Crystals are the building blocks of most minerals, and their geometric shapes provide important clues for identification. Understanding crystal systems and habits helps geologists interpret a mineral's atomic structure and formation conditions.
The Seven Crystal Systems
All minerals crystallize into one of seven fundamental crystal systems, determined by the arrangement of atoms within the crystal lattice. Each system is defined by the lengths of its crystallographic axes and the angles between them.
| Crystal System | Axes Lengths | Axes Angles | Common Minerals |
|---|---|---|---|
| Isometric (Cubic) | a = b = c | α = β = γ = 90° | Halite, Pyrite, Galena, Diamond |
| Tetragonal | a = b ≠ c | α = β = γ = 90° | Rutile, Zircon, Cassiterite |
| Orthorhombic | a ≠ b ≠ c | α = β = γ = 90° | Topaz, Olivine, Sulfur, Barite |
| Hexagonal | a = b ≠ c | α = β = 90°, γ = 120° | Quartz, Calcite, Corundum, Apatite |
| Trigonal | a = b = c | α = β = γ ≠ 90° (but < 120°) | Quartz (trigonal variety), Rhodochrosite, Tourmaline |
| Monoclinic | a ≠ b ≠ c | α = γ = 90°, β ≠ 90° | Muscovite, Biotite, Gypsum, Orthoclase |
| Triclinic | a ≠ b ≠ c | α ≠ β ≠ γ ≠ 90° | Plagioclase feldspar, Turquoise, Kyanite |
Crystal Habits
Crystal habit refers to the characteristic shape in which a mineral typically grows. This can include the overall form of individual crystals or the way multiple crystals grow together.
Common Crystal Habits
Individual Crystal Habits
- Prismatic: Long, slender crystals with parallel sides (e.g., tourmaline)
- Tabular: Flat, tablet-like crystals (e.g., barite)
- Acicular: Needle-like crystals (e.g., rutile)
- Bladed: Elongated crystals with flat, blade-like shapes (e.g., kyanite)
- Dodecahedral: Twelve-sided crystals (e.g., garnet)
- Octahedral: Eight-sided crystals with triangular faces (e.g., fluorite, diamond)
- Cubic: Cube-shaped crystals with six equal faces (e.g., halite, pyrite)
Aggregate Habits
- Granular: Aggregates of small, anhedral crystals (e.g., chert)
- Fibrous: Composed of fine, thread-like crystals (e.g., asbestos, gypsum)
- Botryoidal: Rounded, grape-like masses (e.g., hematite, malachite)
- Stalactitic: Cylindrical or cone-shaped deposits formed by dripping (e.g., calcite, aragonite)
- Massive: Compact, formless aggregates with no visible crystal structure (e.g., chalcedony)
- Dendritic: Tree-like or fern-like branching patterns (e.g., native copper, manganese oxides)
- Drusy: Surface covered with small, sparkling crystals (e.g., quartz druse)
Factors Affecting Crystal Growth
Several factors influence the development of crystal systems and habits:
- Chemical composition: Determines the atomic arrangement
- Temperature: Affects the rate of crystal growth
- Pressure: Can alter the stable crystal structure
- Available space: Restricted growth can lead to distorted crystals
- Impurities: Can interfere with crystal growth patterns
- Growth medium: Crystals grow differently in water, magma, or vapor
Importance in Mineral Identification
Crystal system and habit are fundamental properties used in mineral identification. When combined with other properties like hardness, color, and cleavage, they can help distinguish between similar-looking minerals. Crystal shape also provides insights into the mineral's formation environment and geological history.
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