Minerals are the building blocks of rocks, and their atomic structure determines their properties. From the way atoms bond to form crystals to how these structures influence physical characteristics, understanding mineral composition is key to grasping Earth's materials.

Crystal systems and habits provide a framework for classifying minerals based on their internal structure and external form. These systems, ranging from cubic to triclinic, help geologists identify and categorize minerals, while crystal habits describe the typical shapes minerals take as they grow.

Atomic Arrangement and Crystal Structure

Atomic arrangement in minerals

• Atomic bonds in minerals dictate mineral properties and behavior
  • Ionic bonds form between oppositely charged ions (NaCl)
  • Covalent bonds share electrons between atoms (diamond)
  • Metallic bonds allow free-flowing electrons (gold)
  • Van der Waals forces weak attractions between molecules (graphite)
• Periodic arrangement of atoms creates orderly structure
  • Unit cell smallest repeating structural unit in crystal
  • Repeating patterns in three dimensions form crystal lattice
• Coordination number surrounding atoms affects mineral structure
  • Higher coordination typically results in denser packing (cubic close-packed)
• Atomic packing efficiency influences mineral density
  • Close-packed structures maximize space utilization
  • Hexagonal close-packed (HCP) common in metals (magnesium)
  • Cubic close-packed (CCP) found in face-centered cubic structures (copper)
• Polymorphism same chemical composition crystallizes differently
  • Diamond and graphite both pure carbon different structures
• Isomorphism different chemical composition similar crystal structures
  • Olivine series (Mg,Fe)2SiO4 varying Mg/Fe ratios

Crystal structure vs physical properties

• Cleavage planes weak bonds between atomic layers
  • Mica cleaves into thin sheets due to layered structure
• Hardness resistance to scratching depends on bond strength
  • Diamond hardest natural material due to strong covalent bonds
• Optical properties light interaction with crystal structure
  • Refraction light bending (calcite double refraction)
  • Birefringence splitting of light rays (quartz)
• Thermal properties heat transfer and expansion
  • Quartz expands anisotropically due to crystal structure
• Electrical properties conductivity and piezoelectricity
  • Quartz generates electric charge under pressure (piezoelectric)
• Magnetic properties electron spin alignment
  • Magnetite strongly magnetic due to aligned electron spins
• Density mass per unit volume related to atomic packing
  • Gold dense due to efficient atomic packing and heavy atoms
• Solubility ease of dissolving related to bond strength
  • Halite (rock salt) highly soluble due to weak ionic bonds

Crystal Systems and Habits

Six main crystal systems

• Cubic (isometric) system highest symmetry
  • Four threefold axes diagonal through cube
  • Three fourfold axes through face centers (pyrite)
• Tetragonal system elongated or compressed cube
  • One fourfold axis vertical through crystal (rutile)
• Orthorhombic system three unequal axes at right angles
  • Three twofold axes perpendicular to each other (topaz)
• Monoclinic system one axis oblique to other two
  • One twofold axis perpendicular to other two (gypsum)
• Triclinic system lowest symmetry no right angles
  • No symmetry elements all axes oblique (feldspar)
• Hexagonal system six-sided prism with six-fold symmetry
  • One sixfold axis vertical through crystal (quartz)

Crystal habits and forms

• Prismatic habit elongated crystals common in many minerals
  • Tourmaline forms long slender crystals
• Tabular habit flattened crystals often plate-like
  • Mica forms thin sheets or books
• Cubic habit cube-shaped crystals in isometric system
  • Halite (rock salt) forms perfect cubes
• Octahedral habit eight-faced crystals common in garnets
  • Diamond often occurs in octahedral form
• Dodecahedral habit twelve-faced crystals in isometric system
  • Garnet can form rhombic dodecahedrons
• Rhombohedral habit parallelogram-faced crystals
  • Calcite often crystallizes in rhombohedral form
• Acicular habit needle-like crystals very thin and elongated
  • Rutile forms slender needle-like crystals
• Fibrous habit thread-like crystals often in parallel groups
  • Asbestos minerals form fibrous masses
• Dendritic habit tree-like branching patterns
  • Manganese oxides often form dendritic patterns
• Botryoidal habit grape-like clusters rounded surfaces
  • Hematite can form botryoidal masses
• Massive habit no distinct crystal faces aggregates
  • Jade typically occurs in massive form