Unit 3: Crystallography and Mineralogy

Concept of crystalline and amorphous state of matters

Solids can be broadly classified into two categories based on the arrangement of their constituent atoms or molecules.

Crystalline State

• Definition: A solid in which the atoms, ions, or molecules are arranged in a definite, repeating, three-dimensional pattern called a crystal lattice.
• Properties:
  • They have a highly ordered internal structure.
  • This internal order is expressed externally as crystal faces (flat, geometric surfaces).
  • They have a sharp, well-defined melting point.
  • Example: Quartz, Halite (salt), Diamond, ice.

Amorphous State

• Definition: A solid in which the atoms or molecules lack an ordered internal structure. The arrangement is random, like that of a liquid.
• The name comes from the Greek a- (without) and morphē (form).
• Properties:
  • They do not have a crystal lattice.
  • They do not form geometric crystal faces.
  • They do not have a sharp melting point; they soften over a range of temperatures.
  • Example: Glass (volcanic glass is called Obsidian), plastic, rubber.

Types of Crystals

Crystals are classified based on the type of chemical bond that holds their atoms together. This bond type determines many of their physical properties (like hardness and melting point).

1. Ionic Crystals: Formed by electrostatic attraction between positive and negative ions. (e.g., Halite, NaCl). They are typically hard but brittle.
2. Covalent Crystals: Atoms are bonded by sharing electrons, forming a strong network. (e.g., Diamond, C). They are extremely hard.
3. Metallic Crystals: Formed by metal atoms "floating" in a "sea" of shared electrons. (e.g., Gold, Copper). They are good conductors of electricity and are malleable.
4. Molecular Crystals: Held together by weak intermolecular forces (like van der Waals forces). (e.g., Ice, H₂O). They are typically soft and have low melting points.

Basics of Crystal Systems

Crystallography is the study of crystals. All crystals can be classified into 7 Crystal Systems (also called 6 systems in some classifications, lumping Hexagonal and Trigonal) based on their symmetry.

This symmetry is defined by imaginary lines called crystallographic axes (labeled a, b, c) and the angles between them (alpha, beta, gamma).

Crystal System	Axial Relationships (Lengths)	Angular Relationships (Angles)	Example Mineral
Isometric (Cubic)	a₁ = a₂ = a₃	α = β = γ = 90°	Halite, Diamond, Garnet
Tetragonal	a₁ = a₂ ≠ c	α = β = γ = 90°	Zircon, Chalcopyrite
Orthorhombic	a ≠ b ≠ c	α = β = γ = 90°	Olivine, Topaz, Sulphur
Hexagonal	a₁ = a₂ = a₃ ≠ c	α = β = 90°, γ = 120°	Beryl, Apatite
Trigonal (Rhombohedral)	a₁ = a₂ = a₃	α = β = γ ≠ 90°	Calcite, Quartz
Monoclinic	a ≠ b ≠ c	α = γ = 90°, β ≠ 90°	Gypsum, Orthoclase
Triclinic	a ≠ b ≠ c	α ≠ β ≠ γ ≠ 90°	Plagioclase Feldspar, Kyanite

Mineral and its Physical Properties

Definition of a Mineral: A mineral is a (1) naturally occurring, (2) inorganic, (3) solid substance with a (4) definite chemical composition and a (5) ordered internal (crystalline) structure.

All five parts of this definition are essential. For example, coal is not a mineral because it's organic, and glass is not a mineral because it lacks a crystalline structure.

Physical Properties of Minerals

These are the properties used to identify minerals in a hand specimen (i.e., without a microscope). They are a direct result of the mineral's chemical composition and crystalline structure.

1. Colour: The most obvious property, but often the least reliable. Some minerals (like Quartz) come in many colors (Amethyst, Rose Quartz, Citrine) due to tiny impurities.
2. Streak: The colour of a mineral's powder when rubbed against an unglazed porcelain plate (a "streak plate"). Streak is much more reliable than colour. For example, Hematite can be black or red, but its streak is *always* cherry-red.
3. Lustre: The way a mineral's surface reflects light.
   • Metallic: Looks like polished metal (e.g., Galena, Pyrite).
   • Non-metallic:
     • Vitreous: Glassy (e.g., Quartz).
     • Pearly: Like a pearl (e.g., Talc, Muscovite).
     • Resinous: Like resin or wax (e.g., Sphalerite).
     • Silky: Has a fibrous look (e.g., Asbestos, satin-spar Gypsum).
     • Dull / Earthy: No shine at all (e.g., Kaolinite clay).
4. Hardness: The mineral's resistance to being scratched. It is measured using the Mohs Scale of Hardness, which ranks 10 common minerals from softest (1) to hardest (10).
   Mohs Scale of Hardness (MEMORIZE THIS):

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

   Mnemonic: "Tall Girls Can Fight And Obtain Quiet Things Constantly, Darling?"
   Common Tools: Fingernail (H=2.5), Copper Penny (H=3.5), Steel Knife/Nail (H=5.5), Glass (H=5.5).
5. Cleavage: The tendency of a mineral to break along flat, parallel planes of weakness in its crystal structure.
   • Described by quality (Perfect, Good, Poor) and number of directions (e.g., Mica has 1 perfect direction, Halite has 3 perfect directions at 90°).
6. Fracture: How a mineral breaks when it does not have a cleavage plane.
   • Conchoidal: Smooth, curved surfaces like broken glass (e.g., Quartz, Obsidian).
   • Fibrous/Splintery: Breaks into splinters.
   • Uneven: A rough, irregular surface.
7. Structure (or Habit): The characteristic shape a mineral or group of minerals grows in.
   • e.g., Fibrous (like asbestos), Bladed (like kyanite), Botryoidal (grape-like bunches), Acicular (needle-like), Massive (no distinct shape).
8. Forms: The geometric shape a crystal takes, like a Cube (Halite) or Hexagonal Prism (Quartz).