Midterm Review_S25

Midterm Review: Minerals, Rocks, and Geologic Structures

The Basics

• Mineral

  • Naturally occurring, homogeneous, inorganic solid substances.

  • Has a definite chemical composition and a characteristic crystalline structure.

  • Components of rocks; rocks may be unconsolidated (e.g., sand, clay, mud) or consolidated (e.g., granite, limestone, coal).

  • Three main types of rocks: igneous, sedimentary, metamorphic.

Minerals

• Classification Traits:

  • Luster:

    • The way light is reflected from a mineral.

    • Types: Nonmetallic, Metallic.

  • Streak:

    • The color of the mineral when powdered (tested on a streak plate). Harder minerals may not produce a streak.

  • Hardness:

    • Measured using Mohs scale (1-10).

    • Example: A mineral can be between two hardness values based on its ability to scratch or be scratched by objects like glass, a penny, or a fingernail.

Characteristics of Minerals

• Cleavage vs. Fracture:

  • Cleavage:

    • Planes along which a mineral breaks, generally parallel to a crystal face (e.g., biotite has cleavage in one direction).

  • Fracture:

    • Any break not along a cleavage plane; most surfaces are irregular (e.g., quartz can exhibit conchoidal fracture).

  • Other Features:

    • Crystal Form: The shape a mineral takes when growing in open space.

    • Taste: Halite tastes salty.

    • Smell: Sulfur smells like rotten eggs.

    • Reactivity: Calcite reacts with HCl acid.

    • Magnetism: Examples include magnetite.

    • Specific Gravity: The relative weight of a mineral (e.g., galena is heavy).

Igneous Rocks

• Key Concepts: Formation environments, cooling histories.

• Classification Criteria:

  • Composition: FELSIC (light), INTERMEDIATE (50/50 light/dark), MAFIC (dark), ULTRAMAFIC (black/green).

  • Note: Obsidian can appear dark but is felsic.

• Cooling Textures:

  • Intrusive: Formed within Earth, displaying PHANERITIC texture (large crystals).

  • Extrusive: Formed at the surface, displaying APHANITIC texture (small crystals).

  • Vesicular: Contains vesicles (holes).

  • Pyroclastic: Composed of volcanic rock fragments.

  • Glassy: Rapid cooling, no crystal formation.

• Porphyritic Texture: Combo of large intrusive crystals and a fine-grained matrix.

Sedimentary Rocks

• Formation Processes: Weathering, transportation, deposition, diagenesis.

• Classifications:

  • Clastic: Visible grains; classified by grain size:

    • Gravel (conglomerate/breccia)

    • Sand (sandstone)

    • Silt (siltstone)

    • Clay (shale)

  • Chemical: Inorganic rocks formed from chemical precipitation (e.g., limestone, gypsum).

  • Biochemical: Form from the remains of organisms (e.g., coal, chalk).

  • Sorting/Rounding: Opposite states (angular/poorly sorted = immature, rounded/sorted = mature).

Metamorphic Rocks

• Formation: Parent rock (Protolith) undergoes increased pressure, temperature, fluid volume.

• Metamorphic Agents: Heat, pressure, hydrothermal fluids.

• Textures:

  • Foliated: Minerals align under pressure (e.g., slate, phyllite, schist, gneiss).

  • Non-Foliated: No directional pressures (e.g., hornfels, quartzite).

Geologic Structures

• Faults: Breaks in rock units with movement. Types include:

  • Normal Fault: Hanging wall moves down; extensional environment.

  • Reverse/Thrust Fault: Hanging wall moves up; compressional environment.

  • Strike Slip Fault: Lateral motion only, without vertical movement.

  • Oblique Slip Fault: Both vertical and horizontal motions.

• Folds: Bends in rock units from applied forces over time. Types include:

  • Syncline: U-shaped, youngest rocks in center.

  • Anticline: Ripple-shaped, oldest rocks in center.

  • Basin/Dome: Respective 360° formations with youngest/oldest rocks in center.

Example Questions

• Identification: Questions might require determining cleavage, rock composition, texture, metamorphic grade.

• Multiple Choice: Covering igneous textures, erosional and metamorphic agents.

• Lab Skills: Recognize and identify minerals and rock samples, use tools for classification, understand fault and fold types and associated movements.