Chemical weathering includes all the changes which alter the chemical composition of materials at the surface. It is mainly the result of interactions between the reactive atmospheric gases (oxygen, water and carbon dioxide) and the minerals in the rocks. The primary chemical reactions that take place are: a) oxidation, b) solution and leaching, and c) hydrolysis and hydration. 

Oxidation

In oxidation, metallic elements in the rocks combine with oxygen (or oxygen and water) to form oxides and hydroxides. For example, iron, a common constituent of mafic igneous rocks will oxidize to form hematite or limonite. Although oxides account for only 5% of all weathering products, they are economically important because they are a primary source for metallic ores. Mixed in with the surface materials, they provide the colors for much of our scenery. Who could visualize the splendid panoramas of our western deserts without all the colors provided by those oxides? 

Solution and leaching

Solution and leaching both involve the action of water. In solution, materials are completely dissolved. Leaching is selective, and only the more soluble components of the original material is dissolved and removed. Both are the result of the action of either by water itself, or by carbonic acid, a mild acid formed when water combines with C02 from the air. Some minerals, such as calcite, are easily dissolved. Solution produces dissolved ions which, in turn, are carried off by water in the ground. Some 15% of all weathering products are dissolved materials. These ions contribute greatly to the chemical content of rivers and ground water. Depending on their presence or absence, they affect the fertility of soils and the health of people who drink the water. Ultimately, they find their way to the ocean, these are the salts that make the oceans salty. 

Hydrolysis and hydration

Most silicates weather by hydrolysis and hydration. In these processes, rather than acting as a simple solvent, water reacts with other chemicals present to form acids and bases and then further reacts with the minerals to form a major group of new minerals: clays. Clays make up about half of all the weathering products and are an extremely important constituent of all soils. 

Inert materials

In addition to the minerals formed during weathering, there are minerals in the original rocks which are relatively unaffected by the various chemical processes and simply survive unchanged. These relatively stable inert minerals, chiefly quartz, are a major part, some 30% of the weathering products. They are commonly sand sized. 

Weathering products

In sum, after weathering, we are left with   rock fragments, clays, sands, and oxides. Collectively, these loose weathering products that mantle the surface of continents are called regolith. The dissolved materials are also present in the soil waters to some extent, although most of them are carried off by surface and ground water which, in wet climates at least, continually infiltrates and seeps through the regolith, and continues to react with it chemically.