If a crystal is struck with a hammer, it will probably shatter into many pieces. Some minerals, such as calcite, break into well-defined blocky shapes with flat surfaces. These are called cleavage fragments (Figure 10b) and the flat surfaces are called cleavage planes (Figure 10a). Note that cleavage planes, which occur within a crystal, are not the same as crystal faces. Cleavage arises when the crystal structure contains repeated parallel planes of weakness (due to weak chemical bonds), along which the crystal will preferentially break.
The mineral mica (which includes biotite and muscovite) has such perfect cleavage in one direction that it can be readily split, or cleaved, into wafer-thin sheets, as shown in Video 2.
Some minerals break into irregular fragments that lack flat surfaces (except for any remnants of original crystal faces). In the case of quartz (Figure 11), which has no cleavage, the broken pieces have a curved pattern and this is called a conchoidal fracture ('conchoidal' is pronounced 'con-koi-dal').
Some minerals have only one set of cleavage planes, others have two sets, and a few (such as calcite) have three. In minerals with only one or two sets of cleavage planes, some broken surfaces will show just fracture.