CHEMICAL WEATHERING (CONTINUED)
Solution results in the removal of ions from a mineral, other types of chemical weathering result in replacement of some ions to produce new minerals. One of the two common ways this happens is called OXIDATION. In this type of weathering the anion in a mineral is replaced by a more reactive anion called an "oxidant". Oxygen itself is the most commonly substituted anion, though others can work under certain conditions. In effect a mineral is changed into an oxide by this process. Usually the anion being replaced is removed from the system by solution in water.
Most oxidation reactions do, in fact, occur faster in the presence of water than they do in its absence. In fact the water usually introduces the oxygen, which dissolves in it very much as CO2 is.
The reactions work best on minerals that contain cations from the middle part of the periodic table, where the ions can exist in multiple oxidation states (i.e., they can enter into bonds by giving or receiving different numbers of electrons depending upon exact conditions).
Iron, for example, exists in most silicate minerals in a reduced state -- it is a +2 cation. However, in a wet, oxygen-rich environment such as the Earth's surface iron prefers to bond as a +3 cation. This equation gives one possible example, but virtually any ferromagnesian mineral would behave more or less in the same way.
2FeSiO4 + 3O2 + H2O ---> 2Fe2O3 + 2SiO4-4 (dissolved silica) + H2O
(Olivine + oxygen dissolved in water alters to hematite + silica dissolved in water).
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