Hexagonal close-packed crystal structure. Computer artwork of a hexagonal close-packed crystal lattice, a common arrangement of atoms in solids. This is a type of lattice in which each layer of atoms lies in the holes between atoms in the layer below it, and directly above the atoms in the next layer down. It is as tightly packed together as spheres can be, occupying 74% of the available space. It is an arrangement favoured by the atoms of several metals, including magnesium and zinc.