A rock's susceptibility to Chemical Weathering depends on the stability of the minerals in it. Rock is composed of minerals that are generally quite stable at the time of formation. When there is variation in the surrounding environment, some minerals tend to become unstable. Chemical reactions take place when the rock comes into contact primarily with water, resulting in formation of new stable compounds and secondary residual materials.
Following are the types of Chemical Weathering that occur in common.
This chemical process happens in case of silicate minerals reacting with water. Granite and sandstone commonly undergo hydrolysis. The H+ and OH- ions in water replace mineral ions (calcium, potassium etc.) to form clay and weak acids. Clay is a weak mineral which crumbles easily and hence withering of rock takes place.
This process involves attachment of the H+ or OH- ions to the crystal lattice structure of minerals. Hence it tends to increase the volume, thereby leading to softening or decomposition. Hematite and Bauxite are common minerals that are hydrated under humid conditions. Though the physical appearance of rock changes, it does not cause much chemical transformation to the substance.
Also known commonly as rusting, this process combines oxygen molecules with the mineral ions such as iron, calcium or magnesium. Oxygen is normally taken from the moisture in soil or air. Subsequently, it results in reduction of the mineral to a less stable compound. A typical example is conversion of iron (Fe) radicals into rust or iron oxide (Fe2O3) which is weaker.