What causes the characteristic crystal shape and cleavage (breaking along flat surfaces) of the mineral halite?
Its cubic lattice structure.
Halite is an ionic compound with the formula
The sodium and chloride ions are bonded together in a lattice structure. Each ion is bonded to six others of the opposite charge, so each sodium is bonded to six chloride ions and vice versa.
This arrangement is very stable and happens to give the lattice a cubic structure. That means that if you cleave a piece of halite, it fractures along smooth planes that are at right angles to each other. We call this cubic cleavage.
Halite isn't the only mineral with this distinctive cleavage. Other examples include pyrite (