Question

How are common crystals categorized

Answer 1

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:

1. **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).

2. **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).

3. **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.

4. **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).

5. **Hardness**: The Mohs scale of hardness categorizes minerals from 1 (talc) to 10 (diamond) based on their ability to scratch one another.

6. **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.