Look at the atomic model of flourite and calcite and then study the cleaved fragments of these minerals under the binocular microscope. Note how for a given mineral, each fragment shows the same cleavage planes and angles. Its irregular pattern of breaking is called conchoidal fracture. This lack of cleavage is often very useful in identifying quartz. Feldspars are an important group of silicates and the most common minerals in the crust of the earth.
All feldspars have two cleavage directions.
It is often important to determine the relative amounts of the two feldspars in a rock. The cleavages of orthoclase and plagioclase are too similar to be useful in distinguishing between them. However, a crystal phenomenon called twinning helps greatly in distinguishing the feldspars. During their growth, plagioclase crystals often repeat, or twin, in a plane parallel to one of the cleavage planes. The twinning causes one set of the cleavage planes to be oriented at slight angles to each other.