Mineral Cleavage Types

Why This Matters

Cleavage is one of the most diagnostic physical properties you'll use to identify minerals in lab practicals and on exams. When a mineral breaks along planes of weakness in its crystal structure, it's revealing something fundamental about how its atoms are arranged internally. You're being tested not just on what cleavage looks like, but on why different minerals break the way they do—and that comes down to bond strength and atomic geometry.

Understanding cleavage connects directly to crystallography, crystal systems, and the relationship between internal structure and external properties. When you see terms like "three directions at 90°" or "parallel to the base," you should immediately visualize the underlying lattice. Don't just memorize that calcite has rhombohedral cleavage—know that this reflects its trigonal crystal system and the orientation of its weakest bonds. That conceptual understanding is what separates a good exam answer from a great one.

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Cleavage Quality: How Cleanly Does It Break?

The quality of cleavage describes how easily and smoothly a mineral splits along its planes of weakness. This depends on how much bond strength varies between different directions in the crystal lattice—greater contrast means cleaner breaks.

Perfect Cleavage

• Splits effortlessly along planes—produces mirror-smooth, highly reflective surfaces with minimal force
• Indicates dramatic bond strength differences between cleavage planes and other directions in the crystal
• Classic examples: mica (peels into flexible sheets) and calcite (breaks into rhombs)

Good Cleavage

• Requires moderate effort to split—surfaces are relatively smooth but may show minor irregularities
• Bond strength contrast is significant but not extreme, allowing reliable but imperfect separation
• Common in chain silicates: amphibole and pyroxene demonstrate good cleavage along their elongated crystal axes

Imperfect Cleavage

• Cleavage planes are present but indistinct—splitting produces somewhat uneven surfaces
• Bond strengths are more uniform across the crystal, making preferential breaking less pronounced
• Feldspar is the key example, showing cleavage in two directions but with surfaces that aren't as clean as mica

Poor Cleavage

• Breaking occurs with difficulty along vaguely defined planes, often producing rough, irregular surfaces
• Nearly uniform bond strength means the mineral may fracture instead of cleaving
• Quartz exhibits poor cleavage—it typically displays conchoidal fracture rather than true cleavage planes

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Cleavage Geometry: Direction and Number of Planes

The geometry of cleavage reflects the crystal system and the spatial arrangement of weak bond planes. Counting cleavage directions and measuring angles between them are essential identification skills.

Basal Cleavage

• One direction parallel to the crystal base—minerals peel off in thin, flat sheets
• Characteristic of sheet silicates where strong bonds exist within layers but weak bonds hold layers together
• Mica is the textbook example, separating into flexible, transparent sheets you can write on

Prismatic Cleavage

• Two cleavage directions producing elongated, prism-shaped fragments
• Common in minerals with chain or columnar structures where bonds are weaker perpendicular to the chain axis
• Seen in tourmaline and beryl—fragments maintain their characteristic elongated habit

Cubic Cleavage

• Three directions at 90° angles—produces perfect cube-shaped fragments
• Reflects the isometric crystal system with equivalent weak bond planes along all three axes
• Halite and galena are classic examples—break a halite crystal and you get smaller cubes

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Complex Cleavage Geometries: Beyond 90°

Some minerals display cleavage in multiple directions at angles other than 90°, producing distinctive fragment shapes that reflect their unique crystal symmetries.

Rhombohedral Cleavage

• Three directions not at 90°—produces rhombohedron-shaped fragments (like a squashed cube)
• Characteristic of the trigonal crystal system where the three-fold symmetry axis creates non-orthogonal weak planes
• Calcite is the definitive example—every fragment, no matter how small, maintains the rhombohedral shape

Octahedral Cleavage

• Four directions corresponding to the faces of an octahedron (8-sided shape)
• Found in isometric minerals where bond weakness occurs along planes connecting opposite corners of the cube
• Fluorite and diamond display this pattern—diamond's octahedral cleavage is exploited by gem cutters

Dodecahedral Cleavage

• Six directions producing 12-faced dodecahedral fragments
• Rare and complex geometry reflecting specialized bonding arrangements in certain isometric minerals
• Sphalerite demonstrates this pattern—the cleavage creates distinctive multi-faceted pieces

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Cleavage Angles: The Diagnostic Detail

For minerals with two cleavage directions, the angle between planes is often the key to identification—especially for distinguishing similar-looking minerals.

Amphibole Cleavage

• Two directions at approximately 120° and 60°—produces diamond-shaped cross-sections
• Reflects the double-chain silicate structure where chains are offset, creating non-perpendicular weak planes
• Hornblende is the common example—look for the characteristic 120° angle in hand samples

Pyroxene Cleavage

• Two directions at approximately 90°—produces nearly square or rectangular cross-sections
• Single-chain silicate structure creates cleavage planes that intersect at right angles
• Augite is the typical example—the 90° angle distinguishes it from amphibole at a glance

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Quick Reference Table

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Self-Check Questions

1. Both calcite and halite have three cleavage directions—what geometric property distinguishes their cleavage, and how does this reflect their different crystal systems?

2. You're examining two dark, elongated silicate minerals in lab. What single measurement would definitively distinguish amphibole from pyroxene?

3. Compare and contrast mica and quartz: both are silicates, but one has perfect cleavage and one has poor cleavage. Explain how their atomic structures account for this difference.

4. If an FRQ asks you to explain how internal crystal structure relates to physical properties, which mineral would best demonstrate the connection between sheet silicate structure and cleavage behavior?

5. Fluorite and halite are both isometric minerals—why does fluorite display octahedral cleavage while halite displays cubic cleavage?