Minerals: Definition and Classification

Definition of a Mineral

This is a fundamental concept. A substance must meet all five criteria to be considered a mineral.

A mineral is a (1) naturally occurring, (2) inorganic, (3) solid, with a (4) definite (but not fixed) chemical composition and a (5) ordered internal (crystalline) structure.

1. Naturally Occurring: Must be formed by natural geological processes. Synthetic diamonds are not minerals.
2. Inorganic: Not formed by living organisms. A seashell (calcite) is biogenic, but after burial and recrystallization into limestone, it is a mineral.
3. Solid: Must be solid at Earth's surface temperatures. Ice is a mineral; liquid water is not.
4. Definite (but not fixed) Chemical Composition: Can be expressed by a chemical formula. "Not fixed" means substitution is allowed (e.g., Olivine is (Mg,Fe)₂SiO₄, where Mg and Fe can substitute for each other).
5. Ordered Internal Structure: Atoms are arranged in a repeating 3D pattern (a crystal lattice). Substances lacking this, like volcanic glass (Opal, Obsidian), are called mineraloids.

Classification of Minerals

Minerals are classified chemically, based on their dominant anion (negative ion) or anionic group. This is the most important classification scheme.

• Silicates: Dominant group, (SiO₄)⁴⁻ anion. E.g., Quartz, Feldspar.
• Carbonates: (CO₃)²⁻ anion. E.g., Calcite.
• Sulphides: S²⁻ anion. E.g., Pyrite.
• Oxides: O²⁻ anion. E.g., Hematite.
• Sulphates: (SO₄)²⁻ anion. E.g., Gypsum.
• Halides: Cl⁻, F⁻, etc. anions. E.g., Halite.

Physical and Chemical Properties

These are the tools used to identify minerals, especially in the field (hand-specimen identification).

Moh's Scale of Hardness

Hardness is the resistance of a mineral to being scratched. Moh's scale is a relative scale from 1 (softest) to 10 (hardest).

Other Physical Properties

• Cleavage: The tendency of a mineral to break along flat, parallel planes of weak atomic bonding. Described by its quality (perfect, good, poor) and number of directions (e.g., 1 direction in Mica, 2 in Feldspar, 3 in Calcite/Halite).
• Fracture: How a mineral breaks when it does not have cleavage. E.g., Conchoidal (curved, like glass; typical of Quartz), uneven, fibrous.
• Lustre: How light reflects off the mineral's surface.
  • Metallic: Looks like metal (e.g., Pyrite, Galena).
  • Non-Metallic: (e.g., Vitreous (glassy, like Quartz), Pearly (like Mica), Silky, Greasy, Resinous, Dull/Earthy).
• Colour: The observed color. Can be unreliable (e.g., Quartz can be many colors).
• Streak: The color of the mineral's powder when scraped on a porcelain "streak plate". Very reliable for metallic minerals (e.g., Hematite is always cherry-red, Pyrite is greenish-black).
• Specific Gravity (Density): How "heavy" it feels. (e.g., Galena and Barite are unusually heavy for their size).

Chemical Properties

Some simple chemical tests can be diagnostic:

• Reaction with Acid: Carbonates (like Calcite) will effervesce (fizz) with dilute HCl.
• Taste: Halides (like Halite) have a salty taste. (Use with caution!)
• Smell: Sulphides (like Pyrite) may give a "rotten egg" smell when struck or powdered.

Composition of Common Rock-Forming Minerals

These are the minerals that make up >90% of the Earth's crust.

Felsic Minerals (Light-colored, rich in Si, Al, K, Na)

• Quartz: SiO₂ (Silicon Dioxide). Very stable, hard, no cleavage.
• Potassium Feldspar (K-Feldspar): KAlSi₃O₈ (Orthoclase, Microcline). Often pink, 2 cleavages at ~90°.
• Plagioclase Feldspar: (NaAlSi₃O₈ - CaAl₂Si₂O₈). A solid-solution series. Often white/grey, 2 cleavages at ~90°, may show striations.
• Muscovite (Mica): KAl₂(AlSi₃O₁₀)(OH)₂. Light-colored (silvery), perfect 1-direction cleavage (peels in sheets).

Mafic Minerals (Dark-colored, rich in Mg, Fe)

• Biotite (Mica): K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂. Dark-colored (black/brown), perfect 1-direction cleavage.
• Amphibole Group (e.g., Hornblende): Complex formula. Dark green/black, 2 cleavages NOT at 90° (at 60°/120°).
• Pyroxene Group (e.g., Augite): Complex formula. Dark green/black, 2 cleavages at ~90°.
• Olivine: (Mg,Fe)₂SiO₄. Olive-green, glassy lustre, conchoidal fracture (no cleavage).

Silicate and Non-Silicate Structures

Silicate Structures (Polymerization)

All silicate minerals are built from the Silica Tetrahedron: (SiO₄)⁴⁻. This is one Silicon (Si⁴⁺) atom bonded to four Oxygen (O²⁻) atoms. This unit has a -4 charge.

The classification of silicates is based on how these tetrahedra link together by sharing oxygen atoms—a process called polymerization.

Non-Silicate Structures (CCP and HCP)

Non-silicate minerals have different building blocks. Many simple structures (like in native metals or simple sulphides/halides) can be described by the way atoms (as spheres) are packed together.

• Hexagonal Close Packing (HCP): The most efficient way to pack spheres. Layers are stacked in an "ABABAB..." pattern. The third layer sits directly above the first.
• Cubic Close Packing (CCP): Also maximally efficient. Layers are stacked in an "ABCABC..." pattern. The fourth layer sits above the first. This structure has a face-centered cubic (FCC) unit cell.
• Application: In Halite (NaCl), the large Cl⁻ ions form a CCP framework, and the small Na⁺ ions fit into the spaces (octahedral voids) between them.