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6 <br />• <br />C. Oxidation - Reduction Reaction <br />1. Iron Silicates - dissolution to Fe(II) <br />Fe2SiO4 + 4COz + 2H20 -} Fe 2+ + 4HC03 + SiO2(s) (10) <br />Fe - olivine <br />• <br />0 <br />2. Iron Carbonate - dissolution to Fe(II) <br />FeCO3 + C02(g) + HzO Z Fe2+ + 2HCO3 (11) <br />siderite <br />In reactions 10 and 11 the iron is not oxidized; <br />the oxidation is shown in reaction 12. <br />3. Oxidation of Fe(II) to Ferric hydroxide <br />Fez+ + 202 (g) + H2O t FeO OH(s) + H+ (12) <br />goethite <br />4. Pyrite: Oxidation of sulfide to sulfate <br />FeS2 + 7/2 02(g) + H2O - Fe 2+ + 2SO4 + 2H+ (13) <br />pyrite <br />The above reactions provide an excellent summary of <br />what occurs as water falls as rain, percolates through the <br />soil and rocks, discharges to the rivers and then to the <br />seas. Some conclusions should be made about the above sum- <br />mary. The dissolution of carbonates as shown in reaction 4 <br />consumes hydrogen ions in acidic waters. The dissolution <br />of quartz as shown in reaction 6 consumes hydroxide ions in <br />basic waters. Since quartz and carbonates are ubiquitous <br />on the earth's crust, natural water is usually buffered <br />between a pH of 5 to 9. Dissolved constituents such as Fe2+, <br />Cat+, Na+, K+, HCO3 and S04 occur naturally in water and <br />are essential to human health.