Weathering Part 2: Chemical Weathering

Even though we may think of these types of acids, they are not the most abundant acid on the planet. In fact, the most abundant form of acid is usually found each time it rains. Carbonic acid (H₂CO₃) is the result of the combination of water with carbon dioxide. It is a very weak acid, so much so that we usually don't notice it. Despite being weak, it is the most abundant acid at the earth's surface. Because of this abundance it is the single most effective chemical weathering agent. As the rain falls through our atmosphere it picks up carbon dioxide. This turns the water slightly acidic. When the rainwater reaches the ground, it filters through the soil, picking up more CO₂ from decaying organic matter. By the time the rainwater reaches hard rocks, it is acidic and easily attacks the underlying bedrock.

Chemical weathering by acids can have two affects on a mineral. In a few cases, such as with the mineral calcite (CaCO₃), the acid reacts to the mineral, breaking the chemical bonds and resulting in a solution of calcium ions and bicarbonate ions. The once solid calcite mineral has been dissolved and is now in solution. At a later stage, something may cause the hydrogen ions to be removed from the solution, allowing for the precipitation of calcite. Caves (such as Luray Caverns in Virginia) are the best places to see examples of both the solubility of calcite, and the later precipitation.

The second effect on minerals is for the mineral to be changed into an entirely new mineral by the acid. This is what happens to feldspar, a common mineral found in igneous rocks. As the carbonic acid attacks the feldspar, the very structure of the mineral is altered, physically and chemically. What was once a mineral with a framework structure to the arrangement of its elements is changed into a clay mineral. The clay mineral is an aluminum silicate with a sheet-like structure to the arrangement of its elements. Clay minerals include kaolinite (Al₂Si₂O₅(OH)₄), talc (Mg₃Si₄O₁₀(OH)₂), and pyrophyllite (Al₂Si₄O₁₀(OH)₂).

Many of the other minerals that are associated with igneous rocks weather in the same way as feldspars do. When we study these minerals, we can see that they weather at rates that are proportional to their position in Bowen's reaction series. The only major rock-forming mineral that is

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