definite crystalline structure, have been obtained, although a satisfactory formula has not. For instance, it is not yet possible to state an un objectionable formula for tourmaline, and the formulas of the great triclinic feldspars were long a source of confusion. The reasons are manifold, faulty analysis, impurities, replac ments of one element or group by another, and sometimes, as in the so-called colloid minerals, the originally formed material has taken out other things from solutions which in the dried out mineral remain inextricably admixed.

The characteristic crystalline structure is so frequent that minerals are sometimes defined as natural crystals. But it is now recognized that crystallinity is dependent on conditions during or preceding solidification, and that, theoreti cally, like any other chemical substance a min eral may, under different conditions, form in the crystalline state or the amorphous state.

Not only is this theoretically true, but there are many so-called °gel" minerals which are known only in the amorphous state and other minerals which are known both in the amorphous and the crystalline state.

Historically, mineralogy as a science dates from the 18th century only. While the ancients utilized a very considerable number of minerals, some for the metals they contained, others as pigments, others as ornaments, charms and talismans and still others in medicine and the arts, they knew little as to their composition and nothing as to their molecular structure. They classified them, it is true, for there is still extant part of a work, On Stones,) written by Theophrastus, who died 286 D.C., while Pliny in his great work on natural history, published 77 A.D., devotes five books to °earths, metals, stones and gems." The greatest contribution to mineralogical knowledge prior to the 18th century was made by Georg Agricola (1494-1555), professor of chemistry at Chemnitz, Saxony, who minutely discussed the known important ores, their min ing, concenf ration and metallurgy; and may be said to have summed up and systematized the knowledge of minerals at that period.

As chemical knowledge increased the com positions of minerals were gradually determined and chemical tests began to replace arbitrary distinctions of structure, color and the like. This is clearly shown in the works of Wallerius, 1747, and Werner, 1798; and from this time the composition became the dominating character.

The existence of a characteristic crystalline structure for most minerals was very gradually recognized and, until the publication of the works of Rome de l'Isle in 1783 and Rene Just Hairy (who °raised mineralogy to the rank of a science"), played no part in the study of min erals. From this period the fundamental im portance of the crystalline structure and the part it plays in the interpretation of both phys ical and chemical phenomena have been every where recognized.

Mineralogy may conveniently be considered under the following headings : (1) Crystallog raphy; (2) Physical Mineralogy; (3) Chemi cal Mineralogy; (4) Formation and Occur rence; (5) Uses; (6) Descriptive Mineralogy ; (7) Determinative Mineralogy.

1. Crystallography, al though a distinct science, has developed with mineralogy and is so interwoven with it that the two sciences are usually taught and studied by the same specialists. Crystallography is dis cussed separately in the articles CRYSTALS;