Cleavage vs. Fracture

When we say a mineral exhibits "fracture," not "cleavage," we are talking about how the mineral breaks. "Fracture" in this instance doesn´t mean a hairline crack, the way you might fracture a bone - it means that the mineral breaks, but not along preferred planes. It´s a more random way of breaking - since the bonds between all the atoms are roughly equal, there aren´t layers of weakness, so the mineral when broken will crack and separate more or less randomly. If a mineral has cleavage, on the other hand, we´re saying that there are planes, or flat surfaces, that are more weakly bound - so if the mineral breaks, it will tend to break along that weak plane. That will produce a nice flat surface rather than a rough jagged edge.