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Dissolution
Evaporites, such as halite or sylvite, are minerals formed by the evaporation of mineral-rich water. These minerals can be easily decomposed simply by being dissolved when in contact with water.
Oxidation
The oxidation of reduced materials can also cause decomposition. For example pyrite, FeS2, on exposure to water and oxygen will decompose to give iron(III)hydroxide and sulphuric acid. Depending on pH values the iron(III) may remain in solution or precipitate out as another type of mineral, for example, goethite.
Acid Hydrolysis
Rain water and much surface water is mildly acidic, mainly due to the dissolution of CO2 in the water to give carbonic acid. This acidity can decompose minerals; for example, the reaction of calcium carbonate and carbonic acid gives the soluble species calcium ions and bicarbonate.
Weathering of complex minerals
The examples given above are of the complete dissolution of simple minerals, but in practice minerals can be more complex and rocks can be composed of a range of minerals. This complicates the decomposition process, perhaps giving rise to different minerals decomposing at different rates, and the products of decomposition precipitating out of solution to form new solid minerals.