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16 <br />• SUMMARY OF WEATHERING <br />The dissolution of minerals to produce waters of high <br />Fe(II), S04 , and H+ contents and the recovery of the water <br />to natural conditions are probably as follows: <br />A. In a rock with relatively low amounts of carbonate <br />minerals and relatively high amounts of pyrite, the <br />pyrite somehow dissolves, producing regions in which <br />the water is below pH 3.5 and contains Fe(II). <br />Oxygen has to be supplied for the dissolution. <br />B. Bacteria begin to thrive in an environment caused by <br />Step A and catalyze the reaction of 02 with FeS2 <br />producing more dissolved Fe(II) and S04 and maintain- <br />ing the pH between 2 and 3. Oxygen has to be supplied <br />for this Step B and the pH has to be within 2 to 3 <br />for effective catalysis by microorganisms. <br />C. If other base metal sulfides are present, their dis- <br />solutions are accelerated by the joint action of the <br />bacteria, H+ ions, S04 ions, and the iron species. <br />D. When the water starts flowing above ground, the <br />action of bacteria and 02 oxidize Fe(II) to Fe(OH)3 <br />and the pH is raised above 3.5. Most of the Fe is <br />precipitated as well as Ag and Pb. Other base metals <br />and S04 persist dissolved in the water. The pH of <br />the water rises to 5-6 at this point. <br />E. Interaction with C02 and carbonate minerals raises <br />. the pH of the water to 7-8 and the other base metals <br />are removed from the water. Sulfate remains dissolved