Common crystals are categorized based on several criteria, primarily focusing on their structural, chemical, and physical properties. Here are the primary categories used in crystallography:
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Crystal Systems: Crystals are classified into seven crystal systems based on their symmetry and the lengths and angles of their axes. These systems are:
- Cubic (or Isometric): All sides equal, angles 90° (e.g., diamond, sodium chloride).
- Tetragonal: Two sides equal, one different, angles 90° (e.g., rutile, tin).
- Orthorhombic: All sides different, angles 90° (e.g., topaz, olivine).
- Hexagonal: Two sides equal, with angles of 120° between them and 90° to the third axis (e.g., quartz, beryl).
- Trigonal: Similar to hexagonal but with additional symmetry (e.g., calcite).
- Monoclinic: Two angles at 90°, one not; all sides different (e.g., gypsum, orthoclase).
- Triclinic: All sides and angles different (e.g., kyanite, turquoise).
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Chemical Composition: Crystals can also be categorized based on their chemical composition, including:
- Simple Compounds: Crystals composed of a single type of molecule (e.g., salt, ice).
- Ionic Crystals: Composed of ions held together by ionic bonds (e.g., NaCl, MgO).
- Covalent Crystals: Atoms connected by covalent bonds (e.g., diamond, quartz).
- Metallic Crystals: Lattice of metal ions surrounded by a sea of delocalized electrons (e.g., copper, gold).
- Molecular Crystals: Comprise molecules held together by van der Waals forces (e.g., sugar, ice).
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Habit and Form: Crystals are often described by their external shape or "habit," which can include:
- Prismatic: Column-like.
- Tabular: Tablet-shaped.
- Acicular: Needle-like.
- Dendritic: Tree-like.
- Drusy: Crystalline surface coating.
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Optical Properties: Some classifications are based on optical behavior, such as:
- Uniaxial: Crystals with a single optic axis (e.g., quartz).
- Biaxial: Crystals with two optic axes (e.g., beryl).
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Hardness: The Mohs scale of hardness categorizes minerals from 1 (talc) to 10 (diamond) based on their ability to scratch one another.
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Other Properties: Crystals can also be categorized based on specific features like:
- Color: The apparent color of a crystal can indicate certain impurities or structural features.
- Luster: The way a crystal reflects light (e.g., metallic, glassy, dull).
- Cleavage and Fracture: The way a crystal breaks can be a key identifying factor.
These categories help scientists and gemologists to classify, identify, and study the properties and behaviors of crystals in various contexts.