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Minerals – Flashcards (Lecture 6, GSCI 160)

Minerals: Definition

  • Natural, inorganic, solid with an ordered internal structure and a specific chemical composition.
  • Examples: NaCl, SiO_2.
  • Distinguish crystalline minerals from amorphous solids (glass).

Crystal Structure: Order vs Disorder

  • Crystalline minerals have long-range atomic order.
  • Amorphous = disordered arrangement (glass is a typical example).

Crystal Lattice and Bonding

  • Crystal lattice: 3-D arrangement of atoms.
  • Bond types that build minerals: ionic, covalent, metallic, van der Waals.
  • Bonding type controls mineral properties and crystal form.
  • Diamond vs Graphite illustrate bonding effects:
    • Diamond: strong covalent bonds, close packing, very hard.
    • Graphite: weak van der Waals between layers, soft.

How Minerals Form

  • Begin with atoms; bonds form; crystals grow from nuclei.
  • Growth requires space and time to develop well-formed crystals.

Silicate Minerals and SiO4 Tetrahedron

  • Core building block: Silicon–Oxygen Tetrahedron (SiO$_4$).
  • Key ratio: ext{Si:O} = 1:4.
  • Linkage patterns determine mineral class:
    • Isolated tetrahedra (nesosilicates) e.g., Olivine.
    • Single chains (inosilicates) e.g., Pyroxene.
    • Double chains (inosilicates) e.g., Amphibole.
    • Sheets (phyllosilicates) e.g., Micas and Clay minerals.
    • Framework (tectosilicates) e.g., Quartz and Feldspars.
  • Common silicate minerals (formulas, representative):
    • Olivine: ext{(Mg,Fe)2SiO4}
    • Pyroxene: ext{(Mg,Fe)SiO_3}
    • Amphibole: complex Ca–Na–Fe–Mg–Al silicate; framework examples include hornblende-type structures.
    • Feldspars: Orthoclase KAlSi3O8; Na-plagioclase NaAlSi3O8; Ca-plagioclase CaAl2Si3O_8.
    • Micas: Muscovite KAl2(AlSi3O{10})(OH)2; Biotite K(Mg,Fe)3(AlSi3O{10})(OH)2.
    • Quartz: SiO_2.

Other Common Silicate Minerals and Non-Silicate Salts/Oxides

  • Common dissolved/mineral phases in water:
    • Halides: NaCl (Halite).
    • Carbonates: CaCO_3 (Calcite).
    • Sulfates: CaSO4\cdot 2H2O (Gypsum).
    • Oxides: Fe2O3 (Hematite).
  • Pure silicon usage: silicon-based chips and crystals are used in electronics.

Mineral Growth and Crystal Habit

  • Growth starts from a seed (nucleation).
  • Atoms attach to surfaces; mineral expands outward.
  • Size/quality depend on space and time; room to grow is rare.
  • Crystal habits (shapes) include: cubes, blades, hexagonal prisms, dodecahedra, rhombohedra, octahedra.

Elemental Abundances in Earth's Crust

  • Crustal minerals largely made of eight elements:
    • O: 46.6\%
    • Si: 27.7\%
    • Al: 8.1\%
    • Fe: 5.0\%
    • Ca: 3.6\%
    • Na: 2.8\%
    • K: 2.6\%
    • Mg: 2.1\%
    • All others: 1.5\%
  • Overall, 74.3\% of crustal minerals comprise these eight elements.

Mineral Classes by Dominant Anion

  • Silicates: ext{SiO}_4^{4-} (dominant class in crust).
  • Oxides, Sulfides, Sulfates, Halides, Carbonates, Native Elements.
  • Examples:
    • Oxide: Hematite Fe2O3, Magnetite Fe3O4.
    • Halide: Fluorite CaF_2, Halite NaCl.
    • Carbonate: Calcite CaCO3, Dolomite CaMg(CO3)_2.
    • Sulfate: Gypsum CaSO4\cdot 2H2O.
    • Native elements: Cu, Au, C (graphite).

Silicon–Oxygen Tetrahedra and Silicate Structures

  • Silicates are built from SiO$_4$ tetrahedra linked in various ways.
  • Framework affects lava viscosity and solidification temperature (polymerization increases viscosity).

Crystal Growth and Habit Revisited

  • Growth under space/time yields well-formed crystals with characteristic shapes.
  • In restricted space, crystals become smaller and less perfect.

Physical Properties (Key Diagnostic Features)

  • Crystal habit/form
  • Color
  • Diaphaneity (transparency)
  • Streak (color of powder)
  • Luster (metallic vs nonmetallic)
  • Hardness (Mohs scale)
  • Specific gravity (density)
  • Cleavage (planes of weakness)
  • Fracture

Color: Impurities and Caution

  • Color can be altered by trace impurities (e.g., Fe, Mn, Ti).
  • Quartz color varieties (amethyst, rose quartz, smoky quartz) arise from impurities.

Streak and Luster

  • Streak: color of a mineral powder on unglazed porcelain.
  • Examples:
    • Magnetite: black streak
    • Pyrite: black streak
    • Hematite: red-to-black streak; metallic vs earthy appearance
  • Luster: metallic vs nonmetallic (earthy, glassy, pearly, dull, silky).

Diaphaneity

  • Transparent, translucent, opaque.

Special Properties (Diagnostic Cues)

  • Effervescence with acid (commonly calcite): reaction with HCl is strong.
  • Dolomite reacts with acid only when powdered (weaker reaction).
  • Magnetism: Magnetite is strongly magnetic.

Cleavage and Fracture

  • Cleavage: preferential break along planes of weakness.
  • Fracture: breakage without planar planes (irregular).
  • Cleavage directions: number of unique cleavage planes (1, 2, 3, 4, etc.).
  • Cleavage angle: angle between cleavage planes (e.g., 90° in halite).
  • Methods to identify cleavage:
    • Light reflection from flat cleavage faces.
    • Look for opposite, identical cleavage faces.
    • Breakage along planes yielding smaller identical pieces (note: do not perform destructive tests in lab).

Cleavage Classification (Overview)

  • Based on directions of cleavage and angles between planes. Examples include 1, 2, 3 or more planes with characteristic angles.

No Cleavage / Fracture

  • Some minerals (e.g., quartz) do not have planes of weakness and fracture irregularly.

Mohs Hardness Scale (Reference Points)

  • 1 Talc
  • 2 Gypsum
  • 3 Calcite
  • 4 Fluorite
  • 5 Apatite
  • 6 Orthoclase
  • 7 Quartz
  • 8 Topaz
  • 9 Corundum
  • 10 Diamond

Notes for Identification Practice

  • Combine multiple properties (hardness, streak, cleavage, luster, color stability) for reliable identification.
  • Use silicate structure knowledge to anticipate mineral groups found in rocks.