Minerals, Rocks, and the Rock Cycle

Introduction

• Earth functions as a complex system composed of many interacting subsystems.
• Fundamental building blocks of these subsystems are Earth materials, chiefly minerals and rocks.

Minerals – Definition & Core Characteristics

• Minerals are homogeneous, naturally-occurring, inorganic solids characterized by:
  • A definite chemical composition.
  • An ordered internal crystal structure.
• Key takeaway: if any of these conditions fail (e.g., synthetic origin, organic derivation, no crystalline order) the substance is not a mineral.

Mineral Formation Processes

• Cooling of Molten Material
  • Magma (inside crust) or lava (surface) cools and crystallizes → minerals.
  • Cooling rate controls crystal size:
  • “Intrusive” cooling (slow, deep) → large crystals.
  • “Extrusive” cooling (fast, surface) → small or glassy crystals.
  • Examples visualized: Granite (slow; visible crystals) vs. Rhyolite (fast; microscopic crystals).
• Precipitation / Evaporation from Solutions
  • Dissolved ions left behind as water evaporates precipitate into mineral crystals.

Physical & Chemical Properties for Mineral Identification

• Color
  • Considered the least reliable diagnostic property due to impurities.
• Streak
  • Color of mineral’s powdered form, tested on unglazed porcelain.
• Luster
  • Describes light reflection: Metallic vs. Non-metallic.
• Hardness
  • Resistance to scratching measured via Mohs scale (1–10).
  • Comparative scratch logic: “If mineral A scratches B, A is harder.”
  • Mohs Scale reference (common comparators included):
  • $1$ Talc – “greasy,” scratched by fingernail.
  • $2$ Gypsum – scratched by fingernail ($\approx 2.2$).
  • $3$ Calcite – scratched by copper penny ($\approx 3.2$).
  • $4$ Fluorite – easily cut by knife.
  • $5$ Apatite – knife/glass barely scratches.
  • $6$ Feldspar – scratches window glass ($\approx 5.5$).
  • $7$ Quartz – scratches steel; streak plate ($\approx 7.0$).
  • $8$ Topaz – cuts glass with ease.
  • $9$ Corundum – used as abrasive.
  • $10$ Diamond – glass-cutter.
• Cleavage vs. Fracture
  • Cleavage: tendency to break along flat, weak planes.
  • Fracture: breaks irregularly (conchoidal, uneven, etc.).
• Crystal Form
  • Habit/shape can aid identification (e.g., cubic halite, hexagonal quartz).
• Specific Gravity (Density)
  • Ratio of mineral density to water: e.g., specific gravity $=5$ means 5× denser than water ($1\,\text{g/cm}^3$).
• Special Tests & Properties
  • Acid reaction: fizz of calcite with $\text{HCl}$.
  • Magnetism: magnetite.
  • Taste: halite (salty).
  • Smell: sulfurous odor in sulfide minerals.
  • Malleability: gold can be hammered.
  • Elasticity: mica sheets bend and spring back.

Chemical Classification of Minerals

• Silicates
  • Built on silicon–oxygen tetrahedron $(\text{SiO}_4^{4-})$.
  • Major rock-formers: olivine $((\text{Mg,Fe})<em>2\text{SiO}</em>4)$, quartz $(\text{SiO}_2)$.
• Oxides
  • Metal cations + oxygen anions.
  • Magnetite $(\text{Fe}<em>3\text{O}</em>4)$, hematite $(\text{Fe}<em>2\text{O}</em>3)$.
• Sulfides
  • Metal cations + sulfide $(\text{S}^{2-})$.
  • Galena $(\text{PbS})$, pyrite $(\text{FeS}_2)$.
• Sulfates
  • Metal cations + sulfate group $(\text{SO}_4^{2-})$.
  • Gypsum $(\text{CaSO}<em>4 \cdot 2\text{H}</em>2\text{O})$ precipitates near surface.
• Halides
  • Halogen anion (Cl⁻, F⁻) + metal.
  • Halite $(\text{NaCl})$, fluorite $(\text{CaF}_2)$.
• Carbonates
  • Contain $\text{CO}_3^{2-}$ ion.
  • Calcite $(\text{CaCO}<em>3)$, dolomite $(\text{CaMg(CO}</em>3)_2)$.
• Native Elements
  • Single element composition: copper $(\text{Cu})$, gold $(\text{Au})$, silver $(\text{Ag})$.

Rocks – General Overview

• Rocks = solid aggregates of one or more minerals.
• Three broad categories based on genesis:
  1. Igneous.
  2. Sedimentary.
  3. Metamorphic.

The Rock Cycle – Core Concept

• Any rock type can transform into any other given suitable conditions (melting, cooling, erosion, deposition, heat/pressure).
• Cyclic pathways involve:
  • Magma ⇌ Igneous rock.
  • Weathering → Sediment → Sedimentary rock.
  • Burial/heat/pressure → Metamorphic rock.
  • Re-melting or uplift renews cycle.

Igneous Rocks

• Form by solidification/crystallization of molten rock at high T (≥ $1{,}250\,^{\circ}\text{C}$).

Intrusive (Plutonic) Igneous Rocks

• Crystallize below surface; slow cooling → coarse (phaneritic) texture.
• Examples: granite, diorite, gabbro, pegmatite, peridotite.
• Special cases described:
  • Kimberlite may host diamonds.
  • Pegmatite hosts large gemstones and tungsten minerals.
  • Olivine-rich dunite, pyroxenite common in mantle-derived intrusions.

Extrusive (Volcanic) Igneous Rocks

• Erupt at/near surface; rapid cooling → fine-grained (aphanitic) or glassy.
• Possible textures: pumice (vesicular), obsidian (glassy), tuff (pyroclastic).
• Examples: basalt, andesite, dacite, rhyolite, obsidian, pumice, tuff.
• Cooling-rate diagram:
  • Fast (surface lava) → obsidian/rhyolite.
  • Slow (underground magma) → granite.

Sedimentary Rocks

• Formed by deposition + lithification of material at Earth’s surface.
• Agents transporting sediment: water, wind, ice, biological activity.

Formation Stages

1. Weathering/Erosion – rock breakdown; particles mobilized.
2. Transportation – movement via streams, wind, glaciers.
3. Deposition – settling when energy decreases.
4. Compaction – overburden pressure expels water/air, presses grains.
5. Cementation – precipitation of cements (calcite $\text{CaCO}<em>3$, silica $\text{SiO}</em>2$, iron oxides) binding grains.

Key Traits

• Commonly stratified (layered) with variations in color, grain size, cement type.

Examples

• Conglomerate, sandstone, shale, limestone, chert, rock salt.

Metamorphic Rocks

• Originate when existing rocks undergo metamorphism—alteration by heat, pressure, stress, and fluids, typically deep below surface.
• Changes include mineralogy, texture, and chemistry.

Textural Types

• Foliated (layered/banded): slate → phyllite → schist → gneiss (increasing grade).
• Non-foliated: marble (from limestone/dolostone), quartzite (from sandstone), hornfels, serpentinite, novaculite.

Summary: Transformation Pathways

• Igneous rock → weathering → sediments → sedimentary rock → burial/heat → metamorphic rock → melting → magma → igneous rock.
• Arrows also exist for direct melt of metamorphic → igneous, or metamorphism of sedimentary → metamorphic, etc.

Performance Task – Mineral Conservation Magazine Cover

• Cross-subject (Earth & Life Science × Personal Development).
• Objective: advocate mineral resource conservation; link to personal birthstones/gemstones.
• Requirements:
  • Create a visually appealing magazine cover (any editing software allowed).
  • Save as PDF: SURNAME, FIRST NAMESECTIONLEARNING TASK #.
• Assessment Rubric
  • Visual Message (15–0 pts): clarity, accuracy, relevance.
  • Visual Appeal (10–0 pts): design balance & aesthetics.
  • Creativity: uniqueness, originality.
  • Output Quality (5–0 pts): conceptual depth and design execution.
• Sample provided: Moonstone-themed cover emphasizing compassion, feminine balance, natural cycles.

Ethical & Practical Implications

• Conservation messaging underscores finite nature of mineral resources, responsible consumption, and connection between human well-being (birthstones) and geologic materials.
• Recognizing properties and classifications aids mining, environmental impact assessments, and sustainable material usage.