What typically causes an ionic crystal to cleave or break relatively easily when scratched?

Answer

Electrostatic repulsion between similarly charged ions placed adjacent to each other

When a sharp object drags across an ionic crystal, shifting planes can place ions of the same charge next to each other, causing electrostatic repulsion that results in cleavage or breakage.

What typically causes an ionic crystal to cleave or break relatively easily when scratched?
mineralstructurecontrolhardnessbond