Crystal Chemistry

How atoms are held together in minerals: Ionic Bonds: Electrons are transferred from one atom to another, creating ions. The attraction between positive and negative ions holds the structure together. Common in many minerals, especially those with metal cations and non-metal anions. Covalent Bonds: Atoms share electrons. Strong bonds. Common in framework silicates like quartz, where silicon and oxygen share electrons. Metallic Bonds: Electrons are delocalized (free to move). Creates properties like electrical conductivity and metallic luster. Found in native metals and some sulfides. Van der Waals Forces: Weak attractions between molecules or layers. Important in some minerals like graphite, where layers are held together weakly (explains why graphite is soft and can be used as a lubricant). Hydrogen Bonds: Weak bonds involving hydrogen. Important in some hydrated minerals and affects properties like cleavage in micas. Mixed Bonding: Most minerals have mixed bonding types. The dominant bond type determines many properties. For example, ionic bonds create cleavage, covalent bonds create hardness.

How different elements can replace each other: Solid Solution: When minerals can have variable composition because similar-sized ions can substitute for each other. For example, olivine can have varying amounts of iron and magnesium: (Mg,Fe)₂SiO₄. Substitution Rules: - Ions must be similar in size (within about 15%) - Ions should have similar charge (or charge balance must be maintained) - Crystal structure must accommodate the substitution Common Substitutions: - Fe²⁺ ↔ Mg²⁺: Very common (olivine, pyroxene, amphibole) - Al³⁺ ↔ Fe³⁺: Common in many silicates - Na⁺ ↔ Ca²⁺: Requires charge balance (plagioclase feldspar) - K⁺ ↔ Na⁺: In feldspars and micas Complete Solid Solution: Some mineral pairs can substitute completely (plagioclase feldspar: NaAlSi₃O₈ to CaAl₂Si₂O₈). Others have limited substitution. Effects on Properties: Substitution affects color, density, and other properties. For example, more iron in olivine makes it darker and denser. Geological Significance: Substitution patterns can indicate formation conditions (temperature, pressure) and help understand geological processes.

How atoms are arranged in three dimensions: Unit Cell: The smallest repeating unit that defines the entire crystal structure. Like a building block that repeats in three dimensions. Coordination: How many nearest neighbors an atom has. For example, in many silicates, silicon is coordinated by 4 oxygens (tetrahedral coordination), while aluminum can be coordinated by 4 or 6 oxygens. Polyhedra: Atoms are often arranged in geometric shapes (polyhedra). SiO₄ tetrahedra in silicates, AlO₆ octahedra in some structures. These polyhedra link together to form the crystal structure. Close Packing: Many structures are based on close packing of atoms (like stacking spheres). This maximizes density and stability. Framework Structures: Three-dimensional networks (like quartz, feldspars). Very stable, hard, and resistant to chemical attack. Layer Structures: Two-dimensional sheets (like micas, clays). Have perfect cleavage parallel to layers. Can expand or contract between layers. Chain Structures: One-dimensional chains (like pyroxenes, amphiboles). Form prismatic crystals with cleavage parallel to chains. Island Structures: Isolated groups (like olivine, garnet). No preferred cleavage direction, often form equant crystals.

How mineral compositions are written: Standard Formulas: Written with elements and subscripts showing ratios. For example, quartz is SiO₂ (one silicon, two oxygens). Parentheses: Used to show groups or substitutions. (Mg,Fe)₂SiO₄ means magnesium and/or iron can be present. Water of Hydration: Shown with a dot: CaSO₄·2H₂O (gypsum has 2 water molecules per formula unit). Variable Composition: Some minerals have variable composition. Formulas show ranges or use parentheses for substitutions. End Members: Pure compositions at the ends of solid solution series. For example, forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄) are end members of olivine. Structural Formulas: Show how atoms are arranged, not just ratios. More complex but more informative. Reading Formulas: Understanding formulas helps predict properties. More oxygen usually means more complex structure. Metal content affects density and color.

Graphical representations of mineral stability: What They Show: Phase diagrams show which minerals are stable under different conditions (temperature, pressure, composition). Temperature-Pressure Diagrams: Show how minerals change with temperature and pressure. Important for understanding metamorphism and igneous processes. Composition Diagrams: Show how composition affects which minerals form. Important for understanding solid solutions and mineral associations. Stability Fields: Regions on diagrams where specific minerals are stable. Boundaries show where one mineral converts to another. Geological Applications: Help understand: - What minerals form at different depths (pressure) - What minerals form at different temperatures - How minerals change during metamorphism - What minerals crystallize from magmas For Rockhounds: Understanding phase diagrams helps predict what minerals you might find in different rock types and geological settings.

How mineral compositions are determined: X-ray Fluorescence (XRF): Bombards sample with X-rays, measures emitted X-rays to determine elements present. Non-destructive, good for major elements. Electron Microprobe: Uses focused electron beam to analyze small areas. Can analyze individual mineral grains. Very precise. Inductively Coupled Plasma (ICP): Dissolves sample and analyzes solution. Very sensitive, can detect trace elements. X-ray Diffraction (XRD): Determines crystal structure, which reveals composition. Essential for identifying minerals and understanding structures. Energy Dispersive X-ray (EDX): Often attached to electron microscopes. Quick analysis of elements present. Less precise but very useful. Wet Chemical Methods: Traditional methods using chemical reactions. Less common now but still used for some analyses. For Collectors: Most rockhounds don't need chemical analysis, but understanding that it's possible and what it reveals helps appreciate the science behind mineral identification.