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 (C). Manufacturers mix graphite with clay to create "pencil lead" for writing and drawing. But what makes graphite so soft? Strong covalent bonds hold carbon atoms together horizontally in layers. However, the distance between the carbon atoms on different layers exceeds that between the carbon atoms on the same layer. As a result, a cleavage plane forms between the layers or "sheets" of carbon atoms.

The type of bond that holds the carbon sheets together also differs from the bond that holds carbon atoms within a sheet. Weak van der Waals bonds hold the carbon sheets together. As a result, graphite crystals split easily between those layers or sheets of carbon. This makes graphite "soft," with a Mohs hardness of 1 to 2.

Mineralogists can describe a mineral's cleavage with specific terms. For example, graphite has perfect basal cleavage. Halite has perfect cubic cleavage. These terms describe the planes of the pieces created when minerals break. Learning what these descriptions mean will help you identify minerals.

How Can Cleavage Testing Help Mineral Identification?

Cleavage testing is important for mineral identification. For example, by carefully observing cleavage planes, you can distinguish quartz from feldspar crystals in rocks. In combination with other tests, cleavage testing can also help you identify other common minerals, such as calcite, gypsum, fluorite, halite, and galena.

Can Cleavage Testing Help Gemologists with Identification?

Since cleavage testing involves breaking a specimen, gemologists won't use this destructive technique for gem identification. Nevertheless, understanding how mineral cleavage works can save gem faceters and jewelers time, effort, and money. Most gemstones are minerals, and some gemstones — like fluorite, topaz, spodumene (kunzite), kyanite, gypsum (selenite), zoisite (tanzanite), lepidolite, celestite, moonstone, muscovite, sunstone, and even diamond — have perfect cleavage in one or more direction. This means they can break easily if mechanical force is applied along their cleavage planes. Anyone working with these gemstones, whether faceting or setting them, must know this.

Jewelry owners should also understand the cleavage properties of their gemstones. Some gems need protective settings to prevent breaks. Mechanical cleaning systems like ultrasonic devices may also shatter some gemstones with perfect cleavage. However, this doesn't mean these gemstones don't belong in jewelry. Some of these gems are very popular and, like diamonds, have other very durable properties that still make them well-suited for jewelry use. You simply need to be aware of their cleavage properties and handle them appropriately.

Describing Mineral Cleavage

Mineralogists must consider three key properties when describing mineral cleavage: cleavage quality, cleavage planes (the number of sides that exhibit cleavage), and cleavage habit. Observing and recording these properties ensures a thorough and accurate description of a mineral's cleavage.

Cleavage Quality

Mineralogists usually use the following terms to describe cleavage quality.

• Perfect (or Easy)
• Good (or Imperfect, Distinct)
• Fair (or Moderate)
• Poor (or Weak, Difficult, Indistinct) 
• None

These terms are arranged in descending order, from minerals that split easily along cleavage planes ("Perfect") to minerals that have no cleavage planes and are hardest to split ("None"). When cleavage testing minerals, mineralogists look for specific features on the cleavage planes that indicate cleavage quality. We'll cover that below.

Cleavage Planes

Many minerals have one or more cleavage directions or cleavage planes. For example:

• One cleavage direction: graphite, topaz, muscovite
• Two cleavage directions: feldspar, pyroxene, amphiboles
• Three cleavage directions: halite, calcite
• Four cleavage directions: fluorite, diamond
• Six cleavage directions: sphalerite

Mineralogists combine a mineral's cleavage quality and planes to describe its cleavage properly. For example, mineralogists would describe muscovite's cleavage as perfect in one direction.

Cleavage Habit

Mineralogists also describe the forms created by the cleavage planes in crystallographic terms. These forms are known as cleavage habits.

Basal (Pinacoidal or Planar) Cleavage

Occurs when there is only one cleavage direction (cleavage plane). Examples are micas (muscovite) and topaz.

Prismatic Cleavage

Occurs when there are two directions of cleavage. Cleavage planes can intersect at different angles: for example, at right angles for orthoclase, nearly 90° for pyroxenes, and 56° and 124° for amphiboles. Cleavage habit helps distinguish between the pyroxene and amphibole mineral groups.

Cubic Cleavage

Occurs when three cleavage planes intersect at 90°. Examples include halite and galena.

Rhombohedral Cleavage

Occurs when three cleavage planes intersect at non-right angles, forming rhombohedral shapes. Calcite is an example.

Octahedral Cleavage

Occurs when four cleavage planes intersect. Examples include fluorite and diamond.

Dodecahedral Cleavage

Occurs when six cleavage planes intersect. Sphalerite is an example.

Table of Cleavage Properties

Consult the following table for cleavage direction and habit visualizations with notable mineral examples.

Mineral Cleavage Testing Procedures

Testing mineral cleavage doesn't require complex procedures. In most cases, you can observe the mineral surface without breaking the mineral. The following step-by-step instructions should suffice. We'll also offer some hints for successful cleavage testing.

Step 1: Sample Cleaning and Preparation

Clean and dry your mineral sample before testing. Additionally, look for signs of weathering, which can give samples misleading appearances.

Conduct cleavage tests under well-lit conditions. Light will help you determine if a surface is reflective.

Step 2: Breaking the Mineral Sample

Breaking your mineral sample may be unnecessary. Your mineral sample may already exhibit cleavage planes, or you may simply want to avoid destroying it. In these cases, skip this step and go to Step 3.

If you must break your sample, get ready to have some fun!

• Always take precautions during cleaving testing. Always wear protective eyewear and gloves.
• Conduct the test on a stable, hard surface.
• If you're testing on a floor, pick up all the fragments after the test.
• Avoid using tiled surfaces. Your tests might damage them. If possible, use wooden surfaces.
• Use a hammer and chisel to control the blows to the sample.

Breaking a mineral sample will create new cleavage planes you can distinguish from crystal faces.

Step 3: Identifying Cleavage Quality

Look for smooth, reflective surfaces on your sample. These are the cleavage planes you'll need to identify and describe.

Distinguishing Crystal Faces from Cleavage Planes

Crystal faces form during a mineral's growth, while cleavage planes only form when a mineral breaks. You must distinguish the sample's original crystal faces from any cleavage planes. Cleavage planes are even, highly reflective surfaces created along a mineral's planes of atomic weakness.

However, crystal faces may also appear even and reflective. One of the best ways to distinguish crystal faces from cleavage planes is to look for striations on their surfaces. Only crystal faces show striations. For example, tourmaline crystals have vertical striations on their faces, and quartz crystals have horizontal striations. Cleavage planes lack these naturally occurring grooves.

Describing Cleavage Quality

After finding the cleavage planes, you have to describe the mineral's cleavage quality. Examine the surfaces carefully.

• Mirror-like, even surfaces indicate perfect cleavage.
• Even surfaces with some rough spots indicate good cleavage.
• Surfaces more irregular than even indicate fair cleavage.
• Barely any surfaces with some patterns still evident indicate poor cleavage.
• Irregular surfaces or no planes indicate no cleavage.

Don't be discouraged if your sample has "no cleavage." This identification can still help you identify your mineral. For example, quartz is a very common mineral with either no or poor cleavage. This can help you readily distinguish quartz from many lookalike minerals with perfect cleavage — such as calcite, gypsum, and topaz.

Step 4: Look for Multiple Cleavage Planes or Directions

Try to find more cleavage planes. Experiment with breaking the mineral in different directions or rotating your sample to uncover more cleavage planes. Please note: if you can count several parallel planes, they are still considered one cleavage direction.

If you find multiple cleavage planes, repeat Step 3 and identify the cleavage quality for each cleavage plane. Some minerals, such as anhydrite, have different cleavage qualities in different directions.

Step 5: Identifying the Cleavage Habit

You can now describe your sample's cleavage habit, the form created by the intersections of its cleavage planes. You should also note the angles created by these intersections because they create different forms, which can help with mineral identification.

• A single cleavage direction creates basal (pinacoidal) cleavage.
• Two cleavage directions create prismatic cleavage.
• Three cleavage planes that intersect at non-right angles create rhombohedral cleavage.
• Three cleavage planes that intersect at right angles create cubic cleavage.
• Four cleavage planes that intersect create octahedral cleavage.

Step 6: Interpreting Your Cleavage Testing Results

To describe your mineral sample's cleavage properly, review your observations and answer these three questions:

• What is the quality of the cleavage plane surface? Perfect, good, fair, poor, or none?
• How many cleavage directions do you see? One, two, three, four, or six? 
• What is the cleavage habit? Basal, prismatic, cubic, rhombohedral, or octahedral?

Next, consult reference works that list minerals and their cleavage. You can find mineral identification tables and reference documents online or in print. Find the mineral or minerals that match your test results.

Remember that you can't identify a mineral based solely on cleavage or any other single property. Conduct more mineralogical tests until you can identify your sample.

Mineral Cleavage Testing Examples

Let's go through some cleavage testing examples. These examples will help you get familiar with conducting the tests. Since you're a beginner, we're starting off knowing the mineral's identity. However, with practice, you'll soon be interpreting your cleavage tests as you identify unknown mineral samples.

Example 1: Muscovite

Take a look at the muscovite crystal in the following photos.

As you can see, muscovite is a sheet-like mineral. Like most minerals in the mica group, muscovite has a single cleavage direction. Let's try to split the muscovite crystal so we can describe the cleavage quality and confirm the cleavage direction.

Muscovite is so easy to split that you won't need any special tools. You can easily split a muscovite crystal along its cleavage plane with just your fingernail. You could continue to split this crystal into multiple layers, but they all count as a single cleavage plane because they are parallel.

Every time you split the muscovite, you'll get a thin, smooth, and reflective flake, as shown in the following photo. The smooth and reflecting cleavage plane indicates perfect cleavage.

If you combine all your observations, you can say your sample has perfect basal cleavage in one direction. Of course, this matches what you would expect from muscovite.

Example 2: Calcite (Iceland Spar)

In the following video, I will examine the cleavage of a variety of calcite known as Iceland spar. This mineral has multiple cleavage planes.

Observe this crystal's rhombohedral shape at the beginning before any splitting. These surfaces are not crystal faces. In this case, you can already see the crystal's cleavage planes. However, we will continue splitting the crystal to confirm the number of cleavage directions.

The crystal breaks cleanly along its cleavage plane by applying a little force with just a gentle hammer tap to the chisel. Notice how the resulting surfaces are smooth and flat, perfectly reflecting light. This is a clear indication of calcite's perfect cleavage.

Next, we will reveal two additional cleavage directions. The three cleavage planes intersect at non-right angles, creating perfect rhombohedron pieces.

As you can see from the test, calcite has perfect rhombohedral cleavage in three directions.

FAQ about Mineral Cleavage Testing

Do All Minerals Have Cleavage?

Most minerals have no cleavage. Thus, finding minerals with cleavage is an important step towards identifying them.

What Minerals Lack Cleavage?

The most common minerals without cleavage are garnet, tourmaline, olivine, pyrite, magnetite, ilmenite, chromite, and hematite. Quartz may have no or poor cleavage. When split, these minerals break unevenly.

Can You Use Cleavage Testing to Identify Minerals?

Although cleavage testing is helpful, you can't make any conclusions about a mineral's identity based solely on its cleavage or any other single property. Cleavage testing may help you eliminate some possibilities. However, you can't make identifications based on cleavage. You should conduct as many mineralogical tests as needed before positively identifying a sample.

How Does Cleavage Affect the Overall Durability of a Mineral?

A mineral's cleavage properties result from atomic weaknesses in its crystal structure. Thus, minerals more susceptible to splitting along cleavage planes will disintegrate faster than those resistant to splitting or those without cleavage. For example, plagioclase has perfect cleavage in two directions. Consequently, plagioclase crystals rarely endure in placer deposits. In contrast, quartz crystals have no or poor cleavage and frequently occur in placer deposits.

Olena Rybnikova, PhD

Olena Rybnikova is a gemologist and mineralogist. She has a PhD in mineralogy and petrology specializing in beryllium minerals and is a certified Applied Jewelry Professional accredited by the Gemological Institute of America. Her passion is actively promoting knowledge and appreciation of nature, geology, and gemstones.

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