Mineral crystallography is the study of the atomic structure and geometric properties of mineral crystals. This fascinating field reveals how atoms arrange themselves into repeating patterns to form the crystals we observe in nature. Understanding crystallography is essential for identifying minerals, determining their physical properties, and interpreting their formation conditions.
Fundamentals of Crystal Structure
Atomic Arrangement in Minerals
All minerals are crystalline substances, meaning their atoms are arranged in a regular, repeating pattern extending in all three dimensions. This ordered arrangement gives minerals their characteristic physical properties.
Unit Cells and Crystal Lattices
The basic building block of a crystal structure is the unit cell:
- Unit cell: The smallest repeating unit that contains all the structural information of the crystal
- Crystal lattice: The three-dimensional array of points representing the positions of atoms in the crystal
- Lattice parameters: The lengths (a, b, c) and angles (α, β, γ) that define the unit cell
Atomic Bonding in Minerals
Different types of chemical bonds determine the structure and properties of minerals:
- Ionic bonds: Formed by the attraction between positively and negatively charged ions (e.g., Halite - NaCl)
- Covalent bonds: Formed by sharing of electrons between atoms (e.g., Diamond - C)
- Metallic bonds: Formed by a sea of delocalized electrons surrounding metal ions (e.g., Native copper)
- Van der Waals bonds: Weak attractive forces between molecules (e.g., Graphite)
- Hydrogen bonds: Special type of dipole-dipole interaction involving hydrogen (e.g., Ice)
The Seven Crystal Systems
All minerals crystallize in one of seven crystal systems, which are distinguished by their unit cell dimensions and symmetry:
| Crystal System | Axial Lengths | Axial Angles | Examples |
|---|---|---|---|
| Cubic (Isometric) | a = b = c | α = β = γ = 90° | Halite, Galena, Diamond, Pyrite |
| Tetragonal | a = b ≠ c | α = β = γ = 90° | Rutile, Zircon, Cassiterite |
| Orthorhombic | a ≠ b ≠ c | α = β = γ = 90° | Barite, Topaz, Sulfur, Olivine |
| Hexagonal | a = b ≠ c | α = β = 90°, γ = 120° | Quartz, Calcite, Corundum, Graphite |
| Trigonal | a = b ≠ c | α = β = 90°, γ = 120° | Quartz (trigonal variety), Rhodochrosite |
| Monoclinic | a ≠ b ≠ c | α = γ = 90°, β ≠ 90° | Muscovite, Biotite, Gypsum, Orthoclase |
| Triclinic | a ≠ b ≠ c | α ≠ β ≠ γ ≠ 90° | Plagioclase feldspar, Kyanite, Turquoise |
Crystal Symmetry
Symmetry Elements
Crystals exhibit various types of symmetry, which can be described by symmetry elements:
- Center of symmetry (inversion center): A point through which each atom can be reflected to an equivalent position on the opposite side
- Mirror plane (plane of symmetry): A plane that divides the crystal into two mirror-image halves
- Rotation axis: A line around which the crystal can be rotated by a certain angle and appear unchanged
- Rotatory-inversion axis: A combination of rotation and inversion through a center
Bravais Lattices
Bravais lattices describe all possible ways to arrange points in three-dimensional space with translational symmetry. There are 14 Bravais lattices grouped into the seven crystal systems:
- Cubic: Primitive (P), Body-centered (I), Face-centered (F)
- Tetragonal: Primitive (P), Body-centered (I)
- Orthorhombic: Primitive (P), Body-centered (I), Face-centered (F), Base-centered (C)
- Hexagonal: Primitive (P)
- Trigonal: Primitive (P)
- Monoclinic: Primitive (P), Base-centered (C)
- Triclinic: Primitive (P)
Crystal Forms and Habits
Crystal Forms
A crystal form is a set of crystal faces related by the symmetry of the crystal. There are two main types of crystal forms:
- Closed forms: Complete sets of faces that can enclose space (e.g., cube, octahedron)
- Open forms: Sets of faces that cannot enclose space and must be combined with other forms (e.g., prism, pyramid)
Common Crystal Habits
Crystal habit refers to the characteristic shape of a crystal or aggregate of crystals. Common habits include:
| Habit | Description | Examples |
|---|---|---|
| Acicular | Slender, needle-like crystals | Natrolite, Actinolite |
| Bladed | Elongated, flat crystals like blades | Kyanite, Stibnite |
| Dendritic | Tree-like or fern-like branching | Dendritic quartz, Manganese oxides |
| Euhedral | Well-formed crystals with sharp edges | Perfect quartz crystals, Pyrite cubes |
| Anhedral | Poorly formed crystals with no obvious faces | Most minerals in rocks |
| Granular | Composed of small grains | Chalcopyrite, Galena |
| Massive | No visible crystal structure | Hematite, Fluorite |
| Prismatic | Long, prism-shaped crystals | Tourmaline, Beryl |
| Tabular | Flat, tablet-shaped crystals | Muscovite, Barite |
| Botryoidal | Grape-like clusters | Hemimorphite, Malachite |
| Stalactitic | Cone or cylinder-shaped hanging deposits | Calcite, Aragonite |
X-ray Crystallography
Principles of X-ray Diffraction
X-ray crystallography is a powerful technique used to determine the atomic structure of crystals:
- X-rays are directed at a crystal
- The x-rays are diffracted by the crystal lattice
- The resulting diffraction pattern is recorded and analyzed
- From the diffraction pattern, the arrangement of atoms in the crystal can be determined
Importance in Mineralogy
X-ray crystallography has revolutionized mineralogy by:
- Providing definitive information about crystal structures
- Enabling accurate mineral identification
- Revealing the relationships between structure and properties
- Helping discover new mineral species
Polymorphism and Isomorphism
Polymorphism
Polymorphism is the phenomenon where a single chemical compound can exist in more than one crystal structure:
- Allotropy: Polymorphism in elements (e.g., diamond and graphite are both forms of carbon)
- Phase transitions: Changes in crystal structure due to changes in temperature or pressure
- Example: Quartz can exist in different polymorphic forms (α-quartz, β-quartz, tridymite, cristobalite) depending on temperature
Isomorphism
Isomorphism occurs when different chemical compounds have the same crystal structure:
- Elements can substitute for one another in the crystal structure
- Creates solid solution series (e.g., the plagioclase feldspar series)
- Allows for the formation of mixed crystals with variable composition
Defects in Crystal Structures
Perfect crystals are rare in nature; most contain various types of defects:
- Point defects: Missing atoms (vacancies), extra atoms (interstitials), or substitution of one atom for another
- Line defects: Dislocations in the crystal lattice
- Plane defects: Stacking faults or grain boundaries
- Volume defects: Inclusions of other minerals or gases
Importance of Crystal Defects
Defects significantly influence a mineral's properties:
- Affect mechanical properties like hardness and brittleness
- Influence electrical conductivity
- Control diffusion rates within crystals
- Create color centers that produce distinctive colors
- Provide sites for chemical reactions to occur
Practical Applications
Understanding crystallography has numerous practical applications:
- Mineral identification: Crystal structure is a fundamental diagnostic property
- Materials science: Designing new materials with specific properties
- Gemology: Cutting and polishing gemstones to maximize their optical properties
- Mineral processing: Developing efficient methods to extract valuable minerals
- Pharmaceutical industry: Studying the crystal structures of drugs
- Earth sciences: Interpreting geological processes and conditions