Tag Archives: crystallography

Geo-pic of the week: Cleavage

clip_image002Calcite at atomic resolution

The mineral in the above pictures is calcite, a common mineral in earth’s crust that is the main component of the sedimentary rock limestone.  The stack of samples (top) exhibit a physical characteristic known as cleavage.  The cleavage of calcite causes it to break into a rhombus-shape (see picture).

Cleavage is the tendency of a crystalline substance, such as a mineral, to break along parallel planes that reflect the internal arrangement of the atoms in the crystal.  All crystals, by definition, have a uniform atomic arrangement.  To illustrate this property, I’ve included a second picture (bottom), borrowed from Dr. Cathy Sutton, that shows an extremely magnified calcite crystal.  The repeating rhombus-shapes in the picture are individual calcite molecules.  Basically, cleavage is the outward expression of the internal structure of a mineral.

The samples on the left were collected from Midwest Lime Quarry, Batesville, Arkansas.