Exploring Rocks: Cleavage

In most crystals, the forces that hold the atoms together are not equally strong in all directions, causing planes of weakness in the structure. Crystals split most easily along these weak planes, a property known as cleavage. Because minerals have unique structures, they cleave in distinctive ways.


In micas, the atoms are arranged in sheets. Planes of weakness parallel to the sheets cause the mica to cleave into paper-thin flakes. These crystals of mica have split along their cleavage and the gaps have been filled with quartz.


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Phlogopite schist, Hamilton County, New York
Crossed polarizers

Pyroxenes have two cleavage directions, almost at right angles to each other. The cleavage appears as a rectangular pattern in this thin section cut perpendicular to both these directions.


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Diopside Marble, Essex County, New York
Crossed polarizers

Amphiboles (green) also have two cleavage directions, about 60 degrees apart. The cleavage angle in some of these grains is less than 60 degrees because the thin section is not perpendicular to the cleavage planes. Amphiboles and pyroxenes can be easily confused, and the angle between the cleavage directions helps to identify them.


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Amphibolite, North-East Greenland
Plane Polarized Light

Look at Phengite to see another example of a single cleavage, and Diopside in Marble for two cleavages.

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