💎Mineralogy Unit 4 – Crystal Systems and Mineral Habits

Key Concepts and Definitions

• Minerals are naturally occurring, inorganic solids with a definite chemical composition and an ordered atomic arrangement
• Crystals are solid materials whose atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions
• Crystal systems are categories of crystal structures that are defined by the symmetry of their unit cell (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic)
• Unit cells are the smallest repeating unit that shows the full symmetry of the crystal structure
• Symmetry describes the balanced proportions or similarity of parts in a crystal's structure (reflection, rotation, inversion)
• Habits refer to the characteristic external shape of a crystal (prismatic, tabular, equant, acicular, fibrous, bladed, botryoidal)
• Polymorphism occurs when a mineral can crystallize in more than one form or crystal system (graphite and diamond)
• Pseudomorphism happens when a mineral chemically changes into another mineral but retains its original external shape

Crystal Structure Fundamentals

• Crystals are composed of atoms, ions, or molecules arranged in a periodic pattern that extends in three dimensions
• The smallest repeating unit of a crystal structure is called the unit cell which is defined by its lattice parameters (lengths of cell edges and angles between them)
• The arrangement of atoms within the unit cell determines the crystal's symmetry and physical properties
• Bravais lattices describe the 14 possible three-dimensional arrangements of points in space (primitive cubic, body-centered cubic, face-centered cubic)
  • Primitive cubic has atoms only at the corners of the unit cell
  • Body-centered cubic has an additional atom at the center of the unit cell
  • Face-centered cubic has additional atoms at the center of each face of the unit cell
• Close packing refers to the arrangement of atoms in a crystal structure where they occupy the maximum amount of space (hexagonal close packing, cubic close packing)
• Coordination number indicates the number of nearest neighbors an atom has in a crystal structure (ranging from 1 to 12)
• Interstitial sites are the spaces or voids between atoms in a close-packed structure where smaller atoms can fit (tetrahedral, octahedral)

The Seven Crystal Systems

• Cubic crystal system has the highest symmetry with equal lengths ($a = b = c$) and all angles equal to 90° ($\alpha = \beta = \gamma = 90°$) (pyrite, galena, halite)
• Tetragonal crystal system has two equal axes ($a = b \neq c$) and all angles equal to 90° ($\alpha = \beta = \gamma = 90°$) (zircon, rutile, cassiterite)
• Hexagonal crystal system has two equal axes ($a = b \neq c$) and angles $\alpha = \beta = 90°, \gamma = 120°$ (quartz, calcite, beryl)
• Trigonal crystal system is similar to hexagonal but with a threefold symmetry axis (tourmaline, corundum, cinnabar)
• Orthorhombic crystal system has three unequal axes ($a \neq b \neq c$) and all angles equal to 90° ($\alpha = \beta = \gamma = 90°$) (topaz, olivine, barite)
• Monoclinic crystal system has three unequal axes ($a \neq b \neq c$) and one angle not equal to 90° ($\alpha = \gamma = 90° \neq \beta$) (gypsum, mica, orthoclase)
• Triclinic crystal system has the lowest symmetry with three unequal axes ($a \neq b \neq c$) and none of the angles equal to 90° ($\alpha \neq \beta \neq \gamma \neq 90°$) (kyanite, plagioclase, rhodonite)

Common Mineral Habits

• Euhedral crystals are well-formed with sharp, easily recognized faces (garnet, pyrite, quartz)
• Subhedral crystals are partially formed with some well-developed faces (olivine, pyroxene, amphibole)
• Anhedral crystals lack well-formed crystal faces and have irregular shapes (feldspar, quartz in granite)
• Prismatic habit is elongated with well-developed crystal faces parallel to the long axis (tourmaline, beryl, quartz)
• Tabular habit is flattened or tablet-shaped with one dimension significantly shorter than the other two (barite, topaz, wulfenite)
• Equant habit has crystal dimensions roughly equal in all directions, often cubic or spherical (garnet, fluorite, pyrite)
• Acicular habit is needle-like with one dimension much longer than the other two (natrolite, rutile, selenite)
  • Fibrous habit consists of fine, thread-like crystals that appear silky (asbestos, ulexite, serpentine)
• Bladed habit is composed of flattened, blade-like crystals (kyanite, actinolite, selenite)
• Botryoidal habit resembles a bunch of grapes with radiating crystals forming rounded masses (hematite, malachite, smithsonite)

Symmetry in Crystals

• Symmetry is the regular repetition of structural features in a crystal (faces, edges, corners)
• Symmetry elements are geometric entities (points, lines, planes) about which symmetry operations can be performed
• Symmetry operations are movements that bring a crystal into a position indistinguishable from its original position (rotation, reflection, inversion)
• Rotation symmetry involves rotating the crystal around an axis by a specific fraction of 360° (2-fold, 3-fold, 4-fold, 6-fold)
  • 2-fold rotation symmetry means the crystal appears the same after a 180° rotation (orthoclase, gypsum)
  • 3-fold rotation symmetry means the crystal appears the same after a 120° rotation (calcite, tourmaline)
• Reflection symmetry involves mirroring the crystal across a plane (muscovite, orthoclase, halite)
• Inversion symmetry involves passing the crystal through a point, turning it inside out (garnet, halite, pyrite)
• Rotoinversion symmetry is a combination of rotation and inversion (quartz, cinnabar, staurolite)

Factors Influencing Crystal Growth

• Temperature affects the rate of crystal growth and the stability of different crystal structures (polymorphs)
  • Higher temperatures generally lead to faster crystal growth rates
  • Some minerals are stable only at specific temperature ranges (diamond vs. graphite)
• Pressure influences the stability and formation of certain crystal structures (graphite vs. diamond)
  • High pressures can favor denser crystal structures (diamond)
  • Low pressures can favor less dense crystal structures (graphite)
• Chemical composition determines which elements are available for crystal formation and their proportions
  • Changes in chemical composition can lead to variations in crystal structure and properties (olivine series)
• Impurities can affect crystal growth, habit, and color (quartz varieties: amethyst, citrine, smoky quartz)
  • Some impurities may inhibit crystal growth, leading to smaller or deformed crystals
  • Other impurities may promote crystal growth or lead to the formation of larger crystals
• Supersaturation is the driving force for crystal growth, occurring when a solution contains more dissolved solute than the equilibrium concentration
  • Higher levels of supersaturation lead to faster crystal growth rates
• Nucleation is the formation of a small crystal nucleus upon which further crystal growth can occur
  • Homogeneous nucleation occurs spontaneously within the bulk of the solution
  • Heterogeneous nucleation occurs on pre-existing surfaces or particles

Identification Techniques

• Visual inspection involves observing the crystal's habit, color, luster, and other physical properties
• Hardness is a mineral's resistance to scratching, measured using the Mohs scale (ranging from 1 to 10)
  • Minerals with higher hardness can scratch those with lower hardness (diamond scratches all other minerals)
• Cleavage is the tendency of a mineral to break along flat, planar surfaces due to weak bonding in specific directions (mica, feldspar, calcite)
• Fracture describes the irregular breakage of a mineral that lacks cleavage (quartz, olivine, garnet)
• Specific gravity is the ratio of a mineral's density to the density of water, used for identification (gold, pyrite, galena)
• Streak is the color of a mineral's powder, obtained by rubbing the mineral against a streak plate (hematite, pyrite, magnetite)
• Luster describes the way a mineral reflects light from its surface (metallic, vitreous, resinous, pearly)
• Chemical tests involve observing reactions with acids or other reagents (calcite reacts with hydrochloric acid)
• Optical properties, such as refractive index, birefringence, and pleochroism, can be used for identification using microscopy techniques (thin sections, immersion oils)

Real-World Applications

• Gemstones are minerals valued for their beauty, rarity, and durability in jewelry and decorative objects (diamond, sapphire, emerald)
  • The quality of a gemstone depends on its color, clarity, cut, and carat weight
• Industrial minerals are used in manufacturing, construction, and other industries (gypsum for drywall, quartz for electronics)
  • The specific properties of industrial minerals determine their suitability for different applications
• Ores are minerals that can be mined and processed economically to extract valuable elements (chalcopyrite for copper, sphalerite for zinc)
  • The grade and tonnage of an ore deposit determine its economic viability
• Environmental remediation involves using minerals to clean up contaminated soil or water (zeolites for heavy metal adsorption)
• Mineral exploration uses knowledge of crystal systems and mineral habits to locate and assess potential mineral deposits
  • Certain crystal habits or mineral associations can indicate specific geological environments or formation conditions
• Materials science and engineering utilize principles of crystallography to design and develop new materials with specific properties (semiconductors, superconductors, biomaterials)
  • Manipulating crystal structures at the atomic level can lead to materials with enhanced performance or novel functionalities
• Geochemistry and petrology rely on understanding crystal structures and mineral compositions to interpret the formation and evolution of rocks and Earth's interior
  • The presence of specific minerals or crystal structures can provide insights into the pressure, temperature, and chemical conditions of rock formation