Mineral Cleavage
Cleavage describes the tendency of a mineral to break in preferred directions along bright, reflective planar surfaces. It is controlled by the internal bonding forces within the mineral structure. Not all mineral have cleavage!
Phlogopite
KMg3(Si3Al)O10(F,OH)2
Cleavage in 1 Direction
Size: 10 cm
Galena
PbS
Cleavage in 3 Directions at right angle to each other
Size: 6 cm
Calcite
CaCO3
Rhombohedral cleavage at 3 Direction cleavage not at right angle to each other
Size: 8 cm