According to Brooklyn College, cleavage occurs when a rock breaks along smooth planes that are parallel to areas in which the mineral has weak bonds. By contrast, rocks are said to fracture if they break irregularly, because the mineral does not form planes of weakness.Continue Reading
Cleavage can occur in several different ways. Some minerals, such as gypsum and muscovite, only exhibit cleavage in one plane. According to Brooklyn College, some minerals, such as halite, cleave to produce cubic particles. This occurs because the rock cleaves in three separate directions and each of the directions is oriented 90 degrees away from the others. Calcite, by comparison, cleaves at angles of 120 degrees and 60 degrees, producing rhombohedral shapes. Quartz does not exhibit cleavage at all, as it fractures irregularly.
Brooklyn College explains that some minerals exhibit perfect cleavage, in which no rough surfaces are produced at all. Others only exhibit good cleavage. Such minerals may exhibit some irregularity in the cleavage, but the amount of roughness is minimal. Poor cleavage occurs when the amount of rough surfaces dominate the amount of smooth surface area. Indistinct cleavage occurs when a mineral cleaves, but the smooth faces are so small that they are not noticeable.Learn more about Geology