Mineral Cleavage Testing: A Practical Guide

What is Mineral Cleavage?

Cleavage is the tendency of minerals to break along specific directions, corresponding to atomic weaknesses in the crystal structure. These directions are known as cleavage planes.

Let me share a personal story from my student days. Imagine a professor entering a classroom with a rock in hand. That’s a familiar sight in a geology class. Suddenly, she drops the rock on the floor. Soon, the room reverberates with the sound of many more rocks crashing onto the floor. We’re all stunned. What is the professor doing? She then tells us to gather perfect cubes of halite from the floor. (Halite is the mineralogical term for salt). The topic for that day’s lecture: cleavage.

A mineral’s cleavage results from its crystal structure properties. Atomic bonds hold atoms together in the crystal structure. If atoms are located far from one another, the bond is weaker. Some minerals may have different types of bonds within one crystal. A cleavage plane is likely to occur between long, weak bonds.

Graphite Cleavage: A Familiar Example

Most people are likely familiar with graphite, a soft mineral made of carbon