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3 <br />i <br />0 <br />THE CHEMICAL NATURE OF MINE DRAINAGES <br />THE WEATHERING OF ROCKS <br />Rocks exposed at the earth's surface are subjected to <br />alteration by the chemical, physical or biological processes. <br />This is called weathering, and chemical weathering is most <br />important here. This section on weathering processes is a <br />summary of what is found in Garrels and McKenzie (16) and <br />Krauskopf (17) . <br />The species in typical rain water and the dissolved <br />species in typical river water for North America are shown <br />in Table I (16). The constituents added between the rain <br />and the river must come from the earth's crust. They are <br />primarily derived from the weathering of the minerals in <br />rocks. The reactions of minerals with air and water are <br />certainly diverse; a study of the different types of igneous <br />rocks, sediments and soils will confirm this. However, some <br />useful generalizations can be made about weathering reactions. <br />The outline below categorizes the most important reactions <br />and provides useful examples (16): <br />A. Congruent Reactions - all the products are soluble in water <br />1. Simple Salts - simple dissolution <br />NaCl(s) -* Na+(aq) + C1-(aq) (1) <br />halite <br />Ca•S04.2HzO(s) 4- Ca2+(aq) + S04 (aq) + 2H2O (2) <br />gypsum <br />0