3.4 Mineral Hardness, Cleavage, and Fracture

Cleavage and Fracture

What does a mineral look like when it is broken? Cleavage and fracture describe the appearance of a mineral when a crystal is broken from an external force such as physical weathering or when you strike it with a hammer. Cleavage is the tendency for crystals to break along planar surfaces that are parallel to atomic planes. Nonetheless, some minerals do not break along smooth planes at all! Such a tendency for a mineral to break unevenly is called fracture. Take a look at the two images below. Can you tell the difference between Cleavage and Fracture?

Conchoidal fracture demonstrated by quartz.
Figure 3.4.1 Conchoidal fracture demonstrated by quartz.
Cubic halite showing three directions of smooth planes of cleavage.
Figure 3.4.2 Cubic halite shows three directions of smooth planes of cleavage. Image by Rob Lavinsky, Wikimedia Commons, CC BY-SA 3.0

Let’s investigate these properties in a little more detail! Watch the video below for a summary of how mineralogists generally classify cleavage.

In summary, the number of non-parallel cleavage planes that can be observed is an identifying characteristic of the mineral. It is typical to observe the following types of cleavage as shown in the table below. [1]

CLEAVAGE TYPE EXAMPLE

Basal: 1 direction of cleavage
Muscovite demonstrating basal cleavage.
Figure 3.4.3 Muscovite demonstrating basal cleavage.
2 directions of cleavage, perpendicular
Orthoclase feldspar demonstrating 2-directional cleavage at 90 angles from one another.
Figure 3.4.4 Orthoclase feldspar demonstrating 2-directional cleavage at 90° angles from one another.
2 directions of cleavage, non-perpendicular
Albite feldspar demonstrating 2-directional cleavage planes at non-90 angles
Figure 3.4.5 Albite feldspar demonstrates 2-directional cleavage planes at non-90° angles.
Cubic: 3 directions of cleavage, perpendicular
Halite demonstrating cubic cleavage
Figure 3.4.6 Halite demonstrating cubic cleavage.
Rhombohedral: 3 directions of cleavage, rhombohedral
Calcite demonstrating rhombohedral cleavage.
Figure 3.4.7 Calcite demonstrating rhombohedral cleavage.
Octahedral: 4 directions of cleavage (rare)
Fluorite demonstrating octahedral cleavage.
Figure 3.4.8 Fluorite demonstrating octahedral cleavage.

We will now review what we have seen of cleavage and fracture by examining some common mineral samples!

A sample of mica with flat smooth sheets
Figure 3.4.9 Sample 1. Image by Pascal Terjan, Wikimedia Commons, CC BY-SA 2.0
Sample 2. Rhomb-shaped cluster of crystals
Figure 3.4.10 Sample 2. Image by Robert M. Lavinsky, Wikimedia Commons, CC BY-SA 3.0
Sample 3. Transparent mineral with curved, rough breakage plane
Figure 3.4.11 Sample 3. Image by James St. John, Wikimedia Commons, CC BY 2.0

Hardness

Mineral hardness is the mineral’s resistance to being scratched. The hardness of the mineral can be defined by a numerical scale, called the Mohs Hardness Scale which goes from 0 to 10. You can test the hardness of a mineral using everyday objects like a penny, fingernail, nail, glass, or file to find out where on the hardness scale the mineral lies.

https://youtube.com/clip/UgkxoXw2Q9GVzb4r1wM2BvZ5R4Fa8s70eUID

If you can scratch the mineral with the object, then the object is harder than the mineral. If the mineral can scratch the object, then the mineral is harder than the object. You can also use other minerals to assign a hardness number to a mineral. In other words, we assign a hardness number to a mineral by making relative observations!

Here is the hardness scale and some objects and minerals that fall along with it for reference:

Mohs Hardness Scale of Minerals: From 1 (softest) to 10 (hardest), the order goes: talc, gypsum, fingernail (2.5), calcite, penny (3.5), fluorite, apatite, glass plate (5.5), feldspar, steel file (6.5), quartz, topaz, corundum, diamond.
Figure 3.4.12 Mohs Hardness Scale of Minerals

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Exploring Physical Geology Lab Online Copyright © 2022 by Charlene Estrada is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book