Is Corundum Cleavage or Fracture? Understanding the Breakage Behavior of a Hard MineralExploring Corundum’s Breakage PropertiesCorundum is a mineral well-known for its exceptional hardness and is the second hardest natural mineral after diamond. It is composed of aluminum oxide (Al₂O₃) and is valued in both industrial and gemstone applications. When discussing how minerals break, two key terms often arise cleavage and fracture. So, when it comes to corundum, which one does it exhibit? This topic will explore the breakage characteristics of corundum and explain what makes this mineral unique.
Cleavage vs. Fracture What’s the Difference?
Before diving into corundum specifically, it’s important to understand the distinction between cleavage and fracture.
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Cleavage refers to the tendency of a mineral to break along flat, even surfaces. These surfaces are aligned with planes of weakness in the mineral’s crystal structure.
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Fracture happens when a mineral breaks in an irregular or uneven way, often due to the absence of natural planes of weakness. Fractures can appear as conchoidal (shell-like), fibrous, or jagged.
These physical properties help geologists and gemologists identify minerals and assess their potential uses.
Corundum’s Breakage Fracture, Not Cleavage
Corundum does not have true cleavage. Instead, it exhibits a type of breakage known as parting and shows conchoidal to uneven fracture. This means corundum does not split cleanly along flat surfaces like some other minerals, such as mica or calcite.
Understanding Parting in Corundum
Parting can sometimes be mistaken for cleavage, but it’s a different process. Parting in corundum occurs along planes of structural weakness that develop due to external stress, deformation, or twinning within the crystal. Unlike cleavage, parting is not inherent to the crystal structure itself and is less predictable.
Parting in corundum may show up during cutting or polishing and is usually visible only under certain conditions. Because of its inconsistency, parting is not considered a reliable property like cleavage is.
Fracture Types Observed in Corundum
Corundum typically shows conchoidal fracture, which means the broken surface resembles the inside of a shell with curved ridges. This type of fracture is also found in glass and quartz. In some specimens, corundum may also display an uneven fracture, with a rough, irregular breakage pattern.
These types of fracture are common in minerals that lack cleavage, especially those with tightly bonded atomic structures.
Why Corundum Has No True Cleavage
The atomic structure of corundum explains why it does not exhibit cleavage. Corundum has a hexagonal crystal system with strong ionic and covalent bonds in all directions. This uniform bonding creates a structure that resists splitting along any particular plane, making cleavage nearly impossible.
Instead of breaking along predictable paths, corundum fractures irregularly under force, despite its hardness.
The Role of Hardness in Corundum’s Breakage
Corundum ranks 9 on the Mohs scale of hardness, just below diamond. This makes it extremely resistant to scratching and wear, which is why it’s used in abrasives and as a gemstone. However, high hardness doesn’t necessarily mean resistance to breaking. While hard, corundum can still fracture if struck with enough force or pressure.
Its lack of cleavage makes it more durable as a gemstone since it’s less likely to split along planes when worn or set into jewelry.
Corundum in the Gemstone World Rubies and Sapphires
Gem-quality corundum comes in two well-known varieties
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Ruby – Red corundum, colored by chromium.
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Sapphire – Corundum of any color other than red, most famously blue.
The fracture properties of both rubies and sapphires are the same as other corundum varieties. Their lack of cleavage makes them suitable for various cuts and settings without the risk of splitting.
However, gem cutters still need to be cautious of parting planes, especially when shaping high-value stones.
Industrial Uses of Corundum and the Importance of Fracture
Beyond gemstones, synthetic corundum is widely used in industry. Its hardness and fracture resistance make it ideal for cutting tools, grinding wheels, and sandpapers.
Fracture behavior is important in these applications because it affects how the material wears down during use. A mineral with true cleavage might split or chip in unwanted directions, but corundum’s predictable fracture makes it more reliable for consistent performance.
Identifying Corundum in the Field or Lab
Geologists and mineral collectors can identify corundum based on several characteristics
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Hardness It scratches almost everything except diamond.
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Crystal Shape Often forms hexagonal or barrel-shaped crystals.
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Luster Exhibits a vitreous to adamantine luster.
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Fracture Shows uneven or conchoidal breakage, not flat cleavage surfaces.
These properties, along with its density and lack of cleavage, help distinguish it from similar-looking minerals.
Comparing Corundum to Other Minerals
To better understand corundum’s breakage, let’s compare it with a few other minerals
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Calcite Has perfect cleavage in three directions, forming rhombohedral fragments.
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Mica Has excellent cleavage in one direction, peeling into sheets.
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Quartz Similar to corundum in that it also shows conchoidal fracture and no cleavage.
These comparisons show how corundum’s structure makes it distinct in both the gem and mineral worlds.
Conclusion Corundum Breaks by Fracture, Not Cleavage
Corundum is a mineral known for its toughness and high resistance to scratching, but it does not exhibit true cleavage. Instead, it breaks through fracture, often appearing conchoidal or uneven. Although it may show parting under certain conditions, this is not the same as cleavage and is not consistently observed.
Understanding how corundum breaks is important in gemology, geology, and industrial applications. Its reliable fracture behavior, combined with extreme hardness, makes it a valuable and versatile mineral in many fields.