The fundamental difference between crystal habit and crystal cleavage lies in the processes they describe: crystal habit refers to how a mineral grows, while crystal cleavage describes how it breaks along specific internal planes of weakness. Understanding both is crucial for identifying and classifying minerals.
Here's a detailed comparison:
| Feature | Crystal Habit | Crystal Cleavage |
|---|---|---|
| Definition | The characteristic external shape or form a mineral assumes as it grows, reflecting its internal atomic structure and growth conditions. | The tendency of a mineral to break along smooth, flat, parallel surfaces due to weaker atomic bonds in those specific directions. |
| Process | Growth | Breaking/Fracturing |
| Origin | Influenced by internal atomic arrangement, growth rate, available space, temperature, pressure, and chemical environment. | Determined by the internal atomic structure and the varying strengths of chemical bonds between atoms. |
| Appearance | Observed as distinct crystal shapes (e.g., cubes, prisms, needles, sheets). | Results in flat, reflective surfaces when broken. |
| Variability | Can vary significantly within the same mineral species depending on growth conditions. | Typically consistent within a given mineral species, occurring in specific directions and qualities. |
| Examples | Prismatic (quartz), cubic (pyrite), tabular (barite), fibrous (asbestos), dendritic (manganese oxides). | Basal (mica), cubic (halite), rhombohedral (calcite), octahedral (fluorite). |
| Diagnostic Use | Useful for initial recognition, especially with well-formed crystals. | A primary diagnostic feature, indicating bond strength and crystal symmetry. |
Understanding Crystal Habit
Crystal habit, also known as crystal form, describes the typical shape a mineral's individual or aggregate crystals take on during their formation. It is a direct reflection of the mineral's internal atomic structure and the environmental conditions present during its growth.
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Factors Influencing Habit:
- Internal Atomic Structure: The fundamental arrangement of atoms dictates the preferred growth directions.
- Growth Rate: Rapid growth can lead to different habits compared to slow, undisturbed growth.
- Available Space: Minerals growing in open cavities (vugs) can develop well-formed, euhedral crystals, while those growing in confined spaces may be anhedral (lacking distinct faces).
- Temperature and Pressure: Variations can influence which crystal faces are preferentially developed.
- Impurities: The presence of other elements or compounds can alter the growth patterns.
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Common Crystal Habits and Examples:
- Prismatic: Elongated, column-like crystals (e.g., quartz, tourmaline).
- Tabular: Flat, plate-like crystals (e.g., barite, wulfenite).
- Cubic: Cube-shaped crystals (e.g., pyrite, galena).
- Octahedral: Eight-faced crystals (e.g., magnetite).
- Acicular: Needle-like crystals (e.g., natrolite).
- Fibrous: Hair-like or thread-like masses (e.g., asbestos minerals).
- Dendritic: Branching, tree-like patterns (e.g., certain manganese oxides).
- Botryoidal: Grape-like aggregates (e.g., hematite, malachite).
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Practical Insight: While habit can be variable, some minerals consistently display a characteristic habit, making it a valuable tool for preliminary identification. For instance, the perfect hexagonal prisms of quartz or the cubic crystals of pyrite are often instantly recognizable. For further reading on various habits, consult resources like Geology.com's mineral habit guide.
Understanding Crystal Cleavage
Crystal cleavage is a mineral's tendency to break along specific internal planes of weakness, resulting in smooth, flat surfaces. This property is directly controlled by the mineral's atomic structure and the relative strengths of the chemical bonds holding the atoms together. Where bonds are weaker, the mineral will preferentially break.
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Underlying Cause:
- Within a crystal lattice, atoms are arranged in a repeating pattern. The strength of the bonds between these atoms varies in different directions. Cleavage occurs along planes where the interatomic bonds are the weakest.
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Describing Cleavage:
- Quality:
- Perfect: Produces very smooth, flat, reflective surfaces (e.g., mica, calcite).
- Good: Produces distinct but less perfectly smooth surfaces (e.g., feldspar).
- Poor/Indistinct: Difficult to observe, may be interrupted by irregular breaks.
- Number of Directions:
- One Direction (Basal): Breaks like sheets (e.g., mica).
- Two Directions: Produces step-like fractures (e.g., feldspar at nearly 90 degrees, hornblende at 56/124 degrees).
- Three Directions:
- Cubic: At 90 degrees (e.g., halite, galena).
- Rhombohedral: Not at 90 degrees (e.g., calcite).
- Four Directions (Octahedral): Produces eight-faced fragments (e.g., fluorite).
- Angle: The angle at which cleavage planes intersect is characteristic of a mineral (e.g., 90 degrees for halite, 75 degrees for calcite).
- Quality:
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Distinction from Fracture:
- Cleavage: Predictable breaking along smooth, flat planes due to structural weakness.
- Fracture: Irregular, uneven breaking that occurs when there are no planes of weakness or when stress is applied in a direction not parallel to cleavage planes (e.g., conchoidal fracture in quartz, resembling broken glass; uneven, hackly).
- Cleavage and fracture are the most important diagnostic features of many minerals, and often the most difficult to understand and identify without practice.
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Practical Insight: Cleavage is a highly reliable diagnostic property because it is an intrinsic characteristic of a mineral's internal structure. Observing how a mineral breaks provides direct evidence of its atomic arrangement and bond strengths. Minerals with perfect cleavage often cleave readily with a light tap, revealing their characteristic flat surfaces. Learn more about mineral cleavage and fracture at Mindat.org's guide to physical properties.
Key Differences Summarized
In essence, crystal habit is an external expression of how a mineral grows, influenced by both internal structure and external conditions, leading to varied shapes. Cleavage, conversely, is an internal property defining how a mineral breaks along precise, inherent planes of weakness, providing consistent and predictable breakage patterns. Both properties, though distinct in their origin and manifestation, serve as fundamental tools for mineral identification and classification.
