As they break, some crystalline minerals always split along a smooth,
flat surface. This property is known as cleavage. Some cleavages are
smooth and perfect; others are not so perfect. The cleavage surfaces,
because of the mineral’s crystalline internal structure, are parallel to
possible crystal faces, even though the mineral itself may occur as a
crystalline mass without a perfect crystal shape.
Some minerals will cleave in only one direction; some, in several
directions. For example, galena, a mineral found in Texas, has perfect
_cubic_ cleavage. It cleaves in three directions that are at right
angles to each other. These cleavage directions are parallel to possible
cubic crystal faces, and some of the cleavage fragments are cubes.