Chemical weathering of igneous minerals leads to the formation of secondary minerals, which constitute the weathering products of the parent minerals. Secondary weathering minerals of igneous rocks can be classified mainly as iron oxides, salts, and phyllosilicates. The chemistry of the secondary minerals is controlled in part by the chemistry of the parent rock. Mafic rocks tends to contain higher proportions of magnesium and ferric and ferrous iron, which can lead to secondary minerals high in abundance of these cations, including serpentine, Al-, Mg- and Ca-rich clays, and iron oxides such as hematite. Felsic rocks tends to have relatively higher proportions of potassium and sodium, which can lead to secondary minerals rich in these ions, including Al-, Na- and K-rich clays such as kaolinite, montmorillonite and illite.