Unit 1: Fundamentals of Mineralogy

Minerals: Definition and Classification

Definition of a Mineral

A mineral is a naturally occurring, inorganic, solid substance that has an orderly internal crystalline structure and a definite (but not fixed) chemical composition.

• Naturally Occurring: Not man-made (e.g., synthetic diamonds are not minerals).
• Inorganic: Not formed by biological processes (e.g., coal is not a mineral). Shells are a grey area, but the calcite they are made of is a mineral.
• Solid: Not a liquid (water) or gas. Ice is a mineral; water is not.
• Orderly Crystalline Structure: The atoms are arranged in a repeating, ordered 3D pattern. This is why minerals form crystals. Volcanic glass (obsidian) is not a mineral because its atoms are disordered (amorphous).
• Definite Chemical Composition: The chemical formula is known (e.g., Quartz is SiO₂; Halite is NaCl). The composition can vary within a limited range (e.g., Olivine (Mg,Fe)₂SiO₄).

Classification of Minerals

Minerals are classified based on their dominant anion or anionic group (the negatively charged part of their chemical formula).

1. Silicates: The largest group. Anion is the (SiO₄)^4- tetrahedron. (e.g., Quartz, Feldspar, Mica).
2. Carbonates: Anion is (CO₃)^2-. (e.g., Calcite, Dolomite).
3. Oxides: Anion is O^2-. (e.g., Hematite Fe₂O₃, Magnetite Fe₃O₄).
4. Sulfides: Anion is S^2-. (e.g., Pyrite FeS₂, Galena PbS).
5. Sulfates: Anion is (SO₄)^2-. (e.g., Gypsum CaSO₄·2H₂O).
6. Halides: Anion is a halogen (Cl^-, F^-, Br^-). (e.g., Halite NaCl, Fluorite CaF₂).
7. Native Elements: Composed of a single element. (e.g., Gold Au, Copper Cu, Diamond C).

Physical and Chemical Properties of Minerals

Physical Properties

These are the diagnostic properties used to identify minerals in hand specimens.

• Lustre: How light reflects from the mineral's surface (e.g., Metallic, Vitreous (glassy), Pearly, Dull).
• Colour: The observed colour. Can be unreliable.
• Streak: The colour of the mineral's powder (tested on a streak plate). More reliable than colour.
• Hardness: Resistance to scratching (see Mohs Scale).
• Cleavage: Tendency to break along flat planes of atomic weakness (e.g., Mica has 1 perfect cleavage; Calcite has 3).
• Fracture: How a mineral breaks when not on a cleavage plane (e.g., Conchoidal - curved, like broken glass).
• Crystal Form/Habit: The natural shape the crystal grows in (e.g., Prismatic, Tabular, Acicular).
• Specific Gravity: The density ("heaviness") of the mineral.

Chemical Properties

These relate to the mineral's composition and bonding.

• Composition: The elements that make up the mineral.
• Solubility: Some minerals dissolve in water (Halite) or acid (Calcite).
• Bonding: The type of atomic bonds (ionic, covalent, metallic) controls properties like hardness and cleavage.

Mohs Scale of Hardness

A relative scale from 1 (softest) to 10 (hardest) that measures scratch resistance. A mineral can scratch any mineral softer than itself and will be scratched by any mineral harder than itself.

Composition of Common Rock-Forming Minerals

These are the minerals that make up the bulk of Earth's crust.

• Quartz: SiO₂ (Pure silica)
• Potassium Feldspar (Orthoclase): KAlSi₃O₈
• Plagioclase Feldspar: (Na,Ca)(Al,Si)AlSi₂O₈ (A solid solution series from Na-rich Albite to Ca-rich Anorthite)
• Muscovite Mica (White Mica): KAl₂(AlSi₃O₁₀)(OH)₂
• Biotite Mica (Black Mica): K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂
• Amphibole (e.g., Hornblende): Complex (Ca,Na)_2-3(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂
• Pyroxene (e.g., Augite): (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆
• Olivine: (Mg,Fe)₂SiO₄
• Calcite: CaCO₃

Silicate and Non-Silicate Structures

Silicate Structures

This is the most important structural classification, based on how the (SiO₄)^4- tetrahedra (one silicon atom bonded to four oxygen atoms) are linked together by sharing oxygen atoms. This linkage controls the mineral's cleavage and properties.

Non-Silicate Structures

These have simpler structures, often based on packing of atoms/ions. Examples include:

• Oxides: Hematite, Magnetite (often metallic or sub-metallic).
• Carbonates: Calcite (rhombohedral cleavage, fizzes in acid).
• Sulfides: Pyrite (cubic crystals, metallic lustre), Galena (cubic cleavage, metallic).

CCP and HCP Structures

These describe the most efficient ways to pack spherical atoms, common in metals (native elements) and simple ionic compounds (like Halides and Sulfides).

Hexagonal Close Packing (HCP)

• Stacking Sequence: A-B-A-B...
• Description: The third layer is placed directly above the first layer (A).
• Result: Produces hexagonal symmetry.
• Example: Zinc, Magnesium.

Cubic Close Packing (CCP)

• Stacking Sequence: A-B-C-A-B-C...
• Description: The third layer (C) is placed in a new position, different from both A and B. The fourth layer is then placed above the first (A).
• Result: This is also called Face-Centered Cubic (FCC) and produces cubic symmetry.
• Example: Copper, Gold, Silver, Galena (PbS), Halite (NaCl).