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which, under surface influences, give rise to intricate chemical changes. In discussing the subject of the superficial alteration of ore deposits, therefore, we treat a similar, but much less understood, subject than the superficial alteration of rocks.
Technical names of alteration products.—The altered surface outcrop of ore deposits is known by various names in different regions. Among the Cornish miners of England it is known as gossan, a name which has also been adopted into American mining nomenclature, though other special names are given in special classes of deposits. In France it is known as chapeau de fer; in Germany as eisener hut; among the Spanish Americans as pacos or colorados. As almost all deposits contain more or less iron minerals, the outcrops are usually stained brown from their oxidation, and hence the reference to iron in the French and German names. Sometimes, however, the outcrops are stained black by the oxidation of manganese carbonate or silicate, or green by copper minerals, or other colors by the formation of other compounds.
Agents of alteration.—The superficial alteration of ore deposits, as of any rock, results from a combination of mechanical and chemical disintegration, brought about by the combined action of the atmosphere, surface waters, changes in temperature, and the various organic and inorganic materials contained in the air and water. In nature, we never deal with perfectly pure water, but different waters contain different ingredients derived from the air and from the different materials with which they come in contact. Among the most important of these ingredients are oxygen, numerous organic acids like carbonic, oxalic, malic, citric, formic, propionic, butyric, acetic acids, etc., certain inorganic acids, such as sulphuric, nitric, hydrochloric, hydrobromic, etc., etc. Some of the acids mentioned occasionally occur in the free state, but most of them are generally combined with some of the bases present, such as the alkalies, lime, magnesia, iron, alumina, etc. These various ingredients, of course, are not all contained in the same waters, but are found in various associations in different waters. The organic acids mentioned represent