to dissolved carbon dioxide is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls. Hydrolysis and carbonation Hydrolysis (also called incongruent dissolution) is a form of chemical weathering in which only part of a mineral is taken into solution. The rest of the mineral is transformed into a new solid material, such as a clay mineral. For example, forsterite (magnesium olivine) is hydrolyzed into solid brucite and dissolved silicic acid: Mg2SiO4 + 4 H2O ⇌ 2 Mg(OH)2 + H4SiO4 forsterite + water ⇌ brucite + silicic acid Most hydrolysis during weathering of minerals is acid hydrolysis, in which protons (hydrogen ions), which are present in acidic water, attack chemical bonds in mineral crystals. The bonds between different cations and oxygen ions in minerals differ in strength, and the weakest will be attacked first. The result is that minerals in igneous rock weather in roughly the same order in which they were originally formed (Bowen's Reaction Series). Relative bond strength is shown in the following table: This table is only a rough guide to order of weathering. Some minerals, such as illite, are unusually stable, while silica is unusually unstable given the strength of the silicon-oxygen bond. Carbon dioxide that dissolves in water to form carbonic acid is the most important source of protons, but organic acids are also important natural sources of acidity. Acid hydrolysis from dissolved carbon dioxide is sometimes described as carbonation, and can result in weathering of the primary minerals to secondary carbonate minerals. For example, weathering of forsterite can produce magnesite instead of brucite via the reaction: Mg2SiO4 + 2 CO2 + 2 H2O ⇌ 2 MgCO3 + H4SiO4 forsterite + carbon dioxide + water ⇌ magnesite + silicic acid in solution Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate. This is an important reaction in controlling the amount of CO2 in the atmosphere and can affect climate. Aluminosilicates containing highly soluble cations, such as sodium or potassium ions, will release the cations as dissolved bicarbonates during acid