Minerals are the raw material from which all the metals and all the chemical salts except the organic compounds are made. Not all minerals containing a desired element are util ized, and generally only one or two materials are obtained directly from a mineral. These products are themselves used for the manu facture of others, as for instance the mineral halite or common salt is the indirect source of nearly all of the sodium salts, hut is the direct source principally of a crude sodium sulphate from which a multitude of other salts are manufactured.
6. Descriptive Mineralogy.— It may be said that it is the province of descriptive min eralou to sum up all the results of the study of minerals, as already outlined, into orderly form for each mineral species and to so classify the different species that related minerals shall be grouped together.
The basis of classification may be scientific, or economic or genetic; each for certain pur poses being the most satisfactory. Classifica tions until the 18th century were based on dis tinctions of structure, color, use, or some fancied similarity, and as has been said were °chiefly designed to enable amateurs to arrange their collections in a fixed Pliny the natu ralist (23-79 A.D.) classified as metals earths, stones and gems. Avicenna nearly 1,000 years later used a very similar classification with many subdivisions based on either external characteristics or easily ascertained properties.
Scientific classifications based on essential characters began as the increased chemical knowledge brought composition and chemical tests to the front. This was instanced in the systems of Wallerius in 1747 and Werner in 1798. The still later realization that most min erals possessed a characteristic molecular struc ture revealed by its crystals and physical char acters followed naturally the discoveries of de l'Isle, and others of the laws govern ing crystals; and the methods of examining crystals placed crystalline structure alongside chemical composition as the bases of natural scientific classification. The system of James
D. Dana is probably most used throughout the world and °follows first the chemical composi tion and second the crystallographic and other physical characters which indicate more or less clearly the relations of individual species?' Eight principal divisions are made from a chemical standpoint as follows: In subdividing, the chemical composition and crystalline form are considered with the pur pose of assembling in groups those minerals which have analogous compositions and closely similar forms. For instance the barite group under anhydrous sulphates consists of sulphates in which Ba, Ca, Sr and Zn are in the same Mendeleeff group, and which show close sim ilarity in crystal constants.
S b c' Barite BaSO. 0.13152 1 1.3136 Celestite SrSO. 0.7790 1 I . 2801 Angleaite PbSO4 0.7852 1 1.2894 Anhydrite CaSO4 O. 8933 1 1.0008 Zinkosite ZnSO4 0.8923 1 1.4137 Hydrocyasite CuSO4 0.7971 1 1.1300 Descriptive mineralogy also serves to keep in order the nomenclature. Uniformity can only be obtained if after careful consideration the term entitled to priority and otherwise satisfactory is made the name of the species and the host of synonyms and often unessential variety names assembled under it.
7. Determinative Mineralogy.— This sub ject has already been discussed in a separate article. See DETERMINATIVE MINERALOGY.
The following is a partial list of comparatively recent works and may be supplemented by the lists following the articles on Crystals and Crystallography.