Besides the three kinds of bonds just discussed, some minerals include other types of bonds such as van der Waals bonds and hydrogen bonds. These bonds do not involve valence electrons but instead result from weak electrostatic forces due to uneven charge distribution in a crystal structure. Very weak van der Waals bonds are important in graphite and some clay minerals, for example. This explains why graphite is much softer than diamond, which has the same composition but covalent bonds. In the mineral brucite, combinations of hydrogen and van der Waals bonds hold sheets of MgO6 molecules together. Clay minerals have excellent cleavage because covalent and ionic bonds create strongly bonded layers, but weak van der Waals and hydrogen bonds hold the layers together. So, the minerals split easily into sheets. Because most mineral properties are explained by ionic, covalent, or metallic bonds and we can ignore van der Waals and hydrogen bonds for most purposes.