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Encyclopedia Britannica - Main :: MIC-MOL |
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MINERALOGY , the science which describes and classifies the different kinds of mineral
In this article we shall treat only of the general characters of minerals; their special
After a brief historical sketch the subject will be treated under the following headings: I. Characters of Minerals. 1. Morphological Characters. a. Crystalline Form. b. State of Aggregation: Structure. 2. Physical Characters. a. Optical Characters (Colour, &c.). b. Magnetic, Electrical and Thermal Characters. c. Characters depending on Cohesion (Hardness, &c.). d. Specific Gravity. e. Touch , Taste and Smell.Chemical Characters. 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 attention of mankind from the earliest times. The stone and bronze implements of prehistoric man and many of his personal ornaments and charms were directly or indirectly of mineral
late
It was not until the end of the 18th and beginning of the r9th century, when the foundations of crystallography were laid by Rome de l'Isle and R. J. flatly, and chemistry had assumed its 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 Scheele, Klaproth, Charles Hatchett Vauquelin
Berzelius
Brewster and others; of natural crystals. Later, the investigation of rocks in thin section under the microscope led to the exact determination; particularly by A. Des Cloizeaux (1867), of the optical constants of rock-forming minerals.For a detailed account of the history of mineralogy (including crystallography), see F. von Kobell, Geschichte der Mineralogie von 2650z 86o (Munchen, 1864). The recent
I.Characters of Minerals. A distinction is to be made between essential and non-essential characters. Essential characters are those 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 characterssuch 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 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 haematite (" specular iron ore "), with its smooth black faces and brilliant metallic lustre, is strikingly different in appearance from a piece of massive haematite (" red iron ore "), which is dull and earthy and bright red in colour; the former is so hard that it can only with difficulty be scratched with a knife, while the latter is quite soft and soils the fingers. Both specimens will, however, be found on analysis to have the same chemical composition (Fe2O3), the same crystalline structure (as determined by the optical characters under the microscope in the 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 minute crystalline individuals,, which have interfered with each other's growth to such an extent that no crystal-faces have been developed. Such differences do3. not therefore depend on the nature of the material, but only on the conditions which prevailed during its growth. (See e.g. End of Article: MINERALOGY If you wish, you can link directly to this article.
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