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MINERALOGY
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MINERALOGY , the See also:science which describes and classifies the different kinds of See also:mineral See also:matter constituting the material of the See also:earth's crust and of those extra-terrestrial bodies called meteorites. The study of minerals is thus a See also:branch of natural See also:history, but one in which certain of the exact sciences find an application. The determination of the See also:composition and constitution of minerals is a chemical problem; their See also:optical and other See also:physical properties are determined according to the principles of physics; the study of their crystalline See also:form and structure belongs to See also:crystallography; their modes of occurrence, origins, associations and changes come within the See also:province of See also:geology and See also:petrology; while a See also:consideration of the localities at which they are found requires some acquaintance with See also:geography. Finally, there is the economic See also:side, dealing with the See also:mining and application of useful minerals, the extraction of metals from their ores, and • the uses of minerals for See also:building, decoration and See also:jewelry. In this See also:article we shall treat only of the See also:general characters of minerals; their See also:special characters will be found in the articles on the individual minerals. After a brief See also:historical See also:sketch the subject will be treated under the following headings: I. Characters of Minerals. 1. Morphological Characters. a. Crystalline Form. b. See also:State of See also:Aggregation: Structure. 2. Physical Characters. a. Optical Characters (See also:Colour, &c.). b. Magnetic, See also:Electrical and Thermal Characters. c. Characters depending on Cohesion (Hardness, &c.). d. Specific Gravity. e. See also:Touch, See also:Taste and See also:Smell.
Chemical Characters. See also:Synthesis of Minerals.
II. Occurrence and Origin of Minerals.
Alteration of Minerals: Pseudomorphs.
History.—Owing to their numerous applications for useful and decorative purposes, minerals have attracted the See also:attention of mankind from the earliest times. The See also: Some of the Arabian philosophers devoted themselves to the study of minerals, and about 1262 Albertus See also:Magnus wrote his De mineralibus. In the 16th See also:century Georgius See also: Cronstedt (1758)-the external characters of minerals formed the basis of the mixed systems of See also:classification of A. G. See also:Werner (1774) and of other authors, and even as See also:late as the Natural History See also:System of Mineralogy of F. See also:Mohs (1820).
It was not until the end of the 18th and beginning of the r9th century, when the See also:foundations of crystallography were laid by See also:Rome de l'Isle and R. J. flatly, and See also:chemistry had assumed its See also:modern phase, that any real advance was made in scientific mineralogy. It was then recognized that chemical composition and crystalline form were characters of the first importance, and that external (natural history) characters were often more or less accidental. During this period numerous mineral sub-stances were analysed by See also:Scheele, See also:Klaproth, See also: See also:Mitscherlich in 1819 and 1821 respectively cleared up many difficulties encountered in the See also:definition of mineral-species. About the same See also:time also the See also:discovery by E. L. See also:Malus of the polarization of See also:light gave an impetus to the optical examination, by See also:Sir See also:David See also:Brewster and others; of natural crystals. Later, the investigation of rocks in thin See also:section under the See also:microscope led to the exact determination; particularly by A. See also:Des Cloizeaux (1867), of the optical constants of See also:rock-forming minerals. For a detailed See also:account of the history of mineralogy (including crystallography), see F. von See also:Kobell, Geschichte der Mineralogie von 2650–z 86o (Munchen, 1864). The See also:recent history of mineral-specien may be well traced in the six See also:editions of J. D. See also:Dana's System of Mineralogy (1837–1892). I.—Characters of Minerals. A distinction is to be made between essential and non-essential characters. Essential characters are those See also:relating to chemical composition, crystalline form, crystallo-physical properties and specific gravity; these are identical, or vary only within certain defined limits, in all specimens of the same mineral-species. Non-essential characters—such as colour, lustre, hardness, form and structure of aggregates--depend largely on the presence of impurities, or on the state of aggregation of imperfectly formed crystalline individuals. In an absolutely pure,* and perfectly See also:developed crystal all the characters may be said to be essential, but such crystals are of exceptional occurrence in nature, and certain of the characters are subject to modification under different conditions of growth. For example: a well-formed crystal of See also:haematite (" specular See also:iron ore "), with its smooth See also:black faces and brilliant metallic lustre, is strikingly different in See also:appearance from a piece of massive haematite (" red iron ore "), which is dull and earthy and See also:bright red in colour; the former is so hard that it can only with difficulty be scratched with a See also:knife, while the latter is quite soft and soils the fingers. Both specimens will, however, be found on See also:analysis to have the same chemical composition (Fe2O3), the same crystalline structure (as determined by the optical characters under the microscope in the See also:case of the massive variety), and very nearly the same specific gravity (especially if this be determined upon finely powdered material, the effect of cavities being thus eliminated). The essential characters being identical, the difference between the two specimens lies in the state of aggregation of the material: with " specular iron ore " we have a single crystal, while with the " red iron ore " we are dealing with a confused aggregate of See also:minute crystalline individuals,, which have interfered with each other's growth to such an extent that no crystal-faces have been developed. Such See also:differences do 3. not therefore depend on the nature of the material, but only on the conditions which prevailed during its growth. (See e.g. Additional information and CommentsThere are no comments yet for this article.
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