What causes a lack of cleavage in some minerals? what causes a lack of cleavage in some minerals? the chemical composition of the mineral bonds of differing strength in certain directions bonds of equal strength in all directions the shape of the mineral a small impurity in the atomic structure?

The right answer is Bonds of equal strength in all directions.

Cleavage is a preferred fracture plane whose orientation is dictated by the crystalline structure. "It is, therefore, the property of certain minerals to break in directions predetermined by the atomic structure of the mineral ( = to cleave). According to the minerals, there may be one, two, or three directions of cleavage, called cleavage planes and the number of cleavage planes.