Fracture of Minerals: Types & Examples

Fracture describes the characteristic manner in which a mineral breaks when there are no distinct planes of weakness, or cleavage planes, within its crystal structure. When a mineral is subjected to stress in a direction that doesn't correspond with its cleavage planes, or if it lacks cleavage altogether, it will fracture in an irregular fashion.

Definition: A fracture in minerals refers to the way a mineral breaks or cracks when it does not follow well-defined cleavage planes. Unlike cleavage, which results in smooth, flat breaks, fractures are uneven and reveal how the mineral's internal atomic arrangement responds to external forces.

The type of fracture a mineral exhibits is contingent upon its internal atomic structure, its hardness, and the nature of the forces applied during breakage.

Fracture patterns are vital for mineral identification and classification. Recognizing how a mineral fractures can provide insights into its physical properties, helping distinguish it from others that might look similar but fracture differently.

Types of mineral fractures, including conchoidal, hackly, granular, earthy, fibrous, and splintery fractures.

How Fracture Happens in Minerals

Fracture occurs when stress is applied to a mineral in a way that exceeds the strength of the bonds holding its atoms or molecules together. Unlike cleavage, which occurs along specific planes where the atomic bonds are weakest, fractures occur when there is no natural plane of weakness in the crystal structure. This leads to a more random breakage.

The type of fracture depends largely on the atomic structure of the mineral, the arrangement of atoms or ions, and how stress is distributed across the mineral. For example, in minerals like quartz, where the atomic bonds are uniformly strong in all directions, the fracture tends to be conchoidal. In contrast, minerals with more complex or fibrous structures break differently, resulting in splintery or hackly fractures.

Types of Mineral Fracture

Fracture is classified based on the appearance and texture of the surface created when the mineral breaks. The main types of fracture include:

Conchoidal Fracture

Definition: Conchoidal fracture describes a type of breakage where the mineral or material fractures with a curved, smooth surface, reminiscent of the interior surface of a seashell. The term "conchoidal" indeed derives from the Greek word for shell.

Appearance: The fracture surfaces are characteristically smooth and can be either concave or convex, featuring a distinctive rippled or wave-like pattern.

Conchoidal Fracture

Cause:

This fracture occurs in materials where the atomic or molecular bonds are relatively uniform in strength in all directions. When force is applied, the break does not follow any particular crystal plane but instead creates a smooth, curved surface.

Characteristics:

• Results in sharp edges which can be very fine, making materials with conchoidal fracture useful for tools in prehistoric times (like flint or obsidian).
• While the surface is smooth, it often displays concentric or arc-like ridges, which are sometimes referred to as ripple marks or rib marks.

Examples:

• Quartz: A common mineral exhibiting conchoidal fracture, making it useful for stone tools in ancient times.
• Obsidian: A type of volcanic glass that's renowned for its conchoidal fracture, producing extremely sharp edges used in surgical scalpels and ancient weaponry.

Uneven or Irregular Fracture

Definition: An uneven or irregular fracture results in a rough, non-uniform surface where the mineral breaks without following any predictable pattern or plane.

Appearance: The fracture surface is rugged and jagged but tends to be less sharp than hackly fractures. The texture can be described as coarse or bumpy.

Uneven or irregular fracture in minerals

Cause:

• This type of fracture is most common in minerals where:
• There is no pronounced cleavage due to the lack of planes of weakness within the crystal structure.
• The bonding forces between atoms or molecules are relatively uniform in all directions, preventing the fracture from following any specific path.

Characteristics:

• Surface lacks symmetry or smoothness, appearing randomly rough.
• No specific geometric shape or pattern; the fracture paths are unpredictable.

Examples:

• Limonite: This iron ore mineral displays an uneven fracture due to its often amorphous or poorly crystalline nature.
• Magnetite: Known for its strong magnetic properties, magnetite also typically fractures in an uneven manner, reflecting its cubic crystal system but lack of clear cleavage planes.

Fibrous Fracture

Definition: A fibrous fracture occurs when minerals break into fibers or fine threads, showcasing a texture reminiscent of split fabric. The surface of the break appears composed of tiny, elongated fibers.

Appearance: The fracture results in elongated, thread-like or hair-like fibers, which can be parallel to each other, reflecting the mineral's internal crystal structure.

Fibrous fractures in minerals, Asbestos and Byssolite

Cause:

Fibrous fractures are prevalent in minerals with a fibrous or columnar crystal structure. Here, the atomic bonds are significantly weaker in certain directions than in others, leading to a break along these weaker bond lines.

Characteristics:

• Produces needle or thread-like fragments.
• The fracture surface often appears as an assembly of elongated, fibrous masses.
• Fibrous has a more uniform, parallel fiber-like appearance, while splintery looks like the mineral has shattered into sharp, often less uniform splinters.
  

Examples:

• Asbestos: Known for its fibrous nature, it easily separates into thin, flexible fibers.
• Sillimanite: Exhibits a fibrous fracture due to its prismatic to fibrous crystal habit.
  

Splintery Fracture

Definition: Splintery fracture is often considered a subtype of fibrous fracture where the mineral breaks into long, thin, and sharp fragments resembling wooden splinters.

Appearance: A splintery fracture results in the formation of sharp, needle-like, or splintery fragments. Unlike a fibrous fracture, which separates into more uniform fibers, a splintery fracture produces distinctly sharp, elongated, and sometimes brittle pieces.

Cause:

This fracture type occurs in minerals with a crystal structure that has strong bonds in one direction but significantly weaker bonds in others, causing the mineral to split into splinter-like fragments.

Characteristics:

• Produces thin, elongated, and sharp fragments, often more pointed and brittle than those in a simple fibrous fracture.
• The fracture surface can appear jagged with an array of splinter-like projections.

Examples:

• Kyanite: Known for its bladed habit, kyanite can exhibit splintery fractures along certain planes due to its anisotropic crystal structure.
• Tremolite: A member of the amphibole group, it can show splintery fracture when its long, prismatic crystals are broken.

Hackly Fracture

Definition: Hackly fracture (also known as jagged fracture) refers to a break in a mineral or metal that results in a rough, jagged surface with sharp and often thorn-like protrusions. This fracture is characteristic of materials that can deform plastically before breaking.

Appearance: The fractured surface is distinctly uneven, displaying numerous small, sharp points, and ridges, giving it a hacked or torn appearance.

Hackly fractures in metallic minerals, including gold and copper

Cause:

Hackly fractures occur in minerals or metals that are malleable or ductile, meaning they can be stretched or bent without breaking initially. When these materials finally fracture, they do so in a way that reflects this ductility, tearing apart unevenly rather than cleaving cleanly.

Characteristics:

• Surface is rough with highly irregular, sharp, and often pointed projections, resembling the texture of broken metal.
• Unlike conchoidal fracture, it does not show smooth curves but rather looks as if the material was ripped or torn.

Examples:

• Copper: When fractured, native copper often shows a hackly surface due to its ductile nature.
• Silver and Gold: These precious metals, especially in their native form, exhibit hackly fractures because of their high malleability and ductility.

Earthy Fracture

Definition: An earthy (Powdery) fracture is characterized by a mineral breaking into a substance that looks and feels like compacted earth or powder, often crumbly in nature.

Appearance: The fracture surface has a dull, chalky, or soil-like texture, lacking the sheen or smoothness found in other fracture types.

Kaolinite and Bauxite specimens displaying earthy fractures

Cause: This type of fracture occurs in minerals that are:

• Soft or have a low hardness scale rating.
• Composed of loosely bonded, fine particles or have a granular structure where the cohesive forces between particles are weak.

Characteristics:

• Surface appears dull and has a powdery or crumbly consistency.
• Upon breaking, these minerals do not form sharp edges but instead disintegrate into soft fragments or powder.

Examples:

Kaolinite: As a clay mineral, it naturally exhibits an earthy fracture due to its layered silicate structure which does not hold together firmly when broken.

Granular Fracture

Definition: A granular fracture occurs when a mineral breaks into small, usually equidimensional grains or crystals, giving the fracture surface a grainy or sugary appearance.

Description: This type of fracture is typical in minerals that are aggregates of small crystals or grains, where the cohesion between these individual units is weaker than the internal structure of the grains themselves.

Appearance: The surface looks like it's made up of numerous tiny granules, similar to a handful of sand or sugar, reflecting the mineral's internal structure.

Granular fractures mineral, showing small, irregular fragments.

Cause:

Granular fractures are prevalent in polycrystalline minerals where individual crystals are not strongly bonded to each other, leading to breakage along grain boundaries rather than through the crystals themselves.

Characteristics:

• Surface texture is grainy, often resembling fine to coarse sand.
• The mineral tends to disintegrate into its constituent grains upon fracture, rather than forming sharp edges or smooth surfaces.

Examples:

• Chromite: As an iron chromium oxide, chromite often shows a granular fracture due to its typical formation as an aggregate of small crystals.
• Quartz: While quartz is often known for its conchoidal fracture, it can also exhibit granular fracture when it breaks along the boundaries of its quartz grains rather than through them.
• Olivine: While known for its conchoidal fracture in gem quality peridot, in its more common rock-forming occurrences, olivine can exhibit granular fracture when composed of fine grains.

Subconchoidal Fracture

Definition: Subconchoidal fracture refers to a breakage pattern in minerals that is somewhat similar to conchoidal fracture but with less pronounced curvature. The surfaces are smoother and the curves less distinct, indicating a transition towards an uneven fracture.

Description: This type of fracture displays surfaces that are gently curved, lacking the deep, shell-like concavities of a true conchoidal fracture. It's as if the mineral begins to show conchoidal tendencies but does not fully develop them.

Appearance:

Surfaces might appear wavy or with very shallow curves, suggesting an intermediate stage between a perfectly smooth conchoidal fracture and a more irregular, uneven fracture.

Subconchoidal fractures chert, showing an irregular, curved pattern.

Cause:

Subconchoidal fractures occur in materials where the atomic or molecular bonds are strong but not entirely uniform in all directions, leading to fractures that don't quite reach the smooth circularity of conchoidal breaks but are not entirely irregular either.

Characteristics:

• Less defined ripple marks or concentric rings compared to conchoidal.
• The fracture edge can still be somewhat sharp but typically not as sharp as with a true conchoidal fracture.

Examples:

• Chalcedony: This microcrystalline form of quartz might exhibit subconchoidal fractures due to its fine, fibrous structure that doesn't allow for the deep curves of conchoidal fractures but still breaks more smoothly than an uneven fracture.
• Jasper and Agate: Both are varieties of chalcedony and can display subconchoidal fractures. Their breakage often lacks the clear conchoidal patterns seen in larger quartz crystals but still shows some rounding.
• Flint: A variety of quartz that sometimes shows subconchoidal fracture along with conchoidal breaks.

Factors Influencing Fracture

Several factors affect how a mineral fractures:

Crystal Structure: The internal arrangement of atoms in a mineral determines how it breaks. Minerals with a highly ordered and regular structure, such as quartz, tend to have characteristic fractures like conchoidal fractures. In contrast, minerals with a more granular or fibrous structure might exhibit uneven or splintery fractures.

Bonding Strength: The strength of the atomic bonds in a mineral also plays a key role. Minerals with strong bonds that are equally distributed in all directions will tend to fracture rather than cleave, as there are no planes of weakness. For example, quartz, which has strong Si-O bonds in all directions, exhibits conchoidal fracture.

Hardness and Brittleness: Minerals that are hard and brittle, like quartz or obsidian, are more likely to fracture. Soft or ductile minerals, like native metals (copper, gold), may exhibit hackly fracture because they deform before breaking.

Impurities: Impurities in a mineral can create zones of weakness or stress, leading to uneven or irregular fracture surfaces.

Hardness Vs. Fracture

While fracture refers to how a mineral breaks, hardness refers to how resistant a mineral is to scratching or indentation. These two properties are related but distinct. A mineral can be hard but still exhibit a specific type of fracture when subjected to force. For example, quartz is hard (7 on the Mohs hardness scale) but exhibits conchoidal fracture.

Cleavage Vs. Fracture

While cleavage refers to the tendency of a mineral to break along flat planes of weakness within its crystal structure, fracture occurs when a mineral breaks along surfaces that are not related to any planes of atomic weakness.

• Cleavage is smooth and often flat (e.g., mica, halite), while fracture is rough and irregular (e.g., quartz).
• Cleavage is often predictable and uniform, while fractures are more random and depend on the shape and size of the mineral specimen as well as external forces.
• Understanding fracture patterns is crucial in both academic and practical applications, including gem cutting, mineral identification, and material science.

Fracture in Gemology and Material Science

Gem Cutting: The fracture properties of minerals are important in gem cutting and jewelry design. For instance, the conchoidal fracture of quartz and obsidian is considered when shaping gemstones or creating tools, as it produces sharp, clean edges. Historically, this property was crucial for making stone tools.

Industrial Applications: Understanding how a mineral fractures is critical in various industrial applications, from ore processing to material engineering. Minerals with certain fracture types may be more difficult to work with or may be less durable for specific purposes.

Summary

Fracture is a key characteristic in the study of minerals, revealing a mineral's structural properties and aiding in its identification. Depending on the internal atomic structure, minerals can exhibit conchoidal, fibrous, uneven, hackly, or earthy fractures. Each type of fracture gives a different surface texture and shape, providing clues about the mineral’s composition and formation process.

Read also:
Crystal Habits and Forms
Crystal Systems and Crystal Structure
Luster of Minerals: Types & Examples