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7 <br /> <br /> Many of the reactions are equilibrium reactions which <br /> are capable of being easily reversed. For example, if the <br /> concentration of silicic acid in water becomes too high, <br /> reaction S will go to the left causing the deposition of <br /> quartz. For the case of Fe and Al, these ions are tied up <br /> as solid residues in clays and oxides and their concentra- <br /> tions in natural waters are ultimately quite low. Examples <br /> of this are reactions 9 and 12. This is why there are only <br /> minor amounts of Fe and Al dissolved in rivers as shown in <br /> Table I. Most problems with natural waters arise when water <br /> weathers formations which contain a unique mix of minerals <br /> and this water is needed before the ultimate weathering <br /> reactions have occurred. <br />• <br />THE WEATHERING OF ORE MINERALS <br />The Primary Reactions <br />In the summary of weathering reactions listed above, <br />iron minerals are listed separately. The primary reason for <br />this is because iron exists as Fe(II) in many igneous minerals, <br />but the ultimate weathering product is iron (III) hydroxide. <br />The weathering takes place in two steps. First the release <br />of Fe(II) occurs as shown in reactions 10 and 11, then the <br />Fe(II) is oxidized to iron (III) hydroxide, as shown in <br />reaction 12. The hydroxide product can occur in forms other <br />than the goethite shown in reaction 12 and accounts for the <br />different orange-red and yellow deposits seen around mine <br />• adits. A key feature of reaction 12 which distinguishes it